Table of contents

We consider the polarization of 21 cm line radiation from the epoch of reionization due to both intrinsically polarized emission and secondary mechanisms. We argue that Thomson scattering of the 21 cm quadrupole by the reionized universe is likely to produce the largest effect. The 21 cm quadrupole is sourced by baryonic density fluctuations and the fluctuations in the ionization fraction due to discrete H II regions. Since Thomson scattering produces only E-type polarization and is achromatic, its unique fingerprint could in principle be separated from foregrounds associated with polarized synchrotron emission, which should not be correlated with the cosmic signal. We estimate that Poisson fluctuations of H II regions at the end of reionization (z_R ~ 6-20) produce a brightness temperature-polarization cross-correlation signal of ~0.1-0.3 mK on angular scales of tens of arcminutes. This cross-correlation signal is within the instrument sensitivities of the future Square Kilometre Array (SKA) and close to the sensitivities of the forthcoming Mileura Widefield Array (MWA) and Low Frequency Array (LOFAR).

We compare the peculiar velocity field within 65 h^-1 Mpc predicted from 2MASS photometry and public redshift data to three independent peculiar velocity surveys based on Type Ia supernovae, surface brightness fluctuations in elliptical galaxies, and Tully-Fisher distances to spiral galaxies. The three peculiar velocity samples are each in good agreement with the predicted velocities and produce consistent results for β_K = Ω/b_K. Taken together, the best-fit β_K = 0.49 ± 0.04. We explore the effects of morphology on the determination of β by splitting the 2MASS sample into E+S0 and S+Irr density fields and find that both samples are equally good tracers of the underlying dark matter distribution, but that early types are more clustered, by a relative factor b_E/b_S ~ 1.6. The density fluctuations of 2MASS galaxies in 8 h^-1 Mpc spheres in the local volume is found to be σ_8,K = 0.9. From this result and our value of β_K, we find σ₈(Ω_m/0.3)^0.6 = 0.91 ± 0.12. This is in excellent agreement with results from the IRAS redshift surveys, as well as other cosmological probes. Combining the 2MASS and IRAS peculiar velocity results yields σ₈(Ω_m/0.3)^0.6 = 0.85 ± 0.05.

Near-future Sunyaev-Zel'dovich (SZ) surveys such as ACT, SPT, APEX, and Planck will soon find thousands of galaxy clusters. Multifrequency arcminute-resolution SZ observations can, in principle, determine each cluster's gas temperature (T_e), bulk velocity (v_pec), and optical depth (τ). However, the frequency bands and detector sensitivity employed by upcoming surveys will generally not be sufficient to disentangle the degeneracy between these three cluster parameters, even in the absence of SZ signal contamination from point sources and imperfect primary microwave background subtraction. Assuming contaminants can be removed, we find that near-future SZ surveys will be able to constrain well two cluster gas parameters that are linear combinations of τT_e, τv_z, and τT. Because the SZ intensity shift is nearly a linear function of τT_e, τv_z, and τT, a correspondence exists between the two effective gas parameters that SZ surveys can constrain and simple line-of-sight integrals through the three-dimensional cluster. We illustrate the parameter constraints and correspondence to line-of-sight integrals using three-dimensional N-body + hydro cluster simulations and a Markov-chain Monte Carlo method. We show that adding an independent temperature measurement to upcoming SZ data breaks the parameter degeneracy and that the cluster effective velocity thus constrained is approximately the optical-depth-weighted velocity integrated along the cluster line of sight. A temperature prior with an error as large as 2 keV still gives bulk velocity errors of 100 km s^-1 or less, even for a more typical cluster with an electron temperature of 3 keV, for ACT-like SZ observations in the absence of signal contamination. The Markov-chain constraints on v_pec and τ that we obtain are more encouraging and most likely more accurate than those obtained from Fisher matrices.

When the source in a four-image gravitational lens system lies sufficiently close to a "fold" caustic, two of the lensed images lie very close together. If the lens potential is smooth on the scale of the separation between the two close images, the difference between their fluxes should approximately vanish, R_fold ≡ (F₊ - F_-)/(F₊ + F_-) ≈ 0. (The subscript indicates the image parity.) Violations of this "fold relation" in observed lenses are thought to indicate the presence of structure on scales smaller than the separation between the close images. We present a detailed study of the fold relation in realistic smooth lenses, finding it to be more subtle and rich than was previously realized. The degree to which R_fold can differ from zero for smooth lenses depends not only on the distance of the source from the caustic, but also on its location along the caustic, and then on the angular structure of the lens potential (ellipticity, multipole modes, and external shear). Since the source position is unobservable, it is impossible to say from R_fold alone whether the flux ratios in an observed lens are anomalous or not. Instead, we must consider the full distribution of R_fold values that can be obtained from smooth lens potentials that reproduce the separation d₁ between the two close images and the distance d₂ to the next nearest image. (By reducing the image configuration to these two numbers, we limit our model dependence and obtain a generic analysis.) We show that the generic features of this distribution can be understood, which means that the fold relation provides a robust probe of small-scale structure in lens galaxies. We then compute the full distribution using Monte Carlo simulations of realistic smooth lenses. Comparing these predictions with the data, we find that five of the 12 known lenses with fold configurations have flux ratio anomalies: B0712+472, SDSS 0924+0219, PG 1115+080, B1555+375, and B1933+503. Combining this with our previous analysis revealing anomalies in three of the four known lenses with cusp configurations, we conclude that at least half (8/16) of all four-image lenses that admit generic, local analyses exhibit flux ratio anomalies. The fold and cusp relations do not reveal the nature of the implied small-scale structure, but do provide the formal foundation for substructure studies, and also indicate which lenses deserve further study. Although our focus is on close pairs of images, we show that the fold relation can be used—with great care—to analyze all image pairs in all 22 known four-image lenses and reveal lenses with some sort of interesting structure.

Gravitational lensing effects arise from the light-ray deflection by all of the mass distribution along the line of sight. It is then expected that weak-lensing cluster surveys can provide us with true mass-selected cluster samples. With numerical simulations, we analyze the correspondence between peaks in the lensing convergence κ-map and dark matter halos. In particular, we emphasize the difference between the peak κ-value expected from a dark matter halo modeled as an isolated and spherical one, which exhibits a one-to-one correspondence with the halo mass at a given redshift, and that of the associated κ-peak from simulations. For halos with the same expected κ, their corresponding peak signals in the κ-map present a wide dispersion. At an angular smoothing scale of θ_G = 1', our study shows that for relatively large clusters, the complex mass distribution of individual clusters is the main reason for the dispersion. The projection effect of uncorrelated structures does not play significant roles. The triaxiality of dark matter halos accounts for a large part of the dispersion, especially for the tail on the high-κ side. Thus, lensing-selected clusters are not really mass-selected. To better predict κ-selected cluster abundance for a cosmological model, one has to take into account the triaxial mass distribution of dark matter halos. On the other hand, for a significant number of clusters, their mass distribution is even more complex than that described by the triaxial model. Our analyses find that large substructures affect the identification of lensing clusters considerably. They could show up as separate peaks in the κ-map and cause a misassociation of the whole cluster with a peak resulting only from a large substructure. The lower end dispersion of κ is attributed mostly to this substructure effect. For θ_G = 2', the projection effect can be significant and contributes to the dispersion at both high- and low-κ ends.

We have measured the weak-lensing signal as a function of rest-frame B-, V-, and R-band luminosity for a sample of "isolated" galaxies. These results are based on four-band photometry from the Red-Sequence Cluster Survey, enabling us to determine photometric redshifts for a large number of galaxies. We select a secure sample of lenses with photometric redshifts 0.2 < z < 0.4 and study the relation between the virial mass and baryonic contents. In addition, we discuss the implications of the derived photometric redshift distribution for published cosmic shear studies. The virial masses are derived from a fit to the observed lensing signal. For a galaxy with a fiducial luminosity of 10¹⁰h^-2L_B,☉, we obtain a mass M_vir = 9.9 × 10¹¹M_☉. The virial mass as a function of luminosity is consistent with a power law ∝L^1.5, with similar slopes for the three filters considered here. These findings are in excellent agreement with results from the Sloan Digital Sky Survey and semianalytic models of galaxy formation. We measure the fraction of mass in stars and the baryon fraction in galaxies by comparing the virial mass-to-light ratio to predicted stellar mass-to-light ratios. We find that star formation is inefficient in converting baryons into stars, with late-type galaxies converting ~33% and early-type galaxies converting only ~14% of baryons into stars. Our results imply that the progenitors of early-type galaxies must have low stellar mass fractions, suggestive of a high formation redshift.

We investigate possible signatures of feedback from galactic superwinds (GSWs) on the metallicity of the Lyα forest, using a set of high-resolution hydrodynamic simulations of a ΛCDM model. Simulations produce metals self-consistently, based on one single parameter, the metal yield, which in turn is constrained by metallicity in the intracluster gas. We follow metals as a separate density species. The metallicity of Lyα clouds having column density of N ~ 10^14.5-10^15.5 cm^-2 at z = 2-4 is correctly predicted by simulations, both with and without GSWs, implying an in situ origin for these metals. However, a unique signature and sensitive test of GSWs are provided by lower column density clouds of 10¹²-10¹⁴ cm^-2. We find that the number density of Lyα lines with metallicity Z ≥ 10^-3Z_☉ and neutral hydrogen column density N < 10^13.5 cm^-2 provides a first quantitative measure of the strength of GSWs, because metals in these systems are a contaminant. We predict that the number of such lines per unit redshift at z ~ 3 should be about 0.1 in the absence of GSWs. With the observed GSW strength, we expect to see 20-50 such lines per unit redshift. This is an observational challenge. Furthermore, we find that the difference between simulations with and without GSWs becomes much larger with regard to a subset of such clouds with high Doppler widths, since the contaminated systems are considerably hotter than the normal IGM. We also present preliminary results on C IV and O VI lines as a function of GSW strength. The filling factor of metal-rich regions is a strong function of GSWs. With and without GSWs the volume filling factor is 6.0%, 4.2%, and 1.9% and 1.0%, 0.28%, and 0.08%, respectively, for regions with metallicity greater than 10^-3, 10^-2, and 10^-1Z_☉. Finally, in clouds of N ~ 10^14.5 cm^-2, we predict that the ratio of secondary (e.g., N) to primary metals (e.g., O, C) is expected to be smaller by a factor of 10 than in large galaxies, which better retain metals; this factor increases to ≥50 for N ≤ 10^13.5 cm^-2.

We introduce a very deep, _lim ~ 27, multicolor imaging survey of very faint star-forming galaxies at z ~ 4, 3, 2.2, and 1.7. This survey, carried out on the Keck I telescope, uses the very same U_nGI filter system that is employed by the Steidel team to select galaxies at these redshifts and thus allows us to construct identically selected but much fainter samples. However, our survey reaches ~1.5 mag deeper than the work of Steidel and his group, letting us probe substantially below the characteristic luminosity L* and thus study the properties and redshift evolution of the faint component of the high-z galaxy population. The survey covers 169 arcmin² in three spatially independent patches on the sky and—to ≤ 27—contains 427 GI-selected z ~ 4 Lyman break galaxies, 1481 U_nG-selected z ~ 3 Lyman break galaxies, 2417 U_nG-selected z ~ 2.2 star-forming galaxies, and 2043 U_nG-selected z ~ 1.7 star-forming galaxies. In this paper, the first in a series, we introduce the survey, describe our observing and data reduction strategies, and outline the selection of our z ~ 4, 3, 2.2, and 1.7 samples.

We use the combination of the 2 Ms Chandra X-ray image, new J and H band images, and the Spitzer IRAC and MIPS images of the Chandra Deep Field-North to obtain high spectroscopic and photometric redshift completeness of high and intermediate X-ray luminosity sources in the redshift interval z = 2-3. We measure the number densities of z = 2-3 active galactic nuclei (AGNs) and broad-line AGNs in the rest-frame 2-8 keV luminosity intervals 10⁴⁴-10⁴⁵ and 10⁴³-10⁴⁴ ergs s^-1 and compare these with previous lower redshift results. We confirm a decline in the number densities of intermediate-luminosity sources at z > 1. We also measure the number density of z = 2-3 AGNs in the luminosity interval 10⁴³-10^44.5 ergs s^-1 and compare it with previous low- and high-redshift results. Again, we find a decline in the number densities at z > 1. In both cases, we can rule out the hypothesis that the number densities remain flat to z = 2-3 at above the 5 σ level.

We present the results from a damped Lyα survey of the Sloan Digital Sky Survey, Data Release 3. We have discovered over 500 new damped Lyα systems at z > 2.2, and the complete statistical sample for z > 1.6 has more than 600 damped Lyα galaxies. We measure the H I column density distribution f_{H i}(N, X) and its zeroth and first moments (the incidence ℓ_DLA and gas mass density Ω of damped Lyα systems, respectively) as a function of redshift. The key results include: (1) the full SDSS DR3 f_{H i}(N, X) distribution (z ~ 3.06) is well fit by a Γ function (or double power law) with "break" column density N_γ = 10^21.5±0.1 cm^-2 and "faint-end" slope α = -1.8 ± 0.1; (2) the shape of the f_{H i}(N, X) distributions in a series of redshift bins does not show evolution; (3) the incidence and gas mass density of damped systems decrease by 35% ± 9% and 50% ± 10% during ≈1 Gyr between the redshift intervals z = [3.0, 3.5] and z = [2.2, 2.5]; and (4) the incidence and gas mass density of damped Lyα systems in the lowest SDSS redshift bin (z = 2.2) are consistent with the current values. We investigate a number of systematic errors in damped Lyα analysis and identify only one important effect: we measure 40% ± 20% higher Ω values toward a subset of brighter quasars than toward a faint subset. This effect is contrary to the bias associated with dust obscuration and suggests that gravitational lensing may be important. Comparing the results against several models of galaxy formation in ΛCDM, we find that all of the models significantly underpredict ℓ_DLA at z = 3, and only SPH models with significant feedback can reproduce Ω at high redshift. Based on our results for the damped Lyα systems, we argue that the Lyman limit systems contribute ≈33% of the universe's H I atoms at all redshifts z = 2-5. Furthermore, we infer that the f_{H i}(N, X) distribution for N < 10²⁰ cm^-2 has an inflection with slope d log f/d log N > -1. We advocate a new mass density definition, the mass density of predominantly neutral gas Ω, to be contrasted with the mass density of gas associated with H I atoms. We contend the damped Lyα systems contribute >80% of Ω at all redshifts and therefore are the main reservoirs for star formation.

We consider the spatial clustering of massive black hole (MBH) mergers and discuss possible ways to use gravitational wave (GW) observations in the LISA and DECIGO/BBO range for obtaining cosmological and cosmogonical information. Constraints on large-scale structure (LSS) and merger histories may be possible through the detection of an alignment of the GW polarization direction with principal axes of the LSS. Constraints on the merger physics and the reionization epoch may be obtained by GW measurements of MBH correlation lengths, in the case where the MBH angular momentum loss occurs through gas drag. Such measurements would provide information about the LSS and the reionization epoch, as well as about the astrophysics of MBH mergers, in addition to and independent of that obtained from electromagnetic signals.

We consider the formation and evolution of vortices in a hydrodynamic shearing-sheet model. The evolution is done numerically using a version of the ZEUS code. Consistent with earlier results, an injected vorticity field evolves into a set of long-lived vortices, each of which has a radial extent comparable to the local scale height. But we also find that the resulting velocity field has a positive shear stress, ⟨Σδv_rδv_ϕ⟩. This effect appears only at high resolution. The transport, which decays with time as t^-1/2, arises primarily because the vortices drive compressive motions. This result suggests a possible mechanism for angular momentum transport in low-ionization disks, with two important caveats: a mechanism must be found to inject vorticity into the disk, and the vortices must not decay rapidly due to three-dimensional instabilities.

In this work we show that Bardeen-Petterson accretion disks can exhibit unique, detectable features in relativistically broadened emission line profiles. Some of the unique characteristics include inverted line profiles with sharper red horns and softer blue horns and even profiles with more than two horns from a single rest-frame line. We demonstrate these points by constructing a series of synthetic line profiles using simple two-component disk models. We find that the resulting profiles are very sensitive to the two key parameters one would like to constrain, namely, the Bardeen-Petterson transition radius r_BP and the relative tilt β between the two disk components over a range of likely values [10 ≤ r_BP/(GM/c²) ≤ 40, 15° ≤ β ≤ 45°]. We use our findings to show that some of the "extra" line features observed in the spectrum of the Seyfert 1 galaxy MCG -6-30-15 may be attributable to a Bardeen-Petterson disk structure. Similarly, we apply our findings to two likely Bardeen-Petterson candidate Galactic black holes, GRO J1655-40 and XTE J1550-564. We provide synthetic line profiles of these systems using observationally constrained sets of parameters. Although we do not formally fit the data for any of these systems, we confirm that our synthetic spectra are consistent with current observations.

We investigate the iron Kα fluorescent line produced by hard X-ray photons from the magnetic reconnection-heated corona. The hot corona with temperature of ~10⁹ K can irradiate the underlying disk with a continuum X-ray spectrum produced via thermal Comptonization. Then the iron atoms in the disk photoelectrically absorb X-ray photons and radiate Kα line photons. Therefore, the activity of the corona is responsible for the iron line emission from the underlying disk. In previous studies, oversimplified X-ray photon sources were often assumed above the disk in order to compute the iron line profile or power-law line emissivity profiles were assumed with an index as a free parameter. We adopt the more realistic corona model constructed by Liu et al. in which the corona is heated by magnetic energy released through the reconnection of magnetic flux loops and which has no free parameter. Then the accretion energy is dominantly dissipated in the corona, in which X-ray photons are efficiently produced and irradiate the underlying disk. We find that the local emissivity of the iron line on the disk is approximated as F_Kα(r) ∝ r^-5. The iron line profiles derived from this model give excellent fits to the observational data of MCG -6-30-15 with the profiles derived theoretically for i ~ 30° for the 4-7 keV energy band. Possible origins of line variability are briefly discussed.

The HST snapshot imaging survey of 110 BL Lac objects (Urry et al.) has clearly shown that the host galaxies are massive and luminous ellipticals. The dispersion of the absolute magnitudes is sufficiently small that the measurement of the galaxy brightness becomes a valuable way of estimating their distance. This is illustrated by constructing a Hubble diagram of the 64 resolved objects with known redshift. By means of this relationship, we estimate the redshift of five resolved BL Lac objects of the survey that have no spectroscopic z. The adopted method also allows us to evaluate lower limits to the redshift for 13 objects of still unknown z using the lower limit on the host galaxy magnitude. This technique can be applied to any BL Lac object for which an estimate or a lower limit of the host galaxy magnitude is available. Finally, we show that the distribution of the nuclear luminosity of all the BL Lac objects of the survey indicates that the objects for which both the redshift and the host galaxy are undetected are among the most luminous, and possibly the most highly beamed.

We have characterized the energy-dependent X-ray variability properties of the Seyfert 1 galaxy NGC 3783 using archival XMM-Newton and Rossi X-Ray Timing Explorer data. The high-frequency fluctuation power spectral density function (PSD) slope is consistent with flattening toward higher energies. Light-curve cross-correlation functions yield no significant lags, but peak coefficients generally decrease as energy separation of the bands increases on both short and long timescales. We have measured the coherence between various X-ray bands over the temporal frequency range of 6 × 10^-8-1 × 10^-4 Hz; this range includes the temporal frequency of the low-frequency PSD break tentatively detected by Markowitz et al. and includes the lowest temporal frequency over which coherence has been measured in any active galactic nucleus to date. Coherence is generally near unity at these temporal frequencies, although it decreases slightly as energy separation of the bands increases. Temporal frequency-dependent phase lags are detected on short timescales; phase lags are consistent with increasing as energy separation increases or as temporal frequency decreases. All of these results are similar to those obtained previously for several Seyfert galaxies and stellar mass black hole systems. Qualitatively, these results are consistent with the variability models of Kotov et al. and Lyubarskii, wherein the X-ray variability is due to inwardly propagating variations in the local mass accretion rate.

We present results based on XMM-Newton observations of the nearby spiral galaxy M51 (NGC 5194 and NGC 5195). We confirm the presence of the seven known ultraluminous X-ray sources (ULXs) with luminosities exceeding the Eddington luminosity for a 10 M_☉ black hole, a low-luminosity active galactic nucleus (LLAGN) with 2-10 keV luminosity of 1.6 × 10³⁹ ergs s^-1, and soft thermal extended emission from NGC 5194 detected with Chandra. In addition, we also detected a new ULX with luminosity of ~10³⁹ ergs s^-1. We have studied the spectral and temporal properties of the LLAGN and eight ULXs in NGC 5194 and an ULX in NGC 5195. Two ULXs in NGC 5194 show evidence for short-term variability, and all but two ULXs vary on long timescales (over a baseline of ~2.5 yr), providing strong evidence that these are accreting sources. One ULX in NGC 5194, source 69, shows possible periodic behavior in its X-ray flux. We derive a period of 5925 ± 200 s at a confidence level of 95% on the basis of three cycles. This period is lower than the period of 7620 ± 500 s derived from a Chandra observation in 2000. The higher effective area of XMM-Newton enables us to identify multiple components in the spectra of ULXs. Most ULXs require at least two components, a power law and a soft X-ray excess component that is modeled by an optically thin plasma or a multicolor disk blackbody (MCD). However, the soft excess emissions inferred from all ULXs except source 69 are unlikely to be physically associated with the ULXs, as their strengths are comparable to that of the surrounding diffuse emission. The soft excess emission of source 69 is well described either by a two-temperature MEKAL plasma or a single-temperature MEKAL plasma (kT ~ 690 eV) and an MCD (kT ~ 170 eV). The MCD component suggests a cooler accretion disk compared to those in Galactic X-ray binaries, consistent with those expected for intermediate-mass black holes (IMBHs). An iron Kα line (EW ~ 700 eV) or K absorption edge at ~7.1 keV is present in the EPIC pn spectrum of source 26. The spectrum of the ULX in NGC 5195, source 12, is consistent with a simple power law. The LLAGN in NGC 5194 shows an extremely flat hard X-ray power law (Γ ~ 0.7), a narrow iron Kα line at 6.4 keV (EW ~ 3 keV), and strong soft X-ray excess emission. The full-band spectrum is well described by a two-component MEKAL plasma and reflection from cold material such as a putative torus.

Using the Chandra Advanced CCD Imaging Spectrometer Imaging array (ACIS-I), we have carried out a deep hard X-ray observation of the Galactic plane region at (l,b) ≈ (285,00), where no discrete X-ray source had been reported previously. We have detected 274 new point X-ray sources (4 σ confidence), as well as strong Galactic diffuse emission within two partially overlapping ACIS-I fields (~250 arcmin² in total). The point-source sensitivity was ~3 × 10^-15 ergs s^-1 cm^-2 in the hard X-ray band (2-10 keV) and ~2 × 10^-16 ergs s^-1 cm^-2 in the soft band (0.5-2 keV). The sum of all the detected point-source fluxes accounts for only ~10% of the total X-ray flux in the field of view. Even hypothesizing a new population of much dimmer and numerous Galactic point sources, the total observed X-ray flux cannot be explained. Therefore, we conclude that X-ray emission from the Galactic plane has a truly diffuse origin. Removing point sources brighter than ~3 × 10^-15 ergs s^-1 cm^-2 (2-10 keV), we have determined the Galactic diffuse X-ray flux to be 6.5 × 10^-11 ergs s^-1 cm^-2 deg^-2 (2-10 keV). Only 26 point sources were detected in both the soft and hard bands, indicating that there are two distinct classes of X-ray sources distinguished by their spectral hardness ratios. The surface number density of the hard sources is only slightly higher than that measured at the high Galactic latitude regions, indicating that the majority of the hard sources are background AGNs. Following up the Chandra observation, we have performed a near-infrared (NIR) survey with SofI at ESO/NTT. Almost all the soft X-ray sources have been identified in the NIR, and their spectral types are consistent with main-sequence stars, suggesting that most of them are nearby X-ray-active stars. On the other hand, only 22% of the hard sources had NIR counterparts, which are presumably Galactic. From X-ray and NIR spectral study, they are most likely to be quiescent cataclysmic variables. Our observation suggests a population of ≳10⁴ cataclysmic variables in the entire Galactic plane fainter than ~2 × 10³³ ergs s^-1. We have carried out a precise spectral study of the Galactic diffuse X-ray emission excluding the point sources. Confirming previous results, we have detected prominent emission lines from highly ionized heavy elements in the diffuse emission. In particular, the central energy of the iron emission line was determined to be 6.52 keV (90% confidence), which is significantly lower than what is expected from a plasma in thermal equilibrium. The downward shift of the iron line center energy suggests nonequilibrium ionization states of the plasma or the presence of a nonthermal process to produce 6.4 keV fluorescent lines.

We explore the evolution of field early-type galaxies in a sample extracted from the ACS images of the southern GOODS field. The galaxies are selected by means of a nonparametric analysis, followed by visual inspection of the candidates with a concentrated surface brightness distribution. We furthermore exclude from the final sample those galaxies that are not consistent with an evolution into the Kormendy relation between surface brightness and size that is observed for z = 0 ellipticals. The final set, which comprises 249 galaxies with a median redshift z_m = 0.71, represents a sample of early-type systems not selected with respect to color, with similar scaling relations as those of bona fide elliptical galaxies. The distribution of number counts versus apparent magnitude rejects a constant number density with cosmic time and suggests a substantial decrease with redshift: n ∝ (1 + z)^-2.5. The majority of the galaxies (78%) feature passively evolving old stellar populations. One-third of those in the upper half of the redshift distribution have blue colors, in contrast to only 10% in the lower redshift subsample. An adaptive binning of the color maps using an optimal Voronoi tessellation is performed to explore the internal color distribution. We find that the red and blue early-type galaxies in our sample have distinct behavior with respect to the color gradients, so that most blue galaxies feature blue cores whereas most of the red early-types are passively evolving stellar populations with red cores, i.e., similar systems to local early-type galaxies. Furthermore, the color gradients and scatter do not evolve with redshift and are compatible with the observations at z = 0, assuming a radial dependence of the metallicity within each galaxy. Significant gradients in the stellar age are readily ruled out. This work emphasizes the need for a careful sample selection, as we found that most of those galaxies that were visually classified as candidate early types—but then rejected based on the Kormendy relation—feature blue colors characteristic of recent star formation.

We have investigated the mass-metallicity (M-Z) relation using galaxies at 0.4 < z < 1.0 from the Gemini Deep Deep Survey (GDDS) and Canada-France Redshift Survey (CFRS). Deep K- and z'-band photometry allowed us to measure stellar masses for 69 galaxies. From a subsample of 56 galaxies, for which metallicity of the interstellar medium is also measured, we identified a strong correlation between mass and metallicity for the first time in the distant universe. This was possible because of the larger baseline spanned by the sample in terms of metallicity (a factor of 7) and mass (a factor of 400) than in previous works. This correlation is much stronger and tighter than the luminosity-metallicity relation, confirming that stellar mass is a more meaningful physical parameter than luminosity. We find clear evidence for temporal evolution in the M-Z relation in the sense that at a given mass, a galaxy at z ~ 0.7 tends to have lower metallicity than a local galaxy of similar mass. We use the z ~ 0.1 Sloan Digital Sky Survey M-Z relation and a small sample of z ~ 2.3 Lyman break galaxies with known mass and metallicity to propose an empirical redshift-dependent M-Z relation. According to this relation the stellar mass and metallicity in small galaxies evolve for a longer time than they do in massive galaxies. This relation predicts that the generally metal-poor damped Lyα galaxies have stellar masses of the order of 10^8.8M_☉ (with a dispersion of 0.7 dex) all the way from z ~ 0.2 to 4. The observed redshift evolution of the M-Z relation can be reproduced remarkably well by a simple closed-box model in which the key assumption is an e-folding time for star formation that is higher or, in other words, a period of star formation that lasts longer in less massive galaxies than in more massive galaxies. Such a picture supports the downsizing scenario for galaxy formation.

We present deep near-infrared images of the Antennae galaxies, taken with the Palomar Wide-Field Infrared Camera (WIRC). The images cover a 433 × 433 (24.7 × 24.7 kpc) area around the galaxy interaction zone. We derive J- and K_s-band photometric fluxes for 172 infrared star clusters and discuss details of the two galactic nuclei and the overlap region. We also discuss the properties of a subset of 27 sources that have been detected with WIRC, HST, and the VLA. The sources in common are young clusters of less than 10 Myr, which show no correlation between their infrared colors and 6 cm radio properties. These clusters cover a wide range in infrared color due to extinction and evolution. The average extinction is about A_V ~ 2 mag, while the reddest clusters may be reddened by up to 10 mag.

We present photometric and kinematic information obtained by measuring 197 planetary nebulae (PNs) discovered in the flattened Fornax elliptical galaxy NGC 1344 (also known as NGC 1340) with an on-band, off-band, and grism+on-band filter technique. We build the PN luminosity function (PNLF) and use it to derive a distance modulus m - M = 31.4 ± 0.18, which is slightly smaller than, but in good agreement with, the surface brightness fluctuation distance. The PNLF also provides an estimate of the specific PN formation rate: (6 ± 3) × 10^-12 PNs yr^-1L. If we combine the positional information from the on-band image with PN positions measured on the grism+on-band image, we can measure the radial velocities of 195 PNs, some of them distant more than three effective radii from the center of NGC 1344. We complement this data set with stellar kinematics derived from integrated spectra along the major and minor axes and parallel to the major axis of NGC 1344. The line-of-sight velocity dispersion profile indicates the presence of a dark matter halo around this galaxy.

Using HST STIS, we have detected far-ultraviolet nuclear activity in the giant elliptical galaxy NGC 1399, the central and brightest galaxy in the Fornax I cluster. The source reached a maximum observed far-UV luminosity of ~1.2 × 10³⁹ ergs s^-1 in 1999 January. It was detectable in earlier HST archival images in 1996 (B band) but not in 1991 (V band) or 1993 (UV). It faded by a factor of ~4 by mid-2000. The source is almost certainly associated with the low-luminosity AGN responsible for the radio emission in NGC 1399. The properties of the outburst are remarkably similar to the UV-bright nuclear transient discovered earlier in NGC 4552 by Renzini and coworkers. The source is much fainter than expected from its Bondi accretion rate (estimated from Chandra high-resolution X-ray images), even in the context of "radiatively inefficient accretion flow" models, and its variability also appears inconsistent with such models. High spatial resolution UV monitoring is a valuable means to study activity in nearby LLAGNs.

We present the detailed spectral analysis of a sample of M33 B-type supergiant stars, aimed at the determination of their fundamental parameters and chemical composition. The analysis is based on a grid of non-LTE metal line-blanketed model atmospheres including the effects of stellar winds and spherical extension computed with the code FASTWIND. Surface abundance ratios of C, N, and O are used to discuss the chemical evolutionary status of each individual star. The comparison of observed stellar properties with theoretical predictions of massive star evolutionary models shows good agreement within the uncertainties of the analysis. The spatial distribution of the sample allows us to investigate the existence of radial abundance gradients in the disk of M33. The comparison of stellar and H II region O abundances (based on direct determinations of the electron temperature of the nebulae) shows good agreement. Using a simple linear radial representation, the stellar oxygen abundances result in a gradient of -0.0145 ± 0.005 dex arcmin^-1 (or -0.06 ± 0.02 dex kpc^-1) up to a distance equal to ~1.1 times the isophotal radius of the galaxy. A more complex representation cannot be completely discarded by our stellar sample. The stellar Mg and Si abundances follow the trend displayed by O abundances, although with shallower gradients. These differences in gradient slope cannot be explained at this point. The derived abundances of the three α-elements yield solar metallicity in the central regions of the disk of M33. A comparison with recent planetary nebula data from Magrini and coworkers indicates that the disk of M33 has not suffered from a significant O enrichment in the last 3 Gyr.

In Sgr A* at the Galactic center, by far the closest and easiest supermassive black hole we can study, the observational evidence is increasingly pointing to the presence of a compact, hot, magnetized disk feeding the accretor. In such low Mach number plasmas, forces arising, e.g., from pressure gradients in the plasma, can altogether negate the warping of disks around Kerr black holes caused by the Bardeen-Petterson effect and can lead to coherent precession of the entire disk. In this paper, we present for the first time highly detailed three-dimensional smoothed particle hydrodynamics (SPH) simulations of the accretion disk evolution in Sgr A*, guided by observational constraints on its physical characteristics, and conclude that indeed the Bardeen-Petterson effect is probably absent in this source. Given what we now understand regarding the emission geometry in this object, we suggest that a ~50-500 days modulation in Sgr A*'s spectrum, arising from the disk precession, could be an important observational signature; perhaps the ~106 days period seen earlier in its radio flux, if confirmed, could be due to this process. On the other hand, if future observations do not confirm this long modulation in Sgr A*'s spectrum, this would be an indication either that the disk size or orientation is very different from current estimates, that the black hole is not spinning at all (unlikely), or that our current understanding of how it produces its radiative output is incorrect.

Many young stars reside within the central half-parsec from Sgr A*, the supermassive black hole in the Galactic center. The origin of these stars remains a puzzle. Recently, Hansen & Milosavljevic (HM) have argued that an intermediate-mass black hole (IMBH) could have delivered the young stars to the immediate vicinity of Sgr A*. Here we focus on the final stages of the HM scenario. Namely, we integrate numerically the orbits of stars that are initially bound to the IMBH but are stripped from it by the tidal field of Sgr A*. Our numerical algorithm is a symplectic integrator designed specifically for the problem at hand; however, we have checked our results with SYMBA, a version of the widely available SWIFT code. We find that the distribution of the postinspiral orbital parameters is sensitive to the eccentricity of the inspiraling IMBH. If the IMBH is on a circular orbit, then the inclinations of numerically computed orbits relative to the inspiral plane are almost always smaller than 10°, and therefore (1) the simulations are in good agreement with the observed motions of stars in a clockwise-moving stellar disk; and (2) the simulations never reproduce the orbits of stars outside this disk, which include those in the second thick ring of stars and the randomly oriented unrelaxed orbits of some of the S stars. If the IMBH's orbital eccentricity is e = 0.6, then approximately half of the stars end up with orbital inclinations below 10°, and another half have inclinations anywhere between 0° and 180°; this is somewhat closer to what is observed. We also show that if the IRS 13 cluster is bound by an IMBH, as has been argued by Maillard et al., then the same IMBH could not have delivered all of the young stars to their present location.

We have compiled composition, luminosity, and binarity information for carbon-enhanced, metal-poor (CEMP) stars reported by recent studies. We divided the CEMP star sample into two classes having high and low abundances, respectively, of the s-process elements and consider the abundances of several isotopes, in particular, ¹²C, ¹³C, and ¹⁴N, as well as the likely evolutionary stages of each star. Despite the fact that objects in both groups were selected from the same surveys (primarily the HK survey), without a priori knowledge of their s-process element abundances, we identify the following remarkable differences between the two classes: s-element-rich CEMP (CEMP-s) stars occupy a wide range of evolutionary states, but do not have a strongly evolved ¹³C/¹⁴N ratio, whereas s-element-normal CEMP stars (CEMP-no) are found only high up the first-ascent giant branch and possess ¹³C/¹⁴N ratios approaching the CN cycle equilibrium value. We argue that these observational constraints can be accommodated by the following scenarios. CEMP-s stars acquire their distinctive surface compositions during their lifetimes when mass is transferred from an AGB companion that has recently synthesized ¹²C and s-process elements. Such mass-accreting stars can be enriched at almost any stage of their evolution and hence are found throughout the H-R diagram. Dilution of transferred surface material as the accretor ascends the giant branch and its surface convective zone deepens may reduce the number of such stars, whose surfaces remain C-rich at high luminosities. Many, but not necessarily all, such stars should currently be in binary systems. Li-preserving CEMP-s stars may require a different explanation. In contrast, a CEMP-no star is proposed to have formed from gas that was enriched in ¹²C from the triple-α process in a previous generation of stars, some of which has been converted to ¹³C and ¹⁴N during the present star's giant branch evolution. The binary fraction of such stars should be the same as that of non-carbon-enhanced, metal-poor stars.

We present Chandra X-ray images and spectra of the most prominent cloud-shock interaction region in the Puppis A supernova remnant. The bright eastern knot (BEK) has two main morphological components: (1) a bright compact knot that lies directly behind the apex of an indentation in the eastern X-ray boundary; and (2) lying 1' westward behind the shock, a curved vertical structure (bar) that is separated from a smaller bright cloud (cap) by faint diffuse emission. Based on hardness images and spectra, we identify the bar and cap as a single shocked interstellar cloud. Its morphology strongly resembles the "voided sphere" structures seen at late times in Klein and coworkers' experimental simulations of cloud-shock interactions, when the crushing of the cloud by shear instabilities is well underway. We infer an interaction time of roughly 3 cloud-crushing timescales, which translates to 2000-4000 yr, based on the X-ray temperature, physical size, and estimated expansion of the shocked cloud. This is the first X-ray-identified example of a cloud-shock interaction in this advanced phase. Closer to the shock front, the X-ray emission of the compact knot in the eastern part of the BEK region implies a recent interaction with relatively denser gas, some of which lies in front of the remnant. The complex spatial relationship of the X-ray emission of the compact knot to optical [O III] emission suggests that there are multiple cloud interactions occurring along the line of sight.

We present the results of an in-depth optical study of the core-collapse supernova remnant G292.0+1.8 using the Rutgers Fabry-Perot (RFP) imaging spectrometer. Our observations provide a detailed picture of the supernova remnant in the emission lines of [O III] λ5007, Hα, and [N II] λ6548. The [O III] Fabry-Perot scans reveal a bright crescent-shaped spur of previously known high-velocity (V_radial ~ 1500 km s^-1) O-rich ejecta located on the eastern side of the remnant. The spur consists of a semicoherent structure of mostly redshifted material, along with several clumps that have apparently broken out of the more orderly shell-like expansion. The high-velocity (≳600 km s^-1) component of the spur also displays a scalloped morphology characteristic of Rayleigh-Taylor instabilities. We also find a large number of fast-moving knots (FMKs) of O-rich ejecta undetected in prior photographic plate images and similar to features seen in Cas A. The FMKs are distributed sparsely in the interior of G292.0+1.8 and are seen mostly in blueshifted emission out to V_radial ≈ -1700 km s^-1. The position-velocity distribution of the FMKs can be kinematically described as a shell 34 in radius expanding at a velocity of 1700 km s^-1. Another feature apparent in the [O III] scans is an equatorial belt consisting of both a barlike structure at zero radial velocity and a clumpy, high-velocity ejecta component seen in projection along the line of sight. Portions of the zero-velocity bar are spatially well correlated with a similar structure seen in the Chandra X-ray image of G292.0+1.8. The bar is also detected in our Hα RFP images at zero radial velocity, providing further evidence that this structure is of circumstellar origin. We find that the optical and X-ray properties of the bar are consistent with incomplete (partially radiative) shocks in material of moderate densities. There are also a number of faint, elongated structures seen in Hα at zero radial velocity across the interior of G292.0+1.8 that lack [O III] and X-ray counterparts. These filaments may be low-density H I clouds photoionized by hard radiation from the interior of the remnant. Overall, these results suggest that G292.0+1.8 is currently interacting with a low-density environment. We find no evidence for high-velocity Hα or [N II] emission over the dynamical range sampled by the RFP. Assuming a distance of 6 kpc for G292.0+1.8, we estimate a kinematic age of (3000-3400)d₆ yr for this remnant.

We present the results of Chandra X-Ray Observatory observations of the planetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-type Wolf-Rayet central stars that are currently driving fast (~1000 km s^-1), massive winds into denser, slow-moving (~10 km s^-1) material ejected during recently terminated asymptotic giant branch (AGB) evolutionary phases. Hence, these observations provide key tests of models of wind-wind interactions in PNs. In NGC 40, we detect faint, diffuse X-ray emission distributed within a partial annulus that lies nested within a ~40'' diameter ring of nebulosity observed in optical and near-infrared images. Hen 2-99 is not detected. The inferred X-ray temperature (T_X ~ 10⁶ K) and luminosity (L_X ~ 2 × 10³⁰ ergs s^-1) of NGC 40 are the lowest measured thus far for any PN displaying diffuse X-ray emission. These results, combined with the ringlike morphology of the X-ray emission from NGC 40, suggest that its X-ray emission arises from a "hot bubble" that is highly evolved and is generated by a shocked, quasi-spherical fast wind from the central star, as opposed to AGB or post-AGB jet activity. In contrast, the lack of detectable X-ray emission from Hen 2-99 suggests that this PN has yet to enter a phase of strong wind-wind shocks.

X-ray absorption lines of highly ionized species such as O VII at about zero redshift have been firmly detected in the spectra of several active galactic nuclei. However, the location of the absorbing gas remains a subject of debate. To separate the Galactic and extragalactic contributions to the absorption, we have obtained Chandra LETG-HRC and Far Ultraviolet Spectroscopic Explorer observations of the black hole X-ray binary LMC X-3. We clearly detect the O VII Kα absorption line with an equivalent width of 20(14, 26) mÅ (90% confidence range). The Ne IX Kα absorption line is also detected, albeit marginally. A joint analysis of these lines, together with the nondetection of the O VII Kβ and O VIII Kα lines, gives the temperature, velocity dispersion, and hot oxygen column density as 1.3(0.7,1.8) × 10⁶ K, 79(62,132) km s^-1, and 1.9(1.2, 3.2) ×10¹⁶ cm^-2, assuming a collisional ionization equilibrium of the X-ray-absorbing gas and a Galactic interstellar Ne/O number ratio of 0.18. The X-ray data allow us to place a 95% confidence lower limit to the Ne/O ratio as 0.14, but the upper limit is not meaningfully constrained. The O VII line centroid and its relative shift from the Galactic O I Kα absorption line, detected in the same observations, are inconsistent with the systemic velocity of LMC X-3 (+310 km s^-1). The far-UV spectrum shows O VI absorption at Galactic velocities, but no O VI absorption is detected at the LMC velocity at greater than 3 σ significance. The measured Galactic O VI column density is higher than the value predicted from the O VII-bearing gas, indicating multiphase absorption. Both the nonthermal broadening and the decreasing scale height with the increasing ionization state further suggest an origin of the highly ionized gas in a supernova-driven galactic fountain. In addition, we estimate the warm and hot electron column densities from our detected O II Kα line in the LMC X-3 X-ray spectra and from the dispersion measure of a pulsar in the LMC vicinity. We then infer the O/H ratio of the gas to be ≳ 8 × 10^-5, consistent with the chemically enriched galactic fountain scenario. We conclude that the Galactic hot interstellar medium should in general substantially contribute to zero-redshift X-ray absorption lines in extragalactic sources.

We present new Keck images at 0.9 μm and OVRO 1.3 mm continuum images of five Class I protostars in the Taurus star-forming region. We analyze these data in conjunction with broadband spectral energy distributions (SEDs) and 8-13 μm spectra from the literature using a Monte Carlo radiative transfer code. By fitting models for the circumstellar dust distributions simultaneously to the scattered light images, millimeter continuum data, and the SEDs, we attempt to distinguish between flared disks, infalling envelopes with outflow cavities, and combinations of disks and envelopes. For each of these circumstellar density distributions, we generate grids of models for varying geometries, dust masses, and accretion rates and determine the best fits by minimizing the residuals between model and data. Comparison of the residuals for best-fit disk, envelope, and disk+envelope models demonstrates that, in general, models incorporating both massive envelopes and massive embedded disks fit the imaging+SED data best. The implied envelope infall rates for these disk+envelope models are generally consistent with infall rates derived by previous investigators, although they are approximately an order of magnitude larger than inner disk accretion rates inferred from recent spectroscopic measurements. In addition, the disk masses inferred from our models are close to or larger than the limit for gravitationally stable disks, indicating that Class I disks may undergo periodic episodes of enhanced accretion, perhaps as a result of gravitational instabilities. An important caveat to these results is that in some cases, no single model can fit all of the imaging and SED data well, suggesting that further refinements to models of the circumstellar dust distributions around Class I sources are necessary. We discuss several potential improvements to the models, as well as new constraints that will become available with upcoming millimeter and infrared facilities.

We argue that classical T Tauri stars (CTTSs) possess significant nonphotospheric excess in the J and H bands (1.25 and 1.66 μm, respectively). We first show that normalizing the spectral energy distributions (SEDs) of CTTSs to the J band leads to a poor fit of the optical fluxes (which are systematically overestimated), while normalizing the SEDs to the I_C band (0.8 μm) produces a better fit to the optical bands and in many cases reveals the presence of a considerable excess at J and H. Near-infrared spectroscopic veiling measurements from the literature support this result. We find that J- and H-band excesses correlate well with the K-band (2.2 μm) excess and that the J - K and H - K colors of the excess emission are consistent with that of a blackbody at the dust sublimation temperature (~1500-2000 K). We propose that this near-IR excess originates at a hot inner rim, analogous to those suggested to explain the "near-IR bump" in the SEDs of Herbig Ae/Be stars. To test our hypothesis, we use the model presented by Dullemond and coworkers to fit the photometry data between 0.5 and 24 μm of 10 CTTSs associated with the Chamaeleon II molecular cloud. We find that simple models that include luminosities calculated from I_C-band magnitudes and an inner rim may account for the reported J- and H-band excesses. The models that best fit the data are those in which the inner radius of the disk is larger than expected for a rim in thermal equilibrium with the photospheric radiation field alone. In particular, we find that large inner rims are necessary to account for the mid-infrared fluxes (3.6-8.0 μm) obtained by the Spitzer Space Telescope (Spitzer). The large radius could be explained if, as proposed by D'Alessio and colleagues, the UV radiation from the accretion shock significantly affects the sizes of the inner holes in disks around CTTSs. Finally, we argue that deriving the stellar luminosities of CTTSs by making bolometric corrections to the J-band fluxes, which is the "standard" procedure for obtaining CTTS luminosities, systematically overestimates these luminosities. The overestimated luminosities translate into underestimated ages when the stars are placed in the H-R diagram. Thus, the results presented herein have important implications for the dissipation timescale of inner accretion disks.

We report on Keck Interferometer observations of the double-lined binary (B) component of the quadruple pre-main-sequence (PMS) system HD 98800. With these interferometric observations, combined with astrometric measurements made by the Hubble Space Telescope (HST) Fine Guidance Sensors (FGS) and published radial velocity observations, we have estimated preliminary visual and physical orbits of the HD 98800 B subsystem. Our orbit model calls for an inclination of 668 ± 32 and allows us to infer the masses and luminosities of the individual components. In particular we find component masses of 0.699 ± 0.064 and 0.582 ± 0.051 M_☉ for the Ba (primary) and Bb (secondary) components, respectively. Spectral energy distribution (SED) modeling of the B subsystem suggests that the B circumstellar material is a source of extinction along the line of sight to the B components. This seems to corroborate a conjecture by Tokovinin that the B subsystem is viewed through circumbinary material, but it raises important questions about the morphology of that circumbinary material. Our modeling of the subsystem component SEDs finds temperatures and luminosities in agreement with previous studies, and coupled with the component mass estimates allows for comparison with PMS models in the low-mass regime with few empirical constraints. Solar abundance models seem to underpredict the inferred component temperatures and luminosities, while assuming slightly subsolar abundances brings the models and observations into better agreement. The current preliminary orbit does not yet place significant constraints on existing PMS stellar models, but prospects for additional observations improving the orbit model and component parameters are very good.

This is a report on detailed modeling of young high-mass protostellar candidates during their most embedded and obscured phases. We performed narrowband mid-infrared imaging of three candidate high-mass protostellar objects in G11.94-0.62, G29.96-0.02, and G45.07+0.13 at Gemini Observatory using the Thermal-Region Camera and Spectrograph (T-ReCS). The sources were imaged through up to 11 narrowband filters, sampling their SEDs over the entire 2-25 μm infrared range. For the first time, we have fit the observed SEDs of massive protostars with models that take into account departures from spherical symmetry in the infalling envelopes. In this way, we have been able to derive from the models the detailed physical parameters for these earliest stages of massive stellar life. Our detailed modeling suggests that massive star formation can proceed in a way very similar to the formation of low-mass stars.

We present infrared (IR) and optical echelle spectra of the classical T Tauri star TW Hydrae. Using the optical data, we perform detailed spectrum synthesis to fit atomic and molecular absorption lines and determine key stellar parameters: T_eff = 4126 ± 24 K, log g = 4.84 ± 0.16, [M/H] = -0.10 ± 0.12, and log g = 5.8 ± 0.6 km s^-1. The IR spectrum is used to look for Zeeman broadening of photospheric absorption lines. We fit four Zeeman-sensitive Ti I lines near 2.2 μm and find that the average value of the magnetic field over the entire surface is B = 2.61 ± 0.23 kG. In addition, several nearby magnetically insensitive CO lines show no excess broadening above that produced by stellar rotation and instrumental broadening, reinforcing the magnetic interpretation for the width of the Ti I lines. We carry out extensive tests to quantify systematic errors in our analysis technique that may result from inaccurate knowledge of the effective temperature or gravity, finding that reasonable errors in these quantities produce a ~10% uncertainty in the mean field measurement.

We study the properties of plasmas containing a low-energy thermal photon component at comoving temperatures of θ ≡ kT'/m_ec² ~ 10^-5 to 10^-2 interacting with an energetic electron component characteristic of, e.g., the dissipation phase of relativistic outflows in gamma-ray bursts (GRBs), X-ray flashes, and blazars. We show that for scattering optical depths larger than a few, the balance between Compton and inverse Compton scattering leads to the accumulation of electrons at values of γβ ~ 0.1-0.3. For optical depths larger than ~100, this leads to a peak in the comoving photon spectrum at 1-10 keV that is very weakly dependent on the values of the free parameters. In particular, these results are applicable to the internal shock model of GRBs, as well as to slow dissipation models, e.g., as might be expected from reconnection, if the dissipation occurs at a subphotospheric radii. For GRB bulk Lorentz factors of ~100, this results in observed spectral peaks clustering in the 0.1-1 MeV range, with conversion efficiencies of electrons into photon energy in the BATSE range of ~30%.

We perform Monte Carlo simulations to study the E_p-E_iso correlation in the context of a multiple-subjet model (or inhomogeneous jet model) for γ-ray bursts (GRBs), X-ray-rich GRBs (XRRs), and X-ray flashes (XRFs). For a single subjet, we find that E_p ∝ E for large viewing angles. For the multiple-subjet model in which all the subjets have the same intrinsic properties, off-axis events show E_p ∝ E with 0.4 < a < 0.5. If the intrinsic properties of the subjets are distributed so that on-axis emission of each subjet follows a correlation E_p ∝ L, we obtain the Amati correlation (E_p ∝ E) over 3 orders of magnitude in E_p. Although the scatter around the Amati correlation is large in the simulation, the results are consistent with the observed properties of GRBs with known redshifts and the BASTE GRBs with pseudoredshifts derived from the lag-luminosity correlation. We also calculate the event rates, the redshift distributions, and the T₉₀ duration distributions of GRBs, XRRs, and XRFs, which can be detected by HETE-2, assuming that the source redshift distribution is in proportion to the cosmic star formation rate. It is found that the event rates of the three classes are comparable, that the average redshift of the XRRs is a little larger than those of the GRBs and the XRFs, and that short XRRs arise when a single subjet is viewed off-axis or viewed on-axis with slightly high redshift.

Multidimensional simulations of the neutrino-driven mechanism behind core-collapse supernovae have long shown that the explosions from this mechanism would be asymmetric. Recently, detailed core-collapse simulations have shown that the explosion may be strongest in a single direction. We present a suite of simulations modeling these "single-lobe" supernova explosions of a 15 M_☉ red supergiant star, focusing on the effect these asymmetries have on the gamma-ray emission and the mixing in the explosion. We discuss how these asymmetries in the explosion mechanism might explain many of the observed "asymmetries" of supernovae, focusing on features of both supernova 1987A and the Cassiopeia A supernova remnant. In particular, we show that single-lobe explosions provide a promising solution to the redshifted iron lines of supernova 1987A. We also show that the extent of mixing for explosive burning products depends sensitively on the angular profile of the velocity asymmetry and may be much more extensive than previously assumed.

We study the K-correction for the case of emission lines formed in the X-ray-illuminated atmosphere of a Roche lobe-filling star. We compute the K-correction as a function of the mass ratio q and the disk flaring angle α using a compact binary code in which the companion's Roche lobe is divided into 10⁵ resolution elements. We also study the effect of the inclination angle in the results. We apply our model to the case of the neutron star low-mass X-ray binary X1822-371 (V691 CrA), where a K-emission velocity K_em = 300 ± 8 km s^-1 has been measured by Casares et al. Our numerical results, combined with a previous determination of system parameters, yields 1.61 M_☉ ≤ M_NS ≤ 2.32 M_☉ and 0.44 M_☉ ≤ M₂ ≤ 0.56 M_☉ for the two binary components (i.e., 0.24 ≤ q ≤ 0.27), which provide compelling evidence for a massive neutron star in this system. We also discuss the implications of these masses into the evolutionary history of the binary.

The loss of linear momentum by gravitational radiation and the resulting gravitational recoil of black hole binary systems may play an important role in the growth of massive black holes in early galaxies. We calculate the gravitational recoil of nonspinning black hole binaries at the second post-Newtonian order (2 PN) beyond the dominant effect, obtaining, for the first time, the 1.5 PN correction term due to tails of waves and the next 2 PN term. We find that the maximum value of the net recoil experienced by the binary due to the inspiral phase up to the innermost stable circular orbit (ISCO) is of the order of 22 km s^-1. We then estimate the kick velocity accumulated during the plunge from the ISCO up to the horizon by integrating the momentum flux using the 2 PN formula along a plunge geodesic of the Schwarzschild metric. We find that the contribution of the plunge dominates over that of the inspiral. For a mass ratio m₂/m₁ = , we estimate a total recoil velocity (due to both adiabatic and plunge phases) of 100 ± 20 km s^-1. For a ratio of 0.38, the recoil is maximum, and we estimate it to be 250 ± 50 km s^-1. In the limit of small mass ratio, we estimate V/c ≈ 0.043(±20%)(m₂/m₁)². Our estimates are consistent with, but span a substantially narrower range than, those of Favata and coworkers.

On 2004 December 27, a giant flare from SGR 1806-20 was detected on Earth. Its thermal spectrum and temperature suggest that the flare resulted from an energy release of about 10⁴⁷ ergs s^-1 close to the surface of a neutron star in the form of radiation and/or pairs. This plasma expanded under its own pressure, producing a fireball, and the observed gamma rays escaped once the fireball became optically thin. The giant flare was followed by a bright radio afterglow, with an observable extended size, implying an energetic relativistic outflow. We revisit here the evolution of relativistic fireballs, and we calculate the Lorentz factor and energy remaining in relativistic outflow once the radiation escapes. We show that pairs that arise naturally in a pure pair-radiation fireball do not carry enough energy to account for the observed afterglow. We consider various alternatives and show that if the relativistic outflow that causes the afterglow is related directly to the prompt flare, then the initial fireball must be loaded by baryons or Poynting flux. While we focus on parameters applicable to the giant flare and the radio afterglow of SGR 1806-20, the calculations presented here could be also applicable to gamma-ray bursts (GRBs).

We propose that the anomalously bright white dwarf luminosity function observed in NGC 6791 (Bedin et al.) is the consequence of the formation of 0.5 M_☉ white dwarfs with helium cores instead of carbon cores. This may happen if mass loss during the ascent of the red giant branch is strong enough to prevent a star from reaching the helium flash. Such a model can explain the slower white dwarf cooling (relative to standard models) and fits naturally with scenarios advanced to explain extreme horizontal branch stars, a population of which are also found in this cluster.

A nonlinear time series analysis is performed on light intensity data for the variable DB white dwarf PG 1351+489. The data were collected during the XCov12 campaign of the Whole Earth Telescope. Two data sets integrated to 30 and 10 s, taken at different times during the campaign, were analyzed. The power spectrum of PG 1351+489 is broadband, with a dominant fundamental frequency f₀, and its harmonics, subharmonics, and other lower amplitude peaks. Our time delay phase-space portrait in 3 dimensions is a rhombohedral shape wrapped around a torus with triangular projections. This differs from earlier reports of a more circular two-dimensional representation. We show that the difference is due to binning and subsequent filtering in the earlier work. A circular phase-space portrait is obtained if only the fundamental frequency is used, the reconstruction subsequently growing in complexity as more and more of the power spectrum is included. Poincaré section return times for the 30 s integrated data correspond to the fundamental period. For the 10 s integrated data, other return times are observed. These are shown to be due to dynamical resetting. An example of dynamical resetting on a theoretical model is given. Since resetting takes the phase-space point off the trajectory, locally projective nonlinear noise reduction was used. Locally projective nonlinear noise reduction results in lower white noise by a factor of ~50. Thus, three more harmonics of the fundamental frequency f₀, four more (n + 0.47)f₀ subharmonics, and at least 12 other peaks are identified.

We examine the influence of the gravity of the companion (the secondary) to the massive primary star η Carinae on the winds blown by the primary and the secondary. The two winds collide with each other after passing through two respective shock waves, and escape the system while strongly emitting in the X-ray band. While during most of the 5.5 yr orbital period the companion's gravity has a negligible effect on the winds, we find that near periastron the companion's gravity may significantly influence the flow, and the companion might accrete from the primary's wind under certain circumstances. Near periastron passage, the collision region of the two winds may collapse onto the secondary star, a process that could substantially reduce the X-ray luminosity. We suggest that such an accretion process produces the long, almost flat, X-ray minimum in η Car.

Eight consecutive low-frequency radial p-modes are identified in the G0 IV star η Bootis based on 27 days of ultraprecise rapid photometry obtained by the MOST (Microvariability and Oscillations of Stars) satellite. The MOST data extend smoothly, to lower overtones, the sequence of radial p-modes reported in earlier ground-based spectroscopy by other groups. The sampling is nearly continuous; hence, the ambiguities in p-mode identifications due to aliases, such as the cycle day^-1 alias found in ground observations, are not an issue. The lower overtone modes from the MOST data constrain the interior structure of the model of η Boo, giving a best fit on a grid of ~300,000 stellar models for a composition of (X,Z) = (0.71,0.04), a mass of M = 1.71 ± 0.05 M_☉, and an age of t = 2.40 ± 0.03 Gyr. The surface temperature and luminosity of this model, which were constrained only by using the oscillation modes, are close (1 σ) to current best estimates of η Boo's surface temperature and luminosity. With the interior fit anchored by the lower overtone modes seen by MOST, standard models are not able to fit the higher overtone modes with the same level of accuracy. The discrepancy, model minus observed frequency, increases from 0.5 μHz at 250 μHz to 5 μHz at 1000 μHz and is similar to the discrepancy that exists between the Sun's observed p-mode frequencies and the p-mode frequencies of the standard solar model. This discrepancy promises to be a powerful constraint on models of three-dimensional convection.

A systematic and automated search of the extensive GLIMPSE mid-infrared survey data of the inner Galaxy was carried out to uncover new star clusters. This search has yielded 59 new clusters. Using our automated search algorithm, these clusters were identified as significant localized overdensities in the GLIMPSE point-source catalog (GLMC) and archive (GLMA). Subsequent visual inspection of the GLIMPSE image mosaics confirmed the existence of these clusters plus an additional 33 heavily embedded clusters missed by our detection algorithm, for a total of 92 newly discovered clusters. These previously uncataloged clusters range in type from heavily embedded to fully exposed clusters. More than half of the clusters have memberships exceeding 35 stars, and nearly all the clusters have diameters of 3' or less. The Galactic latitude distribution of the clusters reveals that the majority are concentrated toward the Galactic midplane. There is an asymmetry in the number of clusters located above and below the midplane, with more clusters detected below the midplane. We also observe an asymmetry in the number of clusters detected in the northern and southern halves of the Galaxy, with more than twice as many clusters detected in the south.

We investigate the linear polarization in the light of extrasolar planetary systems that may arise as a result of an occultation of the star by a transiting planet. Such an occultation breaks any spherical symmetry over the projected stellar disk and thus results in a nonvanishing linear polarization. This polarization will furthermore vary as the occultation progresses. We present both analytical and numerical results for the occultation of G, K, M, and T dwarf stars by planets with sizes ranging from that of Earth to 2 times the size of Jupiter. We find that the occultation polarization may result in an observable signal and provide additional means to characterize various parameters of the system. A particularly interesting result is that, for the later spectral types (i.e., smaller stellar radii), this polarization signature may be observable even for Earth-like planets. This suggests polarization as a possible tool to detect such planets. Departures from symmetry around midtransit in the time dependence of the polarization signature may provide an estimate of the orbital eccentricity.

We analyze the orbital evolution of terrestrial planetary embryos after oligarchic growth, including the effect of the sweeping Jupiter secular resonance combined with tidal drag during the depletion of the protoplanetary gas disk. Previous studies show that the orbits of isolated embryos become unstable through long-term gravitational interaction. However, the planetary systems formed as a result of giant impacts are generally very eccentric, unlike our solar system. Although mechanisms that damp the eccentricity have been proposed, such mechanisms not only damp the eccentricity of embryos strongly but also prevent their orbital crossing and planetary growth. This dilemma can be solved if the protoplanetary collisions occur in an environment where the damping is still working. We consider the stage of gas depletion after the formation of a Jupiter-like planet in the disk. The gas depletion changes the gravitational potential and causes the sweeping of a secular resonance. We find that the secular resonance passes through the terrestrial region from outside to inside, that the resonance excites the eccentricities of isolated embryos, and that it leads to orbit crossing. Because the remnant disk is still present in this stage, the gas drag due to tidal interaction is still effective. The tidal drag effectively damps the eccentricities of the fully grown embryos. This process allows the system to form circular orbits analogous to our solar system. We also find that the tidal drag induces the decay of the semimajor axes of the embryos. As a result of balance between damping and excitation of eccentricity, the embryos migrate along with the secular resonance. This "secular resonance trapping" can lead to rapid collisions and mergers among the embryos as they migrate from the outer to the inner region, concurrent with the disk depletion. Since the induced migration is strongest near the Jovian orbit, the final terrestrial planets tend to be concentrated in a relatively small region (<2 AU). We suggest that this mechanism may be the origin of the severe present-day mass depletion of the asteroid belt.

We investigate the microlensing effects on a source star surrounded by a circumstellar disk, as a function of wavelength. The microlensing light curve of the system encodes the geometry and surface brightness profile of the disk. In the mid- and far-infrared, the emission of the system is dominated by the thermal emission from the cold dusty disk. For a system located at the Galactic center, we find typical magnifications to be of order 10%-20% or higher, depending on the disk surface brightness profile, and the event lasts over 1 yr. At around 20 μm, where the emission for the star and the disk are comparable, the difference in the emission areas results in a chromatic microlensing event. Finally, in the near-infrared and visible, where the emission of the star dominates, the fraction of star light directly reflected by the disk slightly modifies the light curve of the system, which is no longer that of a point source. In each case, the corresponding light curve can be used to probe some of the disk properties. A fraction of 10^-3 to 10^-2 of optical microlensing events are expected to be associated with circumstellar disk systems. We show that the lensing signal of the disk can be detected with sparse follow-up observations of the next generation space telescopes. While direct imaging studies of circumstellar disks are limited to the solar neighborhood, this microlensing technique can probe very distant disk systems living in various environments and has the potential to reveal a larger diversity of circumstellar disks.

We report a detailed spectroscopic abundance analysis for a sample of 18 F-K dwarfs of the young open cluster IC 4665. Stellar parameters and element abundances of Li, O, Mg, Si, Ca, Ti, Cr, Fe, and Ni have been derived using the spectroscopic synthesis tool SME (Spectroscopy Made Easy). Within the measurement uncertainties the iron abundance is uniform, with a standard deviation of 0.04 dex. No correlation is found between the iron abundance and the mass of the stellar convective zone or between the Li abundance and the Fe abundance. In other words, our results do not reveal any signature of accretion and therefore do not support the scenario that stars with planets (SWPs) acquire their on-average higher metallicity compared to field stars via accretion of metal-rich planetary material. Instead, the higher metallicity of SWPs may simply reflect the fact that planetary formation is more efficient in high-metallicity environs. However, since so many details of the planetary system formation processes remain poorly understood, further studies are needed for a final settlement of the problem of the high metallicity of SWPs. The standard deviation of [Fe/H] deduced from our observations, taken as an upper limit on the metallicity dispersion among the IC 4665 member stars, has been used to constrain protoplanetary disk evolution, terrestrial and giant planets formation, and evolution processes. The total reservoir of heavy elements retained by the nascent disks is limited, and high retention efficiency of planet-building material is supported. Under modest surface density, gas giant planets are expected to form in locally enhanced regions or start efficient gas accretion when they only have a small core of a few Earth masses. Our results do not support the possibility that the migration of gas giants and the circularization of terrestrial planets' orbits are regulated by their interaction with a residual population of planetesimals and dust particles.

We present the results of an 850 μm JCMT/SCUBA survey for dust around 13 nearby solar-mass stars. The dust mass sensitivity ranged from 5 × 10^-3 to 0.16 M_⊕. Three sources were detected in the survey, one of which (HD 107146) has been previously reported. One of the other two submillimeter sources, HD 104860, was not detected by IRAS and is surrounded by a cold, massive dust disk with a dust temperature and mass of T_dust = 33 K and M_dust = 0.16 M_⊕, respectively. The third source, HD 8907, was detected by IRAS and ISO at 60-87 μm and has a dust temperature and mass of T_dust = 48 K and M_dust = 0.036 M_⊕, respectively. We find that the deduced masses and radii of the dust disks in our sample are roughly consistent with models for the collisional evolution of planetesimal disks with embedded planets. We also searched for residual gas in two of the three systems with detected submillimeter excesses and place limits on the mass of gas residing in these systems. When the properties measured for the detected excess sources are combined with the larger population of submillimeter excess sources from the literature, we find strong evidence that the mass in small grains declines significantly on a ~200 Myr timescale, approximately inversely with age. However, we also find that the characteristic dust radii of the population, obtained from the dust temperature of the excess and assuming blackbody grains, is uncorrelated with age. This is in contrast to self-stirred collisional models for debris disk evolution, which predict a trend of radius increasing with age t_age ∝ R. The lack of agreement suggests that processes beyond self-stirring, such as giant planet formation, play a role in the evolutionary histories of planetesimal disks.

We present a model of collapsing magnetic traps in magnetic field configurations associated with solar flares. The model is based on a kinematic description of the magnetic field obeying the ideal Ohm's law. The dynamic evolution of the models is given in terms of a time-dependent transformation from Eulerian to Lagrangian coordinates. The transformation can be used to determine the corresponding flow field, the magnetic field, and the electric field from given initial conditions. The theory is formulated for translationally invariant situations, but a fully three-dimensional version is also given. The effect of various transformations and initial conditions is discussed with a view to calculating charged particle orbits in the given electromagnetic fields.

We have used Global Oscillation Network Group (GONG) magnetograms to characterize the changes in the photospheric longitudinal magnetic field during 15 X-class solar flares. An abrupt, significant, and persistent change in the magnetic field occurred in at least one location within the flaring active region during each event. We have identified a total of 42 sites where such field changes occurred. At 75% of these sites, the magnetic field change occurred in less than 10 minutes. The absolute values of the field changes ranged between 30 and almost 300 G, the median being 90 G. Decreases in the measured field component were twice as frequent as increases. The field changes ranged between 1.4 and 20 times the rms noise of the observations. In all but one equivocal case, the field changes occurred after the start of the flare. In all cases, the field changes were permanent. At least two-thirds of the field changes occurred in the penumbrae of sunspots. During three events for which simultaneous Transition Region and Coronal Explorer (TRACE) images are available, we have found excellent spatial and temporal correlation between the change in the magnetic field and an increase in brightness of the footpoints of flare ribbons, but not vice versa. Among many possible explanations for the observations, we favor one in which the magnetic field changes result from the penumbral field relaxing upward by reconnecting magnetic fields above the surface. One of the basic assumptions of flare theories is that the photospheric magnetic field does not change significantly during flares. These results suggest that this assumption needs to be re-examined.

We study statistical properties of small-scale magnetic features around sunspots using time sequences of high-resolution magnetograms of eight sunspots made with the Michelson Doppler Imager (MDI) on board SOHO. Flow maps around the spots are also derived from cross-correlation analysis of MDI continuum or TRACE white light and used for comparison of photospheric flow patterns with the tracks of moving magnetic features. An automated algorithm to find and track unipolar concentrations of magnetic field was developed. Depending on the velocity, size, and distance from the spot, a selected subset of all concentrations can be identified as moving magnetic features (MMFs). Our method finds 4-24 MMFs per hour around the spots, with higher counts for larger sunspots. After being first detected, the MMFs have an average flux content = 2.5 × 10¹⁸ Mx. Their average lifetime is about 1 hr, but it takes a concentration only t_max = 25 minutes to reach its maximum flux content of about = 6.1 × 10¹⁸ Mx. MMFs are found to transport a net flux out of a spot at a rate of (0.4-6.2) × 10¹⁹ Mx hr^-1: if sunspots were to decay only by outflowing MMFs, it would take a sunspot of 10²² Mx one to several weeks to completely disassemble. The MMFs have an initial velocity of = 1.8 km s^-1, faster than the average moat flow. Before merging into the moat region or surrounding network, they travel a distance = 3.5 Mm. The tracks of the individual MMFs correlate with the direction of local plasma flows and sometimes display a spokelike pattern around the sunspots. We find an average initial size = 1.7 Mm², but the distribution of sizes suggests features with a diameter of only 600-1000 km, which would not be recognized by our algorithm. Comparison of a cotemporal, cospatial magnetogram made with the Swedish Vacuum Solar Telescope (SVST) on La Palma (with 12 times the spatial resolution) indicates that unipolar magnetic fluxes in the MDI magnetogram may be comprised of smaller elements with both polarities.

Novel spectropolarimetric observations of the He I multiplet are used to explore the dynamics of the chromospheric oscillation above sunspot umbrae. The results presented here provide strong evidence in support of the two-component model proposed by Socas-Navarro and coauthors. According to this model, the waves propagate only inside channels of subarcsecond width (the "active" component), whereas the rest of the umbra remains nearly at rest (the "quiet" component). Although the observations support the fundamental elements of that model, there is one particular aspect that is not compatible with our data. We find that, contrary to the scenario as originally proposed, the active component remains through the entire oscillation cycle and harbors both the upflowing and the downflowing phase of the oscillation.

We study global inertial modes with the purpose of unraveling the role they play in the tidal dissipation process of Jupiter. For spheres of uniformly rotating, neutrally buoyant fluid, we show that the partial differential equation governing inertial modes can be separated into two ordinary differential equations when the density is constant or when the density has a power-law dependence on radius. For more general density dependencies, we show that one can obtain an approximate solution to the inertial modes that is accurate to the second order in wavevector. Frequencies of inertial modes are limited to ω < 2Ω (Ω is the rotation rate), with modes propagating closer to the rotation axis having higher frequencies. An inertial mode propagates throughout much of the sphere with a relatively constant wavelength and a wave amplitude that scales with density as 1/. It is reflected near the surface at a depth that depends on latitude, with the depth being much shallower near the special latitudes θ = ±ω/2Ω. Around this region, this mode has the highest wave amplitude as well as the sharpest spatial gradient (the "singularity belt"), thereby incurring the strongest turbulent dissipation. Inertial modes naturally cause small Eulerian density perturbations, so they are only weakly coupled to the tidal potential. In a companion paper, we attempt to apply these results to the problem of tidal dissipation in Jupiter.

The process of tidal dissipation inside Jupiter is not yet understood. Its tidal quality factor (Q) is inferred to lie between 10⁵ and 10⁶. Having studied the structure and properties of inertial modes in a neutrally buoyant, coreless, uniformly rotating sphere in a companion paper, we examine here their effects on tidal dissipation. The rate of dissipation caused by resonantly excited inertial modes depends on the following three parameters: how well they are coupled to the tidal potential, how strongly they are dissipated (by the turbulent viscosity), and how densely distributed they are in frequency. We find that as a function of tidal frequency, the Q-value exhibits large fluctuations, with its maximum value set by the group of inertial modes that satisfy δω ~ γ, where δω is the group's typical offset from an exact resonance and γ are the modes' turbulent damping rates. These are intermediate-order inertial modes with wavenumber λ ~ 60, and they are excited to a small surface displacement amplitude of order 10³ cm. The Q-value drops much below the maximum value whenever a lower order mode happens to be in resonance. In our model, inertial modes shed their tidally acquired energy very close to the surface within a narrow latitudinal zone (the "singularity belt"), and the tidal luminosity escapes freely out of the planet. The strength of coupling between the tidal potential and inertial modes is sensitive to the presence of density discontinuities inside Jupiter. In the case of a discreet density jump, as may be caused by the transition between metallic and molecular hydrogen, we find a time-averaged Q ~ 10⁷, with a small but nonnegligible chance (~10%) that the current Q-value falls within the empirically determined range. But when such a jump does not exist, Q ~ 10⁹. Even though it remains unclear whether tidal dissipation due to resonant inertial modes is the correct answer to the problem, it is impressive that even our simple treatment taking planetary rotation into account already leads to a 3-5 orders of magnitude stronger damping than when rotation is ignored. Moreover, our conclusions are not affected by the presence of a small solid core, a different prescription for the turbulent viscosity, or nonlinear mode coupling, but they depend critically on the static stability in the upper atmosphere of Jupiter. This is currently uncertain. Lastly, we compare our results with those from a competing work by Ogilvie & Lin and discuss the prospect of extending this theory to exo-Jupiters, which appear to possess Q-values similar to that of Jupiter.

A body in solar orbit beyond the Kuiper Belt exhibits an annual parallax that exceeds its apparent proper motion by up to many orders of magnitude. Apparent motion of this body along the parallactic ellipse will deflect the angular position of background stars due to astrometric microlensing ("induced parallax"). By synoptically sampling the astrometric position of background stars over the entire sky, constraints on the existence (and basic properties) of a massive nearby body may be inferred. With a simple simulation, we estimate the signal-to-noise ratio for detecting such a body—as a function of mass, heliocentric distance, and ecliptic latitude—using the anticipated sensitivity and temporal cadences from Gaia (launch date 2011). A Jupiter-mass (M_J) object at 2000 AU is detectable by Gaia over the whole sky above 5 σ, with even stronger constraints if it lies near the ecliptic plane. Hypotheses for the mass (~3M_J), distance (~20,000 AU), and location of the proposed perturber ("Planet X"), which gives rise to long-period comets, may be testable.

Characteristic X-ray emission lines are detected from simulants of comet surfaces as they undergo collisions with highly charged ions (HCIs). The HCI projectiles are O⁺²-O⁺⁷. Ion energies are varied in the range (2-7)q keV, where q is the ion charge state. The targets are the insulator minerals olivine, augite, and quartz. It is found that the emission of characteristic K-L, K-M X-rays appears to proceed during positive charging of the surface by the HCI beam. When one uses low-energy, flood-gun electrons to neutralize the surface charge, the X-ray emission is eliminated or greatly reduced, depending on the flood-gun current. Acceleration of background electrons onto the charged surface results in excitation of elemental transitions, including the K-L₂ and K-L₃ target X-ray emission lines of Mg and Si located spectroscopically at 1253.6 and 1739.4 eV, respectively. Also observed are emission lines from O, Na, Ca, Al, and Fe atoms in the target and charge-exchange lines via surface extraction of electrons by the O^+q electric field. Good agreement is found in the ratio of the measured X-ray yields for Mg and Si relative to the ratio of their electron-impact K-shell ionization cross sections. The present study may serve as a guide to astronomers as to specific observing X-ray energies indicative of solar/stellar wind or magnetospheric ion interactions with a comet, planetary surface, or circumstellar dust.

A new code and methodology are introduced for solving the general relativistic magnetohydrodynamic (GRMHD) equations in fixed background spacetimes using time-explicit, finite-volume discretization. The code has options for solving the GRMHD equations using traditional artificial viscosity (AV) or nonoscillatory central difference (NOCD) methods, or a new extended AV (eAV) scheme using artificial viscosity together with a dual energy/flux-conserving formulation. The dual-energy approach allows for accurate modeling of highly relativistic flows at boost factors well beyond what has been achieved to date by standard artificial viscosity methods. It provides the benefit of Godunov methods in capturing high Lorentz boosted flows, but without complicated Riemann solvers, and the advantages of traditional artificial viscosity methods in their speed and flexibility. In addition, the GRMHD equations are solved on an unstructured grid that supports local adaptive mesh refinement using a fully threaded oct-tree (in three dimensions) network to traverse the grid hierarchy across levels and immediate neighbors. A number of tests are presented to demonstrate robustness of the numerical algorithms and adaptive mesh framework over a wide spectrum of problems, boosts, and astrophysical applications, including relativistic shock tubes, shock collisions, magnetosonic shocks, Alfvén wave propagation, blast waves, magnetized Bondi flow, and the magnetorotational instability in Kerr black hole spacetimes.

To date, in almost all strong gravitational lensing analyses for modeling giant arc systems and multiple quasar images, it has been assumed that all the deflecting matter is concentrated in one lens plane at a certain distance—the thin lens approximation. However, in a few observed cases, lenses at more than one redshift have been identified as contributing to the image splitting. Here we report on a quantitative investigation of the importance and frequency of significant multiple lensing agents. We use multilens plane simulations to evaluate how frequently the combination of two or more lens planes is essential for multiple imaging, as compared with the cases for which a single lens plane alone provides enough focusing to be supercritical. We find that the fraction of cases for which more than one lens plane contributes significantly to a multiimage lensing situation is a strong function of source redshift. For sources at redshift unity, 95% of lenses involve only a single mass concentration, but for a more typical scenario with, e.g., a source at a redshift of z_s = 3.8, as many as 38% of the strongly lensed quasars/arcs occur because of a significant matter contribution from one or more additional lens planes. In the roughly 30% of cases when additional planes make a significant contribution, the surface mass density of the primary lens will be overestimated by about 10%-15%, if the additional contributions are not recognized.

We report on Spitzer IRAC observations of the spectroscopically confirmed z = 6.56 lensed Lyα emitting source HCM 6A that was found behind the cluster Abell 370. Detection of the source at 3.6 and 4.5 μm, corresponding to rest-frame optical emission, allows us to study the stellar population of this primeval galaxy. The broadband flux density at 4.5 μm is enhanced compared to the continuum at other wavelengths, likely due to the presence of strong Hα in emission. The derived Hα line flux corresponds to a star formation rate of ≳140 M_☉ yr^-1, more than an order of magnitude larger than estimates from the ultraviolet continuum and Lyα emission line. The dust extinction required to explain the discrepancy is A_V ~ 1 mag. The inference of dust at such high redshifts is surprising and implies that the first epoch of star formation in this galaxy occurred at z ~ 20.

We have analyzed XMM-Newton Optical Monitor UV (180-400 nm) data for a sample of 33 galaxies. Thirty are cluster member galaxies, and nine are central cluster galaxies (CCGs) in cooling flow clusters having mass deposition rates between 8 and 525 M_☉ yr^-1. By comparing the ratio of UV to 2MASS J-band fluxes, we find a significant UV excess in many, but not all, cooling flow CCGs, consistent with several previous studies based on optical imaging data (McNamara & O'Connell; Cardiel et al.; Crawford et al.). This UV excess is a direct indication of the presence of young massive stars and, therefore, recent star formation. Using the Starburst99 model of continuous star formation over a 900 Myr period, we derive star formation rates of 0.2-219 M_☉ yr^-1 for the cooling flow sample. For two-thirds of this sample, it is possible to equate Chandra/XMM cooling flow mass deposition rates with UV-inferred star formation rates, for a combination of starburst lifetime and IMF slope. This is a pilot study of the well-populated XMM UV cluster archive, and a more extensive follow-up study is currently underway.

We study the impact of outflows driven by active galactic nuclei (AGNs) on galaxy formation. Outflows move into the surrounding intergalactic medium (IGM) and heat it sufficiently to prevent it from condensing onto galaxies. In the dense, high-redshift IGM, such feedback requires a highly energetic outflow, driven by a large AGN. However, in the more tenuous low-redshift IGM, equivalently strong feedback can be achieved by less energetic winds (and thus smaller galaxies). Using a simple analytic model, we show that this leads to the antihierarchical quenching of star formation in large galaxies, consistent with current observations. At redshifts prior to the formation of large AGNs, galaxy formation is hierarchical and follows the growth of dark matter halos. The transition between the two regimes lies at the z ≈ 2 peak of AGN activity..

We show that the observational data of extragalactic radio sources tend to support the theoretical relationship between the jet precession period and the optical luminosity of the sources, as predicted by the model in which an accretion disk causes the central black hole to precess.

We have discovered a sample of 17 metal-poor, yet luminous, star-forming galaxies at redshifts z ~ 0.7. They were selected from the initial phase of the DEEP2 survey of 3900 galaxies and the Team Keck Redshift Survey (TKRS) of 1536 galaxies as those showing the temperature-sensitive [O III] λ4363 auroral line. These rare galaxies have blue luminosities close to L*, high star formation rates of 5-12 M_☉ yr^-1, and oxygen abundances of 1/3 to 1/10 solar. They thus lie significantly off the luminosity-metallicity relation found previously for field galaxies with strong emission lines at redshifts z ~ 0.7. The prior surveys relied on indirect, empirical calibrations of the R₂₃ diagnostic and the assumption that luminous galaxies are not metal-poor. Our discovery suggests that this assumption is sometimes invalid. As a class, these newly discovered galaxies are (1) more metal-poor than common classes of bright emission-line galaxies at z ~ 0.7 or at the present epoch; (2) comparable in metallicity to z ~ 3 Lyman break galaxies but less luminous; and (3) comparable in metallicity to local metal-poor extreme blue compact galaxies (XBCGs), but more luminous. Together, the three samples suggest that the most luminous, metal-poor, compact galaxies become fainter over time.

Mid-infrared observations (3.6-24 μm) of normal giant elliptical galaxies with the Spitzer Space Telescope are consistent with pure populations of very old stars with no evidence of younger stars. Most of the stars in giant elliptical galaxies are old, but the mean stellar age determined from Balmer absorption in optical spectra can appear much younger due to a small admixture of younger stars. The mean stellar age can also be determined from the spectral energy distribution in the mid-infrared, which decreases with time relative to the optical emission and shifts to shorter wavelengths. The observed flux ratios F_{8 μm}/F_{3.6 μm} and F_{24 μm}/F_{3.6 μm} for elliptical galaxies with the oldest Balmer line ages are lower than predicted by recent models of single stellar populations. For elliptical galaxies with the youngest Balmer line ages in our sample, 3-5 Gyr, the flux ratios F_{24 μm}/F_{3.6 μm} are identical to those of the oldest stars. When theoretical mid-IR spectra of old (12 Gyr) and young stellar populations are combined, errors in the F_{24 μm}/F_{3.6 μm} observations are formally inconsistent with a mass fraction of young stars that exceeds ~1%. This is less than the fraction of young stars expected in discussions of recent surveys of elliptical galaxies at higher redshifts. However, this inconsistency between Balmer line ages and those inferred from mid-IR observations must be regarded as provisional until more accurate observations and theoretical spectra become available. Finally, there is no evidence to date that central disks or patches of dust commonly visible in optical images of elliptical galaxies contribute sensibly to the mid-IR spectrum.

We have found the u - r color versus g - i color gradient space can be used for highly successful morphology classification of galaxies in the Sloan Digital Sky Survey. In this space, galaxies form well-separated early- and late-type branches. The location of galaxies along the branches reflects the degree and locality of star formation activity, and monotonically corresponds to the sequence of morphological subclasses. When the concentration index is also used, the completeness and reliability of classification reaches about 91% for a training set of SDSS galaxies brighter than r_pet ≈ 15.9. At the faintest magnitudes (r_pet ≈ 17.5) of the SDSS spectroscopic sample, the accuracy still remains at about 88%. The new classification scheme will help us find accurate relations of galaxy morphology with spatial and temporal environments, and help us to understand the origin of morphology of galaxies.

An explanation is given of the low value of R_λ ≡ A_λ/E(B - V), the ratio of absolute to selective extinction deduced from Type Ia supernova observations. The idea involves scattering by dust clouds located in the circumstellar environment or at the highest velocity shells of the supernova ejecta. The scattered light tends to reduce the effective R_λ in the optical but has an opposite effect in the ultraviolet. The presence of circumstellar dust can be tested by ultraviolet to near-infrared observations and by multiepoch spectropolarimetry of Type Ia supernovae.

We present the first detailed spectroscopic and photometric analysis of an eclipsing binary in the Andromeda Galaxy (M31). This is a 19.3 mag semidetached system with late O and early B spectral type components. From the light and radial velocity curves we have carried out an accurate determination of the masses and radii of the components. Their effective temperatures have been estimated by modeling the absorption-line spectra. The analysis yields an essentially complete picture of the properties of the system, and hence an accurate distance determination to M31. The result is d = 772 ± 44 kpc [(m - M)₀ = 24.44 ± 0.12 mag]. The study of additional systems, currently in progress, should reduce the uncertainty of the M31 distance to better than 5%.

We use low-dispersion spectra obtained at the Magellan Observatory to study the broad Hα emission from the reverse shock of the infant supernova remnant SNR 1987A. These spectra demonstrate that the spatiokinematic structure of the reverse shock can be distinguished from that of the circumstellar ring and hot spots, even at ground-based spatial resolution. We measure a total dereddened Hα flux of 1.99(±0.22) × 10^-13 ergs s^-1 cm^-2 at an epoch 18.00 yr after outburst. At 50 kpc, the total reverse shock luminosity in Hα is roughly 15 L_☉, which implies a total flux of neutral hydrogen atoms across the reverse shock of 8.9 × 10⁴⁶ s^-1, or roughly 2.3 × 10^-3M_☉ yr^-1. This represents an increase by a factor of ~4 since 1997. Lyman continuum radiation from gas shocked by the forward blast wave can ionize neutral hydrogen atoms in the supernova debris before they reach the reverse shock. If the inward flux of ionizing photons exceeds the flux of hydrogen atoms approaching the reverse shock, this preionization will shut off the broad Lyα and Hα emission. The observed X-ray emission of SNR 1987A implies that the ratio of ionizing flux to hydrogen atom flux across the reverse shock is presently at least 0.04. The X-ray emission is increasing much faster than the flux of atoms, and if these trends continue, we estimate that the broad Lyα and Hα emission will vanish in ≲ 7 yr.

The central parsec of the Galaxy contains dozens of massive stars with a cumulative mass-loss rate of ~10^-3M_☉ yr^-1. Shocks among these stellar winds produce the hot plasma that pervades the central part of the Galaxy. We argue that these stellar wind shocks also efficiently accelerate electrons and protons to relativistic energies. The relativistic electrons inverse Compton scatter the ambient ultraviolet and far-infrared radiation field, producing high-energy γ-rays with a roughly constant luminosity from ~GeV to ~10 TeV. This can account for the TeV source seen by HESS in the Galactic center. Our model predicts a GLAST counterpart to the HESS source with a luminosity of ≈10³⁵ ergs s^-1 and cooling break at ≈4 GeV. Synchrotron radiation from the same relativistic electrons should produce detectable emission at lower energies, with a surface brightness of ≈10^-14B ergs s^-1 cm^-2 arcsec^-2 from ~THz to ~keV, where B_-3 is the magnetic field strength in units of mG. The observed level of diffuse thermal X-ray emission in the central parsec requires B ≲ 300 μG in our models. Future detection of the diffuse synchrotron background in the central parsec can directly constrain the magnetic field strength, providing an important boundary condition for models of accretion onto Sgr A*.

We report the discovery of multiple two-dimensional rings in the quadrupolar planetary nebula NGC 6881. As many as four pairs of rings are seen in the bipolar lobes, and three rings are seen in the central torus. While the rings in the lobes have the same axis as one pair of the bipolar lobes, the inner rings are aligned with the other pair. The two pairs of bipolar lobes are likely to be carved out by two separate high-velocity outflows from the circumstellar material left over from the asymptotic giant branch (AGB) wind. The two-dimensional rings could be the results of dynamical instabilities or the consequence of a fast outflow interacting with remnants of discrete AGB circumstellar shells.

We searched for steady PeV gamma-ray emission from the Monogem ring region with the Tibet air shower array from 1997 February to 2004 October. No evidence for statistically significant gamma-ray signals was found in a region 111° ≤ R.A. < 114°, 125 ≤ decl. < 155 in the Monogem ring where the MAKET-ANI experiment recently claimed a positive detection of PeV high-energy cosmic radiation, although our flux sensitivity is approximately 10 times better than MAKET-ANI's. We set the most stringent integral flux upper limit at a 99% confidence level of 4.0 × 10^-12 cm^-2 s^-1 sr^-1 above 1 PeV on diffuse gamma rays extended in the 3° × 3° region.

We suggest a plausible interpretation for the twin kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries. We identify the upper kHz QPO frequencies to be the rotational frequency and the lower kHz QPOs as the standing kink modes of loop oscillations at the inner edge of the accretion disk, respectively. Taking into account the interaction between the neutron star magnetic field and the disk, this model naturally relates the twin QPO frequencies with the star's spin frequencies. We have applied the model to four X-ray sources with kHz QPOs detected simultaneously and the known spin frequencies.

It is generally believed that magnetic fields of some neutron stars, the so-called magnetars, are enormously strong, up to 10¹⁴-10¹⁵ G. Recent investigations have shown that the atmospheres of magnetars are possibly composed of helium. We calculate the structure and bound-bound radiative transitions of the He⁺ ion in superstrong fields, including the effects caused by the coupling of the ion's internal degrees of freedom to its center-of-mass motion. We show that He⁺ in superstrong magnetic fields can produce spectral lines with energies of up to ≈3 keV, and it may be responsible for absorption features detected recently in the soft X-ray spectra of several radio-quiet isolated neutron stars. Quantization of the ion's motion across a magnetic field results in a fine structure of spectral lines, with a typical spacing of tens of electron volts in magnetar-scale fields. It also gives rise to ion cyclotron transitions, whose energies and oscillator strengths depend on the state of the bound ion. The bound-ion cyclotron lines of He⁺ can be observed in the UV-optical range at B ≲ 10¹³ G, and they get into the soft X-ray range at B ≳ 10¹⁴ G.

We present a phase-connected timing solution for the nearby isolated neutron star RX J1308.6+2127 (RBS 1223). From dedicated Chandra observations as well as archival Chandra and XMM-Newton data spanning a period of 5 years, we demonstrate that the 10.31 s pulsations are slowing down steadily at a rate of = 1.120(3) × 10^-13 s s^-1. Under the assumption that this is due to magnetic dipole torques, we infer a characteristic age of 1.5 Myr and a magnetic field strength of 3.4 × 10¹³ G. As with RX J0720.4-3125, the only other radio-quiet thermally emitting isolated neutron star for which a timing solution has been derived, the field strength is roughly consistent with what was inferred earlier from the presence of a strong absorption feature in its X-ray spectrum. Furthermore, for both sources the characteristic age is in excess of the cooling age inferred from standard cooling models. The sources differ, however, in their timing noise: while RX J0720.4-3125 showed considerable timing noise, RX J1308.6+2127 appears relatively stable.

We describe the results of numerical simulations of the dynamics of the boundary layer (BL) between the accretion disk and the surface of a nonmagnetic white dwarf (WD) for different viscosities that correspond to different stages for dwarf novae burst cycles. The simulations cover the inner part of the accretion disk, the BL, and the upper atmosphere of the star. The high-viscosity case, which corresponds to a dwarf nova in outburst, shows an optically thick BL that after 1 Keplerian rotation period (t_K = 19 s) extends more than 30° to either side of the disk plane. The BL is optically thick and thus occludes part of the star. The low-viscosity case, which corresponds to a dwarf nova in quiescence, also shows a BL, but it is optically thin.

We examine the blue straggler populations of 13 low-luminosity (M ≳ -6) globular clusters and two old open clusters. These clusters test blue straggler formation in environments intermediate between higher luminosity (and usually higher density) clusters and the Galactic field. The anticorrelation between the relative frequency of blue stragglers (F_BSS = N_BSS/N_HB) and cluster luminosity continues to the lowest luminosity clusters, which have frequencies meeting or exceeding that of field stars. In addition, we find that the anticorrelation between straggler frequency and central density disappears for clusters with density less than about 300 L_{V, ☉} pc^-3, although this appears to be an artifact of the correlation between cluster luminosity and central density. We argue on observational (wide, eccentric binaries containing blue stragglers in M67, and the existence of very bright stragglers in most of the clusters in our sample) and theoretical grounds that stellar collisions still produce a significant fraction of the blue stragglers in low-luminosity star clusters, due to the long-term survival of wide binaries.

We have extracted a 37 day light curve with a precision of 0.0012 mag per point for the Microvariability and Oscillations of Stars (MOST) guide star, HD 163868 (B5 Ve). Its rich frequency spectrum resembles that of a slowly pulsating B (SPB) star but, being a rapid rotator, we designate it SPBe. The 60 most significant periods lie in three distinct groups centered on 8 days and 14 and 7 hr. We demonstrate that the 14 and 7 hr periods can be modeled by two swarms of high-order, prograde sectorial g-modes (m = -1, -2), which are destabilized by the iron opacity bump. Our model also predicts a group of r-modes with periods near 2.3 days, which correspond to frequencies observed in the tail of the 8 day group. The remaining periodicities, between 7 and 11 days, cannot be explained by unstable modes in our model.

We present results from a multiwavelength campaign to monitor the 2005 outburst of the low-mass young star V1118 Ori. Although our campaign covers the X-ray, optical, infrared, and radio regimes, we focus in this Letter on the properties of the X-ray emission in V1118 Ori during the first few months after the optical outburst. Chandra and XMM-Newton detected V1118 Ori at three epochs in early 2005. The X-ray flux and luminosity stayed similar within a factor of 2 and at the same level as in a preoutburst observation in 2002. The hydrogen column density showed no evidence for variation from its modest preoutburst value of N_H ~ 3 × 10²¹ cm^-2. However, a spectral change occurred from a dominant hot plasma (~25 MK) in 2002 and in 2005 January to a cooler plasma (~8 MK) in 2005 February and in 2005 March. We hypothesize that the hot magnetic loops high in the corona were disrupted by the closing in of the accretion disk due to the increased accretion rate during the outburst, whereas the lower cooler loops were probably less affected and became the dominant coronal component.

Recent observations of the ground-state transition of HDO at 464 GHz toward the protoplanetary disk of DM Tau have detected the presence of water vapor in the regions just above the outer disk midplane (Ceccarelli et al.). In the absence of nonthermal desorption processes, water should be almost entirely frozen onto the grain mantles, and HDO undetectable. In this Letter we present a chemical model that explores the possibility that the icy mantles are photodesorbed by FUV (6 eV ≤ hν ≤ 13.6 eV) photons. We show that the average interstellar FUV field is enough to create a layer of water vapor above the disk midplane over the entire disk. Assuming a photodesorption yield of 10^-3, the water abundance in this layer is predicted to be ~3 × 10^-7, and the average H₂O column density is ~1.6 × 10¹⁵ cm^-2. The predictions are very weakly dependent on the details of the model, like the incident FUV radiation field, and on the gas density in the disk. Based on this model, we predict a gaseous HDO/H₂O ratio in DM Tau of ~1%. In addition, we predict the ground-state transition of water at 557 GHz to be undetectable with Odin and/or with the Herschel Space Observatory Heterodyne Instrument for the Far Infrared (HIFI).

The recent discovery by M. Konacki of a "hot Jupiter" in the hierarchical triple star system HD 188753 challenges established theories of giant planet formation. If the orbital geometry of the triple has not changed since the birth of the planet, then a disk around the planetary host star would probably have been too compact and too hot for a Jovian planet to form by the core accretion model or gravitational collapse. This paradox is resolved if the star was initially either single or had a much more distant companion. It is suggested here that a close multistar dynamical encounter transformed this initial state into the observed triple, an idea that follows naturally if HD 188753 formed in a moderately dense stellar system—perhaps an open cluster—that has since dissolved. Three distinct types of encounters are investigated. The most robust scenario involves an initially single planetary host star that changes places with the outlying member of a preexisting hierarchical triple.

Using the Hubble Space Telescope, the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory, and the Spitzer Space Telescope, we have performed deep imaging from 0.8 to 8 μm of the southern subcluster in the Chamaeleon I star-forming region. In these data, we have discovered an object, Cha 110913-773444, whose colors and magnitudes are indicative of a very low mass brown dwarf with a circumstellar disk. In a near-infrared spectrum of this source obtained with the Gemini Near-Infrared Spectrograph, the presence of strong steam absorption confirms its late-type nature (≳M9.5) while the shapes of the H- and K-band continua and the strengths of the Na I and K I lines demonstrate that it is a young, pre-main-sequence object rather than a field dwarf. A comparison of the bolometric luminosity of Cha 110913-773444 to the luminosities predicted by the evolutionary models of Chabrier & Baraffe and Burrows and coworkers indicates a mass of 8M_J, placing it fully within the mass range observed for extrasolar planetary companions (M ≲ 15M_J). The spectral energy distribution of this object exhibits mid-infrared excess emission at λ > 5 μm, which we have successfully modeled in terms of an irradiated viscous accretion disk with ≲ 10^-12M_☉ yr^-1. Cha 110913-773444 is now the least massive brown dwarf observed to have a circumstellar disk, and indeed is one of the least massive free-floating objects found to date. These results demonstrate that the raw materials for planet formation exist around free-floating planetary-mass bodies.

We present the discovery and initial physical and dynamical characterization of the object 2003 UB313. The object is sufficiently bright that for all reasonable values of the albedo it is certain to be larger than Pluto. Prediscovery observations back to 1989 are used to obtain an orbit with extremely small errors. The object is currently at aphelion in what appears to be a typical orbit for a scattered Kuiper Belt object, except that it is inclined by about 44° from the ecliptic. The presence of such a large object at this extreme inclination suggests that high-inclination Kuiper Belt objects formed preferentially closer to the Sun. Observations from Gemini Observatory show that the infrared spectrum is, like that of Pluto and 2005 FY9, dominated by the presence of frozen methane, although visible photometry shows that the object is almost neutral in color compared to Pluto's extremely red color. 2003 UB313 is likely to undergo substantial seasonal change over the large range of heliocentric distances that it travels; at its current distance, Pluto is likely to prove a useful analog for better understanding the range of seasonal changes on this body.

An early result from the Transition Region and Coronal Explorer (TRACE) was that the EUV filter ratios for many narrow coronal loops (widths of a few arcseconds) were found to cluster within the small range 0.50-1.70, as functions of position along loop length. The most common interpretation is that the temperature along the loop is in fact nearly constant with a value between 1.1 and 1.3 MK. This interpretation has resulted in a class of TRACE loop models with heating close to the footpoints. We analyze the filter ratio method to show that the constant TRACE 195 Å/173 Å ratios can be reproduced by multithermal differential emission measures (DEMs) along the line of sight over a wide range of peak temperatures, so long as the distribution is relatively flat and spans the temperature response of both channels. Furthermore, in the limit of flat (i.e., very multithermal) DEMs, the filter ratio method is biased toward the ratio of the integrals of the temperature response functions. This result is general to any measurement of intensity ratios that are formed over a nonzero temperature range (e.g., narrow passbands and ion emission lines).

Roxburgh & Vorontsov have recently proposed the use of ratios of small to large frequency separations of low angular degree p-modes as a means of eliminating from asteroseismic data the unwanted influence of the structure of the near-surface layers of stars. Here we have studied the impact of the solar activity cycle on the magnitude of these so-called frequency separation ratios using data collected by Sun-as-a-star observations. The ratios are observed to change with the shifting level of global solar activity. The effect, which we detect in BiSON Doppler velocity data at a marginal level of significance, is shown to be a consequence of the influence of acoustic asphericity from the surface activity on the azimuthally dependent Sun-as-a-star frequencies. The results suggest that any analysis that makes use of ratios formed from long helioseismic data sets may therefore show effects of bias. While the effect is less significant in shorter data sets, of length similar to what will soon be available from asteroseismic campaigns, an approximate doubling of the effects from the solar asphericity may be sufficient to cause complications for stellar analyses.

The analysis of the Hanle effect in solar molecular lines allows us to obtain empirical information on hidden, mixed-polarity magnetic fields at subresolution scales in the (granular) upflowing regions of the "quiet" solar photosphere. Here we report that collisions seem to be very efficient in depolarizing the rotational levels of MgH lines. This has the interesting consequence that in the upflowing regions of the quiet solar photosphere the strength of the hidden magnetic field cannot be much larger than 10 G, assuming the simplest case of a single-valued microturbulent field that fills the entire upflowing photospheric volume. Alternatively, an equally good theoretical fit to the observed scattering polarization amplitudes can be achieved by assuming that the rate of depolarizing collisions is an order of magnitude smaller than in the previous collisionally dominated case, but then the required strength of the hidden field in the upflowing regions turns out to be unrealistically high. These constraints reinforce our previously obtained conclusion that there is a vast amount of hidden magnetic energy and unsigned magnetic flux localized in the (intergranular) downflowing regions of the quiet solar photosphere.