Table of contents

We have developed a method for measuring higher order weak-lensing distortions of faint background galaxies, namely, the weak gravitational flexion, by fully extending the Kaiser, Squires, and Broadhurst method to include higher order lensing image characteristics (HOLICs) introduced by Okura, Umetsu, and Futamase. We take into account explicitly the weight function in calculations of noisy shape moments and the effect of higher order point-spread function (PSF) anisotropy, as well as isotropic PSF smearing. Our HOLICs formalism allows accurate measurements of flexion from practical observational data in the presence of a noncircular, anisotropic PSF. We test our method using mock observations of simulated galaxy images and actual, ground-based Subaru observations of the massive galaxy cluster A1689 (z = 0.183). From the high-precision measurements of the spin-1 first flexion, we obtain a high-resolution mass map in the central region of A1689. The reconstructed mass map shows a bimodal feature in the central 4' × 4' region of the cluster. The major, pronounced peak is associated with the brightest cluster galaxy and central cluster members, while the secondary mass peak is associated with a local concentration of bright galaxies. The refined, high-resolution mass map of A1689 demonstrates the power of the generalized weak-lensing analysis techniques for quantitative and accurate measurements of the weak gravitational lensing signal.

Sensitive surveys of the cosmic microwave background will detect thousands of galaxy clusters via the Sunyaev-Zel'dovich (SZ) effect. Two SZ observables, the central or maximum and integrated Comptonization parameters y_max and Y, relate in a simple way to the total cluster mass, which allows the construction of mass functions (MFs) that can be used to estimate cosmological parameters such as Ω_M, σ₈, and the dark energy parameter w. However, clusters form from the mergers of smaller structures, events that can disrupt the equilibrium of intracluster gas on which SZ- M relations rely. From a set of N-body/hydrodynamical simulations of binary cluster mergers, we calculate the evolution of Y and y_max over the course of merger events and find that both parameters are transiently "boosted," primarily during the first core passage. We then use a semianalytic technique developed by Randall et al. to estimate the effect of merger boosts on the distribution functions YF and yF of Y and y_max, respectively, via cluster merger histories determined from extended Press-Schechter (PS) merger trees. We find that boosts do not induce an overall systematic effect on YFs, and the values of Ω_M, σ₈, and w were returned to within 2% of values expected from the nonboosted YFs. The boosted yFs are significantly biased, however, causing Ω_M to be underestimated by 15%-45%, σ₈ to be overestimated by 10%-25%, and w to be pushed to more negative values by 25%-45%. We confirm that the integrated SZ effect, Y, is far more robust to mergers than y_max, as previously reported by Motl et al. and similarly found for the X-ray equivalent Y_X, and we conclude that Y is the superior choice for constraining cosmological parameters.

I provide estimates of the ultraviolet and visible light luminosity density at z ∼ 6 after accounting for the contribution from faint galaxies below the detection limit of deep HST and Spitzer surveys. I find that the rest-frame V-band luminosity density is a factor of ∼2-3 below the ultraviolet luminosity density at z ∼ 6. This implies that the maximal age of the stellar population at z ∼ 6, for a Salpeter initial mass function (IMF) and a single, passively evolving burst, must be ≲100 Myr. If the stars in z ∼ 6 galaxies are remnants of the star formation that was responsible for ionizing the intergalactic medium, reionization must have been a brief process that was completed at z < 7. This assumes the most current estimates of the clumping factor and escape fraction and a Salpeter slope extending up to 200 M_☉ for the stellar IMF (dN/dM ∝ M^α, α = −2.3). Unless the ratio of the clumping factor to escape fraction is less than 60, a Salpeter slope for the stellar IMF and reionization redshift higher than 7 are ruled out. In order to maintain an ionized intergalactic medium from redshift 9 onward, the stellar IMF must have a slope of α = −1.65 even if stars as massive as ~200 M_☉ are formed. Correspondingly, if the intergalactic medium was ionized from redshift 11 onward, the IMF must have α = −1.5. The range of stellar mass densities at z ∼ 6 straddled by IMFs which result in reionization at z > 7 is (1.3 ± 0.4) × 10⁷M_☉ Mpc⁻³.

We present a formalism to reveal merging by subtracting the change in the galaxy stellar mass function (MF) due to mass-dependent star formation (SF) from the observed time derivative of the MF. We present the SF rate (SFR) in the FORS Deep Field as a function of stellar mass and time spanning 9 < log M_* < 12 and 0 < z < 5. At z ≳ 3, the average SFR is a power law of stellar mass (_* ∝ M^0.6_*). The average SFR in massive objects at this redshift is 100-500 M_☉ yr⁻¹. At z ∼ 3, the SFR starts to drop at high masses. The break mass at which the SFR deviates from the power-law form decreases smoothly from log M¹_* ≳ 13 at z ∼ 5 to log M¹_* ∼ 10.9 at z ∼ 0.5, according to M¹_*(z) = 2.7 × 10¹⁰(1 + z)^2.1 (downsizing). We directly observe a relationship between SF history and mass. More massive galaxies have steeper and earlier onsets of SF, higher peak SFR, followed by a shorter decay time. The SFR in high-mass galaxies at z ∼ 4 can explain their rapid increase in abundance. Within large uncertainties, at most 0.8 effective major mergers per Gyr are consistent with the data, yet enough to transform most high-mass objects into ellipticals contemporaneously with their major SF episode. In contrast, at z ∼ 1.5 and at M_* ≳ 11, mergers contribute 0.1-0.2 Gyr⁻¹ to the increase in number density. This corresponds to ~1 major merger per object a 1.5 > z > 0. At 10 < log M_* < 11, galaxies are being preferentially destroyed at early times, while at later times the change in their numbers turns positive. This is an indication of the top-down buildup of the red sequence suggested by other recent observations.

Motivated by hydrodynamical simulations, we present results of an attempt to include more detailed gas physics within semianalytic models (SAMs) of galaxy formation, focusing on the role played by environmental effects. The main difference from previous SAMs is that we include "gravitational" heating of the intracluster medium (ICM) by the net surplus of gravitational potential energy released from gas that has been stripped from infalling satellites. Gravitational heating appears to be an efficient heating source able to prevent cooling in environments corresponding to dark matter halos more massive than M_* at any given redshift because of the continued growth by mergers. The energy release by gravitational heating can match that by AGN feedback in massive galaxies and can exceed it in the most massive ones. However, there is a fundamental difference in the way the two processes operate. Gravitational heating becomes important at late times, when the peak activity of AGNs is already over, and it is very mass dependent. This mass dependency and time behavior gives the right trend to recover downsizing in the star formation rate of massive galaxies. We present a number of first-order comparisons of our model to well-established observations of galaxy properties, which can be summarized as follows. The cosmic star formation rate is reproduced well. In addition, the star formation episode of our model galaxies is a strong function of mass. Massive galaxies with M_* > 10¹¹M_☉ make most of their stars at look-back times of roughly 11 Gyr and show very low amounts of residual star formation at late times due to suppression by environmental effects. Furthermore, the luminosity function and color bimodality of the galaxy population are reproduced well.

We report on the first results of a new study aimed at understanding the diversity and evolutionary history of distant galactic bulges in the context of now well-established trends for pure spheroidal galaxies. To this end, bulges have been isolated for a sample of 137 spiral galaxies within the redshift range 0.1 < z < 1.2 in the GOODS fields. Using proven photometric techniques, we determine the characteristic parameters (size, surface brightness, profile shape) of both the disk and bulge components in our sample. In agreement with earlier work that utilized aperture colors, distant bulges show a broader range of optical colors than would be the case for passively evolving populations. To quantify the amount of recent star formation necessary to explain this result, we used DEIMOS to secure stellar velocity dispersions for a sizeable fraction of our sample. This has enabled us to compare the fundamental plane of our distant bulges with that for spheroidal galaxies in a similar redshift range. Bulges of spiral galaxies with a bulge-to-total luminosity ratio (B/T) >0.2 show similar patterns of evolution to those seen for pure spheroidals such that the stellar populations of all spheroids with M > 10¹¹M_☉ are consistent with a single major burst of star formation at high redshift (z_f≳ 2), while bulges with M < 10¹¹M_☉ must have had more recent stellar mass growth (~10% in mass since z ∼ 1). Although further data spanning a wider range of redshift and mass are desirable, the similarity between the assembly histories of bulges and low-mass spheroidals seems difficult to reconcile with the picture whereby the majority of large bulges form primarily via secular processes within spiral galaxies.

Due to the lack of low-redshift long gamma-ray bursts (GRBs), the circular problem has been a severe obstacle for using GRBs as cosmological candles. In this paper, we present a new method to deal with such a problem in Markov chain Monte Carlo (MCMC) global fitting analysis. Assuming a certain type of correlation, for the parameters involved in the correlation relation, we treat them as free parameters and determine them simultaneously with cosmological parameters through MCMC analysis on GRB data together with other observational data. Then the circular problem is naturally eliminated in this procedure. To demonstrate the feasibility of our method, we take the Ghirlanda relation (E_γ ∝ CE^A_peak) as an example, while keeping in mind the debate about its physical validity. Together with SN Ia, WMAP, and SDSS data, we include 27 GRBs with the reported Ghirlanda relation in our study and perform MCMC global fitting. We consider the ΛCDM model and dynamical dark energy models, respectively. We also include the curvature of the universe in our analysis. In each case, in addition to the constraints on the relevant cosmological parameters, we obtain the best-fit values as well as the distributions of the correlation parameters A and C. With CMB+LSS+SNe+GRB data included in the analysis, the results on A and C for different cosmological models are in agreement well within a 1 σ range. It is also noted that the distributions of A and C are generally broader than the priors used in many studies in the literature. Our method can be readily applied to other GRB relations, which might be better physically motivated.

We present a new photometric search for high-z galaxies hosting Population III (Pop III) stars based on deep intermediate-band imaging observations obtained in the Subaru Deep Field (SDF), using Suprime-Cam on the Subaru Telescope. By combining our new data with the existing broadband and narrowband data, we searched for galaxies that emit strongly in both Lyα and He II λ1640 (dual emitters) and are promising candidates for Pop III-hosting galaxies at 3.93≲ z≲ 4.01 and 4.57≲ z≲ 4.65. Although we found 10 dual emitters, most of them turn out to be [O II]-[O III] dual emitters or Hβ-(Hα+[N II]) dual emitters at z < 1, as inferred from their broadband colors and from the ratio of the equivalent widths. No convincing candidate Lyα-He II dual emitter of SFR_PopIII≳ 2 M_☉ yr⁻¹ was found by our photometric search in 4.03 × 10⁵ Mpc³ in the SDF. This result disfavors low-feedback models for Pop III star clusters and implies an upper limit on the Pop III SFR density of SFRD_PopIII < 5 × 10⁻⁶M_☉ yr⁻¹ Mpc⁻³. This new selection method to search for Pop III-hosting galaxies should be useful in future narrowband surveys to achieve the first observational detection of Pop III-hosting galaxies at high redshifts.

Chandra observations of the type 1.8, low-redshift (z = 0.115), red (J − K_S ∼ 2) quasar 2MASSJ 104943+583750 (2M1049+5837) indicated an unusually hard X-ray spectrum (HR ∼ 0.6), suggesting obscuration of N_H ∼ 4 × 10²² cm⁻² and a flat Γ ∼ 0.4 power-law slope. A higher signal-to-noise ratio XMM-Newton observation reported here reveals a more complex spectrum, being extremely hard above ~2 keV, Γ ∼ − 0.6, with a well-defined soft excess similar to the bright, nearby Seyfert 2 galaxy Mrk 3. Such extremely hard quasar spectra cannot be a dominant contributor to the cosmic X-ray background (CXRB; Γ ∼ 1.4). Modeling of 2M1049+5837 shows the observed 2-10 keV spectrum to be the sum of a strongly absorbed "normal" Γ ∼ 1.8 underlying quasar X-ray continuum, and a cold reflection component (R ∼ 2). The strong attenuation of the intrinsic X-ray continuum by a sub-Compton-thick line-of-sight column, N_H ∼ 3.4 × 10²³ cm⁻², reveals a soft X-ray emission component whose spectrum indicates reprocessing/emission from, possibly extended, photoionized gas. The luminosity of the soft X-ray component is similar to Seyfert 2 galaxies, an order of magnitude less than comparable type 1 AGNs, suggesting partial obscuration of core-bright emission. The optical emission is complex, with distinct red and blue scattered light components believed to originate in two extended regions visible in HST imaging data. The unusual combination of properties: optical and X-ray obscuration of the nuclear emission, partially obscured broad but unobscured narrow emission lines, and partially obscured soft X-ray excess, implies an intermediate viewing angle, over or through the edge of an obscuring disk + wind or torus. 2M1049+5837 again demonstrates both the ambiguity of low signal-to-noise X-ray spectra/hardness ratios and the potential of 2MASS-selected red sources for exploring the complex obscuration and geometry of the nuclear regions in AGNs.

We present mid-infrared spectroscopy of a sample of 16 optically faint infrared luminous galaxies obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. These sources were jointly selected from Spitzer and Chandra imaging surveys in the NOAO Deep Wide-Field Survey Boötes field and were selected from their bright X-ray fluxes to host luminous AGNs. None of the spectra show significant emission from polycyclic aromatic hydrocarbons (PAHs; 6.2 μm equivalent widths <0.2 μm), consistent with their infrared emission being dominated by AGNs. Nine of the X-ray sources show 9.7 μm silicate absorption features. Their redshifts are in the range 0.9 < z < 2.6, implying infrared luminosities of log L_IR = 12.5–13.6 L_☉. The average silicate absorption is not as strong as that of previously targeted optically faint infrared luminous galaxies with similar mid-infrared luminosities, implying that the X-ray selection favors sources behind a smaller column of Si-rich dust than non-X-ray selection. Seven of the X-ray sources have featureless power-law mid-infrared spectra. We argue that the featureless spectra likely result from the sources having weak or absent silicate and PAH features rather than the sources lying at higher redshifts, where these features are shifted out of the IRS spectral window. We investigate whether there are any correlations between X-ray and infrared properties and find that sources with silicate absorption features tend to have fainter X-ray fluxes and harder X-ray spectra, indicating a weak relation between the amount of silicate absorption and the column density of X-ray-absorbing gas.

We present the mid-infrared colors of X-ray-detected AGNs and explore mid-infrared selection criteria. Using a statistical matching technique, the likelihood ratio, over 900 IRAC counterparts were identified with a new MUSYC X-ray source catalog that includes ~1000 published X-ray sources in the Chandra Deep Field-South and Extended Chandra Deep Field-South. Most X-ray-selected AGNs have IRAC spectral shapes consistent with power-law slopes, f_ν ∝ ν^α, and display a wide range of colors, -2 ⩽ α ⩽ 2. Although X-ray sources typically fit to redder (more negative α) power laws than non-X-ray-detected galaxies, more than 50% do have flat or blue (galaxy-like) spectral shapes in the observed 3-8 μm band. Only a quarter of the X-ray-selected AGNs detected at 24 μm are well fit by featureless red power laws in the observed 3.6-24 μm, likely the subset of our sample whose infrared spectra are dominated by emission from the central AGN region. Most IRAC color selection criteria fail to identify the majority of X-ray-selected AGNs, finding only the more luminous AGNs, the majority of which have broad emission lines. In deep surveys, these color selection criteria select 10%-20% of the entire galaxy population and miss many moderate-luminosity AGNs.

The residuals about the standard M_bh-σ relation correlate with the effective radius, absolute magnitude, and Sérsic index of the host bulge, giving rise to an apparent black hole "fundamental plane." However, we show that the elliptical galaxies do not define such a plane. Instead, it is a handful of barred galaxies, which are shown to systematically deviate from the M_bh-σ relation by δ log M_bh ≈ − 0.5 to –1.0 dex (their σ-values are too large) and generate much of the aforementioned three-parameter correlations. Removal of the seven barred galaxies from the Tremaine et al. set of 31 galaxies gives a "barless" M_bh-σ relation with an intrinsic scatter of 0.17 dex (vs. 0.27 dex for the 31 galaxies) and a total scatter of 0.25 dex (vs. 0.34 dex for the 31 galaxies). Furthermore, removal of the barred galaxies, or all the disk galaxies, from an expanded and updated set of 40 galaxies with direct black hole mass measurements gives a consistent result, such that log (M_bh/M_☉) = (8.25 ± 0.05) + (3.68 ± 0.25) log (σ/200 km s⁻¹) . In addition, the barless σ-L relation for galaxies with direct black hole mass measurements is found to be consistent with that from the SDSS sample of early-type galaxies, and the barless M_bh-σ relation, the M_bh-n relation, and the K-band M_bh-L relation are all shown to yield SMBH masses less than 2–4 × 10⁹M_☉.

We present the first results from the AGN Multiwavelength Survey of Early-Type Galaxies in the Virgo Cluster (AMUSE-Virgo). This large program targets 100 early-type galaxies with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory and the Multiband Imaging Photometer on board the Spitzer Space Telescope, with the aim of providing an unbiased census of low-level supermassive black hole activity in the local universe. Here we report on the Chandra observations of the first 16 targets, and combine them with results from archival data of another, typically more massive, 16 targets. Pointlike X-ray emission from a position coincident with the optical nucleus is detected in 50% of the galaxies (down to our completeness limit of ~4 × 10³⁸ ergs s⁻¹). Two of the X-ray nuclei are hosted by galaxies (VCC 1178 [N4464] and VCC 1297 [N4486B]) with absolute B magnitudes fainter than –18, where nuclear star clusters are known to become increasingly common. After carefully accounting for possible contamination from low-mass X-ray binaries, we argue that the detected nuclear X-ray sources are most likely powered by low-level accretion on to a supermassive black hole, with a ≲11% chance contamination in VCC 1178, where a star cluster is barely resolvable in archival Hubble Space Telescope images. Based on black hole mass estimates from the global properties of the host galaxies, all the detected nuclei are highly sub-Eddington, with luminosities in the range –8.4 < log (L_{0.3–10 keV}/L_Edd) < − 5.9. The incidence of nuclear X-ray activity increases with the stellar mass M_⋆ of the host galaxy: only between 3% and 44% of the galaxies with M_⋆ < 10¹⁰M_☉ harbor an X-ray active supermassive black hole. The fraction rises to between 49% and 87% in galaxies with stellar mass above 10¹⁰M_☉ (at the 95% confidence level).

We compile black hole (BH) masses for ~60,000 quasars in the redshift range 0.1≲ z≲ 4.5 included in the Fifth Data Release of the Sloan Digital Sky Survey, using virial BH mass estimators based on the Hβ, Mg II, and C IV emission lines. Within our sample, the widths of the three lines follow lognormal distributions, with means and dispersions that do not depend strongly on luminosity or redshift. The Mg II- and Hβ-estimated BH masses are consistent with one another, but there is a positive bias between the C IV- and Mg II-estimated BH masses correlated with the C IV-Mg II blueshift, suggesting that the C IV estimator is more severely affected by a disk wind. If the underlying BH mass distribution decreases with mass and the Eddington ratio distribution at fixed true BH mass has nonzero width, we show that the measured virial BH mass and Eddington ratio distributions within finite luminosity bins are subject to Malmquist bias. We present a model that reproduces the observed virial mass distribution, quasar luminosity function, and line width distribution of our sample; it has an underlying BH mass distribution dN/dlog M ∝ M^−2.6 and a lognormal true Eddington ratio distribution at fixed true mass with dispersion 0.4 dex and mean dependent on BH mass. In this model, the observed virial mass (Eddington ratio) distribution for the SDSS sample is biased high (low) by ~0.6 dex within finite luminosity bins. Finally, we compare virial BH masses of radio and broad absorption line quasars with ordinary quasars matched in redshift and luminosity.

We present multifrequency simultaneous VLBA observations at 15, 22, and 43 GHz toward the nucleus of the nearby radio galaxy NGC 1052. These three continuum images reveal a double-sided jet structure whose relative intensity ratios imply that the jet axis is oriented close to the sky plane. The steeply rising spectra at 15-43 GHz at the inner edges of the jets strongly suggest that synchrotron emission is absorbed by foreground thermal plasma. We detected H₂O maser emission in the velocity range of 1550-1850 km s⁻¹, which is redshifted by 50-350 km s⁻¹ with respect to the systemic velocity of NGC 1052. The redshifted maser gas appears projected against both sides of the jet, in the same manner as the H I seen in absorption. The H₂O maser gas is located where the free-free absorption opacity is large. This probably implies that the masers in NGC 1052 are associated with a circumnuclear torus or disk as in the nucleus of NGC 4258. Such circumnuclear structure could be the source of accretion onto the central engine.

Nongravitational processes, such as feedback from galaxies and their active nuclei, are believed to have injected excess entropy into the intracluster gas, and therefore to have modified the density profiles in galaxy clusters during their formation. Here we study a simple model for this so-called preheating scenario, and ask (1) whether it can simultaneously explain both global X-ray scaling relations and number counts of galaxy clusters, and (2) whether the amount of entropy required evolves with redshift. We adopt a baseline entropy profile that fits recent hydrodynamic simulations, modify the hydrostatic equilibrium condition for the gas by including ≈20% nonthermal pressure support, and add an entropy floor K₀ that is allowed to vary with redshift. We find that the observed luminosity-temperature (L − T) relations of low-redshift (⟨ z⟩ = 0.05) HIFLUGCS clusters and high-redshift (⟨ z⟩ = 0.80) WARPS clusters are best simultaneously reproduced with an entropy floor that evolves from ≈200 h^−1/3 keV cm ² at z ≈ 0.8 to ≳300 h^−1/3 keV cm ² at z < 0.05. This evolution may take place predominantly at low redshift (z≲ 0.2). If we restrict our analysis to the subset of bright (kT≳ 3 keV) clusters, we find that the evolving entropy floor can mimic a self-similar evolution in the L − T scaling relation. This degeneracy with self-similar evolution is, however, lifted when 0.5 keV ≲ kT≲ 3 keV clusters are included. Using the cosmological parameters from the WMAP 3 yr data, but treating σ₈ as a free parameter, our model can reproduce the number counts of the X-ray galaxy clusters in the 158 deg²ROSAT PSPC survey, with a best-fit value of σ₈ = 0.80 ± 0.05.

We present the evolution of the color-magnitude distribution of galaxy clusters from z = 0.45 to z = 0.9 using a homogeneously selected sample of ~1000 clusters drawn from the Red-Sequence Cluster Survey (RCS). The red fraction of galaxies decreases as a function of increasing redshift for all clustercentric radii, consistent with the Butcher-Oemler effect, and suggesting that the cluster blue population may be identified with newly infalling galaxies. We also find that the red fraction at the core has a shallower evolution compared with that at the cluster outskirts. Detailed examination of the color distribution of blue galaxies suggests that they have colors consistent with normal spirals and may redden slightly with time. Galaxies of starburst spectral type contribute less than 5% of the increase in the blue population at high redshift, implying that the observed Butcher-Oemler effect is not caused by a unobscured starbursts, but is more consistent with a normal coeval field population.

We present new constraints on the evolution of the early-type galaxy color-magnitude relation (CMR) based on deep near-infrared imaging of a galaxy protocluster at z = 2.16 obtained using NICMOS on board the Hubble Space Telescope. This field contains a spectroscopically confirmed space overdensity of Lyα- and Hα-emitting galaxies that surrounds the powerful radio galaxy MRC 1138–262. Using these NICMOS data we identify a significant surface overdensity (=6.2 times) of red J₁₁₀ − H₁₆₀ galaxies in the color-magnitude diagram (when compared with deep NICMOS imaging from the HDF-N and UDF). The optical-NIR colors of these prospective red-sequence galaxies indicate the presence of ongoing dust-obscured star formation or recently formed (≲1.5 Gyr) stellar populations in a majority of the red galaxies. We measure the slope and intrinsic scatter of the CMR for three different red galaxy samples selected by a wide color cut and using photometric redshifts both with and without restrictions on rest-frame optical morphology. In all three cases both the rest-frame U − B slope and intrinsic color scatter are considerably higher than corresponding values for lower redshift galaxy clusters. These results suggest that while some relatively quiescent galaxies do exist in this protocluster both the majority of the galaxy population and hence the color-magnitude relation are still in the process of forming, as expected.

We present Hubble Space Telescope NICMOS H-band imaging of 33 ultraluminous infrared galaxies (ULIRGs) at z ∼ 2 that were selected from the 24 μm catalog of the Spitzer Extragalactic First Look Survey. The images reveal that at least 17 of the 33 objects are associated with interactions. Up to one-fifth of the sources in our sample could be minor mergers, whereas only two systems are merging binaries with luminosity ratio ≤ 3:1, which is characteristic of local ULIRGs. The rest-frame optical luminosities of the sources are of the order 10¹⁰-10¹¹L_☉ and their effective radii range from 1.4 to 4.9 kpc. The most compact sources are either those with a strong active nucleus continuum or those with a heavy obscuration in the mid-infrared regime, as determined from Spitzer Infrared Spectrograph data. The luminosity of the 7.7 μm feature produced by polycyclic aromatic hydrocarbon molecules varies significantly among compact systems, whereas it is typically large for extended systems. A bulge-to-disk decomposition performed for the six brightest (m_H < 20) sources in our sample indicates that they are best fit by disklike profiles with small or negligible bulges, unlike the bulge-dominated remnants of local ULIRGs. Our results provide evidence that the interactions associated with ultraluminous infrared activity at z ∼ 2 can differ from those at z ∼ 0.

We report subarcsecond resolution IRAM PdBI millimeter CO interferometry of four z ∼ 2 submillimeter galaxies (SMGs), and sensitive CO(3-2) flux limits toward three z ∼ 2 UV/optically selected star-forming galaxies. The new data reveal for the first time spatially resolved CO gas kinematics in the observed SMGs. Two of the SMGs show double or multiple morphologies, with complex, disturbed gas motions. The other two SMGs exhibit CO velocity gradients of ~500 km s⁻¹ across ≤0.2^'' (1.6 kpc) diameter regions, suggesting that the star-forming gas is in compact, rotating disks. Our data provide compelling evidence that these SMGs represent extreme, short-lived "maximum" star-forming events in highly dissipative mergers of gas-rich galaxies. The resulting high-mass surface and volume densities of SMGs are similar to those of compact quiescent galaxies in the same redshift range and much higher than those in local spheroids. From the ratio of the comoving volume densities of SMGs and quiescent galaxies in the same mass and redshift ranges, and from the comparison of gas exhaustion timescales and stellar ages, we estimate that the SMG phase duration is about 100 Myr. Our analysis of SMGs and optically/UV selected high-redshift star-forming galaxies supports a "universal" Chabrier IMF as being valid over the star-forming history of these galaxies. We find that the ¹²CO luminosity to total gas mass conversion factors at z ∼ 2-3 are probably similar to those assumed at z ∼ 0. The implied gas fractions in our sample galaxies range from 20% to 50%.

We present infrared spectroscopy from the Spitzer Space Telescope at one disk position and two positions at a height of 1 kpc from the disk in the edge-on spiral NGC 891, with the primary goal of studying halo ionization. Our main result is that the [Ne III]/[Ne II] ratio, which provides a measure of the hardness of the ionizing spectrum free from the major problems plaguing optical line ratios, is enhanced in the extraplanar pointings relative to the disk pointing. Using a 2D Monte Carlo-based photoionization code that accounts for the effects of radiation field hardening, we find that this trend cannot be reproduced by any plausible photoionization model and that a secondary source of ionization must therefore operate in gaseous halos. We also present the first spectroscopic detections of extraplanar PAH features in an external normal galaxy. If they are in an exponential layer, very rough emission scale heights of 330-530 pc are implied for the various features. Extinction may be nonnegligible in the midplane and reduce these scale heights significantly. There is little significant variation in the relative emission from the various features between disk and extraplanar environment. Only the 17.4 μm feature is significantly enhanced in the extraplanar gas compared to the other features, possibly indicating a preference for larger PAHs in the halo.

We present results from neutral hydrogen (H I) observations of the tip of the Magellanic Stream (MS), obtained with the Arecibo telescope as a part of the ongoing survey by the consortium for Galactic studies with the Arecibo L-band Feed Array. We find four large-scale, coherent H I streams, extending continuously over a length of 20°, each stream possessing different morphology and velocity gradients. The newly discovered streams provide strong support for the tidal model of the MS formation by Connors et al. (2006), who suggested a spatial and kinematic bifurcation of the MS. The observed morphology and kinematics suggest that three of these streams could be interpreted as a three-way splitting of the main MS filament, while the fourth stream appears much younger and may have originated from the Magellanic Bridge. We find an extensive population of H I clouds at the tip of the MS. Two-thirds of clouds have an angular size in the range 3.5^'-10^'. We interpret this as being due to thermal instability, which would affect a warm tail of gas trailing through the Galactic halo over a characteristic timescale of a few Myr to a few hundred Myr. We show that thermal fragments can survive in the hot halo for a long time, especially if surrounded by a <10⁶ K halo gas. If the observed clumpy structure is mainly due to thermal instability, then the tip of the MS is at a distance of ~70 kpc. A significant fraction of H I clouds at the tip of the MS show multiphase velocity profiles, indicating the coexistence of cooler and warmer gas.

We use available proper-motion measurements of Milky Way (MW) satellite galaxies to calculate their orbital poles and projected uncertainties. These are compared with a set of recent cold dark matter (CDM) simulations tailored specifically to solve the MW satellite problem. We show that the CDM satellite orbital poles are fully consistent with being drawn from a random distribution, while the MW satellite orbital poles indicate that the "disk of satellites" of the Milky Way is rotationally supported. Furthermore, a bootstrap analysis of the spatial distribution of theoretical CDM satellites also shows that they are consistent with being randomly drawn. The theoretical CDM satellite population thus shows a significantly different orbital and spatial distribution from that of the MW satellites, most probably indicating that the majority of the latter are of tidal origin rather than being dark matter-dominated substructures. A statistic is presented that can be used to test a possible correlation of satellite galaxy orbits with their spatial distribution.

We have used data from the Sloan Digital Sky Survey (SDSS) Data Release 5 to explore the overall structure and substructure of the stellar halo of the Milky Way using ~4 million color-selected main-sequence turnoff stars with 0.2 < g − r < 0.4 and 18.5 ⩽ r < 22.5. We fit oblate and triaxial broken power law models to the data, and found a "best-fit" oblateness of the stellar halo 0.5 < c/a < 0.8, and halo stellar masses between galactocentric radii of 1 and 40 kpc of 3.7 ± 1.2 × 10⁸M_☉. The density profile of the stellar halo is approximately ρ ∝ r^−α, where –2 > α > − 4. Yet, we found that all smooth and symmetric models were very poor fits to the distribution of stellar halo stars because the data exhibit a great deal of spatial substructure. We quantified deviations from a smooth oblate/triaxial model using the rms of the data around the model profile on scales ≳100 pc, after accounting for the (known) contribution of Poisson uncertainties. Within the DR5 area of the SDSS, the fractional rms deviation σ/total of the actual stellar distribution from any smooth, parameterized halo model is ≳40%: hence, the stellar halo is highly structured. We compared the observations with simulations of galactic stellar halos formed entirely from the accretion of satellites in a cosmological context by analyzing the simulations in the same way as the SDSS data. While the masses, overall profiles, and degree of substructure in the simulated stellar halos show considerable scatter, the properties and degree of substructure in the Milky Way's halo match well the properties of a "typical" stellar halo built exclusively out of the debris from disrupted satellite galaxies. Our results therefore point toward a picture in which an important fraction of the stellar halo of the Milky Way has been accreted from satellite galaxies.

Hypervelocity stars (HVSs) traverse the Galaxy from the central black hole to the outer halo. We show that the Galactic potential within 200 pc acts as a high-pass filter preventing low-velocity HVSs from reaching the halo. To trace the orbits of HVSs throughout the Galaxy, we construct two forms of the potential which reasonably represent the observations in the range 5-10⁵ pc: a simple spherically symmetric model and a bulge-disk-halo model. We use the Hills mechanism (disruption of binaries by the tidal field of the central black hole) to inject HVSs into the Galaxy and to compute the observable spatial and velocity distributions of HVSs with masses in the range 0.6-4 M_☉. These distributions reflect the mass function in the Galactic center, properties of binaries in the Galactic center, and aspects of stellar evolution and the injection mechanism. For 0.6-4 M_☉ main-sequence stars, the fraction of unbound HVSs and the asymmetry of the velocity distribution for their bound counterparts increase with stellar mass. The density profiles for unbound HVSs decline with distance from the Galactic center approximately as r⁻² (but are steeper for the most massive stars, which evolve off the main sequence during their travel time from the Galactic center); the density profiles for the bound ejecta decline with distance approximately as r⁻³. In a survey with a limiting magnitude of V≲ 23, the detectability of HVSs (unbound or bound) increases with stellar mass.

The Cygnus X-Ray Emission Spectroscopic Survey (CyXESS) sounding rocket payload was launched from White Sands Missile Range on 2006 November 20 and obtained a high-resolution spectrum of the Cygnus Loop supernova remnant in the soft X-ray. The novel X-ray spectrograph incorporated a wire-grid collimator feeding an array of gratings in the extreme off-plane mount that ultimately dispersed the spectrum onto gaseous electron multiplier (GEM) detectors. This instrument recorded 65 s of usable data between 43 and 49.5 Å in two prominent features. The first feature near 45 Å is dominated by the He-like triplet of O VII in second order with contributions from Mg X and Si IX-Si XII in first order, while the second feature near 47.5 Å is first-order S IX and S X. Fits to the spectra give an equilibrium plasma at log (T) = 6.2 (kT_e = 0.14 keV) and near cosmic abundances. This is consistent with previous observations, which demonstrated that the soft X-ray emission from the Cygnus Loop is dominated by interactions between the initial blast wave and the walls of a precursor-formed cavity surrounding the Cygnus Loop and that this interaction can be described using equilibrium conditions.

The magnetohydrodynamic evolution of a dense spherical cloud as it interacts with a strong planar shock is studied, as a model for shock interactions with density inhomogeneities in the interstellar medium. The cloud is assumed to be small enough that radiative cooling, thermal conduction, and self-gravity can be ignored. A variety of initial orientations (including parallel, perpendicular, and oblique to the incident shock normal) and strengths for the magnetic field are investigated. During the early stages of the interaction (less than twice the time taken for the transmitted shock to cross the interior of the cloud), the structure and dynamics of the shocked cloud are fairly insensitive to the magnetic field strength and orientation. However, at late times strong fields substantially alter the dynamics of the cloud, suppressing fragmentation and mixing by stabilizing the interface at the cloud surface. Even weak magnetic fields can drastically alter the evolution of the cloud compared to the hydrodynamic case. Weak fields of different geometries result in different distributions and amplifications of the magnetic energy density, which may affect the thermal and nonthermal X-ray emission expected from shocked clouds associated with, for example, supernova remnants.

CS (2-1) measurements toward a large sample of fourth Galactic quadrant infrared dark clouds (IRDCs) were made with the Australia Telescope National Facility Mopra telescope in order to establish their kinematic distances and Galactic distribution. Due to its large critical density, CS unambiguously separates the dense IRDCs from more diffuse giant molecular clouds. The fourth-quadrant IRDCs show a pronounced peak in their radial galactocentric distribution at R = 6 kpc. The first-quadrant IRDC distribution (traced by ¹³CO emission) also shows a peak, but at a galactocentric radius of R = 5 kpc rather than 6 kpc. This disparity in the peak galactocentric radius suggests that IRDCs trace a spiral arm which lies closer to the Sun in the fourth quadrant. Indeed, the deduced IRDC distribution matches the location of the Scutum-Centaurus arm in Milky Way models dominated by two spiral arms. Since, in external galaxies, OB stars form primarily in spiral arms, the association of IRDCs with a Milky Way spiral arm supports the idea that high-mass stars form in IRDCs. The first-quadrant IRDC distribution also reveals a second peak near the solar circle, possibly due to the fact that ¹³CO could trace somewhat lower density IRDCs. The reliability of the MSX IRDC catalog by Simon and coworkers is estimated by using the CS detection rate of IRDC candidates. The overall reliability is at least 58%, and increases to near 100% for high contrasts, Galactic longitudes within ~30° of the Galactic center, and large mid-IR backgrounds. A significant fraction of our IRDC sample (14%) showed two CS velocity components, which probably represent two distinct IRDCs along the same line of sight.

We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum. Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index. This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed. Analysis of the partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum. In the Galaxy's spiral arms, no rotation measure fluctuations on scales above a few parsecs are measured. In the interarm regions fluctuations on larger scales than in spiral arms are present, and show power-law behavior with a shallow spectrum. These results suggest that in the spiral arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and superbubble explosions are the main sources of energy on scales on the order of 100 pc.

Virtually all detailed gas-phase models of the chemistry of dense interstellar clouds exclude polycyclic aromatic hydrocarbons (PAHs). This omission is unfortunate because from the few studies that have been done on the subject, it is known that the inclusion of PAHs can affect the gas-phase chemistry strongly. We have added PAHs to our network to determine the role they play in the chemistry of cold dense cores. Initially, only the chemistry of neutral and negatively charged PAH species was considered, since it was assumed that positively charged PAHs are of little importance. Subsequently, this assumption was checked and confirmed. In the models presented here, we include radiative attachment to form PAH⁻, mutual neutralization between PAH anions and small positively charged ions, and photodetachment. We also test the sensitivity of our results to changes in the size and abundance of the PAHs. Our results confirm that the inclusion of PAHs changes many of the calculated abundances of smaller species considerably. In TMC-1, the general agreement with observations is significantly improved, unlike in L134N. This may indicate a difference in PAH properties between the two regions. With the inclusion of PAHs in dense cloud chemistry, high-metal elemental abundances give a satisfactory agreement with observations. As a result, we do not need to decrease the observed elemental abundances of all metals, and we do not need to vary the elemental C/O ratio in order to produce large abundances of carbon species in TMC-1 (CP).

We use observations of the C I, C II, H I, and H₂ column densities along lines of sight in the Galactic plane to determine the formation rate of H₂ on grains and to determine chemical reaction rates with polycyclic aromatic hydrocarbons (PAHs). Photodissociation region models are used to find the best-fit parameters to the observed columns. We find the H₂ formation rate on grains has a low rate (R ∼ 1 × 10⁻¹⁷ cm³ s⁻¹) along lines of sight with low column density (A_V≲ 0.25) and low molecular fraction (f_H2≲ 10⁻⁴). At higher column densities (0.25 ⩽ A_V ⩽ 2.13), we find a rate of R ∼ 3.5 × 10⁻¹⁷ cm³ s⁻¹. The lower rate at low column densities could be the result of grain processing by interstellar shocks, which may deplete the grain surface area or process the sites of H + H formation, thereby inhibiting H₂ production. Alternatively, the formation rate may be normal, and the low molecular fraction may be the result of lines of sight that graze larger clouds. Such lines of sight would have a reduced H₂ self-shielding compared to the line-of-sight column. We find the reaction C⁺ + PAH⁻→ C + PAH⁰ is best fit with a rate 2.4 × 10⁻⁷Φ_PAHT^−0.5₂ cm³ s⁻¹ with T₂ = T/100 K, and the reaction C⁺ + PAH⁰→ C + PAH⁺ is best fit with a rate 8.8 × 10⁻⁹Φ_PAH cm³ s⁻¹. In high-column-density gas, we find Φ_PAH ∼ 0.4. In low-column-density gas, Φ_PAH is less well constrained, with Φ_PAH ∼ 0.2–0.4.

We investigate the chemical abundances of NGC 3603 in the Milky Way, of 30 Doradus in the Large Magellanic Cloud, and of N66 in the Small Magellanic Cloud. Mid-infrared observations with the Infrared Spectrograph on board the Spitzer Space Telescope allow us to probe the properties of distinct physical regions within each object, the central ionizing cluster, the surrounding ionized gas, photodissociation regions, and buried stellar clusters. We detect [S III], [S IV], [Ar III], [Ne II], [Ne III], [Fe II], and [Fe III] lines and derive the ionic abundances. Based on the ionic abundance ratio (Ne III/H)/(S III/H), we find that the gas observed in the MIR is characterized by a lower degree of ionization than the gas observed in the optical spectra. We compute the elemental abundances of Ne, S, Ar, and Fe. We find that the α-elements Ne, S, and Ar scale with each other. Our determinations agree well with the abundances derived from the optical. The Ne/S ratio is higher than the solar value in the three giant H II regions and points toward a moderate depletion of sulfur on dust grains. We find that the neon and sulfur abundances display a remarkably small dispersion (0.11 dex in 15 positions in 30 Doradus), suggesting a relatively homogeneous ISM, even though small-scale mixing cannot be ruled out.

Velocity anisotropy induced by MHD turbulence is investigated using computational simulations and molecular line observations of the Taurus molecular cloud. A new analysis method is presented to evaluate the degree and angle of velocity anisotropy using spectroscopic imaging data of interstellar clouds. The efficacy of this method is demonstrated on model observations derived from three-dimensional velocity and density fields from the set of numerical MHD simulations that span a range of magnetic field strengths. The analysis is applied to ¹²CO J = 1–0 imaging of a subfield within the Taurus molecular cloud. Velocity anisotropy is identified that is aligned within ~10° of the mean local magnetic field direction derived from optical polarization measurements. Estimated values of the field strength based on velocity anisotropy are consistent with results from other methods. When combined with new column density measurements for Taurus, our magnetic field strength estimate indicates that the envelope of the cloud is magnetically subcritical. These observations favor strong MHD turbulence within the low-density, subcritical, molecular gas substrate of the Taurus cloud.

We report the results of a 100 deg² survey of the Taurus molecular cloud region in ¹²CO and ¹³CO J = 1→ 0. The image of the cloud in each velocity channel includes ≃3 × 10⁶ Nyquist-sampled pixels on a 20^'' grid. The high sensitivity and large spatial dynamic range of the maps reveal a very complex, highly structured cloud morphology, including filaments, cavities, and rings. The axes of the striations seen in the ¹²CO emission from relatively diffuse gas are aligned with the direction of the magnetic field. We have developed a statistical method for analyzing the pixels in which ¹²CO but not ¹³CO is detected, which allows us to determine the CO column in the diffuse portion of the cloud, as well as in the denser regions in which we detect both isotopologues. Using a column-density-dependent model for the CO fractional abundance, we derive the mass of the region mapped to be 2.4 × 10⁴M_☉, more than twice as large as would be obtained using a canonical fixed fractional abundance of ¹³CO, and a factor of 3 greater than would be obtained considering only the high column density regions. We determine that half the mass of the cloud is in regions having column density below 2.1 × 10²¹ cm⁻². The distribution of young stars in the region covered is highly nonuniform, with the probability of finding a star in a pixel with a specified column density rising sharply for N(H₂) = 6 × 10²¹ cm⁻². We determine a relatively low star formation efficiency (mass of young stars/mass of molecular gas), between 0.3% and 1.2%, and an average star formation rate during the past 3 Myr of 8 × 10⁻⁵ stars yr⁻¹.

This article presents a multiwavelength study toward S87, based on a data set of submillimeter/far-infrared/mid-infrared (submillimeter/FIR/MIR) images and molecular line maps. The submillimeter continuum emission measured with JCMT SCUBA reveals three individual clumps, namely, SMM 1, SMM 2, and SMM 3. The MIR/FIR images obtained by the Spitzer Space Telescope indicate that both SMM 1 and SMM 3 harbor point sources. The J = 1–0 transitions of CO,¹³CO, C¹⁸O, and HCO⁺, measured with the 13.7 m telescope of the Purple Mountain Observatory, exhibit asymmetric line profiles. Our analysis of spectral energy distributions (SEDs) shows that all three of the submillimeter clumps are massive (110-210 M_☉), with average dust temperatures in the range ~20-40 K. A multiwavelength comparison convinces us that the asymmetric profiles of molecular lines should result from two clouds at slightly different velocities, and it further confirms that the star-forming activity in SMM 1 is stimulated by a cloud-cloud collision. The stellar contents and SEDs suggest that SMM 1 and SMM 3 are high-mass and intermediate-mass star-forming sites, respectively. However, SMM 2 has no counterpart below 70 μm, which is likely to be a cold high-mass starless core. These results, as mentioned above, expose multiple phases of star formation in S87.

We have carried out an extensive survey of magnetic field strengths toward dark cloud cores in order to test ambipolar-diffusion-driven and turbulence-driven models of star formation. The survey involved ~500 hr of observing with the Arecibo telescope in order to make sensitive OH Zeeman observations toward 34 dark cloud cores. Nine new probable detections were achieved at the 2.5 σ level; the certainty of the detections varies from solid to marginal, so we discuss each probable detection separately. However, our analysis includes all the measurements and does not depend on whether each position has a detection or just a sensitive measurement. Rather, the analysis establishes mean (or median) values over the set of observed cores for relevant astrophysical quantities. The results are that the mass-to-flux ratio is supercritical by ~2, and that the ratio of turbulent to magnetic energies is also ~2. These results are compatible with both models of star formation. However, these OH Zeeman observations do establish for the first time on a statistically sound basis the energetic importance of magnetic fields in dark cloud cores at densities of order 10³–10⁴ cm⁻³, and they lay the foundation for further observations that could provide a more definitive test.

We present the detection and characterization of a peculiar low-mass protostar (IRAS 22129+7000) located ~0.4 pc from the Cederblad 201 photodissociation region (PDR) and ~0.2 pc from the HH 450 jet. The cold circumstellar envelope surrounding the object has been mapped through its 1.2 mm dust continuum emission with IRAM 30 m/MAMBO. The deeply embedded protostar is clearly detected with Spitzer MIPS (70 μm), IRS (20-35 μm), and IRAC (4.5, 5.8, and 8 μm), but also in the K_s band (2.15 μm). Given the large near- and mid-IR excess in its spectral energy distribution, but large submillimeter-to-bolometric luminosity ratio (≃2%), IRAS 22129+7000 must be a transition Class 0/I source and/or a multiple stellar system. Targeted observations of several molecular lines from CO,¹³CO, C¹⁸O, HCO⁺, and DCO⁺ have been obtained. The presence of a collimated molecular outflow mapped with the CSO telescope in the CO J = 3–2 line suggests that the protostar/disk system is still accreting material from its natal envelope. Indeed, optically thick line profiles from high-density tracers such as HCO⁺J = 1–0 show a redshifted absorption asymmetry reminiscent of inward motions. We construct a preliminary physical model of the circumstellar envelope (including radial density and temperature gradients, velocity field, and turbulence) that reproduces the observed line profiles and estimates the ionization fraction. The presence of both mechanical and (nonionizing) far-ultraviolet (FUV) radiative input makes the region an interesting case to study triggered star formation.

Using interferometric data from BIMA observations, combined with detailed modeling in Fourier space of the physical structures predicted by models, we constrain the circumstellar envelope parameters for four Class 0 young stellar objects, as well as their embedded circumstellar disks. The envelopes of these objects are still undergoing collapse, and theoretical collapse models can be compared to the observations. Since it has been suggested in a previous study that both the Larson-Penston and Shu similarity solutions underestimate the age of the system, we adopt Tassis & Mouschovias' model of the collapse process, which includes all relevant magnetic fields effects. The results of the model fitting show a good consistency between theory and data; furthermore, no age problem exists, since the Tassis & Mouschovias' model is age independent for the first 255 kyr. Although the majority of the continuum dust emission arises from the circumstellar envelopes, these objects have well-known outflows, which suggest the presence of circumstellar disks. At the highest resolution, most of the large-scale envelope emission is resolved out by interferometry, but the small-scale residual emission remains, making it difficult to observe only the compact disk component. By modeling the emission of the envelope and subtracting it from the total emission, we constrain the disk masses in our four systems to be comparable to or smaller than the typical disk masses for T Tauri systems.

We have made a serendipitous discovery of an enigmatic outflow source, IRAS 05506+2414 (hereafter IRAS 05506), as part of a multiwavelength survey of pre-planetary nebulae (PPNs). The HST optical and near-infrared images show a bright compact central source with a jetlike extension, and a fanlike spray of high-velocity (with radial velocities up to 350 km s⁻¹) elongated knots which appear to emanate from it. These structures are possibly analogous to the near-IR "bullets" seen in the Orion Nebula. Interferometric observations at 2.6 mm show the presence of a continuum source and a high-velocity CO outflow, which is aligned with the optical jet structure. IRAS 05506 is most likely not a PPN. We find extended NH₃ (1,1) emission toward IRAS 05506; these data, together with the combined presence of far-IR emission, H₂O and OH masers, and CO and CS J = 2–1 emission, strongly argue for a dense, dusty star-forming core associated with IRAS 05506. IRAS 05506 is probably an intermediate-mass or massive protostar, and the very short timescale (200 yr) of its outflows indicates that it is very young. If IRAS 05506 is a massive star, then the lack of radio continuum and the late G to early K spectral type we find from our optical spectra imply that in this object we are witnessing the earliest stages of its life, while its temperature is still too low to provide sufficient UV flux for ionization.

We present observations of two areas totalling 0.57 deg² in the IC 5146 star-forming region at 3.6, 4.5, 5.8, 8.0, 24, and 70 μm observed with the Spitzer Space Telescope. We reexamine the issue of the distance to this cloud and conclude a value of 950 ± 80 pc is most likely. We compare source counts, colors, and magnitudes in our observed region to a subset of the SWIRE data that was processed through our pipeline. We identify more than 200 young stellar object (YSO) candidates from color-magnitude and color-color diagrams, many of which were previously unknown. We compare the colors of these YSOs to the models of Robitaille et al. and perform simple fits to the SED's to estimate properties of the circumstellar disks likely to surround the Class II and III sources. We also compare the mid-IR disk excesses to Hα emission-line data where available. We present a quantitative description of the degree of clustering, estimate the star formation efficiency, and discuss the fraction of YSOs in the region with disks relative to an estimate of the diskless YSO population. Finally, we compare the YSO distribution to the cold dust distribution mapped by SCUBA and briefly describe the diffuse emission likely due to PAHs associated with the H II region.

The X-ray afterglows (AGs) of gamma-ray bursts (GRBs) and X-ray flashes (XRFs) have, after the fast-decline phase of their prompt emission, a temporal behavior varying between two extremes. A large fraction of these AGs has a canonical light curve which, after an initial shallow-decay plateau phase, breaks smoothly into a fast power-law decline. Very energetic GRBs, contrariwise, appear to not have a break: their AGs decline like a power law from the start of the observations. Breaks and "missing breaks" are intimately related to the geometry and deceleration of the jets responsible for GRBs. In the frame of the cannonball (CB) model of GRBs and XRFs, we analyze the cited extreme behaviors (canonical and pure power law) and intermediate cases spanning the observed range of X-ray AG shapes. We show that the entire panoply of X-ray light-curve shapes—measured with Swift and other satellites—are as anticipated in the CB model. We test the expected correlations between the AG's shape and the peak and isotropic energies of the prompt radiation, strengthening a simple conclusion of the analysis of AG shapes: in energetic GRBs the break is not truly missing, it is hidden under the tail of the prompt emission, or it occurs too early to be recorded. We also verify that the spectral index of the unabsorbed AGs and the temporal indexes of their late power-law decline differ by half a unit, as predicted.

We examine the rest-frame energetics of 76 gamma-ray bursts (GRBs) with known redshift that were detected by the Swift spacecraft and monitored by the satellite's X-Ray Telescope (XRT). Using the bolometric fluence values estimated by Butler and coworkers and the last XRT observation for each event, we set a lower limit to their collimation-corrected energy E_γ and find that 68% of our sample is at high enough redshift and/or low enough fluence to accommodate a jet break occurring beyond the last XRT observation and still be consistent with the pre-SwiftE_γ distribution for long GRBs. We find that relatively few of the X-ray light curves for the remaining events show evidence for late-time decay slopes that are consistent with that expected from post-jet break emission. The breaks in the X-ray light curves that do exist tend to be shallower and occur earlier than the breaks previously observed in optical light curves, yielding a E_γ distribution that is far lower than the pre-Swift distribution. If these early X-ray breaks are not due to jet effects, then a small but significant fraction of our sample have lower limits to their collimation-corrected energy that place them well above the pre-SwiftE_γ distribution. Either scenario would necessitate a much wider post-SwiftE_γ distribution for long cosmological GRBs compared to the narrow standard energy deduced from pre-Swift observations. We note that almost all of the pre-SwiftE_γ estimates come from jet breaks detected in the optical whereas our sample is limited entirely to X-ray wavelengths, furthering the suggestion that the assumed achromaticity of jet breaks may not extend to high energies.

We argue that the dominant radiation mechanism responsible for the early prompt γ-ray emission of gamma-ray bursts could be the resonant inverse Compton scattering of relativistic electrons in an intense magnetic field. By using this mechanism, some problems in the field of gamma-ray bursts could be clarified, e.g., the origin of the Amati relation, the formation of the observed broken power-law spectra, and the related deadline problem, among others. Our model also predicts that the emitted γ-rays could be highly polarized.

The light curve, energy spectra, energetics, and IPN localization of an exceedingly intense, short-duration, hard-spectrum burst, GRB 070201, obtained from Konus-Wind, INTEGRAL (SPI-ACS), and MESSENGER data are presented. The total fluence of the burst and the peak flux are S = 2.00^{+ 0.10}_−0.26 × 10⁻⁵ erg cm⁻² and F_max = 1.61^{+ 0.29}_−0.50 × 10⁻³ erg cm⁻² s⁻¹. The IPN error box has an area of 446 arcmin² and covers the peripheral part of the M31 galaxy. Assuming that the source of the burst is indeed in M31 at a distance of 0.78 Mpc, the measured values of the fluence S and maximum flux F_max correspond to a total energy of Q = 1.5 × 10⁴⁵ erg and a maximum luminosity L = 1.2 × 10⁴⁷ erg s⁻¹. These data are in good agreement with the corresponding characteristics of the previously observed giant flares from other soft gamma repeaters. The evidence for the identification of this event as a giant flare from a soft gamma repeater in the M31 galaxy is presented.

We describe the Wise Observatory Optical Transient Search (WOOTS), a survey for supernovae (SNe) and other variable and transient objects in the fields of redshift 0.06-0.2 Abell galaxy clusters. We present the survey design and data analysis procedures, and our object detection and follow-up strategies. We have obtained follow-up spectroscopy for all viable SN candidates and present the resulting SN sample here. Out of the 12 SNe we have discovered, 7 are associated with our target clusters while 5 are foreground or background field events. All but 1 of the SNe (a foreground field event) are Type Ia SNe. Our noncluster SN sample is uniquely complete, since all SN candidates have been either spectroscopically confirmed or ruled out. This allows us to estimate that the sample of field (noncluster) SNe, found in flux-limited surveys similar to WOOTS, would be dominated (~80%) by SNe Ia. Our spectroscopic follow-up observations also elucidate the difficulty in distinguishing active galactic nuclei from SNe. In separate papers we use the WOOTS sample to derive the SN rate in clusters for this redshift range and to measure the fraction of intergalactic cluster SNe. We also briefly report here on some quasars and asteroids discovered by WOOTS.

We present evidence for the formation of dust grains in an unusual Type Ib supernova (SN) based on late-time spectra of SN 2006jc. The progenitor suffered an outburst qualitatively similar to those of luminous blue variables (LBVs) just 2 yr prior to the SN, and we propose that the dust formation is a consequence of the SN blast wave overtaking that LBV-like shell. The key evidence for dust formation is (1) the appearance of a red/near-infrared continuum emission source that can be fit by T ≈ 1600 K graphite grains, and (2) fading of the redshifted sides of intermediate-width He I emission lines, yielding progressively more asymmetric blueshifted lines as dust obscures receding material. This provides the strongest case yet for dust formation in any SN Ib/c. Both developments occurred between 51 and 75 days after peak brightness, while the few other SNe observed to form dust did so after a few hundred days. Geometric considerations indicate that dust formed in the dense swept-up shell between the forward and reverse shocks, and not in the freely expanding SN ejecta. The rapid cooling leading to dust formation may have been aided by extremely high shell densities of 10¹⁰ cm⁻³, indicated by He I line ratios. The brief epoch of dust formation is accompanied by He II λ4686 emission and enhanced X-ray emission, suggesting a common link. These clues imply that the unusual dust formation in this object was not attributable to properties of the SN itself, but instead—like most peculiarities of SN 2006jc—was a consequence of interaction with the dense environment created by an LBV-like eruption 2 yr before the SN.

UBVRI photometry and medium-resolution optical spectroscopy of the peculiar Type Ia supernova SN 2005hk are presented and analyzed, covering the premaximum phase to around 400 days after explosion. The supernova is found to be underluminous compared to "normal" Type Ia supernovae. The photometric and spectroscopic evolution of SN 2005hk is remarkably similar to the peculiar Type Ia event SN 2002cx. The expansion velocity of the supernova ejecta is found to be lower than normal Type Ia events. The spectra obtained ≳200 days since explosion do not show the presence of forbidden [Fe II], [Fe III], and [Co III] lines, but are dominated by narrow, permitted Fe II, NIR Ca II, and Na I lines with P Cygni profiles. The thermonuclear explosion model with Chandrasekhar mass ejecta and a kinetic energy smaller (E_K = 0.3 × 10⁵¹ ergs) than that of canonical Type Ia supernovae is found to well explain the observed bolometric light curve. The mass of ⁵⁶Ni synthesized in this explosion is 0.18 M_☉. The early spectra are successfully modeled with this less energetic model, with some modifications of the abundance distribution. The late spectrum is explained as a combination of a photospheric component and a nebular component.

A major question in the study of black hole binaries involves our understanding of the accretion geometry when the sources are in the ``hard'' state, with an X-ray energy spectrum dominated by a hard power-law component and radio emission coming from a steady ``compact'' jet. Although the common hard state picture is that the accretion disk is truncated, perhaps at hundreds of gravitational radii (R_g) from the black hole, recent results for the recurrent transient GX 339–4 by Miller and coworkers show evidence for disk material very close to the black hole's innermost stable circular orbit. That work studied GX 339–4 at a luminosity of ~5% of the Eddington limit (L_Edd) and used parameters from a relativistic reflection model and the presence of a thermal component as diagnostics. Here we use similar diagnostics but extend the study to lower luminosities (2.3% and 0.8% L_Edd) using Swift and RXTE observations of GX 339–4. We detect a thermal component with an inner disk temperature of ~0.2 keV at 2.3% L_Edd. At both luminosities, we detect broad features due to iron Kα that are likely related to reflection of hard X-rays off disk material. If these features are broadened by relativistic effects, they indicate that the material resides within 10R_g, and the measurements are consistent with the disk's inner radius remaining at ~4R_g down to 0.8% L_Edd. However, we also discuss an alternative model for the broadening, and we note that the evolution of the thermal component is not entirely consistent with the constant inner radius interpretation.

We have developed a new model for the X-ray spectral fitting package XSPEC that takes into account the effects of both thermal and dynamical (i.e., bulk) Comptonization. The model consists of two components: one is the direct blackbody-like emission due to seed photons that are not subjected to effective Compton scattering, while the other is a convolution of the Green's function of the energy operator with a blackbody-like seed photon spectrum. When combined thermal and bulk effects are considered, the analytical form of the Green's function may be obtained as a solution of the diffusion equation describing Comptonization. Using data from the BeppoSAX, INTEGRAL, and RXTE satellites, we test our model on the spectra of a sample of six bright neutron star low-mass X-ray binaries with low magnetic fields, covering three different spectral states. Particular attention is given to the transient power-law-like hard X-ray (≳30 keV) tails, which we interpret in the framework of the bulk motion Comptonization process. We show that the values of the best-fit δ-parameter, which represents the importance of bulk with respect to thermal Comptonization, can be physically meaningful and can at least qualitatively describe the physical conditions of the environment in the innermost part of the system. Moreover, we show that in fitting the thermal Comptonization spectra to the X-ray spectra of these systems, the best-fit parameters of our model are in excellent agreement with those from compTT, a broadly used and well-established XSPEC model.

We report on optical and X-ray observations of the accretion-powered millisecond pulsar IGR J00291+5934 in quiescence. Time-resolved I-band photometry has been obtained with the 4.2 m William Herschel Telescope, while a 3 ks Chandra observation provided contemporaneous X-ray coverage. We found an unabsorbed 0.5-10 keV X-ray flux of 1 × 10⁻¹³ erg cm⁻² s⁻¹, which implies that the source was in quiescence at the time of the optical observations. Nevertheless, the optical I-band light curve of IGR J00291+5934 shows evidence for strong flaring. After removal of the strongest flares, we find evidence for an orbital modulation in the phase-folded I-band light curve. The overall modulation can be described by effects resulting from the presence of a superhump. Comparing our light curve with that reported recently we find evidence for a change in the quiescent base level. Similar changes have now been reported for four soft X-ray transients, implying that they may be a common feature of such systems in quiescence. Furthermore, the maximum in our folded light curve occurs at a different phase than observed before.

Most of the sources in the Galactic plane detected by the EGRET instrument on board the Compton Gamma Ray Observatory remain unidentified. Since the energy spectra of some of them strongly suggest that the gamma-ray emission originates from pulsars, it is interesting to look for pulsations in the data. However, detecting pulsations in gamma-ray emission by a blind search, that is, without prior knowledge of the pulsar spin parameters, is exceedingly difficult. Previous gamma-ray pulsar blind searches have relied on the execution of very long FFTs, requiring the use of supercomputers. We present a pulsar blind search, based on our time-differencing technique, that can be carried out more economically, in this case, on a standard PC. We present results from the application of this method to a periodicity search on all 271 sources from the third EGRET catalog. No statistically significant new detection was found, for a flux sensitivity of about 10⁻⁶ photons cm⁻² s⁻¹.

We investigate the interaction between alternating magnetic fields with cold current sheets and a relativistic collisionless shock wave. Many kinds of high-energy astrophysical objects may involve such alternating magnetic fields and a relativistic shock. They can be potent sources for the generation of high-energy particles. We found that a precursor wave, propagating upstream from the shock front, accelerates a dense current sheet plasma upstream. In the case that the width of the respective magnetic field reversal is larger than the downstream gyroradius, the current sheet generates a large-amplitude magnetosonic wave downstream by the collision with a shock front. The motional electric field accompanied by the magnetosonic wave can further accelerate the preaccelerated particles, forming a nonthermal energy spectrum. In addition, the current sheet structure is stable against not only the collision but also compression by other current sheets. In the thin current sheet case, which means the case that the width of the alternating magnetic field reversal is smaller than the downstream gyroradius, the magnetic field dissipates and the magnetosonic wave excitation is absent. This result is applied to pulsar wind nebulae. The result of the dissipation could solve the σ problem.

We present constraints on the number of Galactic magnetars, which we have established by searching for sources with periodic variability in 506 archival Chandra observations and 441 archival XMM-Newton observations of the Galactic plane (| b| < 5°). Our search revealed four sources with periodic variability on timescales of 200-5000 s, all of which are probably accreting white dwarfs. We identify 7 of 12 known Galactic magnetars, but find no new examples with periods between 5 and 20 s. We convert this nondetection into limits on the total number of Galactic magnetars by computing the fraction of the young Galactic stellar population that our survey covered. We find that easily detectable magnetars, modeled after persistent anomalous X-ray pulsars (e.g., with L_X = 10³⁵ ergs s⁻¹ [0.5-10.0 keV] and A_rms = 12% ), could have been identified in ≈5% of the Galactic spiral arms by mass. If we assume that three previously known examples randomly fall within our survey, then there are 59^{+ 92}₋₃₂ in the Galaxy. Barely detectable magnetars (L_X = 3 × 10³³ ergs s⁻¹ and A_rms = 15% ) could have been identified throughout ≈0.4% of the spiral arms. The lack of new examples implies that <540 exist in the Galaxy (90% confidence). Similar constraints are found by considering the detectability of transient magnetars in outburst. For assumed lifetimes of 10⁴ yr, the birth rate of magnetars is between 0.003 and 0.06 yr⁻¹. Therefore, the birth rate of magnetars is at least 10% of that for normal radio pulsars, and could exceed that value, unless transient magnetars are active for ≳10⁵ yr.

We present the results of an 80 ks Chandra ACIS-S observation of the double pulsar system J0737–3039. Furthermore, we report on spectral, spatial and timing analysis of the combined X-ray observations performed so far for this system. Fitting a total of ~1100 photons, we show that the X-ray spectrum of the J0737–3039 system is very soft, and not satisfactorily modeled by a simple blackbody or an atmospheric model. However, it is not possible yet to discriminate between a predominantly nonthermal and a predominantly thermal origin for the X-ray emission. Adopting a simple power-law emission model, the photon index (Γ = 3.7 ± 0.4, 90% confidence interval) and the implied conversion efficiency of the rotational energy of PSR J0737–3039A into X-ray emission (4.1 ± 0.5 × 10⁻⁴, for a distance to the source of 500 pc) are compatible with the X-ray photons being emitted in the magnetosphere of PSR J0737–3039A. This hypothesis is also supported by the absence of detectable X-ray orbital modulation (up to ~20%, 3 σ) or any X-ray nebular emission and it is in agreement with the high (≳75%) X-ray pulsed fraction of PSR J0737–3039A. A two blackbody or a Comptonized blackbody model also reproduce the data, and the upper limit to the value of the hydrogen column density, N_H≲ 1 × 10²⁰ cm⁻², is in better agreement (with respect to the power-law model) with the Galactic N_H in that direction and at that distance. For the two blackbody model the implied emission radii and temperatures are also compatible with those seen in other recycled pulsars, calling for the bulk of the X-ray photons being originated from heated regions at the surface of pulsar A. On the other hand, in the Comptonized blackbody model, the electron temperature seems to be significantly smaller than in other similar objects.

In an earlier paper we introduced a model for pulsars in which nonradial oscillations of high spherical degree (ℓ ) aligned to the magnetic axis of a spinning neutron star were able to reproduce subpulses like those observed in single-pulse measurements of pulsar intensity. The model did not address polarization, which is an integral part of pulsar emission. Observations show that many pulsars emit radio waves that appear to be the superposition of two linearly polarized emission modes with orthogonal polarization angles. In this paper we extend our model to incorporate linear polarization. As before, we propose that pulsational displacements of stellar material modulate the pulsar emission, but now we apply this modulation to a linearly polarized mode of emission, as might be produced by curvature radiation. We further introduce a second polarization mode, orthogonal to the first, that is modulated by pulsational velocities. We combine these modes in superposition to model the observed Stokes parameters in radio pulsars.

In this paper we analyze time series measurements of PSR B0943+10 and fit them with a nonradial oscillation model. The model we apply was first developed for total intensity measurements in an earlier paper, and expanded to encompass linear polarization in a companion paper to this one. We use PSR B0943+10 for the initial tests of our model because it has a simple geometry, it has been exhaustively studied in the literature, and its behavior is well documented. As prelude to quantitative fitting, we have reanalyzed previously published archival data of PSR B0943+10 and uncovered subtle but significant behavior that is difficult to explain in the framework of the drifting-spark model. Our fits of a nonradial oscillation model are able to successfully reproduce the observed behavior in this pulsar.

We investigate the hyperon bulk viscosity due to the nonleptonic process n + p⇌ p + Λ in K⁻ condensed matter and its effect on the r-mode instability in neutron stars. We find that the hyperon bulk viscosity coefficient in the presence of antikaon condensate is suppressed compared with the case without the condensate. The suppressed hyperon bulk viscosity in the superconducting phase is still an efficient mechanism to damp the r-mode instability in neutron stars.

We present Chandra HETG observations of SS Cygni in quiescence and outburst. The spectra are characterized by He-like and H-like Kα emission lines from O to Fe, as well as L-shell emission lines from Fe. In quiescence, the spectra are dominated by the H-like Kα lines, whereas in outburst the He-like lines are as intense as the H-like lines. In outburst, the H-like Kα lines from O to Si are broad, with widths of 4-14 eV in Gaussian σ (1800-2300 km s⁻¹). The large line widths, together with line profiles, indicate that the line-emitting plasma is associated with the Keplerian disk and still retains the azimuthal bulk motion. In quiescence, the emission lines are narrower, with a Gaussian σ of 1-3 eV (420-620 km s⁻¹). A slightly larger velocity for lighter elements suggests that the lines in quiescence are emitted from an ionizing plasma at the entrance of the boundary layer, where the bulk motion of the optically thick accretion disk is converted into heat due to friction. Using the line intensity ratio of He-like and H-like Kα lines for each element, we have also investigated the temperature distribution in the boundary layer both in quiescence and outburst. The distribution of SS Cyg is found to be consistent with other dwarf novae investigated systematically with ASCA data.

We report on variations in important X-ray emission lines in a series of Chandra grating spectra of the supermassive colliding wind binary star η Car, including key phases around the X-ray minimum/periastron passage in 2003.5. The X-rays arise from the collision of the slow, dense wind of η Car with the fast, low-density wind of an otherwise hidden companion star. The X-ray emission lines provide the only direct measure of the flow dynamics of the companion's wind along the wind-wind collision zone. We concentrate here on the silicon and sulfur lines, which are the strongest and best-resolved lines in the X-ray spectra. Most of the line profiles can be adequately fit with symmetric Gaussians with little significant skewness. Both the silicon and sulfur lines show significant velocity shifts and correlated increases in line widths through the observations. The = forbidden-to-intercombination ratio from the Si XIII and S XV triplets is near or above the low-density limit in all observations, suggesting that the line-forming region is >1.6 stellar radii from the companion star. We show that simple geometrical models cannot simultaneously fit both the observed centroid variations and changes in line width as a function of phase. We show that the observed profiles can be fitted with synthetic profiles with a reasonable model of the emissivity along the wind-wind collision boundary. We use this analysis to help constrain the line formation region as a function of orbital phase, and the orbital geometry.

We have measured the angular diameters for a sample of 24 exoplanet host stars using Georgia State University's CHARA Array interferometer. We use these improved angular diameters together with Hipparcos parallax measurements to derive linear radii and to estimate the stars' evolutionary states.

We find a hard X-ray (HXR) sigmoidal (-shaped) structure observed by RHESSI between 6 and 150 keV during the initial phase of the X10 flare of 2003 October 29. Its counterparts are seen with the Solar X-Ray Imager and TRACE. The flare evolves from a sigmoid to an arcade phase as observed in TRACE 195 Å images. According to the spatial structure of HXR emission, the flare process can be divided into two different phases. During the first phase, HXR emission in different energy ranges shares a similar sigmoidal evolving structure. The structure appears to contract initially as shown by the time profile of the separation between the two footpoints (FPs) at the ends of the HXR sigmoid. During the second phase, HXR emission in the lower energy range (≲30 keV) evolves into two sources located along the neutral line. Meanwhile, the FPs in the higher energies (≳30 keV) move apart as usual. During the whole flaring process, the value of the flare shear, defined as the angle between the line connecting two FPs and the line perpendicular to the neutral line, decreases steadily. We conclude that the flare was triggered by magnetic reconnection near the center of the sigmoid. The converging and unshearing motion during the sigmoid period is explained by continuing magnetic reconnection driven by the erupting sigmoidal flux rope. The reconnection progresses from highly sheared magnetic field lines to less sheared field lines surrounding the flux rope.

We describe magnetohydrodynamic simulations of a bipolar active region embedded in the Sun's global background field and subjected to twisting footpoint displacements concentrated near its polarity inversion lines to produce strong magnetic shear. The dipole moments of the active region and background field are antiparallel, so that the initially potential magnetic field contains a coronal null. This configuration supports magnetic breakout eruptions in our simulations that exhibit three novel features. First, the eruptions are multiple and homologous: the flare reconnection following each eruption reforms the magnetic null, setting the stage for a subsequent episode of breakout reconnection and eruption driven by the ongoing footpoint motions. Second, the eruptions are confined; that is, their rapidly rising, moderately sheared field lines do not escape the Sun but instead come to rest in the outer corona, comprising a large coronal loop formed by reconnection during the rise phase. Third, the most strongly sheared field lines of the active region are quite flat prior to eruption, expand upward sharply during the event, and lose most of their shear through reconnection with overlying flux, while lower lying field lines survive the eruption and recover their flat configuration within a few hours. These behaviors are consistent with filament disappearance followed by reformation in place. We also find that the upward motion of the erupting sheared flux exhibits a distinct three-phase acceleration profile. All of these features of our simulations—homology, confinement, reformation, and multiphase acceleration—are well established aspects of solar eruptions.

We present a detailed simulation of the evolution of a moderately slow coronal mass ejection (CME; 800 km s⁻¹ at 5 R_☉, where R_☉ is solar radii) in the lower solar corona (2-5 R_☉). The configuration of the Sun's magnetic field is based on the MDI data for the solar surface during Carrington rotation 1922. The pre-CME background solar wind is generated using the Wang-Sheeley-Arge (WSA) model. To initiate a CME, we inserted a modified Titov-Demoulin flux rope in an active region near the solar equator using the Space Weather Modeling Framework (SWMF). After the initiation stage (within 2.5 R_☉), the CME evolves at a nearly constant and slow acceleration of the order of 100 m s⁻², which corresponds to an intermediate-acceleration CME. Detailed analysis of the pressures shows that the thermal pressure accounts for most of the acceleration of the CME. The magnetic pressure contributes to the acceleration early in the evolution and becomes negligible when the CME moves beyond ~3 R_☉. We also present the evolution of the shock geometry near the nose of the CME, which shows that the shock is quasi parallel most of the time.

We examine closely the solar center-to-limb variation of continua and lines and compare observations with predictions from both a three-dimensional (3D) hydrodynamic simulation of the solar surface (provided by M. Asplund and collaborators) and one-dimensional (1D) model atmospheres. Intensities from the 3D time series are derived by means of the new synthesis code ASST, which overcomes limitations of previously available codes by including a consistent treatment of scattering and allowing for arbitrarily complex line and continuum opacities. In the continuum, we find very similar discrepancies between synthesis and observation for both types of model atmospheres. This is in contrast to previous studies that used a "horizontal" and time-averaged representation of the 3D model and found a significantly larger disagreement with observations. The presence of temperature and velocity fields in the 3D simulation provides a significant advantage when it comes to reproducing solar spectral line shapes. Nonetheless, a comparison of observed and synthetic equivalent widths reveals that the 3D model also predicts more uniform abundances as a function of position angle on the disk. We conclude that the 3D simulation provides not only a more realistic description of the gas dynamics, but despite its simplified treatment of the radiation transport, it also predicts reasonably well the observed center-to-limb variation, which is indicative of a thermal structure free from significant systematic errors.

We calculate the scattering effects associated with the interaction of a surface gravity or f-mode with a thin magnetic flux tube embedded in a realistically stratified medium. We find that the dominant scattered wave is an f-mode with amplitude and phase of 1.17% and around 50° relative to the incident wave, compared to the values of 0.13% and 40° estimated from observations. The extent of scattering into high-order acoustic p-modes is too weak to be accurately characterized. We recover the result that the degree of scattering is enhanced as (1) the frequency of the incident wave increases and (2) the flux tube becomes magnetically dominated.

Based on early solar system abundances of short-lived radionuclides (SRs), such as ²⁶Al (T_1/2 = 0.74 Myr) and ⁶⁰Fe (T_1/2 = 1.5 Myr), it is often asserted that the Sun was born in a large stellar cluster, where a massive star contaminated the protoplanetary disk with freshly nucleosynthesized isotopes from its supernova (SN) explosion. To account for the inferred initial solar system abundances of short-lived radionuclides, this supernova had to be close (~0.3 pc) to the young (≤1 Myr) protoplanetary disk. Here we show that massive star evolution timescales are too long, compared to typical timescales of star formation in embedded clusters, for them to explode as supernovae within the lifetimes of nearby disks. This is especially true in an Orion Nebular Cluster (ONC) type of setting, where the most massive star will explode as a supernova ~5 Myr after the onset of star formation, when nearby disks will have already suffered substantial photoevaporation and/or formed large planetesimals. We quantify the probability for any protoplanetary disk to receive SRs from a nearby supernova at the level observed in the early solar system. Key constraints on our estimate are: (1) SRs have to be injected into a newly formed (⩽1 Myr) disk, (2) the disk has to survive UV photoevaporation, and (3) the protoplanetary disk must be situated in an enrichment zone permitting SR injection at the solar system level without disk disruption. The probability of protoplanetary disk contamination by a supernova ejecta is, in the most favorable case, 3 × 10⁻³. We propose instead that ⁶⁰Fe (and possibly ²⁶Al) was inherited from the interstellar medium.

The volatile abundances in comet 17P/Holmes were measured on three dates (UT 2007 October 27.6 and 31.3 and November 2.3) using high-dispersion (λ/Δ λ ∼ 2.5 × 10⁴) infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory and CSHELL at the NASA Infrared Telescope Facility. Compared to other comets, the relative gas production rates in the coma show an enhancement of C₂H₆, HCN, and C₂H₂ with respect to H₂O, by factors of ~2-3. CH₃OH was also detected with an abundance relative to H₂O that is similar to or perhaps slightly enhanced compared to the values observed in other comets. The apparent enrichment of some volatiles in the coma of 17P/Holmes does not necessarily imply an unusual composition for its nucleus because fractionation effects may be important at the relatively large heliocentric distance (R_h = 2.45 AU) at which our observations were performed. Rotational temperatures were determined for H₂O, HCN, C₂H₆, and C₂H₂ in the coma on UT October 27.6 and found to be between 60 and 80 K. We used lines in both the ν₅ and ν₇ bands to obtain the best constraints yet achieved for the rotational temperature of C₂H₆. The spatial distributions of all measured volatiles in the coma are consistent with each other and suggest at most only a minor contribution from sublimating icy grains within our aperture. The overall gas production rate declined by approximately a factor of 7 between UT October 27.6 and November 2.3 with no significant change measured in the relative production rates of C₂H₆ and H₂O during this time.

Published data concerning Stark widths for spectral lines originating from 3s-3p transition arrays of multiply charged ions of different elements are used to demonstrate their dependence on the upper level ionization potential and the rest core charge of the emitter seeing by the electron undergoing transition. The relations found for particular electron temperatures and densities have been used to predict Stark widths of the following highly striped ions where not yet available: Mg VII, Mg IX, Mg X, Na VII, Na VIII, Al VIII, Al X, Al XI, Si XI, Ti XI, Cr XIII, Cr XIV, Fe XV, Fe XVI, Fe XXIII, and Ni XVIII. The accuracy of the data obtained is comparable to the accuracy of the experimental and theoretical values used in establishing these dependences.

With a nonlocal shell model of magnetohydrodynamic turbulence we investigate numerically the turbulent dynamo action for low and high magnetic Prandtl numbers (Pr_m). The results obtained in the kinematic regime and along the way to dynamo saturation are understood in terms of a phenomenological approach based on the local () or nonlocal () nature of the energy transfers. In both cases, the magnetic energy grows at a small scale and saturates as an inverse "cascade."

We derive an expression for the scattering length λ of ionized particles in an initially unmagnetized plasma (B₀ = 0). If λ is small compared to the dimension of the physical system L, the particles will be scattered effectively, which leads to an isotropization of the particle distribution in the physical system. We discuss two applications, nonrelativistic particles in the interstellar medium and highly relativistic particles in the jets of active galactic nuclei. As λ strongly grows with the particle velocity, the ratio λ/L is only small below a critical velocity which is calculated for different cosmic systems.

We use the Allen et al. galaxy cluster gas mass fraction versus redshift data to constrain parameters of three different dark energy models: a cosmological constant dominated one (ΛCDM), the XCDM parameterization of dark energy, and a slowly rolling scalar field model with inverse power-law potential energy density. (Instead of using the Monte Carlo Markov chain method, when integrating over nuisance parameters we use an alternative method of introducing an auxiliary random variable.) The resulting constraints are consistent with, and typically more constraining than, those derived from other cosmological data. A time-independent cosmological constant is a good fit to the galaxy cluster data, but slowly evolving dark energy cannot yet be ruled out.

We present a numerical study of gamma-ray burst-circumburst medium interaction and plasma preconditioning via Compton scattering. The simulation tool employed here—the PhotonPlasma code—is a unique hybrid model; it combines a highly parallelized (Vlasov) particle-in-cell approach with continuous weighting of particles and a sub-Debye Monte Carlo binary particle interaction framework. Our first results from 3D simulations with this new simulation tool suggest that magnetic fields and plasma density filaments are created in the wakefield of prompt gamma-ray bursts, and that the photon flux density gradient has a significant impact on particle acceleration in the burst head and wakefield. We discuss some possible implications of the circumburst medium preconditioning for the trailing afterglow, and also discuss which additional processes will be needed to improve future studies within this unique and powerful simulation framework.

We observed the TeV blazar 1ES 1218+304 with the X-ray astronomy satellite Suzaku in 2006 May. At the beginning of the 2 day continuous observation, we detected a large flare in which the 5-10 keV flux changed by a factor of ~2 on a timescale of 5 × 10⁴ s. During the flare, the increase in the hard X-ray flux clearly lagged behind that observed in the soft X-rays, with the maximum lag of 2.3 × 10⁴ s observed between the 0.3-1 keV and 5-10 keV bands. Furthermore, we discovered that the temporal profile of the flare clearly changes with energy, being more symmetric at higher energies. From the spectral fitting of multiwavelength data assuming a one-zone, homogeneous synchrotron self-Compton model, we obtain a magnetic field strength B ∼ 0.047 G and an emission region size R = 3.0 × 10¹⁶ cm for an appropriate beaming with a Doppler factor of δ = 20. This value of B is in good agreement with an independent estimate through the model fit to the observed time lag ascribing the energy-dependent variability to the differential acceleration timescale of relativistic electrons provided that the gyrofactor ξ is 10⁵.

The origin of the dichotomy of radio loudness among quasars can be explained using recent findings that the mass of the central supermassive black hole (SMBH) in extended radio-loud quasars is systematically a few times that of their counterparts in radio-quiet quasars. This sensitive dependence of radio jet ejection on SMBH mass probably arises from the blockage of jets by the presence of substantial quantities of gas tidally stripped from stars by the central BH. This disruptive gas, however, will only be available around BHs with masses less than M_c≳ 10⁸M_☉, for which the tidal disruption radius lies outside the SMBH's event horizon. Consequently, we find that AGNs with M_BH > M_c can successfully launch jets with a wide range of powers, thus producing radio-loud quasars. The great majority of jets launched by less massive BHs, however, will be truncated in the vicinity of the SMBH due to mass loading from this stellar debris. This scenario also can naturally explain the remarkable dearth of extended radio structures in quasars showing broad absorption line spectra.

In this Letter we present clear evidence that Mrk 573 is an obscured narrow-line Seyfert 1 (NLSy1) galaxy and not an archetypal Seyfert 2, as it has been classified until now. Only three galaxies have been proposed as members of this class, prior to this work. Here we report near-infrared spectroscopic data taken with LIRIS on the 4.2 m William Herschel Telescope (WHT). Our high-quality near-infrared nuclear spectrum in the 0.88-1.35 μm wavelength range shows the permitted O I λλ1.128, 1.317 narrow lines, the Fe II 9200 Å lines, and the Fe II 1 μm lines, together with a relatively broad component (~1700 km s⁻¹) of Paβ. These features can originate only in an optically thick high-density region, similar to those observed in the broad-line region (BLR).

We report the detection of CO(2→1) emission from the z = 1.2 ultraluminous infrared galaxy (ULIRG) GOODS J123634.53+621241.3 (also known as the submillimeter galaxy GN 26). These observations represent the first discovery of high-redshift CO emission using the new Combined Array for Research in Millimeter-Wave Astronomy (CARMA). Of all high-redshift (z > 1) galaxies within the GOODS-North field, this source has the largest far-infrared (FIR) flux observed in the Spitzer 70 and 160 μm bands. The CO redshift confirms the optical identification of the source, and the bright CO(2→1) line suggests the presence of a large molecular gas reservoir of about 7 × 10¹⁰M_☉. The infrared-to-CO luminosity ratio of L(IR)/L^'(CO) = 80 ± 30 L_☉ (K km s⁻¹ pc²)⁻¹ is slightly smaller than the average ratio found in local ULIRGs and high-redshift submillimeter galaxies. The short star formation timescale of about 70 Myr is consistent with a starburst associated with the merger event and is much shorter than the timescales for spiral galaxies and estimates made for high-redshift galaxies selected on the basis of their B − z and z − K colors.

We show that most particle and subhalo orbits in simulated cosmological cold dark matter halos are surprisingly regular and periodic: the phase-space structure of the outer halo regions shares some of the properties of the classical self-similar secondary infall model. Some of the outer branches are clearly visible in the radial velocity-radius plane at certain epochs. However, they are severely broadened in realistic, triaxial halos with nonradial, clumpy, mass accretion. This prevents the formation of high-density caustics: even in the best cases there are only broad, very small (<10%) enhancements in the spherical density profile. Larger fluctuations in ρ (r) caused by massive satellites are common. Infall caustics are therefore too weak to affect lensing or dark matter annihilation experiments. Their detection is extremely challenging, as it requires a large number of accurate tracer positions and radial velocities in the outer halo. The stellar halo of the Milky Way is probably the only target where this could become feasible in the future.

Recent observations have reported that some gas-rich dwarf irregular (dIrr) galaxies appear to have spherical distributions in the outer underlying old and intermediate-age stellar populations (e.g., NGC 6822). These observations imply that some dIrr's have two distinct (or "two-component") structures, i.e., inner disky and outer spherical ones, although the number fraction of dIrr's with such structures remains observationally unclear. We discuss how such two distinct structures are formed during dIrr formation on the basis of observations and simulations. Our numerical simulations show that the remnants of mergers between two gas-rich dIrr's with initially extended gas disks can have both extended spheroids composed of older stellar populations and disks composed mostly of gas and young stars. The simulated remnants with two distinct structures can be still identified as dIrr's owing to the presence of star-forming regions. The structural properties of outer spherical structures in dIrr's formed from dIrr-dIrr merging depend on initial conditions of merging, which suggests that outer structures in dIrr's can be diverse. We also discuss other possible physical mechanisms for the formation of outer spherical structures composed of older stars in dIrr's.

We propose a new method to measure the metallicity of Type Ia supernova progenitors using Mn and Cr lines in the X-ray spectra of young supernova remnants. We show that the Mn-to-Cr mass ratio in Type Ia supernova ejecta is tightly correlated with the initial metallicity of the progenitor, as determined by the neutron excess of the white dwarf material before thermonuclear runaway. We use this correlation, together with the flux of the Cr and Mn Kα X-ray lines in the Tycho supernova remnant recently detected by Suzaku, to derive a metallicity of log (Z) = − 1.32^{+ 0.67}_−0.33 for the progenitor of this supernova, which corresponds to log (Z/Z_☉) = 0.60^{+ 0.31}_−0.60 according to the latest determination of the solar metallicity by Asplund and coworkers. The uncertainty in the measurement is large, but metallicities much smaller than the solar value can be confidently discarded. We discuss the implications of this result for future research on Type Ia supernova progenitors.

We present an XMM-Newton observation of the long-overlooked radio source G350.1–0.3. The X-ray spectrum of G350.1–0.3 can be fit by a shocked plasma with two components: a high-temperature (1.5 keV) region with a low ionization timescale and enhanced abundances, plus a cooler (0.36 keV) component in ionization equilibrium and with solar abundances. The X-ray spectrum and the presence of nonthermal, polarized, radio emission together demonstrate that G350.1–0.3 is a young, luminous supernova remnant (SNR), for which archival H I and¹²CO data indicate a distance of 4.5 kpc. The diameter of the source then implies an age of only ≈900 years. The SNR's distorted appearance and small size and the presence of¹²CO emission along the SNR's eastern edge all indicate that the source is interacting with a complicated distribution of dense ambient material. An unresolved X-ray source, XMMU J172054.5–372652, is detected a few arcminutes west of the brightest SNR emission. The thermal X-ray spectrum and lack of any multiwavelength counterpart suggest that this source is a neutron star associated with G350.1–0.3, most likely a "central compact object," as seen coincident with other young SNRs such as Cassiopeia A.

Our 50 ks Chandra observation of the small radio supernova remnant (SNR) G1.9+0.3 shows a complete shell structure with strong bilateral symmetry, about 100'' in diameter. The radio morphology is also shell-like, but only about 84'' in diameter, based on observations made in 1985. We attribute the size difference to expansion between 1985 and our Chandra observations of 2007. Expansion is confirmed in comparing radio images from 1985 and 2008. We deduce that G1.9+0.3 is of order 100 years old—the youngest supernova remnant in the Galaxy. Based on a very high absorbing column density of 5.5 × 10²² cm⁻², we place G1.9+0.3 near the Galactic center, at a distance of about 8.5 kpc, where the mean remnant radius would be about 2 pc, and the required expansion speed about 14,000 km s⁻¹. The X-ray spectrum is featureless and well described by the exponentially cut off synchrotron model srcut. With the radio flux at 1 GHz fixed at 0.9 Jy, we find a spectral index of 0.65 and a rolloff frequency of 1.4 × 10¹⁸ Hz. The implied characteristic rolloff electron energy of about 94(B/10 μ G)^−1/2 TeV is the highest ever reported for a shell supernova remnant. It can easily be reached by standard diffusive shock acceleration, given the very high shock velocities; it can be well described by either age-limited or synchrotron-loss-limited acceleration. Not only is G1.9+0.3 the youngest known Galactic remnant, it is also only the fourth Galactic X-ray-synchrotron-dominated shell supernova remnant.

We present a 3D numerical simulation of the recently discovered cometary structure produced as Mira travels through the Galactic ISM. In our simulation, we consider that Mira ejects a steady, latitude-dependent wind, which interacts with a homogeneous, streaming environment. The axisymmetry of the problem is broken by the lack of alignment between the direction of the relative motion of the environment and the polar axis of the latitude-dependent wind. With this model, we are able to produce a cometary head with a "double bow shock" which agrees well with the structure of the head of Mira's comet. We therefore conclude that a time dependence in the ejected wind is not required for reproducing the observed double bow shock.

We use archival Spitzer Space Telescope photometry of the old, supersolar-metallicity, massive open cluster NGC 6791 to look for evidence of enhanced mass loss, which has been postulated to explain the optical luminosity function and low white dwarf masses in this benchmark cluster. We find a conspicuous lack of evidence for prolificacy of circumstellar dust production that would have been expected to accompany such mass loss. We also construct the optical and infrared luminosity functions, and demonstrate that these fully agree with theoretical expectations. We thus conclude that there is no evidence for the mass loss of supersolar metallicity red giants to be sufficiently high that they can avoid the helium flash at the tip of the red giant branch.

Terrestrial planets with abundant water have multiple climate modes, including an ice-free, a partially ice-covered, and a globally ice-covered state. Recent geological studies have revealed that the Earth experienced global glaciations in its history ("snowball Earth" hypothesis). In the snowball glaciations, liquid water is thought to have existed under the ice shell because of geothermal heat flow from the Earth's interior. Here, by analogy with the snowball glaciations, I discuss the conditions for an extrasolar terrestrial planet which is covered with ice but has an internal ocean for the timescale of planetary evolution owing to geothermal heat flow from the planetary interior. I show that liquid water can exist if the planetary mass and the water abundance are comparable to the Earth, although a planet with a mass <0.4 M_⊕ (M_⊕ is the Earth's mass) would not be able to maintain the internal ocean. Liquid water would be absolutely stable for a planet with a mass ≳4 M_⊕ (i.e., super-Earth) either on its surface or beneath the ice, irrespective of planetary orbit and luminosity of the central star. Searches for terrestrial planets in extrasolar planetary systems should consider such a "snowball planet," which is a possible type of water-rich terrestrial planet other than an Earth-like "ocean planet."

Most of the first-discovered extrasolar multiplanet systems were found to lie close to dynamically unstable configurations. However, a few observed multiplanet systems (e.g., HD 74156) did not show this trait. Those systems could share this property if they contain an additional planet in between those that are known. Previous investigations identified the properties of hypothetical planets that would place these systems near instability. The hypothetical planet in HD 74156 was expected to have a mass about equal to that of Saturn, a semimajor axis between 0.9 and 1.4 AU, and an eccentricity less than 0.2. HD 74156d, a planet with a mass of 1.3 Saturn masses at 1.04 AU with an eccentricity of 0.25, was recently announced. We have reanalyzed all published data on this system in order to place tighter constraints on the properties of the new planet. We find two possible orbits for this planet, one close to that already identified and another (with a slightly better fit to the data) at ~0.89 AU. We also review the current status of other planet predictions, discuss the observed single-planet systems, and suggest other systems which may contain planets in between those that are already known. The confirmation of the existence of HD 74156d suggests that planet formation is an efficient process, and planetary systems should typically contain many planets.

We report measurements of eight native (i.e., released directly from the comet nucleus) volatiles (H₂O, HCN, CH₄, C₂H₂, C₂H₆, CO, H₂CO, and CH₃OH) in comet 8P/Tuttle using NIRSPEC at Keck 2. Comet Tuttle reveals a truly unusual composition, distinct from that of any comet observed to date at infrared wavelengths. The prominent enrichment of methanol relative to water contrasts with the depletions of other molecules, especially C₂H₂, HCN, and H₂CO. We suggest that the nucleus of 8P/Tuttle may contain two cometesimals characterized by distinct volatile composition. The relative abundances C₂/CN, C₂/OH, and CN/OH in 8P/Tuttle (measured at optical/near-UV wavelengths) differ substantially from the mixing ratios of their potential parents (C₂H₂/HCN, C₂H₂/H₂O, and HCN/H₂O) found in this work. Based on this comparison, our results do not support C₂H₂ and HCN being the principal precursors for respectively C₂ and CN in Tuttle. The peculiar native composition observed in 8P/Tuttle (compared to other comets) provides new strong evidence for chemical diversity in the volatile materials stored in comet nuclei. We discuss the implications of this diversity for expected variations in the deuterium enrichment of water among comets.

Using radiances acquired with Cassini's Visual and Infrared Mapping Spectrometer (VIMS), we construct high solar phase angle curves for Saturn's second largest moon, Rhea. Ground-based studies of Rhea and Saturnian icy satellites are focussing on low phase angles; to our knowledge, these are the first solar phase curve data for Rhea on phase angles >70°. We compare these data to similar phase curves for Enceladus at near-infrared wavelengths to estimate the amount of water vapor that could possibly be generated and thus set an upper limit on the amount of geologic activity that may be occurring on Rhea. We find that Enceladus's plume manifests itself in the VIMS solar phase curve at a phase angle near 160° and peaks most strongly for λ = 2.017810 μm. No such peak can be found in the Rhea VIMS phase curve. Absence of a forward scattering peak supports the recent determination that particles in Rhea's surrounding ring are not small. We calculate that the maximum water vapor column density that could be supplied from Rhea ranges from 1.52 × 10¹⁴ to 1.91 × 10¹⁵ cm⁻², 2 orders of magnitude less than what is calculated by Cassini UVIS for Enceladus. This implies that for Rhea, the level of active internal (endogenic) processes is exceedingly small, if any.

Impulsive,³He-rich events originate close to the interface between slow solar wind overlying active regions and a faster solar wind coming from small coronal holes. This causes large-scale magnetic compressions to be an interplanetary environment for solar energetic particle (SEP) transport in impulsive events, which is typically ignored by SEP modelers. We have modeled SEP transport in a simplified corotating solar wind structure to estimate the possible effect of the rising wind speed on particle anisotropy and spectra at 1 AU. Along with traditional modeling of SEP transport in the static magnetic field and the field-aligned solar wind flow of the corotating frame of reference, we have formulated and tested a new model that is the first model of focused transport applicable to a general case of SEP propagation in realistic, dynamic, and structured solar wind. Numerical modeling shows that a fast increase of the wind speed by only 200 km s⁻¹ can strongly affect the SEP flux anisotropy at 1 AU. Accurate analysis of impulsive SEP events can be done with the use of solar wind data, SEP flux anisotropy measurements, and the new approach that accounts for the solar wind structures associated with the sources of impulsive events and uses the general solution of the focused transport problem applicable to SEPs in realistic solar wind.

We report on the first stereoscopic observations of polar coronal jets made by the EUVI/SECCHI imagers on board the twin STEREO spacecraft. The significantly separated viewpoints (~11°) allowed us to infer the 3D dynamics and morphology of a well-defined EUV coronal jet for the first time. Triangulations of the jet's location in simultaneous image pairs led to the true 3D position and thereby its kinematics. Initially the jet ascends slowly at ≈10-20 km s⁻¹ and then, after an apparent "jump" takes place, it accelerates impulsively to velocities exceeding 300 km s⁻¹ with accelerations exceeding the solar gravity. Helical structure is the most important geometrical feature of the jet which shows evidence of untwisting. The jet structure appears strikingly different from each of the two STEREO viewpoints: face-on in one viewpoint and edge-on in the other. This provides conclusive evidence that the observed helical structure is real and does not result from possible projection effects of single-viewpoint observations. The clear demonstration of twisted structure in polar jets compares favorably with synthetic images from a recent MHD simulation of jets invoking magnetic untwisting as their driving mechanism. Therefore, the latter can be considered as a viable mechanism for the initiation of polar jets.

A multi-ion kinetic model for a coronal loop is presented, whereby ion heating in the magnetically confined plasma is achieved by absorption of ion-cyclotron waves. We assume that linear Alfvén/cyclotron waves penetrate the loop from its footpoint and directly heat the ions. Then due to electron-ion collisions the electrons can also be heated. Depending on the spatial variation of the mean magnetic field, the model is able to produce warm and hot model loops having features similar to the ones observed in extreme-ultraviolet and soft X-ray emissions in real coronal loops. Furthermore, it is found that a loop with high expansion factor is far from local thermal equilibrium (LTE) and shows remarkable temperature differences between electrons and ions. Also in such a case, the heavy ions (minor ions) are via resonant wave absorption heated more than the protons and helium ions (major background ions), whereby the cyclotron-resonance effect leads to a temperature anisotropy with T_⊥ > T_∥. However, if the flux tube cross section is nearly homogeneous, temperature isotropy of the ions is maintained in most parts of the loop, and the plasma is nearly in LTE.

The kinematics of a globally propagating disturbance (also known as an "EIT wave") is discussed using Extreme UltraViolet Imager (EUVI) data from the Solar Terrestrial Relations Observatory (STEREO). We show for the first time that an impulsively generated propagating disturbance has similar kinematics in all four EUVI passbands (304, 171, 195, and 284 Å). In the 304 Å passband the disturbance shows a velocity peak of 238 ± 20 km s⁻¹ within ~28 minutes of its launch, varying in acceleration from 76 to –102 m s⁻². This passband contains a strong contribution from a Si XI line (303.32 Å) with a peak formation temperature of ~1.6 MK. The 304 Å emission may therefore be coronal rather than chromospheric in origin. Comparable velocities and accelerations are found in the coronal 195 Å passband, while lower values are found in the lower cadence 284 Å passband. In the higher cadence 171 Å passband the velocity varies significantly, peaking at 475 ± 47 km s⁻¹ within ~20 minutes of launch, with a variation in acceleration from 816 to –413 m s⁻². The high image cadence of the 171 Å passband (2.5 minutes compared to 10 minutes for the similar temperature response 195 Å passband) is found to have a major effect on the measured velocity and acceleration of the pulse, which increase by factors of ~2 and ~10, respectively. This implies that previously measured values (e.g., using EIT) may have been underestimated. We also note that the disturbance shows strong reflection from a coronal hole in both the 171 and 195 Å passbands. The observations are consistent with an impulsively generated fast-mode magnetoacoustic wave.

Observations with the Hinode space observatory led to the discovery of predominantly horizontal magnetic fields in the photosphere of the quiet internetwork region. Here we investigate realistic numerical simulations of the surface layers of the Sun with respect to horizontal magnetic fields and compute the corresponding polarimetric response in the Fe I 630 nm line pair. We find a local maximum in the mean strength of the horizontal field component at a height of around 500 km in the photosphere, where, depending on the initial state or the boundary condition, it surpasses the vertical component by a factor of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean horizontal field component that is 1.6 or 4.3 times stronger than the vertical component, depending on the initial state or the boundary condition. This is a consequence of both the intrinsically stronger flux density of and the larger area occupied by the horizontal fields. We find that convective overshooting expels horizontal fields to the upper photosphere, making the Poynting flux positive in the photosphere, whereas the Poynting flux is negative in the convectively unstable layer below it.

Amorphous Mg-bearing silicate grains, which were produced by the coalescence between MgO and SiO_x smoke particles, were crystallized to forsterite (Mg₂SiO₄) by electron-beam irradiation in a transmission electron microscope at room temperature. The crystallization induced by electron beams was accelerated by the presence of CH₄ adsorbed on the surface and incorporated interior of the grains. This experimental result implies the possibility of low-temperature crystallization in a silicate carbon star. In the case of binary stars, since the materials that flow from the stars stationarily exist around the star, the formed amorphous silicate grains will be irradiated by electrons from the star for a long duration. As a result, a significant amount of crystalline silicates can be produced.

The rotational spectra of HC₄OH and its deuterated species have been detected in the 17-39 GHz region by microwave spectroscopy. The spectra were generated after pyrolysis of 2-butynol at 830°C; a deuteration reaction to produce DC₄OH and HC₄OD was caused by mixing the 2-butynol with deuterated water vapor. The rotational constants B and C were precisely determined for HC₄OH and the rotational constant (B + C)/2 was determined for DC₄OH and for HC₄OD. With these rotational constants, the entire radio spectrum of HC₄OH can be accurately estimated to better than 0.42 MHz accuracy at frequencies up to 51.1 GHz. These transition frequencies should be of great use for astronomical identification of this molecule.