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Using high-resolution cosmological simulations, we study hydrogen and helium gravitational cooling radiation from gas accretion by young galaxies. We focus on the He II cooling lines, which arise from gas with a different temperature history (T_max ~ 10⁵ K) than H I line-emitting gas. We examine whether three major atomic cooling lines, H I λ1216, He II λ1640, and He II λ304, are observable, finding that Lyα and He II λ1640 cooling emission at z = 2-3 are potentially detectable with deep narrowband (R > 100) imaging and/or spectroscopy from the ground. While the expected strength of H I λ1216 cooling emission depends strongly on the treatment of the self-shielded phase of the IGM in the simulations, our predictions for the He II λ1640 line are more robust, because the He II emissivity is negligible below T ~ 10^4.5 K and less sensitive to the UV background. Although He II λ1640 cooling emission is fainter than Lyα by at least a factor of 10 and, unlike Lyα, might not be resolved spatially with current observational facilities, it is more suitable to study gas accretion in the galaxy formation process because it is optically thin and less contaminated by the recombination lines from star-forming galaxies. The He II λ1640 line can be used to distinguish among mechanisms for powering the so-called Lyα blobs—including gravitational cooling radiation, photoionization by stellar populations, and starburst-driven superwinds—because (1) He II λ1640 emission is limited to very low metallicity [log(Z/Z_☉) ≲ -5.3] and Population III stars and (2) the blob's kinematics are probed unambiguously through the He II line width, which for cooling radiation is narrower (σ < 400 km s^-1) than typical wind speeds.

In primordial gas, molecular hydrogen forms primarily through associative detachment of H^- and H, thereby destroying the H^-. The H^- anion can also be destroyed by a number of other reactions, most notably by mutual neutralization with protons. However, neither the associative detachment nor the mutual neutralization rate coefficients are well determined: both may be uncertain by as much as an order of magnitude. This introduces a corresponding uncertainty into the H₂ formation rate, which may have cosmological implications. Here we examine the effect that these uncertainties have on the formation of H₂ and the cooling of protogalactic gas in a variety of situations. We show that the effect is particularly large for protogalaxies forming in previously ionized regions, affecting our predictions of whether or not a given protogalaxy can cool and condense within a Hubble time, and altering the strength of the ultraviolet background that is required to prevent collapse.

Optical and X-ray observations of the quadruply imaged quasar 1RXS J1131-1231 show flux ratio anomalies among the images of factors of ~2 in the optical and ~3-9 in X-rays. Temporal variability of the quasar seems an unlikely explanation for the discrepancies between the X-ray and optical flux ratio anomalies. The negative parity of the most affected image and the decreasing trend of the anomalies with wavelength suggest microlensing as a possible explanation; this would imply that the source of optical radiation in RXS J1131 is ~10⁴R_g in size for a black hole mass of ~10⁸M_☉. We also present evidence for different X-ray spectral hardness ratios among the four images.

A 5 arcmin² region around the luminous radio-loud quasar SDSS J0836+0054 (z = 5.8) hosts a wealth of associated galaxies, characterized by very red (1.3 < i₇₇₅ - z₈₅₀ < 2.0) color. The surface density of these z ~ 5.8 candidates is approximately 6 times higher than the number expected from deep ACS fields. This is one of the highest galaxy overdensities at high redshifts, which may develop into a group or cluster. We also find evidence for a substructure associated with one of the candidates. It has two very faint companion objects within 2'', which are likely to merge. The finding supports the results of a recent simulation, which finds that luminous quasars at high redshifts lie on the most prominent dark matter filaments and are surrounded by many fainter galaxies. The quasar activity from these regions may signal the buildup of a massive system.

We present a near-infrared quasar composite spectrum spanning the wavelength range 0.58-3.5 μm. The spectrum has been constructed from observations of 27 quasars obtained at the NASA IRTF telescope and satisfying the criteria K_s < 14.5 and M_i < -23; the redshift range is 0.118 < z < 0.418. The signal-to-noise ratio is moderate, reaching a maximum of 150 between 1.6 and 1.9 μm. While a power-law fit to the continuum of the composite spectrum requires two breaks, a single power-law slope of α = -0.92 plus a 1260 K blackbody provides an excellent description of the spectrum from Hα to 3.5 μm, strongly suggesting the presence of significant quantities of hot dust in this blue-selected quasar sample. We measure intensities and line widths for 10 lines, finding that the Paschen line ratios rule out case B recombination. We compute K-corrections for the J, H, K, and Spitzer 3.6 μm bands, which will be useful in analyzing observations of quasars up to z = 10.

We discuss the physical properties of four quasar jets imaged with the Chandra X-Ray Observatory in the course of a survey for X-ray emission from radio jets (Marshall et al.). These objects have sufficient counts to study their spatially resolved properties, even in the 5 ks survey observations. We have acquired Australia Telescope Compact Array data with resolution matching Chandra. We have searched for optical emission with Magellan, with subarcsecond resolution. The radio to X-ray spectral energy distribution for most of the individual regions indicates against synchrotron radiation from a single-component electron spectrum. We therefore explore the consequences of assuming that the X-ray emission is the result of inverse Compton scattering on the cosmic microwave background. If particles and magnetic fields are near minimum energy density in the jet rest frames, then the emitting regions must be relativistically beamed, even at distances of order 500 kpc from the quasar. We estimate the magnetic field strengths, relativistic Doppler factors, and kinetic energy flux as a function of distance from the quasar core for two or three distinct regions along each jet. We develop, for the first time, estimates in the uncertainties in these parameters, recognizing that they are dominated by our assumptions in applying the standard synchrotron minimum energy conditions. The kinetic power is comparable with, or exceeds, the quasar radiative luminosity, implying that the jets are a significant factor in the energetics of the accretion process powering the central black hole. The measured radiative efficiencies of the jets are of order 10^-4.

We present mid-infrared observations of active galactic nuclei (AGNs) in the GOODS fields, performed with the Spitzer Space Telescope. These are the deepest infrared and X-ray fields to date and cover a total area of ~0.1 deg². AGNs are selected on the basis of their hard (2-8 keV) X-ray emission. The median AGN infrared luminosity is at least 10 times larger than the median for normal galaxies with the same redshift distribution, suggesting that the infrared emission is dominated by the central nucleus. The X-ray-to-infrared luminosity ratios of GOODS AGNs, most of which are at 0.5 ≲ z ≲ 1.5, are similar to the values obtained for AGNs in the local universe. The observed infrared flux distribution has an integral slope of ~1.5, and there are 1000 sources per square degree brighter than ~50 μJy at ~3-6 μm. The counts approximately match the predictions of models based on AGN unification, in which the majority of AGNs are obscured. This agreement confirms that the faintest X-ray sources, which are dominated by the host galaxy light in the optical, are obscured AGNs. Using these Spitzer data, the AGN contribution to the extragalactic infrared background light is calculated by correlating the X-ray and infrared catalogs. This is likely to be a lower limit given that the most obscured AGNs are missed in X-rays. We estimate the contribution of AGNs missed in X-rays, using a population synthesis model, to be ~45% of the observed AGN contribution, making the AGN contribution to the infrared background at most ~2%-10% in the 3-24 μm range, depending on wavelength, lower than most previous estimates. The AGN contribution to the infrared background remains roughly constant with source flux in the IRAC bands but decreases with decreasing flux in the MIPS 24 μm band, where the galaxy population becomes more important.

We present spatially resolved, near-diffraction-limited 10 μm spectra of the nucleus of the Seyfert 2 galaxy NGC 1068, obtained with Michelle, the mid-IR imager and spectrometer on the 8.1 m Gemini North Telescope. The spectra cover the nucleus and the central 60 × 04 of the ionization cones at a spatial resolution of approximately 04 (≈30 pc). The spectra extracted in 04 steps along the slit reveal striking variations in continuum slope, silicate feature profile and depth, and fine-structure line fluxes on subarcsecond scales, illustrating in unprecedented detail the complexity of the circumnuclear regions of NGC 1068 at mid-IR wavelengths. A comparison of photometry in various apertures reveals two distinct components: a compact (radius < 15 pc), bright source within the central 04 × 04 and extended, lower brightness emission. We identify the compact source with the AGN-obscuring torus, and the diffuse component with dust in the ionization cones. While the torus emission dominates the flux observed in the near-IR, the mid-IR flux measured with apertures larger than about 1'' is dominated instead by emission from the ionization cones; despite its higher brightness, the torus contributes <30% of the 11.6 μm flux in the central 12 region. Many previous attempts to determine the torus spectral energy distribution are thus likely to be significantly affected by contamination from the extended emission. The observed spectrum of the compact source is compared with clumpy torus models. The models require most of the clouds to be located within a few parsecs of the central engine, in agreement with recent mid-IR interferometric observations. We also present a UKIRT/CGS4 5 μm spectrum covering the R(0)-R(4) lines of the fundamental vibration-rotation band of ¹²CO. None of these lines was detected, and we discuss these nondetections in terms of the filling factor and composition of the nuclear clouds.

We present spatially resolved mid-infrared (mid-IR) spectra of NGC 1068 with a diffraction-limited resolution of 025 using the Long Wavelength Spectrometer (LWS) at the Keck I telescope. The mid-IR image of NGC 1068 is extended along the north-south direction. Previous imaging studies have shown that the extended regions are located inside the ionization cones, indicating that the mid-IR emission arises perhaps from the inner regions of the narrow-line clouds instead of the proposed dusty torus itself. The spatially resolved mid-IR spectra were obtained at two different slit position angles, +80 and -130 across the elongated regions in the mid-IR. From these spectra, we found only weak silicate absorption toward the northern extended regions but strong absorption in the nucleus and the southern extended regions. This is consistent with a model of a slightly inclined cold obscuring torus that covers much of the southern regions but is behind the northern extension. While a detailed analysis of the spectra requires a radiative transfer model, the lack of silicate emission from the northern extended regions prompts us to consider a dual dust population model as one of the possible explanations in which a different dust population exists in the ionization cones compared to that in the dusty torus. Dust inside the ionization cones may lack small silicate grains, giving rise to only a featureless continuum in the northern extended regions, while dust in the dusty torus has plenty of small silicate grains to produce the strong silicate absorption lines toward the nucleus and the southern extended regions.

The combined resolution and sensitivity of the Advanced Camera for Surveys deep imaging provides the capability of high-accuracy lens modeling of Abell 1689. Originally based on the technique of Broadhurst and coworkers, our software is designed to provide a precise and efficient method of modeling cluster lenses without assumptions relating the large-scale cluster dark matter to the light. Abell 1689 is robustly modeled using a freely varying cluster halo component consisting of an NFW profile, shapelets (Refregier), and a mass sheet, as well as a galaxy component based on the light. Another improvement over previous modeling techniques is the application of magnification-corrected image magnitude constraints. The mass within the ~50'' Einstein radius was found to be 2.04 × 10¹⁴M_☉ and was reliably fitted by an NFW with c = r₂₀₀/r_s = 5.70 and r_s = 239 arcsec (727 kpc), which gives d ln ρ/d ln r = -1.34 at r = 150 kpc. The overall B-band mass-to-light ratio within the Einstein radius is 215M_☉L versus only 32M_☉L for the galaxy component. For the overall surface mass density distribution, there are two regimes where the slope, d log Σ/d log R, is nearly constant. The intermediate slope from 6 to 30 kpc is -0.41, while the outer slope from 80 to 120 kpc is -0.57. In addition, our final model is consistent with the results from Broadhurst and coworkers. Monte Carlo simulations are also performed to explore the modeling systematics related to the image positional errors, the dependence on multiple image systems, and the WMAP predictions.

We use HST images to derive effective radii and effective surface brightnesses of 15 early-type (E+S0) lens galaxies identified by the SLACS Survey. Our measurements are combined with stellar velocity dispersions from the SDSS database to investigate for the first time the distribution of lens galaxies in the fundamental plane (FP) space. Accounting for selection effects (top priority to the largest Einstein radii and thus approximately to the largest velocity dispersions, σ ≳ 240 km s^-1) and for passive evolution, the distribution of the lens galaxies inside the FP is indistinguishable from that of the parent sample of SDSS galaxies. We conclude that SLACS lenses are a fair sample of high velocity dispersion E+S0s. By comparing the central stellar velocity dispersion (σ) with the velocity dispersion that best fits the lensing models (σ_SIE) we find ⟨f_SIE⟩ ≡ ⟨σ/σ_SIE⟩ = 1.01 ± 0.02 with 0.065 rms scatter. We infer that within the Einstein radii (typically R_e/2) the SLACS lenses are very well approximated by isothermal ellipsoids, requiring a fine tuning of the stellar and dark matter distribution (the bulge-halo ``conspiracy''). Interpreting the offset from the local FP in terms of evolution of the stellar mass-to-light ratio, we find d log(M/L<sub>B</sub>)/dz = -0.69 ± 0.08 (rms 0.11) consistent with the rate found for field E+S0s and with most of the stars being old (z<sub>f</sub> > 2) and less than ~10% of the stellar mass having formed below z = 1. We discuss our results in the context of formation mechanisms such as collisionless (``dry'') mergers.

An observational approach is presented to constrain the global structure and evolution of the intracluster medium using the ROSAT and ASCA distant cluster sample. From statistical analysis of the gas density profile and the connection to the L_X-T relation under the β-model, the scaled gas profile is found to be nearly universal for the outer region, and L_X(>0.2r₅₀₀) is tightly related to the temperature through T^~3 rather than T². On the other hand, a large density scatter exists in the core region, and there is clearly a deviation from the self-similar scaling for clusters with a small core. A link between the core size and the radiative cooling timescale, t_cool, and the analysis of X-ray fundamental plane suggest that t_cool is a parameter controlling the gas structure and that the appearance of small cores in regular clusters and may be much connected with the thermal evolution. We derive the luminosity-ambient temperature (T') relation, assuming the universal temperature profile for the clusters with short t_cool, and find that the dispersion around the relation significantly decreases and the slope becomes marginally less steep. We further examined the L_X-Tβ relation and showed a trend that merging clusters segregate from the regular clusters on the plane. Considering a correlation between t_cool and the X-ray morphology, the observational results lead us to draw a phenomenological picture: after a cluster collapses and t_cool falls below the age of the universe, the core cools radiatively with quasi-hydrostatic balancing in the gravitational potential, and the central density gradually becomes higher to evolve from an outer-core-dominant cluster, which follows the self-similarity, to an inner-core-dominant cluster.

We present gas and total mass profiles for 13 low-redshift, relaxed clusters spanning a temperature range 0.7-9 keV, derived from all available Chandra data of sufficient quality. In all clusters, gas-temperature profiles are measured to large radii (Vikhlinin et al.) so that direct hydrostatic mass estimates are possible to nearly r₅₀₀ or beyond. The gas density was accurately traced to larger radii; its profile is not described well by a beta model, showing continuous steepening with radius. The derived ρ_tot profiles and their scaling with mass generally follow the Navarro-Frenk-White model with concentration expected for dark matter halos in ΛCDM cosmology. However, in three cool clusters, we detect a central mass component in excess of the Navarro-Frenk-White profile, apparently associated with their cD galaxies. In the inner region (r < 0.1r₅₀₀), the gas density and temperature profiles exhibit significant scatter and trends with mass, but they become nearly self-similar at larger radii. Correspondingly, we find that the slope of the mass-temperature relation for these relaxed clusters is in good agreement with the simple self-similar behavior, M₅₀₀ ∝ T^α, where α = (1.5-1.6) ± 0.1, if the gas temperatures are measured excluding the central cool cores. The normalization of this M-T relation is significantly, by ≈30%, higher than most previous X-ray determinations. We derive accurate gas mass fraction profiles, which show an increase with both radius and cluster mass. The enclosed f_gas profiles within r₂₅₀₀ ≃ 0.4r₅₀₀ have not yet reached any asymptotic value and are still far (by a factor of 1.5-2) from the universal baryon fraction according to the cosmic microwave background (CMB) observations. The f_gas trends become weaker and its values closer to universal at larger radii, in particular, in spherical shells r₂₅₀₀ < r < r₅₀₀.

Observed X-ray spectra of hot gas in clusters, groups, and individual galaxies are commonly fit with a single-temperature thermal plasma model, although the beam may contain emission from components with different temperatures. Recently, Mazzotta et al. pointed out that thus derived T_spec can be significantly different from commonly used definitions of average temperature, such as emission-weighted or emission measure-weighted T, and found an analytic expression for predicting T_spec for a mixture of plasma spectra with relatively hot temperatures (T ≳ 3 keV). In this paper, we propose an algorithm that can accurately predict T_spec in a much wider range of temperatures (T ≳ 0.5 keV) and for essentially arbitrary abundances of heavy elements. This algorithm can be applied in the deprojection analysis of objects with the temperature and metallicity gradients, for correction of the point-spread function (PSF) effects, for consistent comparison of numerical simulations of galaxy clusters and groups with the X-ray observations, and for estimating how emission from undetected components can bias the global X-ray spectral analysis.

We present the results of a large survey of H I, O VI, and C III absorption lines in the low-redshift (z < 0.3) intergalactic medium (IGM). We begin with 171 strong Lyα absorption lines (W_λ ≥ 80 mÅ) in 31 AGN sight lines studied with the Hubble Space Telescope and measure corresponding absorption from higher order Lyman lines with FUSE. Higher order Lyman lines are used to determine N and b accurately through a curve-of-growth (COG) analysis. We find that the number of H I absorbers per column density bin is a power-law distribution, d/dN ∝ N, with β = 1.68 ± 0.11. We made 40 detections of O VI λλ1032, 1038 and 30 detections of C III λ977 out of 129 and 148 potential absorbers, respectively. The column density distribution of C III absorbers has β = 1.68 ± 0.04, similar to β but not as steep as β = 2.2 ± 0.1. From the absorption-line frequency, d/dz = 12 for W_λ(C ) > 30 mÅ, we calculate a typical IGM absorber size r₀ ~ 400 kpc, similar to scales derived by other means. The COG-derived b-values show that H I samples material with T < 10⁵ K, incompatible with a hot IGM phase. By calculating a grid of CLOUDY models of IGM absorbers with a range of collisional and photoionization parameters, we find it difficult to simultaneously account for the O VI and C III observations with a single phase. Instead, the observations require a multiphase IGM in which H I and C III arise in photoionized regions, while O VI is produced primarily through shocks. From the multiphase ratio N/N, we infer the IGM metallicity to be Z_C = 0.12 Z_☉, similar to our previous estimate of Z_O = 0.09 Z_☉ from O VI.

We present a simple method, based on the deformation of spherically symmetric potentials, to construct explicit axisymmetric and triaxial MOND density-potential pairs. General guidelines to the choice of suitable deformations, so that the resulting density distribution is nowhere negative, are presented. This flexible method offers for the first time the possibility of studying the MOND gravitational field for sufficiently general and realistic density distributions without resorting to sophisticated numerical codes. The technique is illustrated by constructing the MOND density-potential pair for a triaxial galaxy model that, in the absence of deformation, reduces to the Hernquist sphere. Such analytical solutions are also relevant for testing and validating numerical codes. Here we present a new numerical potential solver designed to solve the MOND field equation for arbitrary density distributions: the code is tested with excellent results against the analytic MOND triaxial Hernquist model and the MOND razor-thin Kuzmin disk, and a simple application is finally presented.

A homogeneous sample of ~2200 low-redshift disk galaxies with both high sensitivity long-slit optical spectroscopy and detailed I-band photometry is used to construct average, or template, rotation curves in separate luminosity classes, spanning six magnitudes in I-band luminosity. The template rotation curves are expressed as functions both of exponential disk scale lengths r_d and of optical radii R_opt, and extend out to 4.5r_d-6.5r_d, depending on the luminosity bin. The two parameterizations yield slightly different results beyond R_opt, because galaxies whose Hα emission can be traced to larger extents in the disks are typically of higher optical surface brightness and are characterized by larger values of R_opt/r_d. By either parameterization, these template rotation curves show no convincing evidence of velocity decline within the spatial scales over which they are sampled, even in the case of the most luminous systems. In contrast to some previous expectations, the fastest rotators (most luminous galaxies) have, on average, rotation curves that are flat or mildly rising beyond the optical radius, implying that the dark matter halo makes an important contribution to the kinematics also in these systems. The template rotation curves and the derived functional fits provide quantitative constraints for studies of the structure and evolution of disk galaxies, which aim at reproducing the internal kinematics properties of disks at the present cosmological epoch.

We present results from a short Chandra ACIS-S observation of NGC 7618, the dominant central galaxy of a nearby (z = 0.017309, d = 74.1 Mpc) group. We detect a sharp surface brightness discontinuity 14.4 kpc north of the nucleus subtending an angle of 130° with an X-ray tail extending ~70 kpc in the opposite direction. The temperature of the gas inside and outside the discontinuity is 0.79 ± 0.03 and 0.81 ± 0.07 keV, respectively. There is marginal evidence for a discontinuous change in the elemental abundance (Z_inner = 0.65 ± 0.25, Z_outer = 0.17 ± 0.21 at 90% confidence), suggesting that this may be an "abundance" front. Fitting a two-temperature model to the ASCA GIS spectrum of the NGC 7618/UGC 12491 pair shows the presence of a second, much hotter (T =~ 2.3 keV) component. We consider several scenarios for the origin of the edge and the tail, including a radio lobe/IGM interaction, nonhydrostatic "sloshing," equal-mass merger and collision, and ram pressure stripping. In the last case, we consider the possibility that NGC 7618 is falling into UGC 12491 or that both groups are falling into a gas-poor cluster potential. There are significant problems with the first two models, however, and we conclude that the discontinuity and tail are most likely the result of ram pressure stripping of the NGC 7618 group, as it falls into a larger dark matter potential.

We observed two faint tidal dwarf galaxies (TDGs), NGC 5291 N and NGC 5291 S, with the Infrared Spectrograph on the Spitzer Space Telescope. We detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2, 7.7, 8.6, 11.3, 12.6, and 16.5 μm, which match models of groups of ~100 carbon atoms with an equal mixture of neutral and ionized PAHs. The TDGs have a dominant warm (~140 K) dust component in marked contrast to the cooler (40-60 K) dust found in starburst galaxies. For the first time we detect the low-J rotational lines from molecular hydrogen. Adopting LTE, there is ~10⁵M_☉ of ~400 K gas, which is <0.1% of the cold gas mass inferred from ¹²CO (1-0) measurements. The combination of one-third solar metallicity with a recent (<5 million year) episode of star formation is reflected in the S and Ne ratios. The excitation is higher than typical values for starburst galaxies and similar to that found in BCDs. Using the Infrared Array Camera, we identify an additional 13 PAH-rich candidate TDGs. These sources occupy a distinct region of IRAC color space with [3.6] - [4.5] < 0.4 and [4.5] - [8.0] > 3.2. Their disturbed morphologies suggest past merger events between companions; for example, NGC 5291 S has a projected 11 kpc tail. NGC 5291 N and S have stellar masses of (1.5 and 3.0) × 10⁸M_☉, which is comparable to BCDs, although still roughly 10% of the LMC's stellar mass. The candidate TDGs are an order of magnitude less massive. This system appears to be a remarkable TDG nursery.

We stack Spitzer 24 μm images for ~7000 galaxies with 0.1 ≤ z < 1 in the Chandra Deep Field South to probe the thermal dust emission in low-luminosity galaxies over this redshift range. Through stacking, we can detect mean 24 μm fluxes that are more than an order of magnitude below the individual detection limit. We find that the correlations for low- and moderate-luminosity galaxies between the average L_IR/L_UV and rest-frame B-band luminosity, and between the star formation rate (SFR) and L_IR/L_UV, are similar to those in the local universe. This verifies that oft-used assumption in deep UV/optical surveys that the dust obscuration-SFR relation for galaxies with SFR ≤ 20 M_☉ yr^-1 varies little with epoch. We have used this relation to derive the cosmic IR luminosity density from z = 1 to z = 0.1. The results also demonstrate directly that little of the bolometric luminosity of the galaxy population arises from the faint end of the luminosity function, indicating a relatively flat faint-end slope of the IR luminosity function with a power-law index of 1.2 ± 0.3.

We have observed 10 red giant stars in four old Large Magellanic Cloud globular clusters with the high-resolution spectrograph MIKE on the Magellan Landon Clay 6.5 m telescope. The stars in our sample have up to 20 elemental abundance determinations for the α-, iron peak, and neutron-capture element groups. We have also derived abundances for the light odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be more metal-rich than previous estimates from the Ca II triplet, and we derive [Fe/H] values closer to those obtained from the slope of the red giant branch. However, we confirm previous determinations for Hodge 11 and NGC 1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios are comparable to the values observed in old Galactic globular cluster stars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMC clusters do not share the low-Y behavior observed in some dwarf spheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however, are significantly lower than what is seen in the Galactic globular cluster system. Neither does the behavior of [Cu/Fe] as a function of [Fe/H] in our LMC clusters match the trend seen in the Galaxy, staying instead at a constant value of roughly -0.8. Because not all [α/Fe] ratios are suppressed, these abundance ratios cannot be attributed solely to the injection of Type Ia supernova material and instead reflect the differences in star formation history of the LMC versus the Milky Way. An extensive numerical experimental study was performed, varying both input parameters and stellar atmosphere models, to verify that the unusual abundance ratios derived in this study are not the result of the adopted atomic parameters, stellar atmospheres, or stellar parameters. We conclude that many of the abundances in the LMC globular clusters we observed are distinct from those observed in the Milky Way, and these differences are intrinsic to the stars in those systems.

Generation mechanisms of amino acids in the interstellar medium (ISM) are of great importance in connection with the chemical evolution and the origin of life. Ion-molecule reactions and radical-molecule reactions have been studied as possible reaction mechanisms that can occur in ultracold and high-vacuum conditions in the ISM. In this paper, new barrierless reaction pathways generating glycine and its analogs via reactions between closed-shell species have been predicted based on quantum chemical calculations of potential energy surfaces. The ammonium ylide molecule, which is a higher energy isomer of methylamine, can be produced by radiative association between ammonia and the methyl radical cation followed by recombination dissociation. Glycine and its analogs can be generated without thermal activation via a combination of ammonium ylide and some simple molecules such as carbon dioxide. The present results and discussion suggest that higher lying isomers of well-known molecules can be responsible for molecular evolution as well as amino acid generation in the ISM.

TIMMI2 diffraction-limited mid-infrared images of a multipolar proto-planetary nebula IRAS 16594-4656 and a young [WC] elliptical planetary nebula IRAS 07027-7934 are presented. Their dust shells are for the first time resolved (only marginally in the case of IRAS 07027-7934) by applying the Lucy-Richardson deconvolution algorithm to the data, taken under exceptionally good seeing conditions (≤05). IRAS 16594-4656 exhibits a two-peaked morphology at 8.6, 11.5, and 11.7 μm, which is mainly attributed to emission from PAHs. Our observations suggest that the central star is surrounded by a toroidal structure, observed edge-on, with a radius of 04 (~640 AU at an assumed distance of 1.6 kpc) and with its polar axis at P.A. ~ 80^°, coincident with the orientation defined by only one of the bipolar outflows identified in the HST optical images. We suggest that the material expelled from the central source is currently being collimated in this direction and that the multiple outflow formation has not been coeval. IRAS 07027-7934 shows a bright, marginally extended emission (FWHM = 03) in the mid-infrared with a slightly elongated shape along the north-south direction, consistent with the morphology detected by HST in the near-infrared. The mid-infrared emission is interpreted as the result of the combined contribution of small, highly ionized PAHs and relatively hot dust continuum. We propose that IRAS 07027-7934 may have recently experienced a thermal pulse (likely at the end of the AGB) which has produced a radical change in the chemistry of its central star.

We report high spatial resolution VLA observations of the low-mass star-forming region IRAS 16293-2422 using four molecular probes: ethyl cyanide (CH₃CH₂CN), methyl formate (CH₃OCHO), formic acid (HCOOH), and the ground vibrational state of silicon monoxide (SiO). Ethyl cyanide emission has a spatial scale of ~20'' and encompasses binary cores A and B as determined by continuum emission peaks. Surrounded by formic acid emission, methyl formate emission has a spatial scale of ~6'' and is confined to core B. SiO emission shows two velocity components with spatial scales less than 2'' that map ~2'' northeast of the A and B symmetry axis. The redshifted SiO is ~2'' northwest of blueshifted SiO along a position angle of ~135° which is approximately parallel to the A and B symmetry axis. We interpret the spatial position offset in red- and blueshifted SiO emission as due to rotation of a protostellar accretion disk, and we derive ~1.4 M_☉ interior to the SiO emission. In the same vicinity, Mundy et al. also concluded rotation of a nearly edge-on disk from OVRO observations of much stronger and ubiquitous ¹³CO emission, but the direction of rotation is opposite to the SiO emission findings. Taken together, SiO and ¹³CO data suggest evidence for a counterrotating disk. Moreover, archival BIMA array ¹²CO data show an inverse P Cygni profile with the strongest absorption in close proximity to the SiO emission, indicating unambiguous material infall toward the counterrotating protostellar disk at a new source location within the IRAS 16293-2422 complex. The details of these observations and our interpretations are discussed.

We present a search for nearby (D ≲ 100 Mpc) galaxies in the error boxes of six well-localized short-hard gamma-ray bursts (GRBs). None of the six error boxes reveals the presence of a plausible nearby host galaxy. This allows us to set lower limits on the distances and, hence, the isotropic-equivalent energy of these GRBs. Our lower limits are around 1 × 10⁴⁹ ergs (at a 2 σ confidence level); as a consequence, some of the short-hard GRBs we examine would have been detected by BATSE out to distances greater than 1 Gpc and therefore constitute a bona fide cosmological population. Our search is partially motivated by the 2004 December 27 hypergiant flare from SGR 1806-20, and the intriguing possibility that short-hard GRBs are extragalactic events of a similar nature. Such events would be detectable with BATSE to a distance of 50 Mpc, and their detection rate should be comparable to the actual BATSE detection rate of short-hard GRBs. The failure of our search, in contrast, suggests that such flares constitute less than 15% of the short-hard GRBs (<40% at 95% confidence). We discuss possible resolutions of this discrepancy.

Refined one- and two-dimensional models for the nebular spectra of the hyperenergetic Type Ic supernova (SN) 1998bw, associated with the gamma-ray burst GRB 980425, from 125 to 376 days after B-band maximum are presented. One-dimensional, spherically symmetric spectrum synthesis calculations show that reproducing features in the observed spectra, i.e., the sharply peaked [O I] λ6300 doublet and Mg I] λ4570 emission and the broad [Fe II] blend around 5200 Å, requires the existence of a high-density O-rich core expanding at low velocities (≲8000 km s^-1) and of Fe-rich material moving faster than the O-rich material. Synthetic spectra at late phases from aspherical (bipolar) explosion models are also computed with a two-dimensional spectrum synthesis code. The above features are naturally explained by the aspherical model if the explosion is viewed from a direction close to the axis of symmetry (~30°), since the aspherical model yields a high-density O-rich region confined along the equatorial axis. By examining a large parameter space (in energy and mass), our best model gives the following physical quantities: the kinetic energy E₅₁ ≡ E_K/10⁵¹ ergs ≳8-12 and the main-sequence mass of the progenitor star M_ms ≳ 30-35 M_☉. The temporal spectral evolution of SN 1998bw also indicates mixing among Fe-, O-, and C-rich regions, and highly clumpy structure.

In this paper we present a new mechanism for core-collapse supernova explosions that relies on acoustic power generated in the inner core as the driver. In our simulation using an 11 M_☉ progenitor, an advective-acoustic oscillation à la Foglizzo with a period of ~25-30 ms arises ~200 ms after bounce. Its growth saturates due to the generation of secondary shocks, and kinks in the resulting shock structure funnel and regulate subsequent accretion onto the inner core. However, this instability is not the primary agent of explosion. Rather, it is the acoustic power generated early on in the inner turbulent region stirred by the accretion plumes and, most importantly, but later on, by the excitation and sonic damping of core g-mode oscillations. An ℓ = 1 mode with a period of ~3 ms grows at late times to be prominent around ~500 ms after bounce. The accreting proto-neutron star is a self-excited oscillator, "tuned" to the most easily excited core g-mode. The associated acoustic power seen in our 11 M_☉ simulation is sufficient to drive the explosion >550 ms after bounce. The angular distribution of the emitted sound is fundamentally aspherical. The sound pulses radiated from the core steepen into shock waves that merge as they propagate into the outer mantle and deposit their energy and momentum with high efficiency. The ultimate source of the acoustic power is the gravitational energy of infall, and the core oscillation acts like a transducer to convert this accretion energy into sound. An advantage of the acoustic mechanism is that acoustic power does not abate until accretion subsides, so that it is available as long as it may be needed to explode the star. This suggests a natural means by which the supernova is self-regulating.

We compare a suite of three-dimensional explosion calculations and stellar models incorporating advanced physics with observational constraints on the progenitor of Cassiopeia A. We consider binary and single stars from 16 to 40 M_☉ with a range of explosion energies and geometries. The parameter space allowed by observations of nitrogen-rich high-velocity ejecta, ejecta mass, compact remnant mass, and ⁴⁴Ti and ⁵⁶Ni abundances individually and as an ensemble is considered. A progenitor of 15-25 M_☉ that loses its hydrogen envelope to a binary interaction and undergoes an energetic explosion can match all the observational constraints.

We calculate the vertical structure of a local patch of an accretion disk in which heating by dissipation of MRI-driven MHD turbulence is balanced by radiative cooling. Heating, radiative transport, and cooling are computed self-consistently with the structure by solving the equations of radiation MHD in the shearing-box approximation. Using a fully three-dimensional and energy-conserving code, we compute the structure of this disk segment over a span of more than five cooling times. After a brief relaxation period, a statistically steady state develops. Measuring height above the midplane in units of the scale height predicted by a Shakura-Sunyaev model, we find that magnetic pressure causes the disk atmosphere to stretch upward, with the photosphere rising to ≃7H, in contrast to the ≃3H predicted by conventional analytic models. This more extended structure, as well as fluctuations in the height of the photosphere, may lead to departures from Planckian form in the emergent spectra. Dissipation is distributed across the region within ≃3H of the midplane but is very weak at greater altitudes. As a result, the temperature deep in the disk interior is less than that expected when all heat is generated in the midplane. With only occasional exceptions, the gas temperature stays very close to the radiation temperature, even above the photosphere. Because fluctuations in the dissipation are particularly strong away from the midplane, the emergent radiation flux can track dissipation fluctuations with a lag that is only 0.1-0.2 times the mean cooling time of the disk. Long-timescale asymmetries in the dissipation distribution can also cause significant asymmetry in the flux emerging from the top and bottom surfaces of the disk. Radiative diffusion dominates Poynting flux in the vertical energy flow throughout the disk.

We have computed models for ultraluminous X-ray sources (ULXs) consisting of a black hole accretor of intermediate mass (IMBH; e.g., ~1000 M_☉) and a captured donor star. For each of four different sets of initial donor masses and orbital separations we computed 30,000 binary evolution models using a full Henyey stellar evolution code. To our knowledge, this is the first time that a population of X-ray binaries this large has been carried out with other than approximation methods, and it serves to demonstrate the feasibility of this approach to large-scale population studies of mass transfer binaries. In the present study, we find that in order to have a plausible efficiency for producing active ULX systems with IMBHs having luminosities ≳10⁴⁰ ergs s^-1, there are two basic requirements for the capture of companion/donor stars. First, the donor stars should be massive, i.e., ≳8 M_☉. Second, the initial orbital separations after circularization should be close, i.e., ≲6-30 times the radius of the donor star when on the main sequence. Even under these optimistic conditions, we show that the production rate of IMBH-ULX systems may fall short of the observed values by factors of 10-100.

We investigate the time evolution of luminous accretion disks around black holes by conducting two-dimensional radiation-hydrodynamic simulations. We adopt the α prescription for the viscosity. The radial-azimuthal component of the viscous stress tensor is assumed to be proportional to the total pressure in the optically thick region and the gas pressure in the optically thin regime. The viscosity parameter, α, is taken to be 0.1. We find the limit-cycle variation in luminosity between high and low states. When we set the mass input rate from the outer disk boundary to be 100L_E/c², the luminosity suddenly rises from 0.3L_E to 2L_E, where L_E is the Eddington luminosity. It decays after retaining the high value for about 40 s. Our numerical results can explain the variability amplitude and duration of the recurrent outbursts observed in microquasar GRS 1915+105. We show that multidimensional effects play an important role in the high-luminosity state. In this state, the outflow is driven by the strong radiation force, and some part of the radiation energy dissipated inside the disk is swallowed by the black hole due to the photon-trapping effects. This trapped luminosity is comparable to the disk luminosity. We also calculate two more cases: one with a much larger accretion rate than the critical value for the instability and the other with the viscous stress tensor being proportional to the gas pressure only, even when the radiation pressure is dominant. We find no quasi-periodic light variations in these cases. This confirms that the limit-cycle behavior found in the simulations is caused by the disk instability.

We use new and archival Chandra and ROSAT data to study the time variability of the X-ray emission from the pulsar wind nebula (PWN) powered by PSR B1509-58 on timescales of 1 week to 12 yr. There is variability in the size, number, and brightness of compact knots appearing within 20'' of the pulsar, with at least one knot showing a possible outflow velocity of ~0.6c (assuming a distance to the source of 5.2 kpc). The transient nature of these knots may indicate that they are produced by turbulence in the flows surrounding the pulsar. A previously identified prominent jet extending 12 pc to the southeast of the pulsar increased in brightness by 30% over 9 yr; apparent outflow of material along this jet is observed with a velocity of ~0.5c. However, outflow alone cannot account for the changes in the jet on such short timescales. Magnetohydrodynamic sausage or kink instabilities are feasible explanations for the jet variability, with timescales of ~1.3-2 yr. An arc structure located 30''-45'' north of the pulsar shows transverse structural variations and appears to have moved inward with a velocity of ~0.03c over 3 yr. The overall structure and brightness of the diffuse PWN exterior to this arc and excluding the jet has remained the same over the 12 yr span. The photon indices of the diffuse PWN and possibly the jet steepen with increasing radius, likely indicating synchrotron cooling at X-ray energies.

We have conducted a search for giant pulses from four millisecond pulsars using the 100 m Green Bank Telescope. Coherently dedispersed time series from PSR J0218+4232 were found to contain giant pulses of very short intrinsic duration whose energies follow power-law statistics. The giant pulses are in phase with the two minima of the radio integrated pulse profile but are phase-aligned with the peaks of the X-ray profile. Historically, individual pulses more than 10-20 times the mean pulse energy have been deemed to be "giant pulses." As only 4 of the 155 pulses had energies greater than 10 times the mean pulse energy, we argue the emission mechanism responsible for giant pulses should instead be defined through: (1) intrinsic timescales of microsecond or nanosecond duration; (2) power-law energy statistics; and (3) emission occurring in narrow phase windows coincident with the phase windows of nonthermal X-ray emission. Four short-duration pulses with giant-pulse characteristics were also observed from PSR B1957+20. As the inferred magnetic fields at the light cylinders of the millisecond pulsars that emit giant pulses are all very high, this parameter has previously been considered to be an indicator of giant-pulse emissivity. However, the frequency of giant-pulse emission from PSR B1957+20 is significantly lower than for other millisecond pulsars that have similar magnetic fields at their light cylinders. This suggests that the inferred magnetic field at the light cylinder is a poor indicator of the rate of emission of giant pulses.

The monotonic increase of the radius of low-mass stars during their ascent on the red giant branch halts when they suffer a temporary contraction. This occurs when the hydrogen-burning shell reaches the discontinuity in hydrogen content left from the maximum increase in the convective extension, at the time of the first dredge-up, and produces a well known "bump" in the luminosity function of the red giants of globular clusters. If the giant is the mass-losing component in a binary in which mass transfer occurs on the nuclear evolution timescale, this event produces a temporary stop in the mass transfer, which we call "bump-related" detachment. If the accreting companion is a neutron star, in which the previous mass transfer has spun up the pulsar down to millisecond periods, the subsequent mass transfer phase may be altered by the presence of the energetic pulsar. In fact, the onset of a radio ejection phase produces loss of mass and angular momentum from the system. We show that this sequence of events may be at the basis of the shortage of systems with periods between ~20 and ~60 days in the distribution of binaries containing millisecond pulsars. We predict that systems that would be discovered with periods in this gap should preferentially have magnetic moments either smaller than ~2 × 10²⁶ or larger than ~4 × 10²⁶ G cm³. We further show that this period gap should not be present in Population II.

We present the observed pulsation spectra of all known noninteracting ZZ Ceti stars (hydrogen atmosphere white dwarf variables [DAVs]) and examine changes in their pulsation properties across the instability strip. We confirm the well-established trend of increasing pulsation period with decreasing effective temperature across the ZZ Ceti instability strip. We do not find a dramatic order-of-magnitude increase in the number of observed independent modes in ZZ Ceti stars, traversing from the hot to the cool edge of the instability strip; we find that the cool DAVs have one more mode on average than the hot DAVs. We confirm the initial increase in pulsation amplitude at the blue edge and find strong evidence of a decline in amplitude prior to the red edge. We present the first observational evidence that ZZ Ceti stars lose pulsation energy just before pulsations shut down at the empirical red edge of the instability strip.

I propose that some of the most luminous planetary nebulae (PNs) are actually proto-PNs, where a companion white dwarf (WD) accretes mass at a relatively high rate from the post-asymptotic giant branch star that blew the nebula. The WD sustains a continuous nuclear burning and ionizes the nebula. The WD is luminous enough to make the dense nebula luminous in the [O III] λ5007 line. In young stellar populations these WD accreting systems account for a small fraction of [O III]-luminous PNs, but in old stellar populations these binaries might account for most, or even all, of the [O III]-luminous PNs. This might explain the puzzling constant cutoff (maximum) [O III] λ5007 luminosity of the planetary nebula luminosity function across different galaxy types.

Asymptotic giant branch (AGB) stars have several interesting infrared spectral features. Approximately half the oxygen-rich AGB stars to be investigated spectroscopically exhibit a feature at ~13 μm. The carrier of this feature has not yet been unequivocally identified but has been attributed to various dust species, including corundum (α-Al₂O ₃), spinel (MgAl₂O₄), and silica (SiO ₂). In order to constrain the carrier of the 13 μm feature, we have used the one-dimensional radiative transfer code DUSTY to model the effects of composition and optical depth on the shape and strength of the emerging 13 μm feature from corundum and spinel grains. We have modeled various corundum, spinel, corundum-silicate, and spinel-silicate mixtures in dust shells surrounding O-rich AGB stars. These models demonstrate that (1) if corundum is present in these circumstellar dust shells, even at very low relative abundances, a ~13 μm feature should be observed; (2) corundum's weak ~21 μm feature will not be observed, even if it is responsible for the ~13 μm feature; (3) even at low relative abundances, spinel exhibits a feature at 16.8 μm that is not found in observations; and (4) the grains must be spherical. Other grain shapes (spheroids, ellipsoids, and hollow spheres) shift the features to longer wavelengths for both spinel and corundum. Our models show that spinel is unlikely to be the carrier of the 13 μm feature. The case for corundum as the carrier is strengthened but not yet proven.

We report Very Long Baseline Array (VLBA) observations of 43 GHz v = 1, J = 1-0 SiO masers in the circumstellar envelope of the M-type semiregular variable star VX Sgr at three epochs during 1999 April-May. These high-resolution VLBA images reveal a persistent ringlike distribution of SiO masers with a projected radius of ≈3 stellar radii. The typical angular size of 0.5 mas for individual maser features was estimated from two-point correlation function analysis of maser spots. We found that the apparent size scale of maser features was distinctly smaller than that observed in the previous observations by comparing their fractions of the total power imaged. This change in the size scale of maser emission may be related to stellar activity that caused a large SiO flare during our observations. Our observations confirmed the asymmetric distribution of maser emission, but the overall morphology has changed significantly, with the majority of the masers clustering to the northeast of the star compared to the majority of the masers lying in the southwest in 1992. By identifying 42 matched maser features appearing in all three epochs, we determined the contraction of an SiO maser shell toward VX Sgr at a proper motion of -0.507 ± 0.069 mas yr^-1, corresponding to a velocity of about 4 km s^-1 at a distance of 1.7 kpc to VX Sgr. Such a velocity is on the order of the sound speed and can be easily explained by the gravitational infall of material from the circumstellar dust shell.

CX Cep (WR 151) is the WR+O binary (WN5+O5 V) with the second shortest period known in our Galaxy. To examine the circumstellar matter distribution and to better constraint the orbital parameters and mass-loss rate of the W-R star, we obtained broadband and multiband (i.e., UBVRI) linear polarization observations of the system. Our analysis of the phase-locked polarimetric modulation confirms the high orbital inclination of the system (i.e., i = 65°). Using the orbital solution of Lewis et al. (1993), we obtain masses of 33.9 and 23.9 M_☉ for the O and W-R stars, respectively, which agree with their spectral types. A simple polarimetric model accounting for finite stellar size effects allowed us to derive a mass-loss rate for the W-R star of (0.3-0.5) × 10^-5M_☉ yr^-1. This result was remarkably independent of the model's input parameters and favors an earlier spectral type for the W-R component (i.e., WN4). Finally, using our multiband observations, we fitted and subtracted from our data the interstellar polarization. The resulting constant intrinsic polarization of 3%-4% is misaligned in relation to the orbital plane (i.e., θ_CIP = 26° vs. Ω = 75°) and is the highest intrinsic polarization ever observed for a W-R star. This misalignment points toward a rotational (or magnetic) origin for the asymmetry and contradicts the most recent evolutionary models for massive stars (Meynet & Maeder 2003) that predict spherically symmetric winds during the W-R phase (i.e., CIP = 0%).

We propose collisionless damping of fast MHD waves as an important mechanism for the heating and acceleration of winds from rotating stars. Stellar rotation causes magnetic field lines anchored at the surface to form a spiral pattern, and magnetorotational winds can be driven. If the structure is magnetically dominated, fast MHD waves generated at the surface can propagate almost radially outward and cross the field lines. The propagating waves undergo collisionless damping owing to interactions with particles surfing on magnetic mirrors that are formed by the waves themselves. The damping is especially effective where the angle between the wave propagation and the field lines becomes moderately large (~20°-80°). The angle tends naturally to increase into this range because the field in magnetorotational winds develops an increasingly large azimuthal component. The dissipation of the wave energy produces heating and acceleration of the outflow. We show using specified wind structures that this damping process can be important in both solar-type stars and massive stars that have moderately large rotation rates. This mechanism can play a role in the coronae of young solar-type stars that are rapidly rotating and show X-ray luminosities much larger than that of the Sun. The mechanism could also be important for producing the extended X-ray-emitting regions inferred to exist in massive stars of spectral type middle B and later.

We present spectroscopic and photometric observations of the eclipsing system V1061 Cyg (P = 2.35 days). A third star is visible in the spectrum, and the system is a hierarchical triple. We combine the radial velocities for the three stars, times of eclipse, and intermediate astrometric data from the Hipparcos mission (abscissa residuals) to establish the elements of the outer orbit, which is eccentric and has a period of 15.8 yr. We determine accurate values for the masses, radii, and effective temperatures of the binary components: M_Aa = 1.282 ± 0.015 M_☉, R_Aa = 1.615 ± 0.017 R_☉, and T = 6180 ± 100 K for the primary (star Aa), and M_Ab = 0.9315 ± 0.0068 M_☉, R_Ab = 0.974 ± 0.020 R_☉, and T = 5300 ± 150 K for the secondary (Ab). The mass of the tertiary is determined to be M_B = 0.925 ± 0.036 M_☉ and its effective temperature is T = 5670 ± 150 K. Current stellar evolution models agree well with the properties of the primary but show a very large discrepancy in the radius of the secondary, in the sense that the predicted values are ~10% smaller than observed (a ~5 σ effect). In addition, the temperature is cooler than predicted, by some 200 K. These discrepancies are quite remarkable given that the star is only 7% less massive than the Sun, the calibration point of all stellar models. We identify the chromospheric activity as the likely cause of the effect. Inactive stars agree very well with the models, while active ones such as V1061 Cyg Ab appear systematically too large and too cool.

New boron abundances for seven main-sequence B-type stars are determined from HST STIS spectroscopy around the B III 2066 Å line. Boron abundances provide a unique and critical test of stellar evolution models that include rotational mixing, since boron is destroyed in the surface layers of stars through shallow mixing long before other elements are mixed from the stellar interior through deep mixing. The stars in this study are all on or near the main sequence and are members of young Galactic clusters. They show no evidence of mixing with gas from H-burning layers from their CNO abundances. Boron abundances range from 12 + log(B/H) ≤ 1.0 to 2.2. The boron abundances are compared to the published values of the stellar nitrogen abundances [all have 12 + log(N/H) ≤ 7.8] and to their host cluster ages (4-16 Myr) to investigate the predictions from models of massive star evolution with rotational mixing effects. We find that the variations in boron and nitrogen are generally within the range of the predictions from the stellar evolution models with rotation (where predictions for models with rotation rates from 0 to 450 km s^-1 and μ-barriers are examined), especially given their age and mass ranges. Three stars (of 34 B-type stars with detailed boron abundance determinations) deviate from the model predictions, showing either much larger boron depletions than can be explained by the rotating model predictions or a spectroscopic mass that is lower than expected, given the rotating model predictions for its age and abundances. The results from these three stars suggest that rotational mixing could be more efficient than that currently modeled at the highest rotation rates.

Sun-like stars have stellar, brown dwarf, and planetary companions. To help constrain their formation and migration scenarios, we analyze the close companions (orbital period <5 yr) of nearby Sun-like stars. By using the same sample to extract the relative numbers of stellar, brown dwarf, and planetary companions, we verify the existence of a very dry brown dwarf desert and describe it quantitatively. With decreasing mass, the companion mass function drops by almost 2 orders of magnitude from 1 M_☉ stellar companions to the brown dwarf desert and then rises by more than an order of magnitude from brown dwarfs to Jupiter-mass planets. The slopes of the planetary and stellar companion mass functions are of opposite sign and are incompatible at the 3 σ level, thus yielding a brown dwarf desert. The minimum number of companions per unit interval in log mass (the driest part of the desert) is at M = 31M_J. Approximately 16% of Sun-like stars have close (P < 5 yr) companions more massive than Jupiter: 11% ± 3% are stellar, <1% are brown dwarf, and 5% ± 2% are giant planets. The steep decline in the number of companions in the brown dwarf regime, compared to the initial mass function of individual stars and free-floating brown dwarfs, suggests either a different spectrum of gravitational fragmentation in the formation environment or post-formation migratory processes disinclined to leave brown dwarfs in close orbits.

Using a model for refractory clouds, a novel algorithm for handling them, and the latest gas-phase molecular opacities, we have produced a new series of L and T dwarf spectral and atmosphere models as a function of gravity and metallicity, spanning the T_eff range from 2200 to 700 K. The correspondence with observed spectra and infrared colors for early and mid-L dwarfs and for mid- to late T dwarfs is good. We find that the width in infrared color-magnitude diagrams of both the T and L dwarf branches is naturally explained by reasonable variations in gravity and therefore that gravity is the ``second parameter" of the L-T dwarf sequence. We investigate the dependence of theoretical dwarf spectra and color-magnitude diagrams on various cloud properties, such as particle size and cloud spatial distribution. In the region of the L → T transition, we find that no single combination of cloud particle size and gravity can be made to fit all the observed data. Our results suggest that current ignorance of detailed cloud meteorology renders ambiguous the extraction of various physical quantities such as T_eff and gravity for mid-L to early T dwarfs. Nevertheless, for decreasing T_eff, we capture with some accuracy the major spectral features and signatures observed. We speculate that the subdwarf branch of the L dwarfs would be narrower in effective temperature and that for low enough metallicity the L dwarfs would disappear altogether as a spectroscopic class. Furthermore, we note that the new, lower solar oxygen abundances of Allende-Prieto and coworkers produce better fits to brown dwarf data than do the older values. Finally, we discuss various issues in cloud physics and modeling and speculate on how a better correspondence between theory and observation in the problematic L → T transition region could be achieved.

We propose that the two-armed spiral features seen in visible Hubble Space Telescope images of scattered light in HD 100546's circumstellar disk are caused by the illumination of a warped outer disk. A tilt of 6°-15° from the symmetry plane can cause the observed surface brightness variations, providing the disk is very twisted (highly warped) at radii greater than 200 AU where the spiral features are seen. Dust lanes are due in part to shadowing in the equatorial plane from the inner disk within a radius of 100 AU. HD 100546's outer disk, if viewed edge-on, would appear similar to that of Beta Pictoris. A disk initially misaligned with a planetary system becomes warped due to precession induced by planetesimal bodies and planets. However, the twistedness of HD 100546's disk cannot be explained by precession during the lifetime of the system induced by a single Jovian-mass planet within the clearing at ~13 AU. One possible explanation for the corrugated disk is that precession was induced by massive bodies embedded in the disk at larger radius. This would require approximately a Jupiter mass of bodies well outside the central clearing at 13 AU and within the location of the spiral features or at radii approximately between 50 and 200 AU.

We study dynamical interactions of star-planet binaries with other single stars. We derive analytical cross sections for all possible outcomes and confirm them with numerical scattering experiments. We find that a wide mass ratio in the binary introduces a region in parameter space that is inaccessible to comparable-mass systems, in which the nature of the dynamical interaction is fundamentally different from what has traditionally been considered in the literature on binary scattering. We study the properties of the planetary systems that result from the scattering interactions for all regions of parameter space, paying particular attention to the location of the "hard-soft" boundary. The structure of the parameter space turns out to be significantly richer than a simple statement of the location of the hard-soft boundary would imply. We consider the implications of our findings, calculating characteristic lifetimes for planetary systems in dense stellar environments and applying the results to previous analytical studies, as well as past and future observations. Since we recognized that the system PSR B1620-26 in the globular cluster M4 lies in the "new" region of parameter space, we performed a detailed analysis quantifying the likelihood of different scenarios in forming the system we see today.

We have studied dust evolution in a quiescent or turbulent protoplanetary disk by numerically solving a coagulation equation for settling dust particles, using the minimum mass solar nebula model. As a result, if we assume an ideally quiescent disk, the dust particles settle toward the disk midplane to form a gravitationally unstable layer within 2 × 10³ to 4 × 10⁴ yr at 1-30 AU, which is in good agreement with an analytic calculation by Nakagawa et al., although they did not take the particle size distribution into account explicitly. In an opposite extreme case of a globally turbulent disk, on the other hand, the dust particles fluctuate owing to turbulent motion of the gas and most particles become large enough to move inward very rapidly within 70 to 3 × 10⁴ yr at 1-30 AU, depending on the strength of the turbulence. Our result suggests that global turbulent motion should cease for planetesimal formation to be possible in protoplanetary disks.

Recent spectral observations by the Spitzer Space Telescope reveal that some disks around young (~a few times 10⁶ yr old) stars have remarkably sharp transitions to a low-density inner region in which much of the material has been cleared away. It has been recognized that the most plausible mechanism for the sharp transition at a specific radius is the gravitational influence of a massive planet. This raises the question of whether the planet can also account for the hole extending all the way to the star. Using high-resolution numerical simulations, we show that Jupiter-mass planets drive spiral waves that create holes on timescales ~10 times shorter than viscous or planet migration times. We find that the theory of spiral wave-driven accretion in viscous flows by Takeuchi et al. can be used to provide a consistent interpretation of the simulations. In addition, although the hole surface densities are low, they are finite, allowing mass accretion toward the star. Our results therefore imply that massive planets can form extended, sharply bounded spectral holes that can still accommodate substantial mass accretion rates. The results also imply that holes are more likely than gaps for Jupiter-mass planets around solar-mass stars.

In a protoplanetary disk, the inner edge of the region where the temperature falls below the condensation temperature of water is referred to as the snow line. Outside the snow line, water ice increases the surface density of solids by a factor of 4. The mass of the fastest growing planetesimal (the isolation mass) scales as the surface density to the 3/2 power. It is thought that ice-enhanced surface densities are required to make the cores of the gas giants (Jupiter and Saturn) before the disk gas dissipates. Observations of our solar system's asteroid belt suggest that the snow line occurred near 2.7 AU. In this paper we revisit the theoretical determination of the snow line. In a minimum-mass disk characterized by conventional opacities and a mass accretion rate of 10^-8 M_☉ yr^-1, the snow line lies at 1.6-1.8 AU, just past the orbit of Mars. The minimum-mass disk, with a mass of 0.02 M_☉, has a lifetime of 2 million years with the assumed accretion rate. Moving the snow line past 2.7 AU requires that we increase the disk opacity, accretion rate, and/or disk mass by factors ranging up to an order of magnitude above our assumed baseline values.

An asymmetric solar wind termination shock (TS) model is used to study the effects on the modulation of cosmic-ray protons for different scenarios of the solar wind speed (V) in the heliosheath. This two-dimensional model is applied using predictions for V in the heliosheath that were calculated with a time-dependent three-dimensional hydrodynamic model. Decreases stronger than the generally assumed V ∝ 1/r² in the heliosheath are studied, as well as an extreme case, V ∝ r², where r is the radial distance.The effect of the TS is enhanced under certain circumstances, and "barrier"-type modulation in the heliosheath also depends on the chosen V-profiles. Significant changes occur mostly for the A < 0 solar magnetic polarity cycle, at all distances in the equatorial plane, when the V is changed from an incompressible fluid (V ∝ 1/r², ∇ V = 0) in the heliosheath, to V ∝ 1/r⁸, in a symmetrical model. For the asymmetrical case the TS is predicted to be more effective in the heliospheric nose than in the tail, especially for the A < 0 cycle during solar minimum conditions. The different profiles for V do not have a significant effect on the intensities inside the TS, but in the heliosheath the difference can be quite significant. It is found that V ∝ 1/r² in the heliosheath is an oversimplification. The consequent effects of having ∇ V ≠ 0 in the heliosheath prove to be relevant for cosmic-ray modulation and acceleration, especially now that the Voyager 1 spacecraft encountered the TS and entered the heliosheath.

The coronal magnetic field, computed from synoptic maps of the magnetic field and a potential field source surface (PFSS) model, reveals special configurations related to the active regions that were associated with most, if not all, fast halo coronal mass ejections (CMEs) in 2003 October-November. It was shown that these active regions emerged in an open field area, produced a large open field area after emerging, or sat on a boundary of two open field areas with the same polarity. This type of boundary is also known as a "plasma sheet." Such magnetic structures appear to be favorable for the propagation of the disturbance. MHD simulations were performed here to explore the behavior of the propagation of the disturbance against these special configurations of the background magnetic field. It is demonstrated that without the presence of open flux, the speed of the CMEs would have been only 78% of that with open flux present. It is also found that the CMEs from a heliospheric current sheet have a speed only 67% of the CMEs' speed from a plasma sheet, the source boundary with the same polarity.

The Ca II infrared triplet lines around 8540 Å are good candidates for observing chromospheric magnetism. Model spectra of these lines are obtained by combining a radiation hydrodynamic simulation with a Stokes synthesis code. The simulation shows interesting time-varying behavior of the Stokes V profiles as waves propagate through the formation region of the lines. Disappearing and reappearing lobes in the Stokes V profiles as well as profile asymmetries are closely related to the atmospheric velocity gradients.

Velocity oscillations in sunspot umbrae have been measured simultaneously in two spectral lines: the photospheric Si I λ10827 line and the chromospheric He I λ10830 multiplet. From the full Stokes inversion of temporal series of spectropolarimetric observations, we retrieved, among other parameters, the line-of-sight velocity temporal variations at photospheric and chromospheric heights. Chromospheric velocity oscillations show a 3 minute period with a clear sawtooth shape typical of propagating shock wave fronts. Photospheric velocity oscillations have basically a 5 minute period, although the power spectrum also shows a secondary peak in the 3 minute band that has been proven to be a predecessor for its chromospheric counterpart. The derived phase spectra yield a value of the atmospheric cutoff frequency around 4 mHz and give evidence for the upward propagation of higher frequency oscillation modes. The phase spectrum has been reproduced with a simple model of linear vertical propagation of slow magnetoacoustic waves in a stratified magnetized atmosphere that accounts for radiative losses through Newton's cooling law. The model explains the main features in the phase spectrum and allows us to compute the theoretical time delay between the photospheric and chromospheric signals, which happens to have a strong dependence on frequency. We find a very good agreement between this and the time delay obtained directly from the cross-correlation of photospheric and chromospheric velocity maps filtered around the 6 mHz band. This allows us to infer that the 3 minute power observed at chromospheric heights comes directly from the photosphere by means of linear wave propagation, rather than from nonlinear interaction of 5 minute (and/or higher frequency) modes.

Two explanations exist for the short-lived radionuclides (T_1/2 ≤ 5 Myr) present in the solar system when the calcium-aluminum-rich inclusions (CAIs) first formed. They originated either from the ejecta of a supernova or by the in situ irradiation of nebular dust by energetic particles. With a half-life of only 53 days, ⁷Be is then the key discriminant, since it can be made only by irradiation. Using the same irradiation model developed earlier by our group, we calculate the yield of ⁷Be. Within model uncertainties associated mainly with nuclear cross sections, we obtain agreement with the experimental value. Moreover, if ⁷Be and ¹⁰Be have the same origin, the irradiation time must be short (a few to tens of years), and the proton flux must be of order F ~ 2 × 10¹⁰ cm^-2 s^-1. The X-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, ²⁶Al, ³⁶Cl, and ⁵³Mn are also generated at the measured levels within model uncertainties, provided that irradiation occurs under conditions reminiscent of solar impulsive events (steep energy spectra and high ³He abundance). The decoupling of the ²⁶Al and ¹⁰Be observed in some rare CAIs receives a quantitative explanation when rare gradual events (shallow energy spectra and low ³He abundance) are considered. The yields of ⁴¹Ca are compatible with an initial solar system value inferred from the measured initial ⁴¹Ca/⁴⁰Ca ratio and an estimate of the thermal metamorphism time (from Young et al.), alleviating the need for two-layer proto-CAIs. Finally, we show that the presence of supernova-produced ⁶⁰Fe in the solar accretion disk does not necessarily mean that other short-lived radionuclides have a stellar origin.

In the present work we calculate energy levels, oscillator strengths, A-values, and electron-ion excitation collision strengths for high-energy configurations for the iron ions Fe XVII to Fe XXIII. Collision rate coefficients are calculated using the distorted-wave approximation; the contribution of resonant excitation is taken into account by using the isolated-resonance approximation for configurations with principal quantum numbers n = 2 and 3. Results are compared with R-matrix calculations available in the literature for the lowest configurations; our results are the first ever published for highly excited configurations with n ≥ 4 for most ions. Synthetic spectra are calculated from the resulting rate coefficients and compared to available spectral codes. Some applications of the results of the present calculation to solar physics are discussed. These data will be part of the next version of the CHIANTI database.

Galaxies are not distributed randomly throughout space but are instead arranged in an intricate "cosmic web" of filaments and walls surrounding bubble-like voids. There is still no compelling observational evidence of a link between the structure of the cosmic web and how galaxies form within it. However, such a connection is expected on the basis of our understanding of the origin of galaxy angular momentum: disk galaxies should be highly inclined relative to the plane defined by the large-scale structure surrounding them. Using the two largest galaxy redshift surveys currently in existence (2dFGRS and SDSS), we show at the 99.7% confidence level that these alignments do indeed exist: spiral galaxies located on the shells of the largest cosmic voids have rotation axes that lie preferentially on the void surface.

Velocities close to the speed of light are a robust observational property of the jets observed in microquasars and active galactic nuclei, and they are expected to be behind much of the phenomenology of gamma-ray bursts (GRBs). Yet the mechanism boosting relativistic jets to such large Lorentz factors is not fully known. Building on recent general relativistic, multidimensional simulations of progenitors of short GRBs, we discuss a new effect in relativistic hydrodynamics that can act as an efficient booster in jets. This effect is purely hydrodynamical and occurs when large velocities tangential to a discontinuity are present in the flow, yielding Lorentz factors Γ ~ 10²-10³ or larger in flows with moderate initial Lorentz factors. Although without a Newtonian counterpart, this effect can be explained easily through the most elementary hydrodynamical flow (i.e., a relativistic Riemann problem).

Future weak-lensing measurements of cosmic shear will reach such high accuracy that second-order effects in weak-lensing modeling, such as the influence of baryons on structure formation, become important. We use a controlled set of high-resolution cosmological simulations to quantify this effect by comparing pure N-body dark matter runs with corresponding hydrodynamic simulations, carried out both in nonradiative form and in dissipative form with cooling and star formation. In both hydrodynamic simulations, the clustering of the gas is suppressed while that of dark matter is boosted at scales k > 1 h Mpc^-1. Despite this counterbalance between dark matter and gas, the clustering of the total matter is suppressed by up to 1% at 1 h Mpc^-1 ≲ k ≲ 10 h Mpc^-1, while for k ≈ 20 h Mpc^-1 it is boosted, up to 2% in the nonradiative run and 10% in the run with star formation. The stellar mass formed in the latter is highly biased relative to the dark matter in the pure N-body simulation. Using our power spectrum measurements to predict the effect of baryons on the weak-lensing signal at scales corresponding to multipole moments 100 < l < 10,000, we find that baryons may change the lensing power spectrum by less than 0.5% at l < 1000, but by 1% to 10% at 1000 < l < 10,000. The size of the effect exceeds the predicted accuracy of future lensing power spectrum measurements and will likely be detected. Precise determinations of cosmological parameters with weak lensing, and studies of small-scale fluctuations and clustering, therefore rely on properly including baryonic physics.

We present the results of intermediate-resolution (~2 Å) spectroscopy of a sample of 37 candidate Lyα blobs and emitters at redshift z = 3.1 using the DEIMOS spectrograph on the 10 m Keck telescope. The emission lines are detected for all 37 objects and show variety in their line profiles. The Lyα velocity widths (FWHM) of the 28 objects with higher quality spectra, measured by fitting a single Gaussian profile, are in the range of 150-1700 km s^-1 and correlate with the Lyα spatial extents. All 12 Lyα blobs (≥16 arcsec²) have large velocity widths of ≳500 km s^-1. While there are several possible physical interpretations of the Lyα velocity widths (the motion of gravitationally bound gas clouds, inflows, the merging of clumps, or outflows from superwinds), the large velocity widths of the Lyα blobs suggest that they are the sites of massive galaxy formation. If we assume gravitationally bound gas clouds, the dynamical masses of the Lyα blobs are estimated to be ~10¹²-10¹³M_☉. Even for the case of outflows, the outflow velocities are likely to be comparable to the rotation velocities as inferred from the observational evidence for local starburst galaxies.

The Westerbork Synthesis Radio Telescope finds a weak 21 cm line emission feature at the coordinates (R.A., decl., velocity) of the sub-damped Lyα (DLA) absorber observed at z_abs = 0.00632 in the spectrum of PG 1216+069. The emission feature, WSRT J121921+0639, lies within 30'' of the quasar sight line, is detected at a 99.8% (3 σ) confidence level, has M between 5 and 15 × 10⁶M_☉, and has a velocity spread between 20 and 60 km s^-1. Other H I emitters in the field include VCC 297 at a projected distance of 86 h kpc and a previously unreported H I cloud, WSRT J121919+0624, at 112 h kpc with M ~ 3 × 10⁸M_☉. The optically identified foreground galaxy that is closest to the quasar sight line appears to be VCC 339 (~L*/25) at 29 h kpc with a velocity offset of 292 km s^-1. A low surface brightness galaxy with the H I mass of the sub-DLA absorber WSRT J121921+0639 would likely have m_B ~ 17, and its diffuse optical emission would need to compete with the light of both the background QSO and a brighter foreground star ~10'' from the QSO sight line.

It has been suggested that cloudlets of cold (≤10 K) H₂ and dense (≥10⁷ cm^-3) molecular gas constitute the dark halos of galaxies. Such gas is extremely difficult to detect because the classical tracers of molecular gas, CO and/or dust grains, have very low abundances and because their emission is exceedingly weak. For this reason, the cloudlet hypothesis remains substantially unproven so far. In this Letter, we propose a new method to probe the presence of cold H₂ clouds in galactic halos: the ground transition of ortho-H₂D⁺ at 372 GHz. We discuss why the H₂D⁺ is abundant under the physical conditions appropriate for the cloudlets, and we present a chemical model that predicts the H₂D⁺ abundance as a function of four key parameters: gas density and metallicity, the cosmic-ray ionization rate, and dust grain size. We conclude that current ground-based instruments might detect the ortho-H₂D⁺ line emitted by the cloudlets halo and therefore prove the existence of large quantities of dark baryonic matter around galaxies.

We present the results of the first HCO⁺ survey probing the dense molecular gas content of a sample of 16 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN (1-0) observations, had shown that LIRGs and ULIRGs possess a significantly higher fraction of dense molecular gas compared to normal galaxies. While the picture issued from HCO⁺ partly confirms this result, we have discovered an intriguing correlation between the HCN (1-0)/HCO⁺ (1-0) luminosity ratio and the IR luminosity of the galaxy (L_IR). This trend casts doubts on the use of HCN as an unbiased quantitative tracer of the dense molecular gas content in LIRGs and ULIRGs. A plausible scenario explaining the observed trend implies that X-rays coming from an embedded active galactic nucleus may play a dominant role in the chemistry of molecular gas at L_IR ≥ 10¹²L_☉. We discuss the implications of this result for the understanding of LIRGs, ULIRGs, and high-redshift gas-rich galaxies.

Almost all the X-ray afterglows of γ-ray bursts (GRBs) observed by the Swift satellite have a shallow decay phase in their first few thousand seconds. We show that in an inhomogeneous-jet model (multiple-subjet or patchy-shell), the superposition of the afterglows of off-axis subjets (patchy shells) can produce the shallow decay phase. The necessary condition for obtaining the shallow decay phase is that γ-ray-bright subjets (patchy shells) have γ-ray efficiencies higher than previously estimated and that they be surrounded by γ-ray-dim subjets (patchy shells) with low γ-ray efficiency. Our model predicts that events with dim prompt emission will have a conventional afterglow light curve without a shallow decay phase, like GRB 050416A.

We analyze 26 luminous compact blue galaxies (LCBGs) in the Hubble Space Telescope ACS Ultra Deep Field (UDF) at z ~ 0.2-1.3, to determine whether these truly are small galaxies or, rather, bright central starbursts within existing or forming large disk galaxies. Surface brightness profiles from UDF images reach fainter than rest-frame 26.5 B mag arcsec^-2 even for compact objects at z ~ 1. Most LCBGs show a smaller, brighter component that is likely star-forming, and an extended, roughly exponential component with colors suggesting stellar ages from ≳100 Myr to a few gigayears. Scale lengths of the extended components are mostly ≲2 kpc, more than 1.5-2 times smaller than those of nearby large disk galaxies like the Milky Way. Larger, very low surface brightness disks can be excluded down to faint rest-frame surface brightnesses (≳26 B mag arcsec^-2). However, one or two of the LCBGs are large, disklike galaxies that meet LCBG selection criteria because of a bright central nucleus, possibly a forming bulge. These results indicate that ≳90% of high-z LCBGs are small galaxies that will evolve into small disk galaxies, or low-mass spheroidal or irregular galaxies in the local universe, assuming passive evolution and no significant disk growth. The data do not reveal signs of disk formation around small, H II galaxy-like LCBGs, nor do they suggest a simple inside-out growth scenario for larger LCBGs with a disklike morphology. Irregular blue emission in distant LCBGs is relatively extended, suggesting that nebular emission lines from star-forming regions sample a major fraction of an LCBG's velocity field.

Overdensities in the distribution of low-latitude, 2MASS giant stars are revealed by systematically peeling away from sky maps the bulk of the giant stars conforming to "isotropic" density laws generally accounting for known Milky Way components. This procedure, combined with a higher resolution treatment of the sky density of both giants and dust, allows us to probe to lower Galactic latitudes than previous 2MASS giant star studies. While the results show the swath of excess giants previously associated with the Monoceros ring system in the second and third Galactic quadrants at distances of 6-20 kpc, we also find a several times larger overdensity of giants in the same distance range concentrated in the direction of the ancient constellation Argo. Isodensity contours of the large structure suggest that it is highly elongated and inclined by about 3° to the disk, although details of the structure—including the actual location of highest density, overall extent, true shape—and its origin remain unknown because only a fraction of it lies outside highly dust-obscured, low-latitude regions. Nevertheless, our results suggest that the 2MASS M giant overdensity previously claimed to represent the core of a dwarf galaxy in Canis Major (l ~ 240°) is an artifact of a dust extinction window opening to the overall density rise to the more significant Argo structure centered at larger longitude (l ~ 290° ± 10°, b ~ -4° ± 2°).

The dust in the Small Magellanic Cloud (SMC), an ideal analog of primordial galaxies at high redshifts, differs markedly from that in the Milky Way by exhibiting a steeply rising far-ultraviolet extinction curve, an absence of the 2175 Å extinction feature, and a local minimum at ~12 μm in its infrared emission spectrum, suggesting the lack of ultrasmall carbonaceous grains (i.e., polycyclic aromatic hydrocarbon molecules), which are ubiquitously seen in the Milky Way. While current models for the SMC dust all rely heavily on silicates, recent observations of the SMC line of sight toward Sk 155 have indicated that Si and Mg are essentially undepleted and that the depletions of Fe range from mild to severe, suggesting that metallic grains or iron oxides, instead of silicates, may dominate the SMC dust. However, here we apply the Kramers-Kronig relation to demonstrate that neither metallic grains nor iron oxides are capable of accounting for the observed extinction; silicates remain as an important contributor to the extinction, consistent with current models for the SMC dust.

The attenuation of very high energy γ-rays by pair production on the Galactic interstellar radiation field has long been thought of as negligible. However, a new calculation of the interstellar radiation field consistent with multiwavelength observations by the DIRBE and FIRAS instruments indicates that the energy density of the Galactic interstellar radiation field is higher, particularly in the Galactic center, than previously thought. We have made a calculation of the attenuation of very high energy γ-rays in the Galaxy, using this new interstellar radiation field, that takes into account its nonuniform spatial and angular distributions. We find that the maximum attenuation occurs around 100 TeV at the level of about 25% for sources located at the Galactic center and is important for both Galactic and extragalactic sources.

A strong case has been made that radio waves from sources within about half a degree of the Galactic center undergo extreme diffractive scattering. However, problems arise when standard ("Kolmogorov") models of electron density fluctuations are employed to interpret the observations of scattering in conjunction with those of free-free radio emission. Specifically, the outer scale of a Kolmogorov spectrum of electron density fluctuations is constrained to be so small that it is difficult to identify an appropriate astronomical setting. Moreover, an unacceptably high turbulent heating rate results if the outer scale of the velocity field coincides with that of the density fluctuations. We propose an alternative model based on folded magnetic field structures that have been reported in numerical simulations of small-scale dynamos. Nearly isothermal density variations across thin current sheets suffice to account for the scattering. There is no problem of excess turbulent heating, because the outer scale for the velocity fluctuations is much larger than the widths of the current sheets. We speculate that interstellar magnetic fields could possess geometries that reflect their origins: fields maintained by the Galactic dynamo could have large correlation lengths, whereas those stirred by local energetic events might exhibit folded structures.

We have observed a bright flare of Sgr A* in the near-infrared with the adaptive optics-assisted integral-field spectrometer SINFONI. Within the uncertainties, the observed spectrum is featureless and can be described by a power law. Our data suggest that the spectral index is correlated with the instantaneous flux and that both quantities experience significant changes within less than 1 hour. We argue that the near-infrared flares from Sgr A* are due to synchrotron emission of transiently heated electrons, the emission being affected by orbital dynamics and synchrotron cooling, both acting on timescales of ≈20 minutes.

Recent Chandra X-Ray Observatory observations have revealed a large population of faint X-ray point sources in the Galactic center. The observed population consists of ≳2000 faint sources in the luminosity range ~10³¹-10³³ ergs s^-1. The majority of these sources (70%) are described by hard spectra, while the rest are rather soft. The nature of these sources still remains unknown. Belczynski & Taam demonstrated that X-ray binaries with neutron star or black hole accretors may account for most of the soft sources, but are not numerous enough to account for the observed number and X-ray properties of the faint hard sources. A population synthesis calculation of the Galactic center region has been carried out. Our results indicate that the numbers and X-ray luminosities of intermediate Polars are consistent with the observed faint hard Galactic center population.

A high level of complex structure, or "granularity," has been observed in the distribution of infrared-obscuring material toward the Galactic center (GC), with a characteristic scale of 5''-15'', corresponding to 0.2-0.6 pc at a GC distance of 8.5 kpc. This structure has been observed in ISAAC images, which have a resolution of ~06, significantly higher than that of previous studies of the GC. We have discovered granularity throughout the GC survey region, which covers an area of 16 × 08 in longitude and latitude, respectively (300 pc × 120 pc at 8.5 kpc), centered on Sgr A*. This granularity is variable over the whole region, with some areas exhibiting highly structured extinction in one or more wave bands and other areas displaying no structure and a uniform stellar distribution in all wave bands. The granularity does not appear to correspond to longitude, latitude, or radial distance from Sgr A*. We find that regions exhibiting high granularity are strongly associated with high stellar reddening.

The Goldreich-Sridhar model of incompressible turbulence provides us with an elegant approach to describing strong MHD turbulence. It relies on the fact that interacting Alfvénic waves are independent and have random polarization. However, in case of strong interaction, a spontaneous local axial asymmetry can arise. We used direct numerical simulations to show that polarization alignment occurs and that it grows larger at smaller scales. Assuming critical balance, this effect could lead to a shallower spectrum and stronger anisotropy. Even small changes in these two properties will have important astrophysical consequences, e.g., for the cosmic-ray physics.

We have discovered an axially symmetric, well-defined shell of material in the constellation of Cepheus, based on imaging acquired as part of the Galactic First Look Survey with the Spitzer Space Telescope. The 86'' × 75'' object exhibits brightened limbs on the minor axis and is clearly visible at 24 μm, but it is not detected in the 3.6, 4.5, 5.8, 8.0, 70, or 160 μm images. Follow-up 7.5-40 μm spectroscopy reveals that the shell is composed entirely of ionized gas and that the 24 μm imaging traces [O IV] 25.89 μm emission solely. The spectrum also exhibits weaker [Ne III] and [S III] emission, and very weak [Ne V] emission. No emission from warm dust is detected. Spectral cuts through the center of the shell and at the northern limb are highly consistent with each other. The progenitor is not readily identified, but with scaling arguments and comparison to well-known examples of evolved stellar objects, we find the observations to be most straightforward to interpret in terms of a young supernova remnant located at a distance of at least 10 kpc, some 400 pc above the Galactic disk. If confirmed, this would be the first supernova remnant discovered initially at infrared wavelengths.

We report the detection of probable optical counterparts for two intermediate-mass binary pulsar (IMBP) systems, PSR J1528-3146 and PSR J1757-5322. Recent radio pulsar surveys have uncovered a handful of these systems with putative massive white dwarf companions, thought to have an evolutionary history different from that of the more numerous class of low-mass binary pulsars with He white dwarf companions. The study of IMBP companions via optical observations offers us several new diagnostics: the evolution of main-sequence stars near the white dwarf-neutron star boundary, the physics of white dwarfs close to the Chandrasekhar limit, and insights into the recycling process by which old pulsars are spun up to high rotation frequencies. We were unsuccessful in our attempt to detect optical counterparts of PSR J1141-6545, PSR J1157-5112, PSR J1435-6100, and PSR J1454-5846.

Most, perhaps all, stars go through a phase of vigorous outflow during formation. We examine, through three-dimensional MHD simulation, the effects of protostellar outflows on cluster formation. We find that the initial turbulence in the cluster-forming region is quickly replaced by motions generated by outflows. The protostellar outflow-driven turbulence ("protostellar turbulence" for short) can keep the region close to a virial equilibrium long after the initial turbulence has decayed away. We argue that there exist two types of turbulence in star-forming clouds: a primordial (or "interstellar") turbulence and a protostellar turbulence, with the former transformed into the latter mostly in embedded clusters such as NGC 1333. Since the majority of stars are thought to form in clusters, an implication is that the stellar initial mass function is determined to a large extent by the stars themselves, through outflows that individually limit the mass accretion onto forming stars and collectively shape the environments (density structure and velocity field) in which most cluster members form. We speculate that massive cluster-forming clumps supported by protostellar turbulence gradually evolve toward a highly centrally condensed "pivotal" state, culminating in rapid formation of massive stars in the densest part through accretion.

We present two-dimensional MHD simulations of the radiatively driven outflow from a rotating hot star with a dipole magnetic field aligned with the star's rotation axis. We focus primarily on a model with moderately rapid rotation (half the critical value) and also a large magnetic confinement parameter, η_* ≡ BR/V_∞ = 600. The magnetic field channels and torques the wind outflow into an equatorial, rigidly rotating disk extending from near the Kepler corotation radius outward. Even with fine-tuning at lower magnetic confinement, none of the MHD models produce a stable Keplerian disk. Instead, material below the Kepler radius falls back onto the stellar surface, while the strong centrifugal force on material beyond the corotation escape radius stretches the magnetic loops outward, leading to the episodic breakout of mass when the field reconnects. The associated dissipation of magnetic energy heats material to temperatures of nearly 10⁸ K, high enough to emit hard (several keV) X-rays. Such centrifugal mass ejection represents a novel mechanism for driving magnetic reconnection and seems a very promising basis for modeling X-ray flares recently observed in rotating magnetic Bp stars like σ Ori E.

On the basis of three-dimensional time-dependent numerical simulations, we find that compressible magnetohydrodynamic fluids describing super-Alfvénic, supersonic, and strongly magnetized space and laboratory plasmas decay progressively to a state of near-incompressibility, characterized by a subsonic turbulent Mach number. This transition is mediated dynamically by disparate spectral energy dissipation rates in compressible magnetosonic and shear Alfvénic modes. Dissipation leads to super-Alfvénic turbulent motions decaying to a sub-Alfvénic regime that couples weakly with (magneto-) acoustic cascades. Consequently, the supersonic plasma motion dissipates into highly subsonic motion and density fluctuations experience a passive convection.

Current-free double layers (CFDLs) have been recently discovered in a number of laboratory devices, when a low collisional plasma is forced to expand from a high magnetic field source region to a low magnetic field diffusion region. This experimental setup bears a striking resemblance to the natural conditions prevailing in the magnetic funnels of the solar corona. It was commonly thought that magnetic-field-aligned potential disruptions were driven by electron currents, although the theoretical possibility of a CDFL has been known of for some time. Given its recent experimental verification, we make here a contribution to solar plasma physics by investigating the possibility of CFDLs in coronal funnels, which have much in common with the laboratory experiments. Therefore, CFDLs may play an important role in supplying and accelerating plasma in coronal funnels.