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Motivated by the Wilkinson Microwave Anisotropy Probe (WMAP) results indicating an early epoch of reionization, we consider alternative cosmic star formation models that are capable of reionizing the early intergalactic medium. We develop models that include an early burst of massive stars (with several possible mass ranges) combined with standard star formation. We compute the stellar ionizing flux of photons, and we track the nucleosynthetic yields for several elements: D, ⁴He, C, N, O, Si, S, Fe, and Zn. We compute the subsequent chemical evolution as a function of redshift, both in the intergalactic medium and in the interstellar medium of forming galaxies, starting with the primordial objects that are responsible for the reionization. We apply constraints from the observed abundances in the Lyα forest and in damped Lyα clouds in conjunction with the ability of the models to produce the required degree of reionization. We also consider possible constraints associated with the observations of the two extremely metal-poor stars HE 0107-5240 and CS 22949-037. We confirm that an early top-heavy stellar component is required, since a standard star formation model is unable to reionize the early universe and reproduce the abundances of the very metal-poor halo stars. A bimodal (or top-heavy) initial mass function (IMF; 40-100 M_☉) is our preferred scenario, compared with the extreme mass range (≳100 M_☉) often assumed to be responsible for the early stages of reionization. A mode of even more extreme stellar masses in the range ≥270 M_☉ has also been considered. All massive stars in this mode collapse entirely into black holes, and as a consequence, chemical evolution and reionization are decorrelated. The ionizing flux from these very massive stars can easily reionize the universe at z ~ 17. However, the chemical evolution in this case is exactly the same as in the standard star formation model, and the observed high-redshift abundances are not reproduced. We show that the initial top-heavy mode, which originally was introduced to reionize the early universe, produces rapid initial metal pollution. The existence of old, C-rich halo stars with high [O/Fe] and [C/Fe] ratios is predicted as a consequence of these massive stars. The recently observed abundances in the oldest halo stars could trace this very specific stellar population. The extreme mass range is disfavored, and there is no evidence, nor any need, for a hypothesized primordial population of very massive stars in order to account for the chemical abundances of extremely metal-poor halo stars or of the intergalactic medium. The combined population of early-forming normal (0.1-100 M_☉) and massive (40-100 M_☉) stars can simultaneously explain the cosmic chemical evolution and the observations of extremely metal-poor halo stars and also account for early cosmological reionization.

We present Keck Low Resolution Imaging Spectrometer optical spectra of 17 Lyα-emitting galaxies and one Lyman break galaxy at z ≈ 4.5 discovered in the Large Area Lyman Alpha survey. The survey has identified a sample of ~350 candidate Lyα-emitting galaxies at z ≈ 4.5 in a search volume of 1.5 × 10⁶ comoving Mpc³. We targeted 25 candidates for spectroscopy; hence, the 18 confirmations presented herein suggest a selection reliability of 72%. The large equivalent widths (median W ≈ 80 Å) but narrow physical widths (Δv < 500 km s^-1) of the Lyα emission lines, along with the lack of accompanying high-ionization state emission lines, suggest that these galaxies are young systems powered by star formation rather than by active galactic nucleus activity. Theoretical models of galaxy formation in the primordial universe suggest that a small fraction of Lyα-emitting galaxies at z ≈ 4.5 may still be nascent, metal-free objects. Indeed, we find with 90% confidence that three to five of the confirmed sources show W > 240 Å, exceeding the maximum Lyα equivalent width predicted for normal stellar populations. Nonetheless, we find no evidence for He II λ1640 emission in either individual or composite spectra, indicating that although these galaxies are young, they are not truly primitive, Population III objects.

We present results from an analysis of FUSE spectroscopy of the z_em = 0.57 quasar PKS 0405-123. We focus on the intervening metal-line systems identified along the sight line and investigate their ionization mechanism, ionization state, and chemical abundances. Including Hubble Space Telescope STIS spectroscopy, we survey the entire sight line and identify six O VI absorbers to a 3 σ equivalent width (EW) limit of 60 mÅ. This implies an incidence dN/dz = 16 consistent with previous O VI studies. In half of the O VI systems we report positive detections of C III, suggesting that the gas is predominantly photoionized, has multiple ionization phases, or is in a nonequilibrium state. This contrasts with the general description of the warm-hot intergalactic medium (WHIM) as described by numerical simulations in which the gas is predominantly in collisional ionization equilibrium. An appreciable fraction of O VI absorbers may therefore have a different origin. We have also searched the sight line for the Ne VIII doublet (a better probe of the WHIM at T > 10⁶ K) over the redshift range 0.2 < z < z_em. We report no positive detections to an EW limit of 80 mÅ, giving dN/dz < 40 at 95% c.l. The photoionized metal-line systems exhibit a correlation between the ionization parameter (U ≡ Φ/cn_H, with Φ the flux of hydrogen ionizing photons) and H I column density for N(H ) = 10¹⁴-10¹⁶ cm^-2. Both the slope and normalization of this correlation match the prediction inferred from the results of Davé and Tripp for the low-z Lyα forest. In turn, the data show a tentative, unexpected result: five out of the six photoionized metal-line systems show a total hydrogen column density within a factor of 2 of 10^18.7 cm^-2. Finally, the median metallicity [M/H] of twelve z ~ 0.3 absorbers with N(H ) > 10¹⁴ cm^-2 is [M/H] > -1.5, with large scatter. This significantly exceeds the median metallicity of C IV and O VI systems at z ~ 3 and indicates enrichment of the intergalactic medium over the past ≈10 Gyr.

A simple two-color selection based on B-, z-, and K-band photometry is proposed for culling galaxies at 1.4 ≲ z ≲ 2.5 in K-selected samples and classifying them as star-forming or passive systems. The method is calibrated on the highly complete spectroscopic redshift database of the K20 survey, verified with simulations and tested on other data sets. Requiring BzK = (z - K)_AB - (B - z)_AB > -0.2 allows us to select actively star-forming galaxies at z ≳ 1.4, independently of their dust reddening. On the other hand, objects with BzK < -0.2 and (z - K)_AB > 2.5 colors include passively evolving galaxies at z ≳ 1.4, often with spheroidal morphologies. Simple recipes to estimate the reddening, star formation rates (SFRs), and masses of BzK-selected galaxies are derived and are calibrated on K < 20 galaxies. These K < 20 galaxies have typical stellar masses of ~10¹¹M_☉ and sky and volume densities of ~1 arcmin^-2 and ~10^-4 Mpc^-3, respectively. Based on their UV (reddening-corrected), X-ray, and radio luminosities, the BzK-selected star-forming galaxies with K < 20 turn out to have average SFR ≈ 200 M_☉ yr^-1 and median reddening E(B - V) ~ 0.4. This SFR is a factor of 10 higher than that of z ~ 1 dusty extremely red objects, and a factor of 3 higher than found for z ~ 2 UV-selected galaxies, both at similar K limits. Besides missing the passively evolving galaxies, the UV selection appears to miss some relevant fraction of the z ~ 2 star-forming galaxies with K < 20, and hence of the (obscured) SFR density at this redshift. The high SFRs and masses add to other existing evidence that these z = 2 star-forming galaxies may be among the precursors of z = 0 early-type galaxies. A V/V_max test suggests that such a population may be increasing in number density with increasing redshift. Theoretical models cannot reproduce simultaneously the space density of both passively evolving and highly star-forming galaxies at z = 2. In view of Spitzer Space Telescope observations, an analogous technique based on RJL photometry is proposed to complement the BzK selection and to identify massive galaxies at 2.5 ≲ z ≲ 4.0. By selecting passively evolving galaxies as well as actively star-forming galaxies (including strongly dust-reddened ones), these color criteria should help in completing the census of the stellar mass and of the SFR density at high redshift.

When applied to deep photometric catalogs, the two-point angular correlation function, ω(θ), is a sensitive probe of the evolution of galaxy clustering properties. Here we present measurements of ω(θ) as a function of I_AB magnitude and (R - I) color to a depth of I_AB = 24 on scales θ = 7''-3', using a sample of ~350,000 galaxies covering 5 deg² in total over five separate fields. Using redshifts of 2954 galaxies in early DEEP2 Galaxy Redshift Survey data, we construct robust galaxy redshift distributions as a function of I_AB magnitude and (R - I) color for galaxies in the range 0 < z < 2. We constrain models of the redshift evolution of galaxy clustering and find that significant growth of clustering has occurred from z ≥ 1 to z = 0. A model in which the comoving scale length, x₀, evolves linearly with redshift, x₀(z) = x₀(0)(1 - Bz), fits the data better than the model proposed by Groth & Peebles. The clustering properties depend strongly on observed (R - I) color, with both the reddest and bluest galaxies exhibiting large clustering amplitudes and steeper slopes. Different observed (R - I) color ranges are sensitive to very disparate redshift regimes. Red galaxies with (R - I) ~ 1.5 lie in a narrow redshift range centered at z ~ 0.85 and have a comoving scale length of clustering at z = 0.85 of x₀ = 5.0 ± 0.3 h^-1 Mpc. These galaxies have early-type spectra and are likely progenitors of massive local ellipticals. The bluest galaxies with (R - I) ~ 0 appear to be a mix of star-forming galaxies, both relatively local (z ~ 0.3-0.6) dwarfs and bright z > 1.4 galaxies, as well as broad-line active galactic nuclei. We find that local blue dwarfs are relatively unclustered, with x₀ = 1.6 ± 0.2 h^-1 Mpc. The z > 1.4 blue galaxies have a larger clustering scale length, x₀ ≳ 5 h^-1 Mpc.

We present the first determination of the pairwise velocity dispersion (PVD) for galaxies in different luminosity intervals using the final release of the Two-Degree Field Galaxy Redshift Survey (2dFGRS). We have discovered quite surprisingly that the relative velocities of the faint galaxies at small separation are very high, around 700 km s^-1, reaching similar values as the brightest galaxies. At intermediate luminosities M* - 1 (M* is the characteristic luminosity of the Schechter function), the relative velocities exhibit a well-defined steep minimum near 400 km s^-1. This result has been derived using a novel method to determine the real-space power spectrum and the PVD from the redshift-space power spectrum of the 2dFGRS. Combined with the observed luminosity dependence of clustering, our result implies that quite a fraction of faint galaxies, as well as the brightest ones, are in massive halos of galaxy cluster size, but most of the M* galaxies are in galactic halos. Our observed result is compared with the current halo model of galaxies of Yang et al., which was obtained by matching the clustering and luminosity functions of the 2dFGRS. With the model parameters they favored most, the halo model seems to be unable to reproduce the luminosity dependence of the PVD, because it predicts a monotonically increasing PVD with the luminosity. We discuss a possible solution to this model by raising the faint-end slope of the conditional luminosity function in rich clusters. The PVD luminosity dependence may also be an important constraint in general on theories of galaxy formation, such as semianalytical models and hydro/N-body simulations of galaxy formation.

We present the first measurement of weak lensing by large-scale structure on scales of 1°-4° based on radio observations. We utilize the FIRST radio survey, a quarter-sky, 20 cm survey produced with the NRAO Very Large Array (VLA). The large angular scales afforded by the FIRST survey provide a measurement in the linear regime of the matter power spectrum, thus avoiding the necessity of applying uncertain nonlinear corrections. Moreover, since the VLA interferometer has a well-known and deterministic beam, our measurement does not suffer from the irreproducible effects of atmospheric seeing that limit ground-based optical surveys. We use the shapelet method described in an earlier paper to estimate the shear from the shape of radio sources derived directly from the interferometric measurements in the Fourier (u,v)-plane. With realistic simulations we verify that the method yields unbiased shear estimators. We study and quantify the systematic effects that can produce spurious shears, analytically and with simulations, and carefully correct for them. We measure the shear correlation functions on angular scales of 05-40° and compute the corresponding aperture mass statistics. On 1°-4° scales, we find that the B-modes are consistent with zero and detect a lensing E-mode signal significant at the 3.0 σ level. After removing nearby radio sources with an optical counterpart, the E-mode signal increases by 10%-20%, as expected for a lensing signal derived from more distant sources. We use the E-mode measurement on these scales to constrain the mass power spectrum normalization σ₈ and the median redshift z_m of the unidentified radio sources. We find σ₈(z_m/2)^0.6 ≃ 1.0 ± 0.2, where the 1 σ error bars include statistical errors, cosmic variance, and systematics. This is consistent with earlier determinations of σ₈ from cosmic shear, the cosmic microwave background, and cluster abundance, and with our current knowledge of the redshift distribution of radio sources. Taking the prior σ₈ = 0.9 ± 0.1 (68% CL) from the WMAP experiment, this corresponds to z_m = 2.2 ± 0.9 (68% CL) for radio sources without optical counterparts, consistent with existing models for the radio source luminosity function. Our results offer promising prospects for precision measurements of cosmic shear with future radio interferometers such as LOFAR and the SKA.

We present results from a spectroscopic study of ~4000 galaxies in a ~6.2 deg² field in the direction of the Aquarius supercluster and a smaller typical field region in Cetus, down to R < 19.5. Galaxy redshifts were measured using the Two Degree Field system on the Anglo-Australian Telescope and form part of our wider efforts to conduct a spectrophotometric and weak gravitational lensing study of these regions. At the magnitude limit of the survey, we are capable of probing L_* galaxies out to z ~ 0.4. We construct median spectra as a function of various survey parameters as a diagnostic of the quality of the sample. We use the redshift data to identify galaxy clusters and groups within the survey volume. In the Aquarius region, we find a total of 48 clusters and groups, of which 26 are previously unknown systems, and in Cetus we find 14 clusters and groups, of which 12 are new. We estimate centroid redshifts and velocity dispersions for all these systems. In the Aquarius region, we see a superposition of two strong superclusters at z = 0.08 and 0.11, both of which have estimated masses and overdensities similar to those in the Corona Borealis supercluster.

We present new measurements of the Sunyaev-Zel'dovich (SZ) effect from clusters of galaxies using the second-generation Sunyaev-Zel'dovich Infrared Experiment (SuZIE II). We combine these new measurements with previous cluster observations with the SuZIE instrument to form a sample of 15 clusters of galaxies. For this sample we calculate the central Comptonization y₀ and the integrated SZ flux decrement S for each of our clusters. We find that the integrated SZ flux is a more robust observable derived from our measurements than the central Comptonization because of inadequacies in the spatial modeling of the intracluster gas with a standard β-model. This is highlighted by comparing our central Comptonization results with values calculated from measurements using the BIMA and OVRO interferometers. On average, the SuZIE-calculated central Comptonizations are ~60% higher in the cooling flow clusters than the interferometric values, compared to only ~12% higher in the non-cooling flow clusters. We believe this discrepancy to be in large part due to the spatial modeling of the intracluster gas. From our cluster sample we construct y₀-T and S-T scaling relations. The y₀-T scaling relation is inconsistent with what we would expect for self-similar clusters; however, this result is questionable because of the large systematic uncertainty in y₀. The S-T scaling relation has a slope and redshift evolution consistent with what we expect for self-similar clusters, with a characteristic density that scales with the mean density of the universe. We rule out zero-redshift evolution of the S-T relation at ~90% confidence.

With the triaxial density profile of dark matter halos and the corresponding equilibrium gas distribution, we derive two-dimensional Sunyaev-Zel'dovich (SZ) effect and X-ray surface brightness profiles for clusters of galaxies. It is found that the contour map of these observables can be well approximated by a series of concentric ellipses with scale-dependent eccentricities. The statistical distribution of their eccentricities (or, equivalently, axial ratios) is analyzed by taking into account the orientation of clusters with respect to the line of sight and the distribution of the axial ratios and the concentration parameters of dark matter halos. For clusters of mass 10¹³h^-1M_☉ at redshift z = 0, the axial ratio is peaked at η ~ 0.9 for both SZ and X-ray profiles. For larger clusters, the deviation from circular distributions is more apparent, with η peaked at η ~ 0.85 for M = 10¹⁵h^-1M_☉. To be closer to observations, we further study the axial-ratio distribution for mass-limited cluster samples with the number distribution of clusters at different redshifts described by a modified Press-Schechter model. For a mass limit of value M_lim = 10¹⁴h^-1M_☉, the average axial ratio is ⟨η⟩ ~ 0.84, with a tail extended to η ~ 0.6. With the fast advance of high-quality imaging observations of both SZ effect and X-ray emissions, our analyses provide a useful way to probe cluster halo profiles and therefore to test theoretical halo formation models.

The confirmation of the cluster soft excess (CSE) by XMM-Newton has rekindled interest as to its origin. The recent detections of CSE emission at large cluster radii together with reports of O VII line emission associated with the CSE has led many authors to conjecture that the CSE is, in fact, a signature of the warm-hot intergalactic medium (WHIM). In this paper we test the scenario by comparing the observed properties of the CSE with predictions based on models of the WHIM. We find that emission from the WHIM in current models is 3-4 orders of magnitude too faint to explain the CSE. We discuss different possibilities for this discrepancy, including issues of simulation resolution and scale and the role of small density enhancements or galaxy groups. Our final conclusion is that it is unlikely that the WHIM alone is able to account for the observed flux of the CSE.

We have formed "composite spectra" by combining the integrated-light spectra of individual galaxies in eight intermediate-redshift and 12 low-redshift clusters of galaxies. Because these composite spectra have much higher signal-to-noise ratios than individual galaxy spectra, they are particularly useful in quantifying general trends in star formation for galaxy populations in distant clusters, z > 0.3. By measuring diagnostic features that represent stellar populations of very different ages, a grand composite spectrum can reflect the fractions of those populations as accurately as if excellent spectral measurements were available for each galaxy. Such composite spectra can also be useful in the study of finer spectral signatures, for example, spectral indices that break the age-metallicity degeneracy, and the shape of the Hδ absorption line as an indicator of the age and duration of an epoch of starbursting galaxies in a cluster. Measuring the equivalent widths of spectral features in composite spectra is especially well suited for comparing the cosmic variance of star formation in clusters at a given redshift or comparing clusters over a range of redshifts. When we do this we find that [O II] emission and especially Balmer absorption is strong in each of our intermediate-redshift clusters and completely separable from a sample of 12 present-epoch clusters, where these features are weak. Cluster-to-cluster variations at a given epoch seem to be smaller than the strong trend in redshift, which suggests that cosmic evolution is the major factor in the star formation histories of cluster galaxies. Specifically, we show by comparison with the Hδ strengths of present-epoch populations of continuously star-forming galaxies that the higher redshift samples must contain a much higher fraction of starburst galaxies than are found today in any environment.

We investigate the near-infrared K-band properties of the brightest cluster galaxies (BCGs) in a sample of 93 X-ray galaxy clusters and groups, using data from the Two Micron All Sky Survey. Our cluster sample spans a factor of 70 in mass, making it sensitive to any cluster mass-related trends. We derive the cumulative radial distribution for the BCGs in the ensemble and find that 70% of the BCGs are centered in the cluster to within 5% of the virial radius r₂₀₀; this quantifies earlier findings that BCG position coincides with the cluster center as defined by the X-ray emission peak. We study the correlations between the luminosity of the BCGs (L_b) and the mass and the luminosity of the host clusters, finding that BCGs in more massive clusters are more luminous than their counterparts in less massive systems and that the BCGs become less important in the overall cluster light (L₂₀₀) as cluster mass increases. By examining a large sample of optically selected groups, we find that these correlations hold for galactic systems less massive than our clusters (<3 × 10¹³M_☉). From the differences between luminosity functions in high- and low-mass clusters, we argue that BCGs grow in luminosity mainly by merging with other luminous galaxies as the host clusters grow hierarchically; the decreasing BCG luminosity fraction (L_b/L₂₀₀) with cluster mass indicates that the rate of luminosity growth in BCGs is slow compared to the rate at which clusters acquire galaxy light from the field or other merging clusters. Utilizing the observed correlation between the cluster luminosity and mass and a merger tree model for cluster formation, we estimate that the amount of intracluster light (ICL) increases with cluster mass; our calculations suggest that in 10¹⁵M_☉ clusters more than 50% of total stellar mass is in ICL, making the role of ICL very important in the evolution and thermodynamic history of clusters. The cluster baryon fraction accounting for the ICL is in good agreement with the value derived from cosmic microwave background observations. The inclusion of ICL reduces the discrepancy between the observed cluster cold baryon fraction and that found in hydrodynamical simulations. Based on the observed iron abundance in the intracluster medium, we find that the ICL predicted by our model, together with the observed galaxy light, match the iron mass-to-light ratio expected from simple stellar population models, provided that the Salpeter initial mass function is adopted. The ICL also makes it easier to produce the "iron excess" found in the central regions of cool-core clusters.

One of the challenges that models of active galactic nucleus (AGN) heating of the intracluster medium (ICM) face is to explain how the mechanical luminosity of the AGN is tuned to the radiative losses of the ICM. Here we implement a simple one-dimensional model of a feedback mechanism that links the luminosity of the AGN to the accretion rate. We demonstrate how this simple feedback mechanism leads to a quasi-steady state for a broad range of parameters. Moreover, within this feedback model, we investigate the effect of thermal conduction and find that its relative importance depends strongly on the cluster mass.

We measured the stellar velocity dispersions of 15 active galactic nucleus (AGN) host galaxies at redshifts as high as ~0.34. Combining these with published velocity dispersion measurements from the literature, we study the fundamental plane of AGN host galaxies and its evolution. BL Lac object hosts and radio galaxies seem to lie on the same fundamental plane as normal early-type galaxies. The evolution of the mass-to-light ratio of AGN host galaxies shows a similar trend to that observed in normal early-type galaxies, consistent with single-burst passive evolution models with formation redshifts z ≳ 1. The lack of a significant difference between normal and AGN host galaxies in the fundamental plane supports the "grand unification" picture wherein AGNs are a transient phase in the evolution of normal galaxies. The black hole masses of BL Lac objects and radio galaxies, derived using the mass dispersion relation, are similar. The black hole mass is independent of BL Lac object type. The local black hole mass-host galaxy luminosity relation of our subsample at z < 0.1 is similar to that of local normal and radio galaxies, but is less well defined at higher redshift owing to the luminosity evolution of the host galaxies.

We present X-ray observations of the nuclear region of 25 Fanaroff-Riley type I (FR I) radio galaxies from the 3CRR and B2 catalogs, using data from the Chandra and XMM-Newton archives. We find the presence of a X-ray central compact core (CCCX) in 13/25 sources; in 3/25 sources the detection of a CCCX is uncertain, while in the remaining 9/25 sources no CCCX is found. All the sources are embedded in a diffuse soft X-ray component, generally on kiloparsec scales, which is in agreement with the halo of the host galaxy and/or with the intracluster medium. The X-ray spectra of the cores are described by a power law with photon indices Γ = 1.1-2.6. In eight sources excess absorption over the Galactic value is detected, with rest-frame column densities N ~ 10²⁰-10²¹ cm^-2; thus, we confirm the previous claim, based on optical data, that most FR I radio galaxies lack a standard optically thick torus. We find significant correlations between the X-ray core luminosity and the radio and optical luminosities, suggesting that at least a fraction of the X-ray emission originates in a jet; however, the origin of the X-rays remains ambiguous. If the X-ray emission is entirely attributed to an isotropic, accretion-related component, we find very small Eddington ratios, L_bol/L_Edd ~ 10^-3 to 10^-8, and we calculate the radiative efficiency to be η ~ 10^-2 to 10^-6 on the basis of the Bondi accretion rates from the spatial analysis. This suggests that radiatively inefficient accretion flows are present in the cores of low-power radio galaxies.

We present results from a half-day observation by XMM-Newton of the nucleus of the nearby Seyfert 2 galaxy NGC 6300. The X-ray spectrum of the nucleus consists of a heavily absorbed hard component dominating the 3-10 keV band and a soft component seen in the 0.2-2 keV band. In the hard band, the spectrum is well fitted by a power-law model with photon index of 1.83 ± 0.08 attenuated by a Compton-thin absorber (N_H ≃ 2.2 × 10²³ cm^-2). A narrow iron line is detected at 6.43 keV with an equivalent width of ~150 eV; the line velocity width is marginally resolved to be σ ~ 60 eV. The soft emission can be modeled as a power law and may be emission scattered by surrounding plasma. Rapid and high-amplitude variability is observed in the hard X-ray band, whereas both the iron line and the soft emission show no significant variability. It is suggested that the nucleus has experienced an overall long-term trend of decreasing hard X-ray intensity on a timescale of years. We discuss the origins of the spectral components.

The first seven years of RXTE monitoring of Seyfert 1 active galactic nuclei have been systematically analyzed to yield five homogeneous samples of 2-12 keV light curves, probing hard X-ray variability on successively longer durations from ~1 day to ~3.5 yr. The 2-10 keV variability on timescales of ~1 day, as probed by ASCA, is included. All sources exhibit stronger X-ray variability toward longer timescales, but the increase is greater for relatively higher luminosity sources. Variability amplitudes are anticorrelated with X-ray luminosity and black hole mass, but amplitudes saturate and become independent of luminosity or black hole mass toward the longest timescales. The data are consistent with the models of power spectral density (PSD) movement described by Markowitz and coworkers and McHardy and coworkers, whereby Seyfert 1 galaxies' variability can be described by a single, universal PSD shape whose break frequency scales with black hole mass. The best-fitting scaling relations between variability timescale, black hole mass, and X-ray luminosity imply an average accretion rate of ~5% of the Eddington limit for the sample. Nearly all sources exhibit stronger variability in the relatively soft 2-4 keV band compared to the 7-12 keV band on all timescales. There are indications that relatively less luminous or less massive sources exhibit a greater degree of spectral variability for a given increase in overall flux.

Our current understanding of ultraluminous infrared galaxies suggests that they are recent galaxy mergers in which much of the gas in the former spiral disks, particularly that located at distances less than 5 kpc from each of the premerger nuclei, has fallen into a common center, triggering a huge starburst phenomenon. This large nuclear concentration of molecular gas has been detected by many groups, and estimates of molecular mass and density have been made. Not surprisingly, these estimates were found to be orders of magnitude larger than the corresponding values found in our Galaxy. In this paper, a self-consistent model of the high-energy emission of the superstarburst galaxy Arp 220 is presented. The model also provides an estimate of the radio emission from each of the components of the central region of the galaxy (western and eastern extreme starbursts and molecular disk). The predicted radio spectrum is found as a result of the synchrotron and free-free emission and absorption of the primary and secondary steady population of electrons and positrons. The latter is the output of charged pion decay and knock-on leptonic production, subject to a full set of losses in the interstellar medium. The resulting radio spectrum is in agreement with subarcsecond radio observations, which is what allows us to estimate the magnetic field. In addition, the FIR emission is modeled with dust emissivity, and the computed FIR photon density is used as a target for inverse Compton process as well as to give an account of losses in the γ-ray escape. Bremsstrahlung emission and neutral pion decay are also computed, and the γ-ray spectrum is finally predicted. Future possible observations with GLAST and the ground-based Cerenkov telescopes are discussed.

We examine the feasibility of using dust-scattered X-rays for direct determination of distances to nearby galaxies with bright background AGNs, QSOs, or GRBs. We show how the Chandra X-Ray Observatory could be used to determine the distance to M31 to an unprecedented absolute accuracy of ~1%.

We use the KPNO International Spectroscopic Survey (KISS) for emission-line galaxies to identify and describe a sample of local analogs to the luminous compact blue galaxies (LCBGs) that are observed to be abundant at intermediate and high redshift. The sample is selected using criteria believed effective at isolating true examples of LCBGs: SB_e(B band) < 21.0 mag arcsec^-2, M_B < -18.5 (for H₀ = 75 km s^-1 Mpc^-1), and B - V < 0.6. In addition, all LCBG candidates presented are selected to have star formation as their dominant form of activity. We examine the properties of our LCBGs and compare them with those of other KISS star-forming galaxies of the same absolute magnitude range. We find that the KISS LCBGs lie on the extreme end of a fairly continuous distribution of "normal" star-forming galaxies in the plane of surface brightness versus color. This result differs from the results of previous studies that show LCBGs at higher z to be more separate from the "normal" (usually nonactive) galaxies with which they are compared. On average, LCBGs have a higher tendency to emit detectable flux in the radio continuum; have higher Hα luminosities by a factor of 1.6, indicating strong star formation activity; and have slightly lower than expected metal abundances based on the luminosity-metallicity relation for KISS galaxies. We calculate the volume density of our low-z (z < 0.045) sample to be 5.4 × 10^-4h Mpc^-3, approximately 4 times lower than the volume density of the LCBGs at 0.4 < z < 0.7 and ~10 times lower than the volume density of the population at 0.7 < z < 1.0.

Using well-defined selection criteria derived from Zaritsky et al. applied to the LEDA galaxy catalog, we have constructed a sample of elliptical galaxies that can be taken to lie in the field. Such criteria can easily be applied to theoretical simulations for direct comparison with observations. The variation of the number of "isolated" ellipticals with selection criteria is also investigated. A preliminary study of the environment of the field ellipticals shows that, in the mean, they are surrounded by a population of dwarf galaxies, out to projected radii of at least 500 kpc, with a radial density profile of r^-0.6±0.2 and a luminosity function slope of α ~ -1.8. The results are compared and contrasted to the satellite population around isolated spiral galaxies.

We present results of new Monte Carlo calculations made with the DIRTY code of radiative transfer of stellar and scattered radiation for a dusty giant late-type galaxy like the Milky Way, which illustrate the effect of the attenuation of stellar light by internal dust on the integrated photometry of the individual bulge and disk components. Here we focus on the behavior of the attenuation function, the color excess, and the fraction of light scattered or directly transmitted toward the outside observer as a function of the total amount of dust and the inclination of the galaxy, and the structure of the dusty interstellar medium (ISM) of the disk. We confirm that dust attenuation produces qualitatively and quantitatively different effects on the integrated photometry of bulge and disk, whatever the wavelength. In addition, we find that the structure of the dusty ISM affects more sensitively the observed magnitudes than the observed colors of both bulge and disk. Finally, we show that the contribution of the scattered radiation to the total monochromatic light received by the outside observer is significant, particularly at UV wavelengths, even for a two-phase, clumpy, dusty ISM. Thus, understanding dust scattering properties is fundamental for the interpretation of extragalactic observations in the rest-frame UV.

Using a new Chandra ACIS-S3 observation of the normal, isolated, moderate-L_X lenticular galaxy NGC 1332, we resolve the emission into ~75 point sources and a significant diffuse component. We present a detailed analysis of the spectral properties of the diffuse emission, constraining both the temperature profile and the metal abundances in the hot gas. The characteristics of the point-source population and the spatial properties of the diffuse emission are discussed in two companion papers. The diffuse component comprises hot gas with an ~isothermal temperature profile (~0.5 keV) and emission from unresolved point sources. In contrast with the cool cores of many groups and clusters, we find a small central temperature peak. We obtain emission-weighted abundance constraints within 20 kpc for several key elements: Fe, O, Ne, Mg, and Si. The measured iron abundance (Z_Fe = 1.1 in solar units; >0.53 at 99% confidence) strongly excludes the very subsolar values often historically reported for early-type galaxies. This continues, in a lower L_X system, a trend in recent observations of bright galaxies and groups. The abundance ratios, with respect to Fe, of the other elements were also found to be ~solar, with the exception of Z_O/Z_Fe, which was significantly lower (<0.4), as seen in several bright galaxies, groups, and clusters. Such a low O abundance is not predicted by simple models of ISM enrichment by Type Ia and Type II supernovae (SNe) and may indicate a significant contribution from primordial hypernovae. Revisiting Chandra observations of the moderate-L_X, isolated elliptical galaxy NGC 720, we obtain similar abundance constraints (Z_Fe = 0.71, 90% confidence; Z_O/Z_Fe = 0.23 ± 0.21). Adopting standard SNe Ia and SNe II metal yield models, our abundance ratio constraints imply that 73% ± 5% and 85% ± 6% of the Fe enrichment in NGC 1332 and NGC 720, respectively, arises from SNe Ia. Although these results are sensitive to the considerable systematic uncertainty in the SNe yields, they are in good agreement with observations of more massive systems. These two cases of moderate-L_X early-type galaxies reveal a consistent pattern of metal enrichment from cluster scales to moderate L_X/L_B galaxies.

We address the issue of accuracy in recovering density profiles from observations of rotation curves of galaxies. We "observe" and analyze our models in much the same way as observers do the real galaxies. Our models include stellar disks, disks with bars, and small bulges. We find that the tilted-ring model analysis produces an underestimate of the central rotational velocity. In some cases the galaxy halo density profile seems to have a flat core, while in reality it does not. We identify three effects that explain the systematic biases: inclination, small bulge, and bar. Inclination effects are due to the finite thickness of the disk, bar, or bulge. Admixture of a nonrotating bulge component reduces the rotational velocity. A small (200-500 pc) bulge may be overlooked, leading to systematic bias even on relatively large (~1 kpc) distances. In the case of a disk with a bar, the underestimate of the circular velocity is larger because of a combination of noncircular motions and random velocities. The effect of the bar depends on the angle that the bar makes with the line of sight. Signatures of bars can be difficult to detect in the surface brightness profiles of the model galaxies. The variations of inclination angle and isophote position angle with radius are more reliable indicators of bar presence than the surface brightness profiles. The systematic biases in the central ~1 kpc of galaxies are not large. Each effect separately gives typically a few km s^-1 error, but the effects add up. In some cases the error in circular velocity was a factor of 2, but typically we get about a 20% effect. The result is the false inference that the density profile of the halo flattens in the central parts. Our observations of real galaxies show that for a large fraction of galaxies the velocity of gas rotation (as measured by emission lines) is very close to the rotation of the stellar component (as measured by absorption lines). This implies that the systematic effects discussed in this paper are also applicable both for the stars and emission-line gas.

We examine the efficiency with which supernova-enriched gas may be ejected from dwarf disk galaxies, using a methodology previously employed to study the self-enrichment efficiency of dwarf spheroidal systems. Unlike previous studies that focused on highly concentrated starbursts, in the current work we consider discrete supernova events spread throughout various fractions of the disk. We model disk systems having gas masses of 10⁸ and 10⁹M_☉ with supernova rates of 30, 300, and 3000 Myr^-1. The supernova events are confined to the midplane of the disk but distributed over radii of 0%, 30%, and 80% of the disk radius, consistent with expectations for Type II supernovae. In agreement with earlier studies, we find that the enriched material from supernovae is largely lost when the supernovae are concentrated near the nucleus, as expected for a starburst event. In contrast, however, we find the loss of enriched material to be much less efficient when the supernovae occur over even a relatively small fraction of the disk. The difference is due to the ability of the system to relax following supernova events that occur over more extended regions. Larger physical separations also reduce the likelihood of supernovae going off within low-density "chimneys" swept out by previous supernovae. We also find that for the most distributed systems, significant metal loss is more likely to be accompanied by significant mass loss. A comparison with theoretical predictions indicates that when undergoing self-regulated star formation, galaxies in the mass range considered will efficiently retain the products of Type II supernovae.

From newly obtained high-resolution, high signal-to-noise ratio spectra the abundances of the elements La and Eu have been determined over the stellar metallicity range -3 < [Fe/H] < +0.3 in 159 giant and dwarf stars. Lanthanum is predominantly made by the s-process in the solar system, while Eu owes most of its solar system abundance to the r-process. The changing ratio of these elements in stars over a wide metallicity range traces the changing contributions of these two processes to the Galactic abundance mix. Large s-process abundances can be the result of mass transfer from very evolved stars, so to identify these cases we also report carbon abundances in our metal-poor stars. Results indicate that the s-process may be active as early as [Fe/H] = -2.6, although we also find that some stars as metal-rich as [Fe/H] = -1 show no strong indication of s-process enrichment. There is a significant spread in the level of s-process enrichment even at solar metallicity.

The distribution of stellar abundances along the Galactic disk is an important constraint for models of chemical evolution and Galaxy formation. In this study we derive radial gradients of C, N, O, Mg, Al, and Si, as well as S, from abundance determinations in young OB stars. Our database is composed of a sample of 69 members of 25 open clusters, OB associations, and H II regions with Galactocentric distances between 4.7 and 13.2 kpc. An important feature of this abundance database is the fact that the abundances were derived self-consistently in non-LTE using a homogeneous set of stellar parameters. Such an uniform analysis is expected to reduce the magnitude of random errors, as well as the influence of systematics in the gradients defined by the abundance and Galactocentric distance. The metallicity gradients obtained in this study are, in general, flatter than the results from previous recent abundance studies of early-type stars. The slopes are found to be between -0.031 (for oxygen) and -0.052 dex kpc^-1 (for magnesium). The gradients obtained for the studied elements are quite similar, and if averaged they can be represented by a single slope of -0.042 ± 0.007 dex kpc^-1. This value is generally consistent with an overall flattening of the radial gradients with time.

We report measurements of the light curve of the variable Galactic center source IRS 16SW. The light curve is not consistent with an eclipsing binary or any other obvious variable star. The source may be an example of a high-mass variable predicted theoretically but not observed previously.

Interplanetary dust particles (IDPs) contain enigmatic submicron components called GEMS (glass with embedded metal and sulfides). The compositions and structures of GEMS indicate that they have been processed by exposure to ionizing radiation, but details of the actual irradiation environment(s) have remained elusive. Here we propose a mechanism and astrophysical site for GEMS formation that explains for the first time the following key properties of GEMS: they are stoichiometrically enriched in oxygen and systematically depleted in S, Mg, Ca, and Fe (relative to solar abundances); most have normal (solar) oxygen isotopic compositions; they exhibit a strikingly narrow size distribution (0.1-0.5 μm diameter); and some of them contain "relict" crystals within their silicate glass matrices. We show that the compositions, size distribution, and survival of relict crystals are inconsistent with amorphization by particles accelerated by diffusive shock acceleration. Instead, we propose that GEMS are formed from crystalline grains that condense in stellar outflows from massive stars in OB associations, are accelerated in encounters with frequent supernova shocks inside the associated superbubble (SB), and are implanted with atoms from the hot gas in the SB interior. We thus reverse the usual roles of target and projectile. Rather than being bombarded at rest by energetic ions, grains are accelerated and bombarded by a nearly monovelocity beam of atoms as viewed in their rest frame. Meyer, Drury, and Ellison have proposed that Galactic cosmic rays (GCRs) originate from ions sputtered from such accelerated dust grains. We suggest that GEMS are surviving members of a population of fast grains that constitute the long-sought source material for GCRs. Thus, representatives of the GCR source material may have been awaiting discovery in cosmic dust labs for the last 30 yr.

We present 1'' resolution interferometric maps of the CO J = 2-1 and HC₃N J = 25-24 emission in the protoplanetary nebula CRL 618 obtained with the millimeter array of the Owens Valley Radio Observatory (OVRO). Our CO data trace with high accuracy the spatio-kinematic structure of different molecular components: (1) the fast, bipolar outflow, which is observed up to axial distances of ±25 (±2.7 × 10¹⁶ cm) from the nebula center with a maximal deprojected expansion velocity of V_exp ~ 340 km s^-1; (2) the roughly round outer halo, which is slowly expanding at V_exp ~ 17 km s^-1; (3) an extended structure that is elongated in the polar direction ±6'' (±8.1 × 10¹⁶ cm), surrounding the optical lobes, and that expands at V_exp ~ 22 km s^-1 (slow axial component); and (4) a dense, inner torus-like core (R_out ~ 1.6 × 10¹⁶ cm) expanding at V_exp ≲ 12 km s^-1. Components (3) and (4) have been revealed for the first time by our high angular resolution data. The geometry, kinematics, density, and temperature distribution of the individual components of the molecular envelope of CRL 618 have been constrained by fitting a spatio-kinematic model to the data. We interpret our results in terms of nebular evolution as follows. In the last ≳2500 yr, there have been at least two distinct episodes of mass loss at a large scale in the form of a slow wind. The first took place at a rate of ≲ 10^-5M_☉ yr^-1 and gave place to the tenuous outer halo. The second mass-loss event started ~400 yr ago at a higher rate of ≈ 10^-4M_☉ yr^-1, resulting in the formation of the inner dense core. More recently, the interaction between fast axial post-asymptotic giant branch (AGB) winds and the preexisting circumstellar envelope has shaped (i) the fast bipolar outflow, which is probably composed of shocked material that was originally in the inner dense core, and (ii) the slow axial component, which could represent material in the interface between the shocked and unshocked AGB envelope. Finally, we report a recent increase of the free-free continuum emission at 1.3 mm (2.1 ± 0.3 Jy) and 3 mm (2.3 ± 0.3 Jy), which could reflect changes in the activity of the fast post-AGB winds on a timescale of a few years.

The NGC 1333 IRAS 4 region was observed in the HCN and HCO⁺J = 1 → 0 lines using a single-dish telescope and in the 2.1 mm continuum and the H₂CO J = 2₁₂ → 1₁₁ line using an interferometer. The single-dish maps show that there are at least two velocity components in emission: one at V_LSR = 6.7 km s^-1 associated with the IRAS 4 core, and the other at ~8 km s^-1 associated with a cloud extended from the SVS 13 complex. In addition, there is a foreground cold layer at ~8 km s^-1 that causes absorption over most of the mapped area. The cloud structure suggests that the blue-skewed line profile of IRAS 4A/B may not be a sign of protostellar collapse. Examinations of both single-dish and interferometric maps suggest that the dip previously seen in the interferometric spectra toward IRAS 4A/B may be caused mostly by the large-scale foreground layer and partly by missing short-spacing flux. Absorption by an infalling envelope with an unusual velocity profile cannot be ruled out. The HCO⁺ map revealed other molecular cores, one associated with SK 1, and the other with SK 10/14. They are probable sites of star formation.

High-resolution (λ/Δλ = 50,000) K-band spectra of massive, embedded, young stellar objects are presented. The present sample consists of four massive young stars located in nascent clusters powering Galactic giant H II regions. Emission in the 2.3 μm 2-0 vibrational-rotational band head of CO is observed. A range of velocity-broadened profiles seen in three of the objects is consistent with the emission arising from a circumstellar disk seen at various inclination angles. Brγ spectra that support an accretion disk or torus model for massive stars of the same spectral and spatial resolution are also presented. In the fourth object, Br emission suggesting a rotating torus is observed, but the CO profile is narrow, indicating that there may be different CO emission mechanisms in massive stars; this is consistent with earlier observations of the BN object and MWC 349. To date, only young massive stars of late O or early B types have been identified with clear accretion disk signatures in such embedded clusters. Often such stars are found in the presence of other, more massive stars that are revealed by their photospheric spectra but exhibit no disk signatures. This suggests that the timescale for dissipating their disks is much faster than that of the less massive OB stars or that the most massive stars do not form with accretion disks.

We model how the mid-IR colors of young stellar objects (YSOs) vary with stellar temperature. The spectral energy distribution (SED) of each object has contributions from the thermal emission of circumstellar dust, from direct stellar photospheric emission, and from scattered stellar emission. We first isolate the effects of stellar contributions (direct + scattered) to the SED using homologous "Class I" models: the distribution of circumstellar matter is chosen to scale with stellar temperature T_* such that the shape of the thermal contribution to the SED remains constant. The relative contribution of stellar direct and scattered light varies with T_*, changing the 1-10 μm (MIR) colors. Stellar light contributes more to the mid-IR emission of YSOs with lower temperature stars (T_* ~ 4000 K) because the emission peak wavelength of the star is closer to that of the thermal radiation. In YSOs with hotter central stars, since the peak of the stellar and thermal spectra are more separated in wavelength, the 1-10 μm spectrum is closer to a pure thermal spectrum and the objects are redder. Next we consider realistic Class 0, I, and II source models and find that the other dominant effect of varying stellar temperature on YSO SEDs is that of the inner disk wall: in high-T_* models, the dust destruction radius is much farther out, with a consequently larger inner disk wall that contributes relatively more to the 2-10 μm flux. This effect partially offsets that of the stellar contribution, leading to varying behaviors of the 2-10 μm flux: In Class 0 sources, the trend is for higher T_* models to have redder colors. In Class I sources, the trend applies with some exceptions. In Class II sources, 2-10 μm colors become redder, going from T_* = 4000 to 8000 K because of decreasing stellar contribution at T_* = 8000 K, and then become blue again from 8000 to 31,500 K because of an increasing inner disk wall contribution. Near edge-on inclinations, the color behavior is completely different. Our modeled mid-IR protostellar colors have implications for interpretations of Spitzer IRAC observations of star formation regions. It is commonly assumed that the slope of the SED at 1-10 μm is directly related to evolutionary state. We show that inclination effects, aperture size, scattered light, and stellar temperature cause a broad spread in the colors of a source at a single evolutionary state. Color-magnitude diagrams can help sort out these effects by separating sources with different T_* on the basis of their different brightness (for sources at the same distance).

A deep [S II] λλ6717/6731 wide-field survey of Herbig-Haro (HH) objects has been carried out in two fields toward the R CrA molecular cloud using the ESO/MPG 2.2 m Wide Field Imager. Twelve new HH objects, many of which consist of several condensations or knots, have been discovered, and new details of the known HH objects have been revealed. Combining the results of previous optical, infrared, and millimeter-wavelength observations, the possible exciting sources of HH objects in the region are discussed. On the basis of the previously known and newly discovered HH objects, at least five HH flows in the region around R CrA and at least two outflows in the region around VV CrA can be identified. In combination with the previously detected molecular outflows, the HH flows in the R CrA region indicate rather active star formation in the R CrA core in the past 10⁵-10⁶ yr.

High-resolution optical and infrared (IR) echelle spectra of the naked (diskless) T Tauri star Hubble 4 are presented. The K-band IR spectra include four Zeeman-sensitive Ti I lines along with several magnetically insensitive CO lines. Detailed spectrum synthesis combined with modern atmospheric models is used to fit the optical spectra of Hubble 4 in order to determine its key stellar parameters: T_eff = 4158 ± 56 K; log g = 3.61 ± 0.50; [M/H] = -0.08 ± 0.05; v sin i = 14.6 ± 1.7 km s^-1. These stellar parameters are used to synthesize K-band spectra to compare with the observations. The magnetically sensitive Ti I lines are all significantly broadened relative to the lines produced in the nonmagnetic model, while the magnetically insensitive CO lines are well matched by the basic nonmagnetic model. Models with magnetic fields are synthesized and fitted to the Ti I lines. The best-fit models indicate a distribution of magnetic field strengths on the stellar surface characterized by a mean magnetic field strength of 2.51 ± 0.18 kG. The mean field is a factor of 2.0 greater than the maximum field strength predicted by pressure equipartition arguments. To confirm the reality of such strong fields, we attempt to refit the observed profiles using a two-component magnetic model in which the field strength is confined to the equipartition value representing plagelike regions in one component and the field is allowed to vary in a cooler component representing spots. It is shown that such a model is inconsistent with the optical spectrum of the TiO bandhead at 7055 Å.

By combining infrared photometry from the Two Micron All Sky Survey with new optical imaging and spectroscopy, I have performed a search for young low-mass stars and brown dwarfs in two regions encompassing a total area of 4 deg² in the Taurus star-forming region (τ ~ 1 Myr). From this work I have discovered 15 new members of Taurus. In addition, I present seven new members outside of these areas from the initial stage of a survey of all of Taurus. These 22 objects exhibit spectral types of M4.5-M9.25 and masses of 0.3-0.015 M_☉ according to the theoretical evolutionary models of Baraffe & Chabrier, seven of which are likely to be brown dwarfs. Emission in Hα, He I, Ca II, [O I], and [S II] and excess emission in optical and near-infrared bands among some of these objects suggest the presence of accretion, outflows, and circumstellar disks. These results add to the body of work—initiated by the first detections of brown dwarf disks by Comerón and coworkers in 1998 and Luhman in 1999—indicating that disks around young brown dwarfs are relatively common. The results from the 4 deg² survey have been combined with previous studies of Taurus to arrive at an initial mass function (IMF) for a total area of 12.4 deg². As in the previous IMFs for Taurus, the updated IMF peaks at a higher mass (0.8 M_☉) than the mass functions in IC 348 and Orion (0.1-0.2 M_☉). Meanwhile, the deficit of brown dwarfs in Taurus appears to be less significant (×1.4-1.8) than that found in earlier studies (×2) because of a slightly higher brown dwarf fraction in the new IMF for Taurus and a lower brown dwarf fraction in the new spectroscopic IMF for the Trapezium from Slesnick and coworkers. The spatial distribution of the low-mass stars and brown dwarfs discovered in the two new survey areas closely matches that of the more massive members. Thus, on the degree size scales (~3 pc) probed to date, there is no indication that brown dwarfs form through ejection.

Results of numerical calculations of Type II supernova light curves are presented. The model progenitor stars have 6 M_☉ cores and various envelopes, originating from a numerically evolved 20 M_☉ star. Five parameters that affect the light curves are examined: the ejected mass, the progenitor radius, the explosion energy, the ⁵⁶Ni mass, and the extent of ⁵⁶Ni mixing. The following effects have been found: (1) the larger the progenitor radius, the brighter the early-time light curve, with little effect on the late-time light curve; (2) the larger the envelope mass, the fainter the early light curve and the flatter the slope of the late light curve; (3) the larger the explosion energy, the brighter the early light curve and the steeper the slope of the late light curve; (4) the larger the ⁵⁶Ni mass, the brighter the overall light curve after 20-50 days, with no effect on the early light curve; and (5) the more extensive the ⁵⁶Ni mixing, the brighter the early light curve and the steeper the late light curve. The primary parameters affecting the light-curve shape are the progenitor radius and the ejected mass. The secondary parameters are the explosion energy, ⁵⁶Ni mass, and ⁵⁶Ni mixing. I find that while in principle the general shape and absolute magnitude of a light curve indicate a unique set of parameters, in practice it is difficult to avoid some ambiguity in the parameters. I find that the nickel-powered diffusion wave and the recombination of helium produce a prominent secondary peak in all our calculations. The feature is less prominent when compositional mixing, both ⁵⁶Ni mixing and mixing between the hydrogen and helium layers, occurs. The model photospheric temperatures and velocities are presented, for comparison to observation.

An exceptionally intense gamma-ray burst, GRB 030329, was detected and localized by the instruments on board the High Energy Transient Explorer satellite (HETE) at 11:37:14 UT on 2003 March 29. The burst consisted of two ~10 s pulses of roughly equal brightness and an X-ray tail lasting more than 100 s. The energy fluence in the 30-400 keV energy band was S_γ = 1.2 × 10^-4 ergs cm^-2, making GRB 030329 one of the brightest GRBs ever detected. Communication of a 2' error box 73 minutes after the burst allowed the rapid detection of a counterpart in the optical, X-ray, and radio and the ensuing discovery of a supernova with most unusual characteristics. Analyses of the burst light curves reveal the presence of a distinct, bright, soft X-ray component underlying the main GRB; the 2-10 keV fluence of this component is ~7 × 10^-6 ergs cm^-2. The main pulses of GRB 030329 were preceded by two soft, faint, nonthermal bumps. We present details of the HETE observations of GRB 030329.

Late-time near-infrared and optical spectra of the normal-bright Type Ia supernova 2003du about 300 days after the explosion are presented. At this late epoch, the emission profiles of well-isolated [Fe II] lines (in particular that of the strong 1.644 μm feature) trace out the global kinematic distribution of radioactive material in the expanding supernova ejecta. In SN 2003du, the 1.644 μm [Fe II] line seems to show a flat-topped profile, indicative of a thick but hollow-centered expanding shell, rather than a strongly peaked profile that would be expected from a "center-filled" distribution. Based on detailed models for exploding Chandrasekhar-mass white dwarfs, we show that the feature is consistent with spherical explosion models. Our model predicts a central region of nonradioactive electron capture elements up to 2500-3000 km s^-1 as a consequence of burning under high density and an extended region of radioactive ⁵⁶Ni up to 9000-10,000 km s^-1. Furthermore, our analysis indicates that the 1.644 μm [Fe II] line profile is not consistent with strong mixing between the regions of electron-capture isotopes and the ⁵⁶Ni layers, as is predicted by detailed three-dimensional models for nuclear deflagration fronts. We discuss the possibility that the flat-topped profile could be produced as a result of an infrared catastrophe and conclude that such an explanation is unlikely. We discuss the limitations of our analysis and place our results into context by comparison with constraints on the distribution of radioactive ⁵⁶Ni in other SNe Ia and briefly discuss the potential implications of our result for the use of SNe Ia as cosmological standard candles.

There have been claims in the literature that accretion disks in which a centrifugally driven wind is the dominant mode of angular momentum transport are inherently unstable. This issue is considered here by applying an equilibrium-curve analysis to the wind-driving, ambipolar diffusion dominated, magnetic-disk model of Wardle and Königl. The equilibrium solution curves for this class of models typically exhibit two distinct branches. It is argued that only one of these branches represents unstable equilibria and that a real disk/wind system likely corresponds to a stable solution.

We report on simultaneous radio and X-ray observations of the black hole candidate XTE J1650-500 during the course of its 2001-2002 outburst. The scheduling of the observations allowed us to sample the properties of XTE J1650-500 in different X-ray spectral states, namely, the hard state, the steep power-law state, and the thermal dominant state, according to the recent spectral classification of McClintock & Remillard. The hard state is consistent with a compact jet dominating the spectral energy distribution at radio frequencies; however, the current data suggest that its contribution as direct synchrotron emission at higher energies may not be significant. In that case, XTE J1650-500 may be dominated by Compton processes (either inverse Comptonization of thermal disk photons and/or synchrotron self-Compton radiation from the base of the compact jet) in the X-ray regime. We surprisingly detect a faint level of radio emission in the thermal dominant state that may be consistent with the emission of previously ejected material interacting with the interstellar medium, similar (but on a smaller angular scale) to what was observed in XTE J1550-564 by Corbel and coworkers. Based on the properties of radio emission in the steep power-law state of XTE J1650-500 and taking into account the behavior of other black hole candidates (namely, GX 339-4, XTE J1550-564, and XTE J1859+226) while in the intermediate and steep power-law states, we are able to present a general pattern of behavior for the origin of radio emission in these two states that could be important for understanding the accretion-ejection coupling very close to the black hole event horizon.

We have observed the persistent but optically unidentified X-ray source X1908+075 with the Proportional Counter Array and High-Energy X-Ray Timing Experiment instruments on the Rossi X-Ray Timing Explorer (RXTE). The binary nature of this source was recently established by Wen et al., who found a 4.4 day orbital period in results from the RXTE All-Sky Monitor. We report the discovery of 605 s pulsations in the X-ray flux. The Doppler delay curve is measured and provides a mass function of 6.1 M_☉, which is a lower limit to the mass of the binary companion of the neutron star. The degree of attenuation of the low-energy end of the spectrum is found to be a strong function of orbital phase. A simple model of absorption in a stellar wind from the companion star fits the orbital phase dependence reasonably well and limits the orbital inclination angle to the range 38°-72°. These measured parameters lead to an orbital separation of ~60-80 lt-s, a mass for the companion star in the range 9-31 M_☉, and an upper limit to the size of the companion of ~22 R_☉. From our analysis, we also infer a wind mass-loss rate from the companion star of ≳1.3 × 10^-6M_☉ yr^-1 and, when the properties of the companion star and the effects of photoionization are considered, likely ≳4 × 10^-6M_☉ yr^-1. Such a high rate is inconsistent with the allowed masses and radii that we find for a main-sequence or modestly evolved star unless the mass-loss rate is enhanced in the binary system relative to that of an isolated star. We discuss the possibility that the companion might be a Wolf-Rayet star that could evolve to become a black hole in 10⁴ to 10⁵ yr. If so, this would be the first identified progenitor of a neutron star-black hole binary.

We used high-resolution Hubble Space Telescope (HST)-WFPC2 and wide-field ground-based observations to construct a catalog of blue straggler stars (BSSs) that spans the entire radial extent of the globular cluster NGC 6752. The BSS sample is the most extensive ever obtained for this cluster. Although NGC 6752 is a high-density cluster with a large binary population, we found that its BSS content is surprisingly low: the specific number of BSSs is among the lowest ever measured in a cluster. The BSS distribution is highly peaked in the cluster center, shows a rapid decrease at intermediate radii, and rises again at larger distances. This distribution closely resembles those observed in M3 and 47 Tuc by Ferraro and coworkers. To date, BSS surveys covering the central regions with HST and the outer regions with wide-field CCD ground-based observations have been performed for only these three clusters. Despite the different dynamical properties, a bimodal radial distribution has been found in each. A detailed comparison of observed BSS luminosity and temperature distributions with theoretical models reveals a population of luminous, hot BSSs that is not easily interpreted.

Microlensing is the only known direct method to measure the masses of stars that lack visible companions. In terms of microlensing observables, the mass is given by M = (c²/4G)_Eθ_E and so requires the measurement of both the angular Einstein radius θ_E and the projected Einstein radius _E. Simultaneous measurement of these two parameters is extremely rare. Here we analyze OGLE-2003-BLG-238, a spectacularly bright (I_min = 10.3), high-magnification (A_max = 170) microlensing event. Pronounced finite-source effects permit a measurement of θ_E = 650 μas. Although the timescale of the event is only t_E = 38 days, one can still obtain weak constraints on the microlens parallax: 4.4 AU < _E < 18 AU at the 1 σ level. Together these two parameter measurements yield a range for the lens mass of 0.36 M_☉ < M < 1.48 M_☉. As was the case for MACHO-LMC-5, the only other single star (apart from the Sun) whose mass has been determined from its gravitational effects, this estimate is rather crude. It does, however, demonstrate the viability of the technique. We also discuss future prospects for single-lens mass measurements.

Recent theoretical predictions for low-metallicity massive stars predict that these stars should have drastically reduced equatorial winds (mass loss) while on the main sequence, and so should retain most of their angular momentum. Observations of both the Be/(B + Be) ratio and the blue-to-red supergiant ratio appear to have a metallicity dependence that may be caused by high rotational velocities. We have analyzed 39 archival Hubble Space Telescope Imaging Spectrograph (STIS), high-resolution, ultraviolet spectra of O-type stars in the Magellanic Clouds to determine their projected rotational velocities V sin i. Our methodology is based on a previous study of the projected rotational velocities of Galactic O-type stars using International Ultraviolet Explorer (IUE) short-wavelength prime (SWP) camera high-dispersion spectra, which resulted in a catalog of V sin i values for 177 O-type stars. Here we present complementary V sin i values for 21 Large Magellanic Cloud and 22 Small Magellanic Cloud O-type stars based on STIS and IUE UV spectroscopy. The distribution of V sin i values for O-type stars in the Magellanic Clouds is compared to that of Galactic O-type stars. Despite the theoretical predictions and indirect observational evidence for high rotation, the O-type stars in the Magellanic Clouds do not appear to rotate faster than their Galactic counterparts.

We discovered a low-mass companion to the M dwarf GJ 164 with the CCD-based imaging system of the Stellar Planet Survey astrometric program. The existence of GJ 164B was confirmed with Hubble Space Telescope NICMOS imaging observations. A high-dispersion spectral observation in V sets a lower limit of Δm > 2.2 mag between the two components of the system. Based on our parallax value of 82 ± 8 mas, we derive the following orbital parameters: P = 2.04 ± 0.03 yr, a = 1.03 ± 0.03, and M_total = 0.265 ± 0.020 M_☉. The component masses are M_A = 0.170 ± 0.015 M_☉ and M_B = 0.095 ± 0.015 M_☉. Based on its mass, colors, and spectral properties, GJ 164B has spectral type M6-M8 V.

We present first results from the Palomar Adaptive Optics Survey of Young Stars conducted at the Hale 5 m telescope. Through direct imaging we have discovered a brown dwarf and two low-mass stellar companions to the young solar-type stars HD 49197, HD 129333 (EK Dra), and V522 Per and confirmed a previously suspected companion to RX J0329.1+0118 (Sterzik et al.), at respective separations of 095 (43 AU), 074 (25 AU), 209 (400 AU), and 378 (380 AU). Physical association of each binary system is established through common proper motion and/or low-resolution infrared spectroscopy. Based on the companion spectral types, we estimate their masses at 0.06, 0.20, 0.13, and 0.20 M_☉, respectively. From analysis of our imaging data combined with archival radial velocity data, we find that the spatially resolved companion to HD 129333 is potentially identical to the previously identified spectroscopic companion to this star (Duquennoy & Mayor). However, a discrepancy with the absolute magnitude suggests that the two companions could also be distinct, with the resolved one being the outermost component of a triple system. The brown dwarf HD 49197B is a new member of a growing list of directly imaged substellar companions at 10-1000 AU separations from main-sequence stars, indicating that such brown dwarfs may be more common than initially speculated.

Spectra of the heliospheric EUV and X-ray emission induced in the charge transfer collisions of the highly charged solar wind ions with the interstellar gas have been calculated. Cascading photon spectra of individual O^q+, C^q+, N^q+, and Ne^q+ ions have been constructed using recent data on ion radiative transition probabilities and the state-selective population cross sections for charge transfer collisions of the most abundant heavy solar wind ions with H and He atoms. Emission spectra have been calculated for slow and fast solar winds interacting with the heliospheric H and He gas. Relative intensities of the brightest lines have been predicted. The volume power distribution of the charge transfer EUV and X-ray emission has been computed for simplified models of the solar winds and the interstellar gas. X-ray images of the heliosphere have been composed for the region inside 10 AU from the Sun.

We consider the problem of reconnection in weakly collisional plasmas in the strong guide field limit. In this regime the standard resistive Ohm's law is modified to include electron compressibility and electron inertia effects. Despite the increased complexity of the governing equations, we show that analytic steady state solutions, like those discovered by Craig and Henton for the purely resistive case, can be developed for this new system. The resulting solutions are somewhat richer than those of Craig and Henton, and there are various different regimes in parameter space to consider that exhibit multiple length scales, boundary layer effects, and other features. We also examine the dynamical behavior of these new solutions by solving the time-dependent problem numerically.

When electrons that are trapped by strong electrostatic waves are carried across a magnetic field, the Lorentz force can in principle accelerate them to ultrahigh energies. This wave accelerator known as the electron surfing acceleration is thus a potential generation mechanism for ultrarelativistic electrons at astrophysical shocks. Here, we present for the first time a self-consistent simulation that follows the growth and saturation of strong electrostatic waves that are triggered by proton beams moving at relativistic speeds relative to a background plasma. We find in our simulation that proton beams moving at a Lorentz factor of 7 can accelerate electrons to 1 GeV by means of electron surfing acceleration. Thereafter the wave collapses, and it scatters some electrons to energies in excess of 10 GeV. The plasma charge density modulations give rise to a strong growth of the fast extraordinary (X) wave in frequency intervals for which its group velocity is comparable to the beam speed.

We measure the angular power spectrum of the Wilkinson Microwave Anisotropy Probe (WMAP) first-year temperature anisotropy maps. We use the Spatially Inhomogeneous Correlation Estimator (SpICE) to estimate C_l's for multipoles l = 2-900 from all possible cross-correlation channels. Except for the map-making stage, our measurements provide an independent analysis of that by G. Hinshaw et al. Despite the different methods used, there is virtually no difference between the two measurements for l ≲ 700; the highest l's are still compatible within 1 σ errors. We use a novel intrabin variance method to constrain C_l errors in a model-independent way. Simulations show that our implementation of the technique is unbiased within 1% for l ≳ 100. When applied to WMAP data, the intrabin variance estimator yields diagonal errors ~10% larger than those reported by the WMAP team for 100 < l < 450. This translates into a 2.4 σ detection of systematics, since no difference is expected between the SpICE and the WMAP team estimator window functions in this multipole range. With our measurement of the C_l's and errors, we get a χ² per degree of freedom (dof) of 1.042 for a best-fit ΛCDM model, which has a 14% probability, whereas the WMAP team obtained χ²/dof = 1.066, which has a 5% probability. We assess the impact of our results on cosmological parameters using Markov chain Monte Carlo simulations. From WMAP data alone, assuming spatially flat power-law ΛCDM models, we obtain the reionization optical depth τ = 0.145 ± 0.067, spectral index n_s = 0.99 ± 0.04, Hubble constant h = 0.67 ± 0.05, baryon density Ω_bh² = 0.0218 ± 0.0014, cold dark matter density Ω_CDMh² = 0.122 ± 0.018, and σ₈ = 0.92 ± 0.12, consistent with a reionization redshift z_re = 16 ± 5 (68% CL).

We present the first results from a Bayesian analysis of the Wilkinson Microwave Anisotropy Probe (WMAP) first-year data using a Gibbs sampling technique. Using two independent, parallel supercomputer codes, we analyze the WMAP Q, V, and W bands. The analysis results in a full probabilistic description of the information the WMAP data set contains about the power spectrum and the all-sky map of the cosmic microwave background anisotropies. We present the complete probability distributions for each C_ℓ, including any non-Gaussianities of the power spectrum likelihood. While we find good overall agreement with the previously published WMAP spectrum, our analysis uncovers discrepancies in the power spectrum estimates at low-ℓ multipoles. For example, we claim that the best-fit ΛCDM model is consistent with the C₂ inferred from our combined Q+V+W analysis, with a 10% probability of an even larger theoretical C₂. Based on our exact analysis, we can therefore attribute the "low-quadrupole issue" to a statistical fluctuation.

Numerical simulations play an important role in the study of the structure formation of the universe. However, the mass resolution in current simulations is still poor. Due to technical difficulties, it is necessary to use both a greatly reduced number density of particles and a greatly raised unit particle mass. Consequently, the particle masses used in cosmological simulations are about 10⁷⁰ times larger than the GeV candidates in particle physics. This is a huge physical bias that cannot be neglected when interpreting the results of the simulations. Here we discuss how such a bias affects cold dark matter (CDM) cosmological simulations. We find that the small-scale properties of the CDM particle system are changed in two aspects. (1) An upper limit is imposed on the spatial resolution of the simulation results. (2) Most importantly, an unexpected short mean free path is produced, and the corresponding two-body scattering cross section is close to the value expected in the self-interaction dark matter model. Since the mean free path of real CDM particle systems is much longer than that in the simulations, our results imply that (1) there is probably no "cusp problem" in real CDM halos and (2) a much longer time is needed to form new virialized halos in real CDM particle systems than in the simulations. The last result can help us understand the "substructure problem." Our discussion can also explain why the massive halos in the simulations may have smaller concentration coefficients.

Collisionless plasma shock theory, which applies, for example, to the afterglow of gamma-ray bursts, still contains key issues that are poorly understood. In this Letter, we study charged particle dynamics in a highly relativistic collisionless shock numerically using ~10⁹ particles. We find a power-law distribution of accelerated electrons, which upon detailed investigation turns out to originate from an acceleration mechanism that is decidedly different from Fermi acceleration. Electrons are accelerated by strong filamentation instabilities in the shocked interpenetrating plasmas and coincide spatially with the power-law-distributed current filamentary structures. These structures are an inevitable consequence of the now well-established Weibel-like two-stream instability that operates in relativistic collisionless shocks. The electrons are accelerated and decelerated instantaneously and locally: a scenery that differs qualitatively from recursive acceleration mechanisms such as Fermi acceleration. The slopes of the electron distribution power laws are in concordance with the particle power-law spectra inferred from observed afterglow synchrotron radiation in gamma-ray bursts, and the mechanism can possibly explain more generally the origin of nonthermal radiation from shocked interstellar and circumstellar regions and from relativistic jets.

We find that the fraction of early-type galaxies in poor groups (containing from four to 10 members) is a weakly increasing function of the number of the group members and is about 2 times higher than in a sample of isolated galaxies. We also find that the group velocity dispersion increases weakly with the fraction of early-type galaxies. Early-type galaxies in poor groups are brighter in the near-infrared with respect to isolated ones by ΔM_K ~ 0.75 mag, and to a lesser degree also in the blue (ΔM_B ~ 0.5 mag). We also find early-type galaxies in groups to be redder than those in the field. These findings suggest that the formation history for early-type galaxies in overdense regions is different from that in underdense regions, and that their formation in groups is triggered by merging processes.

A substantial fraction of barred galaxies host additional nuclear bars that tumble with pattern speeds exceeding those of the large-scale (primary) stellar bars. We have investigated the mechanism of formation and dynamical decoupling in such nested bars that include gaseous (secondary) nuclear bars within the full-size galactic disks, hosting a double inner Lindblad resonance. Becoming increasingly massive and self-gravitating, the nuclear bars lose internal (circulation) angular momentum to the primary bars and increase their strength. Developing chaos within these bars triggers a rapid gas collapse—bar contraction. During this time period, the secondary bar pattern speed Ω_s ~ a^-1, where a stands for the bar size. As a result, Ω_s increases dramatically until a new equilibrium is reached (if at all), while the gas specific angular momentum decreases—demonstrating the dynamical decoupling of nested bars. Viscosity, and therefore the gas presence, appears to be a necessary condition for the prograde decoupling of nested bars. This process maintains an inflow rate of ~1 M_☉ yr^-1 over ~10⁸ yr across the central 200 pc and has important implications for fueling the nuclear starbursts and active galactic nuclei.

We have found evidence for the presence of two distinct ancient stellar components (both ≥10 Gyr old) in the Sculptor dwarf spheroidal galaxy. We used the ESO Wide Field Imager in conjunction with the Very Large Telescope/FLAMES spectrograph to study the properties of the resolved stellar population of Sculptor out to and beyond the tidal radius. We find that two components are discernible in the spatial distribution of horizontal branch stars in our imaging and in the [Fe/H] and v_hel distributions for our large sample of spectroscopic measurements. They can be generally described as a "metal-poor" component ([Fe/H] < -1.7) and a "metal-rich" component ([Fe/H] > -1.7). The metal-poor stars are more spatially extended than the metal-rich stars, and they also appear to be kinematically distinct. These results provide insight into the formation processes of small systems in the early universe and the conditions found there. Even this simplest of galaxies appears to have had a surprisingly complex early evolution.

The HESS (High Energy Stereoscopic System) collaboration recently reported highly significant detection of TeV γ-rays coincident with Sgr A*. In the context of other Galactic center (GC) observations, this points to the following scenario: In the extreme advection-dominated accretion flow (ADAF) regime of the GC black hole (BH), synchrotron radio/submillimeter emission of ~100 MeV electrons emanates from an inefficiently radiating turbulent magnetized corona within 20R_S (Schwarzschild radii) of the GCBH. These electrons are accelerated through second-order Fermi processes by MHD turbulence, as suggested by Liu et al. Closer to the innermost stable orbit of the ADAF, instabilities and shocks within the flow inject power-law electrons through first-order Fermi acceleration to make synchrotron X-ray flares observed with Chandra, XMM, and INTEGRAL. A subrelativistic MHD wind subtending an ~1 sr cone with power ≳10³⁷ ergs s^-1 is driven by the ADAF from the vicinity of the GCBH. As in pulsar-powered plerions, electrons are accelerated at the wind termination shock at ≳10^16.5 cm from the GCBH, and Compton-scatter the ADAF and the far-infrared dust radiation to TeV energies. The synchrotron radiation of these electrons forms the quiescent X-ray source resolved by Chandra. The radio counterpart of this TeV/X-ray plerion, formed when the injected electrons cool on timescales of ≳10⁴ yr, could explain the origin of nonthermal radio emission in the parsec-scale bar of the radio nebula Sgr A West.

NGC 6543 is one of the few planetary nebulae whose X-ray emission has been shown to be extended and to originate from hot interior gas. Using Far Ultraviolet Spectroscopic Explorer observations, we have now detected nebular O VI emission from NGC 6543. Its central star, with an effective temperature of ~50,000 K, is too cool to photoionize O V, so the O VI ions must have been produced by thermal collisions at the interface between the hot interior gas and the cool nebular shell. We modeled the O VI emission incorporating thermal conduction but find that simplistic assumptions for the asymptotic giant branch and fast wind mass-loss rates overproduce X-ray emission and O VI emission. We have therefore adopted the pressure of the interior hot gas for the interface layer and find that expected O VI emission to be comparable to the observations.

We demonstrate that nucleosynthesis in rapid, high-entropy expansions of proton-rich matter from high temperature and density can result in a wider variety of abundance patterns than heretofore appreciated. In particular, such expansions can produce iron-group nuclides, p-process nuclei, or even heavy, neutron-rich isotopes. Such diversity arises because the nucleosynthesis enters a little explored regime in which the free nucleons are not in equilibrium with the abundant ⁴He. This allows nuclei significantly heavier than iron to form in the presence of abundant free nucleons early in the expansion. As the temperature drops, nucleons increasingly assemble into ⁴He and heavier nuclei. If the assembly is efficient, the resulting depletion of free neutrons allows disintegration flows to drive nuclei back down to iron and nickel. If this assembly is inefficient, then the large abundance of free nucleons prevents the disintegration flows and leaves a distribution of heavy nuclei after reaction freezeout. For cases in between, an intermediate abundance distribution, enriched in p-process isotopes, is frozen out. These last expansions may contribute to the solar system's supply of the p-process nuclides if mildly proton-rich, high-entropy matter is ejected from proto-neutron stars winds or other astrophysical sites. Also significant is the fact that, because the nucleosynthesis is primary, the signature of this nucleosynthesis may be evident in metal-poor stars.

We construct numerical models of the newly discovered binary pulsar J0737-3039A, both with a fully relativistic, uniformly rotating equilibrium code that handles arbitrary spins and in the relativistic slow-rotation approximation. We compare results for a representative sample of viable nuclear equations of state (EOSs) that span three qualitatively different classes of models for the description of nuclear matter. A future dynamical measurement of the neutron star's moment of inertia from pulsar timing data will impose significant constraints on the nuclear EOS. Even a moderately accurate measurement (≲ 10%) may be able to rule out some of these competing classes. Using the measured mass, spin, and moment of inertia to identify the optimal model computed from different EOSs, one can determine the pulsar's radius.

We investigate the pulsar birthrate from a sample of 815 nonrecycled pulsars detected by the Parkes multibeam survey, accounting as accurately as possible for all known selection effects. We find that pulsars with magnetic fields greater than 2.5 × 10¹² G account for more than half of the total birthrate in spite of comprising only about 5%-10% of the total Galactic population. While we do not find evidence for a significant population of pulsars "injected" into the population with spin periods of ~0.5 s, we do find that many, perhaps 40%, are born with periods in the range 0.1-0.5 s. The absolute number and birthrate of Galactic pulsars is strongly dependent on the assumed models for pulsar beaming and Galactic electron distribution. Adopting the most recent models, we find the total pulsar birthrate to be between 0.9 and 1.9 pulsars per century for 1400 MHz luminosities greater than 1 mJy kpc², and the total Galactic population of active radio pulsars above this luminosity limit to be between 70,000 and 120,000.

HeH⁺ is found to be the dominant positive ion over a wide range of temperatures and densities relevant to helium-rich white dwarfs. The inclusion of HeH⁺ in ionization equilibrium computations increases the abundance of free electrons by a significant factor. For temperatures below 8000 K, He^- free-free absorption is increased by up to a factor of 5 by the inclusion of HeH⁺. Illustrative model atmospheres and spectral energy distributions are computed, which show that HeH⁺ has a strong effect on the density and pressure structure of helium-rich white dwarfs with T_eff < 8000 K. The inclusion of HeH⁺ significantly reddens spectral energy distributions and broadband color indices for models with T_eff < 5500 K. This has serious implications for existing model atmospheres, synthetic spectra, and cooling curves for helium-rich white dwarfs.

We present resolved scattered-light images of the debris disk around HD 107146, a G2 star 28.5 pc from the Sun. This is the first debris disk to be resolved in scattered light around a solar-type star. We observed it with the Hubble Space Telescope/Advanced Camera for Surveys coronagraph, using a 18 occulting spot and the F606W (broad V) and F814W (broad I) filters. Within 2'' from the star, the image is dominated by point-spread function subtraction residuals. Outside this limit, the disk looks featureless, except for a northeast-southwest brightness asymmetry that we attribute to forward scattering. The disk has scattered-light fractional luminosities of (L_sca/L_*)_F606W = × 10^-5 and (L_sca/L_*)_F814W = × 10^-5, and it is detected up to 65 away from the star. To map the surface density of the disk, we deproject it by 25^° ± 5^°, divide by the dust-scattering phase (g_F606W = 0.3 ± 0.1, g_F814W = 0.2 ± 0.1), and correct for the geometric dilution of starlight. Within the errors, the surface density has the same shape in each bandpass, and it appears to be a broad (85 AU) ring with most of the opacity concentrated at 130 AU. The ratio of the relative luminosity in F814W to that in F606W has the constant value of 1.3 ± 0.3, with the error dominated by uncertainties in the value of g in each filter. An examination of far-infrared and submillimeter measurements suggests the presence of small grains. The colors and the derived values of g are consistent with the presence of dust particles smaller than the radiation pressure limit. Possible scenarios that may explain the shape of the surface density profile are the creation of a small planet or the scattering and circularization of a large one.

We present high spatial resolution observations of proper motions in the solar NOAA Active Region 10486 using a high-order adaptive optics system, frame selection, and speckle-masking image reconstruction. The data were obtained with the Dunn Solar Telescope of the National Solar Observatory/Sacramento Peak on 2003 October 29. The resolution of the images approaches the diffraction-limited resolution of the Dunn Solar Telescope of about 014 at 527 nm. We analyzed a 2 hr time series with a 1 minute cadence prior to an X10 white-light flare. Local correlation tracking was used to measure the photospheric proper motions. We find specific evidence of strong shear flows along the magnetic neutral line; these shear flows are well defined and correlated with white-light flare kernels in the visible and infrared. The speed along the flow channels can reach up to 1.6 km s^-1, and the separation of channels with head-on flows can be less than 1''. Counterstreaming and complex flow patterns have been distinguishing characteristics of this extraordinarily flare-productive active region.

We present evidence that solar structure changes with changes in solar activity. We find that the adiabatic index, Γ₁, changes near the second helium ionization, i.e., at a depth of about 0.98 R_☉. We believe that this change is a result of the change in the effective equation of state caused by magnetic fields. Inversions should be able to detect the changes in Γ₁ if mode sets with reliable and precise high-degree modes are available.