Table of contents

We present new analytic calculations of the coupling between ultraviolet resonance photons and the population of the hyperfine states in the light elements (H, D, and ³He⁺), which include several previously neglected physical processes. Among these are the backreaction of resonant scattering on the pumping radiation, the scattering of Lyβ photons, and the effect of local departure from pure Hubble flow. The application of the new treatment to the redshifted hydrogen 21 cm and deuterium 92 cm lines from the high-redshift universe results in an amplitude correction of up to an order of magnitude. We further show that the standard assumption that ultraviolet pumping drives the spin temperature toward the kinetic temperature does not hold for deuterium, whose spin temperature is generally negative.

In a previous paper, we outlined a new Bayesian method for inferring the properties of extended gravitational lenses given data in the form of resolved images. This method holds the most promise for optimally extracting information from the observed image, while providing reliable uncertainties in all parameters. Here, we apply the method to the well studied optical Einstein ring 0047-2808. Our results are in broad agreement with previous studies, showing that the density profile of the lensing galaxy is aligned within a few degrees of the light profile, and suggesting that the source galaxy (at redshift 3.6) is a binary system, although its size is only of order 1-2 kpc. We also find that the mass of the elliptical lensing galaxy enclosed by the image is (2.91 ± 0.01) × 10¹¹M_☉. Our method is able to achieve improved resolution for the source reconstructions, although we also find that some of the uncertainties are greater than has been found in previous analyses, due to the inclusion of extra pixels and a more general lens model.

We investigate how well the redshift distributions of galaxies sorted into photometric redshift bins can be determined from the galaxy angular two-point correlation functions. We find that the uncertainty in the reconstructed redshift distributions depends critically on the number of parameters used in each redshift bin and the range of angular scales used, but not on the number of photometric redshift bins. Using six parameters for each photometric redshift bin, and restricting ourselves to angular scales over which the galaxy number counts are normally distributed, we find that errors in the reconstructed redshift distributions are large; i.e., they would be the dominant source of uncertainty in cosmological parameters estimated from otherwise ideal weak lensing or baryon acoustic oscillation data. However, either by reducing the number of free parameters in each redshift bin or by (unjustifiably) applying our Gaussian analysis into the non-Gaussian regime, we find that the correlation functions can be used to reconstruct the redshift distributions with moderate precision; e.g., with mean redshifts determined to ~0.01. We also find that dividing the galaxies into two spectral types, thereby doubling the number of redshift distribution parameters, can result in a reduction in the errors in the combined redshift distributions.

Using the deep Spitzer Infrared Array Camera (IRAC) observations of the Great Observatories Origins Deep Survey (GOODS), we study the stellar masses and star formation histories of galaxies at z ≈ 6 based on the i₇₇₅-band dropout sample selected from the GOODS fields. In total, we derive stellar masses for 53 i₇₇₅-band dropouts that have robust IRAC detections. These galaxies have typical stellar masses of ~10¹⁰M_☉ and typical ages of a couple of hundred million years, consistent with earlier results based on a smaller sample of z ≈ 6 galaxies. The existence of such massive galaxies at z ≈ 6 can be explained by at least one set of N-body simulations of the hierarchical paradigm. We also study 79 i₇₇₅-band dropouts that are invisible in the IRAC data and find that they are typically less massive by a factor of 10. These galaxies are much bluer than those detected by the IRAC, indicating that their luminosities are dominated by stellar populations with ages ≲40 Myr. Based on our mass estimates, we derive a lower limit to the global stellar mass density at z ≈ 6, which is 1.1-6.7 × 10⁶M_☉ Mpc^-3. The prospect of detecting the progenitors of the most massive galaxies at yet higher redshifts is explored. We also investigate the implication of our results for reionization and find that the progenitors of the galaxies comparable to those in our sample, even in the most optimized (probably unrealistic) scenario, cannot sustain the reionization for a period longer than ~2 Myr. Thus most of the photons required for reionization must have been provided by other sources, such as the progenitors of the dwarf galaxies that are far below our current detection capability.

We report on the results of a search for an intergalactic X-ray dust scattering halo in a deep observation of the bright, high-redshift quasar QSO 1508+5714 with the Chandra X-Ray Observatory. We do not detect such a halo. Our result implies an upper limit on the density of diffuse, large-grained intergalactic dust of Ω_d < 2 × 10^-6, assuming a characteristic grain size of ~1 μm. The result demonstrates the sensitivity of this technique for detecting very small amounts of intergalactic dust, which are very hard to detect otherwise. This will allow us to put important constraints on systematic effects induced by extinction on the interpretation of the Hubble diagram for Type Ia supernovae, as well as on the amount and properties of cosmological dust being expelled into the intergalactic medium at early (z ≥ 2) times.

We present a detection of [C I] ³P₁-³P₀ emission in the lensed quasar APM 08279+5255 at z = 3.91 using the IRAM Plateau de Bure interferometer. The [C I] line velocity and width are similar to the values of previously detected high-J CO and HCN lines in this source, suggesting that the emission from all of these species arises from the same region. The apparent luminosity of the [C I] line is L = (3.1 ± 0.4) × 10¹⁰ K km s^-1 pc², which implies a neutral carbon mass M = (4.4 ± 0.6)m^-1 × 10⁷M_☉, where m is the lensing magnification factor. The [C I] line luminosity is consistent with the large molecular gas mass inferred from the nuclear CO line luminosity (~10¹¹m^-1M_☉). We also present an upper limit on the H₂O 1₁₀-1₀₁ line luminosity in APM 08279+5255 of L < 1.8 × 10¹⁰ K km s^-1 pc² (3 σ).

We estimate the intrinsic neutral gas density in damped Lyα (DLA) systems (Ω) in the redshift range 2.2 ≲ z ≲ 5 from the SDSS DR3 DLA sample of optically selected quasars. We take into account self-consistently the obscuration on background quasars due to the dust present in damped Lyα systems. We model the column density and redshift distribution of these systems by using both a nonparametric and a parametric approach. Under conservative assumptions on the dust content of DLA systems, we show that selection effects lead to underestimating the intrinsic neutral gas density by at least 15% with respect to the observed neutral gas density. Over the redshift range 2.2-5.5, we find Ω = 0.97 × 10^-3, where the first set of error bars gives the 1 σ random errors and the second set gives the modeling uncertainty dependent on the fraction of metals in dust, from 0% to 50%. This value compares with Ω = 0.82 × 10^-3 (1 σ error bars), which is obtained when no correction for dust is introduced. In the model with half of the metals mass in dust, we cannot constraint Ω at a confidence level higher than 90%. In this case there is indeed a probability of about 10% that the intrinsic column density distribution of DLA systems is a power law f(N) ∝ 1/N. In contrast, with 25% of the metals in dust, the most realistic estimate, a power law is ruled out at a 99.5% confidence level.

With close pairs of quasars at different redshifts, a background quasar sight line can be used to study a foreground quasar's environment in absorption. We search 149 moderate-resolution background quasar spectra from Gemini, Keck, the MMT, and the SDSS to survey Lyman limit systems (LLSs) and damped Lyα systems (DLAs) in the vicinity of 1.8 < z < 4.0 luminous foreground quasars. A sample of 27 new quasar-absorber pairs is uncovered with column densities 10^17.2 cm^-2 < N < 10^20.9 cm^-2 and transverse (proper) distances of 22 h^-1 kpc < R < 1.7 h^-1 Mpc from the foreground quasars. If they emit isotropically, the implied ionizing photon fluxes are a factor of ~5-8000 times larger than the ambient extragalactic UV background over this range of distances. The observed probability of intercepting an absorber is very high for small separations: six out of eight projected sight lines with transverse separations R < 150 h^-1 kpc have an absorber coincident with the foreground quasar, of which four have N > 10¹⁹ cm^-2. The covering factor of N > 10¹⁹ cm^-2 absorbers is thus ~50% (4/8) on these small scales, whereas ≲2% would have been expected at random. There are many cosmological applications of these new sight lines: they provide laboratories for studying fluorescent Lyα recombination radiation from LLSs; they constrain the environments, emission geometry, and radiative histories of quasars; and they shed light on the physical nature of LLSs and DLAs.

We discovered Hα absorption in the broad Hα emission line of an unusual broad absorption line quasar, SDSS J083942.11+380526.3, at z = 2.318, through near-infrared spectroscopy with the Cooled Infrared Spectrograph and Camera for OHS (CISCO) on the Subaru telescope. The presence of nonstellar Hα absorption is known only in the Seyfert galaxy NGC 4151 to date; thus, our discovery is the first case for quasars. The Hα absorption line is blueshifted by 520 km s^-1 relative to the Hα emission line, and its redshift almost coincides with those of UV low-ionization metal absorption lines. The width of the Hα absorption (~340 km s^-1) is similar to those of the UV low-ionization absorption lines. These facts suggest that the Hα and low-ionization metal absorption lines are produced by the same low-ionization gas, which has a substantial amount of neutral gas. The column density of the neutral hydrogen is estimated to be ~10¹⁸ cm^-2 by assuming a gas temperature of 10,000 K from the analysis of the curve of growth. The continuum spectrum is reproduced by a reddened [E(B - V) ~ 0.15 mag for the SMC-like reddening law] composite quasar spectrum. Furthermore, the UV spectrum of SDSS J083942.11+380526.3 shows a remarkable similarity to that of NGC 4151 in its low state, suggesting that the physical condition of the absorber in SDSS J083942.11+380526.3 is similar to that of NGC 4151 in the low state. As proposed for NGC 4151, SDSS J083942.11+380526.3 may also be seen through the edge of the obscuring torus.

We present a three-dimensional study of the local (≤100 h^-1 kpc) and the large scale (≤1 h^-1 Mpc) environment of bright IRAS balaxies (BIRGs). For this purpose we use 87 BIRGs located at high Galactic latitudes (with 0.008 ≤ z ≤ 0.018), as well as a control sample of nonactive galaxies having the same morphological, redshift, and diameter size distributions as the corresponding BIRG sample. Using the Center for Astrophysics and the Southern Sky Redshift Survey galaxy catalogs (m_b ≲ 15.5), as well as our own spectroscopic observations (m_b ≲ 19.0), for a subsample of the original BIRG sample, we find that the fraction of BIRGs with a close neighbor is significantly higher than that of their control sample. Comparing with a related analysis of Seyfert 1 and Seyfert 2 galaxies by Koulouridis and coworkers, we find that BIRGs have a similar environment to that of Seyfert 2 galaxies, although the fraction of BIRGs with a bright, close neighbor is even higher than that for Seyfert 2 galaxies. An additional analysis of the relation between FIR colors and the type of activity of each BIRG shows a significant difference between the colors of strongly interacting and noninteracting starbursts and a resemblance between the colors of noninteracting starbursts and Seyfert 2 galaxies. Our results support the view that close interactions can drive molecular clouds toward the galactic center, triggering starburst activity and obscuring the nuclear activity. When the close neighbor moves away, starburst activity is reduced with the simultaneous appearance of an obscured (type 2) active galactic nucleus (AGN). Finally, the complete disentanglement of the pair gives birth to an unobscured (type 1) AGN.

Spectra have been obtained with the low-resolution modules of the Infrared Spectrograph (IRS) on the Spitzer Space Telescope (Spitzer) for 58 sources having f_ν(24 μm) > 0.75 mJy. Sources were chosen from a survey of 8.2 deg² within the NOAO Deep Wide-Field Survey region in Boötes (NDWFS) using the Multiband Imaging Photometer (MIPS) on Spitzer. Most sources are optically very faint (I > 24 mag). Redshifts have previously been determined for 34 sources, based primarily on the presence of a deep 9.7 μm silicate absorption feature, with a median z of 2.2. Spectra are presented for the remaining 24 sources for which we were previously unable to determine a confident redshift because the IRS spectra show no strong features. Optical photometry from the NDWFS and infrared photometry with MIPS and the Infrared Array Camera on Spitzer (IRAC) are given, with K photometry from the Keck I telescope for some objects. The sources without strong spectral features have overall spectral energy distributions (SEDs) and distributions among optical and infrared fluxes that are similar to those for the sources with strong absorption features. Nine of the 24 sources are found to have feasible redshift determinations based on fits of a weak silicate absorption feature. Results confirm that the "1 mJy" population of 24 μm Spitzer sources that are optically faint is dominated by dusty sources with spectroscopic indicators of an obscured AGN rather than a starburst. There remain 14 of the 58 sources observed in Boötes for which no redshift could be estimated, and 5 of these sources are invisible at all optical wavelengths.

The emission spectra of TeV blazars extend up to tens of TeV, and the emission mechanism of the TeV γ-rays is explained by synchrotron self-Compton scattering in leptonic models. In these models the time variabilities of X-rays and TeV γ-rays are correlated. However, recent observations of 1ES 1959+65.0 and Mrk 421 have found the "orphan" TeV γ-ray flares, i.e., TeV γ-ray flares without simultaneous X-ray flares. In this paper we propose a model for the orphan TeV γ-ray flares, employing an inhomogeneous leptonic jet model. After a primary flare that accompanies flare-up both in X-rays and TeV γ-rays, radiation propagates in various directions in the comoving frame of the jet. When a dense region in the jet receives the radiation, X-rays are scattered by relativistic electrons/positrons to become TeV γ-rays. These γ-ray photons are observed as an orphan TeV γ-ray flare. The observed delay time between the primary and orphan flares is about 2 weeks, and this is accounted for in our model for parameters such as Γ = 20, d = 4 × 10¹⁷ cm, α = 3, and η = 1, where Γ is the bulk Lorentz factor of the jet, d is the distance between the central black hole and the primary flare site, α/Γ is the angle between the jet axis and the direction of the motion of the dense region that scatters incoming X-rays produced by the primary flare, and η/Γ is the angle between the jet axis and the line of sight.

We characterize the mass-dependent evolution of more than 8000 galaxies using spectroscopic redshifts from the DEEP2 Galaxy Redshift Survey in the range 0.4 < z < 1.4 and stellar masses calculated from K-band photometry obtained at Palomar Observatory. This sample spans more than 1.5 deg² in four independent fields. Using rest-frame U - B color and [O II] equivalent widths, we distinguish star-forming from passive populations in order to explore the nature of ``downsizing''—a pattern in which the sites of active star formation shift from high-mass galaxies at early times to lower mass systems at later epochs. We identify a mass limit, M_Q, above which star formation appears to be quenched and show that the physical mechanisms responsible for downsizing can thus be empirically quantified by charting the evolution in this threshold mass. We find that M_Q decreases with time by a factor of ~3 across our redshift range according to M_Q ∝ (1 + z)^3.5. To further constrain possible quenching mechanisms, we investigate how downsizing depends on local galaxy environment using the projected third-nearest-neighbor statistic D_p,3. For the majority of galaxies near the median density, there is no significant correlation between downsizing and environment. However, a trend is observed in the comparison between environments that are more than 3 times overdense or underdense relative to the median. Here, downsizing appears accelerated in overdense regions that host higher numbers of massive, early-type galaxies as compared to the underdense regions. Our results significantly constrain recent suggestions for the origin of downsizing and indicate that the process for quenching star formation must, primarily, be internally driven.

Strong constraints on the cosmic star formation history (SFH) have recently been established using ultraviolet and far-infrared measurements, refining the results of numerous measurements over the past decade. The data show a compellingly consistent picture of the SFH out to redshift z ≈ 6, with especially tight constraints for z ≲ 1. We fit these data with simple analytical forms and derive conservative uncertainties. Since the z ≲ 1 SFH data are quite precise, we investigate the sequence of assumptions and corrections that together affect the SFH normalization to test their accuracy, both in this redshift range and beyond. As lower limits on this normalization, we consider the evolution in stellar and metal mass densities, and supernova rate density, finding it unlikely that the SFH normalization is much lower than indicated by our direct fit. As a corresponding upper limit on the SFH normalization, we consider the Super-Kamiokande limit on the electron antineutrino (_e) flux from past core-collapse supernovae, which applies primarily to z ≲ 1. We find consistency with the SFH only if the neutrino temperatures from supernova events are relatively modest. Constraints on the assumed initial mass function (IMF) also become apparent. The traditional Salpeter IMF, assumed for convenience by many authors, is known to be a poor representation at low stellar masses (≲1 M_☉), and we show that recently favored IMFs are also constrained. In particular, somewhat shallow, or top-heavy, IMFs may be preferred, although they cannot be too top-heavy. To resolve the outstanding issues, improved data are called for on the supernova rate density evolution, the ranges of stellar masses leading to core-collapse and type Ia supernovae, and the antineutrino and neutrino backgrounds from core-collapse supernovae.

We study whether integrated optical spectroscopy of a disk galaxy can be used to infer the mean or characteristic gas-phase oxygen abundance in the presence of systematic effects such as spatial abundance variations, contributions to the integrated emission-line spectrum from diffuse ionized gas, and dust attenuation. Our sample consists of 14 nearby disk galaxies with integrated spectrophotometry, and observations of more than 250 individual H II regions culled from the literature. We consider both theoretical and empirical strong-line abundance calibrations based on the R₂₃ ≡ ( + )/Hβ parameter. We find that the integrated oxygen abundance correlates well with the gas-phase abundance measured at a fixed galactocentric radius, as determined by the H II region abundance gradient. The typical scatter in the correlation is ±0.1 dex, independent of the abundance calibration or whether the observed integrated emission-line fluxes, the reddening-corrected fluxes, or the emission-line equivalent widths are used. Integrated abundances based on the observed fluxes or equivalent widths, however, are susceptible to additional systematic effects of order 0.05-0.1 dex, at least for the range of reddenings and stellar populations spanned by our sample. Unlike the integrated R₂₃ parameter, we find that the integrated [N II]/Hα and [S II]/Hα ratios are enhanced with respect to line ratios typical of H II regions, consistent with a modest contribution from diffuse ionized gas emission. We conclude that the R₂₃ parameter can be used to reliably measure the gas-phase abundances of distant star-forming galaxies.

The latest Sloan Digital Sky Survey data reveal a prominent bifurcation in the distribution of debris of the Sagittarius dwarf spheroidal (Sgr) beginning at a right ascension of α ≈ 190^°. Two branches of the stream (A and B) persist at roughly the same heliocentric distance over at least 50° of arc. There is also evidence for a more distant structure (C) well behind the A branch. This paper provides the first explanation for the bifurcation. It is caused by the projection of the young leading (A) and old trailing (B) tidal arms of the Sgr, while the old leading arm (C) lies well behind A. This explanation is only possible if the halo is close to spherical, as the angular difference between the branches is a measure of the precession of the orbital plane.

We investigate the properties of mid- to far-infrared emission in the Large Magellanic Cloud (LMC) based on the COBE DIRBE and IRAS Sky Survey Atlas data sets. We focus on the properties of far-infrared (FIR) thermal emission carried by submicron dust and mid-infrared (MIR) excess emission by very small grains in the LMC in comparison to those in the Milky Way. We carefully estimate the dust temperature by decomposing the various structures including young star clusters, supergiant shells, and CO molecular clouds and examine the MIR to FIR spectral energy distribution (SED) associated with each structure in the LMC. The amount of the MIR excess emission in 12 and 25 μm relative to the total FIR emission (F_FIR) in the LMC is generally smaller than that in the Milky Way, which confirms the scarcity of very small grains suggested by past studies on the extinction curve. The characteristics of 25 and 60 μm excess emission proportional to the square of the incident radiation field strength appear around young star clusters in the LMC. This sequence can be reproduced by the superposition of dust emission under different radiation field strengths ranging from 1 to 10⁵ times of that in the solar vicinity. Finally, the MIR to FIR SEDs in CO molecular clouds in the LMC are discussed. Through these attempts, we succeed in completely explaining the characteristics of mid- to far-infrared SEDs given by the infrared broadband photometric data sets in terms of the superposition of the standard stochastic heating dust model.

In order to provide a firm identification of the newly discovered Galactic TeV sources, a search for counterparts in a broad band from soft X-ray to soft gamma rays is crucial as data in these wave bands allow us to distinguish between different types of suggested models (for example, leptonic vs. hadronic) and, in turn, to disentangle their nature. In this paper we report the results of a set of follow-up observations performed by the Swift X-Ray Telescope (XRT) on seven sources recently discovered by HESS, in the range from a few hundred GeV to about 10 TeV, during the inner Galaxy survey. In all but one case, we detect X-ray sources inside or close by the extended TeV-emitting region. All of these putative X-ray counterparts have an accurate arcsecond location and are consistent with being point sources. The main result of our search is the discovery that three of them are located at the center of the diffuse radio emission of the supernova remnants, which have been spatially associated with these TeV objects. HESS J1640-465, HESS J1834-087, and HESS J1813-178 show this evidence, suggestive of a possible pulsar wind nebula association.

We present a catalog of 1888 fundamental-mode RR Lyrae stars detected in the Galactic bulge fields of the second phase of the Optical Gravitational Lensing Experiment (OGLE). The catalog includes basic parameters of the light curves, identifications of Blazhko frequencies, V - I colors at minimum light (for most stars), and other information for each star. We detect a high rate of incidence of the Blazhko phenomenon (at least 27.6%), including unprecedentedly many frequency triplets, which we attribute to our sensitive search method. We find that the minimum light V - I color (useful as a reddening indicator) grows slowly redder with increasing period and exhibits a star-to-star scatter of approximately 0.07 mag. We use this color to evaluate the zero-point accuracy of the reddening map of the Galactic bulge derived from OGLE data and find that in addition to low-level random errors or resolution effects (responsible for much of the scatter), the map may systematically overrepresent E(V - I) by approximately 0.05 mag in most fields. We present reasonably robust evidence that the RR Lyrae-to-red clump color separation is larger by 0.05-0.08 mag in the bulge than locally, which argues for caution in the use of these stars for reddening determinations. We consider the RR Lyrae constraint on the Galactocentric distance, but uncertainty about the absolute magnitude calibration leaves significant flexibility in the result. In contrast to previous results, we robustly detect the signature of the Galactic bar in the RR Lyrae population within the inner ±3° of longitude, and we highlight the apparent differences between the structures traced by the red clump giants and the more metal-poor RR Lyrae stars.

A Chapman-Kolmogorov equation description of compound transport of cosmic rays due to random walk of the magnetic field lines, and for a range of models for particle transport along the field, is developed. The probability distribution, P_p, for the particle propagation along the field corresponds to either (1) a ballistic or scatter-free model, (2) a parallel diffusion model, or (3) a telegrapher equation model. The probability distribution function (pdf) describing the magnetic field statistics, P_FRW, is assumed to be Gaussian. These models are used to discuss features of the dropout events in the low-energy, solar cosmic-ray intensity observed by Mazur et al. We show that the Chuvilgin and Ptuskin transport equation for compound diffusion, at sufficiently late times, can be written as a fractional Fokker-Planck equation, involving ordinary diffusion parallel to the mean magnetic field and compound diffusion of the particles normal to the field. The Green's function solution of the equation and the corresponding spatial moments of the particle transport, both parallel and perpendicular to the field, are obtained. The two-dimensional pdf for compound diffusion across the field is obtained as an inverse Laplace transform, or as a real integral.

We present a Chandra ACIS observations of N157B, a young supernova remnant (SNR) located in the 30 Doradus star formation region of the Large Magellanic Cloud. This remnant contains the most energetic pulsar known (PSR J053747.39-691020.2; Ė = 4.8 × 10³⁸ ergs s^-1), which is surrounded by a X-ray-bright nonthermal nebula that likely represents a toroidal pulsar wind terminal shock observed edge-on. Two of the eight pointlike X-ray sources detected in the observation are shown to have near-IR and optical counterparts (within 05 offsets), which are identified as massive stellar systems in the Cloud. We confirm the nonthermal nature of the comet-shaped X-ray emission feature and show that the spectral steepening of this feature away from the pulsar is quantitatively consistent with synchrotron cooling of shocked pulsar wind particles flowing downstream at a bulk velocity close to the speed of light. Around the cometary nebula we unambiguously detect a spatially resolved thermal component, which accounts for about 1/3 of the total 0.5-10 keV flux from the remnant. This thermal component is distributed among various clumps of metal-enriched plasma embedded in the low surface brightness X-ray-emitting diffuse gas. The relative metal enrichment pattern suggests that the mass of the supernova progenitor is ≳20 M_☉. A comparison of the X-ray data with Hubble Space Telescope optical images now suggests that the explosion site is close to a dense cloud, against which a reflection shock is launched. The interaction between the reflected material and the nebula has likely produced both its cometary shape and the surrounding thermal emission enhancement. SNR N157B is apparently expanding into the hot low-density interior of the surrounding superbubble formed by the young OB association LH 99, as revealed by Spitzer mid-infrared images. This scenario naturally explains the exceptionally large sizes of both the thermal and nonthermal components, as well as the lack of an outer shell of the SNR. However, the real situation in the region is likely to be more complicated. We find that a partially round soft X-ray-emitting clump with distinct spectral properties may result from a separate oxygen-rich remnant. These results provide a rare glimpse into the SNR structure and evolution in a region of recent star formation.

We present high-resolution X-ray spectra from the young supernova remnant Cas A using a 70 ks observation taken by the Chandra High Energy Transmission Grating Spectrometer (HETGS). Line emission, dominated by Si and S ions, is used for high-resolution spectral analysis of many bright, narrow regions of Cas A to examine their kinematics and plasma state. These data allow a three-dimensional (3D) reconstruction using the unprecedented X-ray kinematic results: we derive unambiguous Doppler shifts for these selected regions, with values ranging between -2500 and +4000 km s^-1 and the typical velocity error less than 200 km s^-1. Plasma diagnostics of these regions, derived from line ratios of resolved He-like triplet lines and H-like lines of Si, indicate temperatures largely around 1 keV, which we model as O-rich reverse-shocked ejecta. The ionization age also does not vary considerably over these regions of the remnant. The gratings analysis was complemented by the nondispersed spectra from the same data set, which provided information on emission measure and elemental abundances for the selected Cas A regions. The derived electron density of X-ray emitting ejecta varies from 20 to 200 cm^-3. The measured abundances of Mg, Si, S, and Ca are consistent with O being the dominant element in the Cas A plasma. With a diameter of 5', Cas A is the largest source observed with the HETGS to date. We therefore describe the technique we use and some of the challenges we face in the HETGS data reduction from such an extended, complex object.

Aliphatic hydrocarbons exhibit an absorption feature at 3.4 μm, observed toward sources that sample diffuse regions of the interstellar medium (ISM). The absorbers responsible for this feature are assumed to reside in some component of interstellar dust, but the physical nature of the particles (size, shape, structure, etc.) is uncertain. Observations of interstellar polarization provide discrimination. Since the grains that carry the silicate absorption feature are known to be aligned, polarization across the 3.4 μm hydrocarbon feature can be used to test the silicate-core organic refractory mantle grain theory. Although the 3.4 μm feature has been observed to be devoid of polarization for one line of sight toward the Galactic center, a corresponding silicate polarization measurement for the same line of sight was not available. Here, we present spectropolarimetric observations of GCS 3-II and GCS 3-IV toward the Galactic center, where the 9.7 μm silicate polarization has been previously observed. We show that polarization is not detected across the 3.4 μm feature to a limit of 0.06% ± 0.13% (GCS 3-II) and 0.15% ± 0.31% (GCS 3-IV), well below the lowest available prediction of polarization on the basis of the core-mantle model. We conclude that the hydrocarbons in the diffuse ISM do not reside on the same grains as the silicates, and likely form a separate population of small grains.

We investigate the influence of the accretion disk magnetic flux profile on the collimation of jets applying axisymmetric MHD simulations. We use the ZEUS-3D code modified for magnetic diffusivity. Our simulations evolve from an initial configuration in hydrostatic equilibrium in a force-free magnetic field. Considering a power law for the disk poloidal magnetic field profile B_P ~ r^-μ and for the disk wind density profile ρ ~ r, we perform a systematic study over a wide range of parameters μ and μ_ρ. We find the degree of collimation (quantified by the axial vs. lateral mass flux ratio) decreasing for steepening disk magnetic field profiles. Variation of the total magnetic flux does not affect the collimation degree substantially but changes the timescale of outflow evolution and the terminal jet speed. Our major result is a general relation between the collimation degree and the disk wind magnetization power-law exponent. Highly collimated outflows resulting from a flat disk magnetic field profile tend to be unsteady, producing axially propagating knots. Slightly depending on the disk wind density profile, this behavior sets in for μ < 0.4. We also perform simulations with artificially enhanced decay of the toroidal magnetic field component probing the idea of a "poloidal jet collimation" previously raised in the literature. These outflows remain weakly collimated and propagate slower. Our large numerical grid (7 × 14 AU for protostars) allows us to compare our simulations to observationally indicated jet rotation, suggesting a flat disk magnetic field profile, μ ≃ 0.5, in DG Tau. Our general results are applicable to both stellar and extragalactic sources of MHD jets.

Asymptotic giant branch (AGB) stars generally have spherically symmetric envelopes, whereas most post-AGB stars and planetary nebulae (PNe) show axisymmetric circumstellar envelopes. While various mechanisms for axisymmetric circumstellar structures may explain the shapes of PNe, they do not address how the shape of the circumstellar shell evolves. Here we address the temporal changes in the axisymmetry of AGB star envelopes, and in particular the development of the torus required in the generalized interacting stellar winds (GISW) model. Assuming that (1) an AGB star rotates with sufficient angular speed at the start of the AGB phase, and (2) the rotational angular momentum of the AGB star is conserved, we demonstrate that some very important observational features of AGB star axisymmetry evolution can be reproduced. We find that, compared to the star's increasing luminosity and decreasing effective temperature, the decreasing mass of the star primarily affects the axisymmetry of the envelope. When a representative mass-loss history is adopted in which most of the mass is lost near the end of the AGB phase, the envelope's axisymmetry increases over time, with the strongest increase occurring near the end of the AGB phase. This may naturally explain why most AGB stars have spherically symmetric envelopes, while axisymmetry seems commonplace in the post-AGB/PNe phase. The degree of axisymmetry at the end of the AGB phase is found to increase with increasing main-sequence mass, and the onset of axisymmetry occurs only after the onset of the superwind phase, in good agreement with the observations.

High-resolution mid-infrared images have been obtained in N and Q band for the proto-planetary nebula IRAS 16594-4656. A bright equatorial torus and a pair of bipolar lobes can clearly be seen in the infrared images. The torus appears thinner at the center than at the edges, suggesting that it is viewed nearly edge-on. The infrared lobes correspond to the brightest lobes of the reflection nebula seen in the Hubble Space Telescope (HST) optical image, but with no sign of the point-symmetric structure seen in the visible image. The lobe structure shows a close correspondence with a molecular hydrogen map obtained with HST, suggesting that the dust emission in the lobes traces the distribution of the shocked gas. The shape of the bipolar lobes shows clearly that the fast outflow is still confined by the remnant circumstellar envelope of the progenitor asymptotic giant branch (AGB) star. However, the nondetection of the dust outside of the lobes suggests that the temperature of the dust in the AGB envelope is too low for it to be detected at 20 μm.

The embedded cores L1448 IRS 3, NGC 1333 IRAS 2, and NGC 1333 IRAS 4 are mapped in emission from the C¹⁸O, H¹³CO⁺, and N₂H⁺J = 1 → 0 transitions. The maps are created by combining BIMA and FCRAO observations and are tuned to resolutions of ~50'', 10'', 5'', and 3''. The higher resolution maps reveal emission structures that are considerably smaller than the characteristic core radius (~0.1 pc) identified in earlier single-dish studies. We focus our study on the kinematics of the envelope material traced by the emission lines. We find that although the FCRAO data show relatively smooth velocity gradients across the cores, the velocity fields seen with higher resolution are more random, with central velocities varying over a range of ~1 km s^-1. In general, the distribution of velocities, as well as the complexity of the fields, increases with resolution. To analyze variations in the widths of the emission lines, we employ a method of gridding the datacubes that was initially developed to quantify properties of turbulent cloud models. The cores exhibit a broad range of line widths even at the smallest measurable scales. Pure thermal broadening at the prevailing envelope temperatures (T ≈ 20 K) is insufficient to produce the measured line widths; the narrowest lines must have a turbulent component at least as great as the thermal component, and for nearly all lines, the turbulent component makes the dominant contribution. Our results suggest that turbulent motions persist down to subcore scales of at least 2400 AU.

We present high-resolution 045 × 032 observations from the BIMA array toward the Herbig Ae system MWC 480 in the λ = 1.4 mm dust continuum. We resolve a circumstellar disk of radius ~170 AU and constrain the disk parameters by comparing the observations to flat-disk models. These results show that the typical fit parameters of the disk, such as the mass, M_D ~ 0.04-0.18 M_☉, and the surface density power-law index, p = 0.5 or 1, are comparable to those of the lower mass T Tauri stars. The dust in the MWC 480 disk can be modeled as processed dust material (β ≈ 0.8), similar to the Herbig Ae star CQ Tau disk; the fitted disk parameters are also consistent with less evolved dust (β ≈ 1.2). The possibility of grain growth in the MWC 480 circumstellar disk is supported by the acceptable fits with β ≈ 0.8. The surface density power-law profiles of p = 0.5 and p = 1 can be easily fitted to the MWC 480 disk; however, a surface density power-law profile similar to the minimum mass solar nebula model p = 1.5 is ruled out at an 80% confidence level.

Gamma-ray burst (GRB) afterglow emission is believed to be produced by synchrotron emission of electrons accelerated to high energy by a relativistic collisionless shock propagating into a weakly magnetized plasma. Afterglow observations have been used to constrain the postshock magnetic field and structure, as well as the accelerated electron energy distribution. Here we show that X-ray afterglow observations on day timescales constrain the preshock magnetic field to satisfy B > 0.2(n/1 cm^-3)^5/8 mG, where n is the preshock density. This suggests that either the shock propagates into a highly magnetized fast, v ~ 10³ km s^-1, wind, or the preshock magnetic field is strongly amplified, most likely by the streaming of high-energy shock-accelerated particles. More stringent constraints may be obtained by afterglow observations at high photon energy at late, >1 day, times.

We consider the interaction of a relativistically moving shell, composed of thermal photons, a reversing magnetic field, and a small admixture of charged particles, with a dense Wolf-Rayet wind. A thin layer of Wolf-Rayet material is entrained at the head of this outflow; it cools and becomes Rayleigh-Taylor unstable, thereby providing an additional source of inertia and variability. The gamma rays streaming across the forward shock load the wind material with electron-positron pairs and push it to relativistic speeds close to the engine. This defines a characteristic radiative compactness at the point where the reverse shock has completed its passage back through the shell. We argue that the prompt gamma-ray emission is triggered by this external braking, at an optical depth ~1 to electron scattering. Torsional MHD waves, excited by the forced reconnection of the reversing magnetic field, carry a fluctuating current and are Landau damped at high frequencies on the parallel motion of the light charges. We show that the heated charges cool primilarly by inverse Compton radiation, which is beamed along the magnetic field. Thermal radiation that is advected out from the base of the jet cools the particles. The observed relation between peak energy and isotropic luminosity—both its amplitude and scaling—is reproduced if the blackbody seeds are generated in a relativistic jet core that is subject to Kelvin-Helmholtz instabilities with the Wolf-Rayet envelope. This relation is predicted to soften below an isotropic luminosity L_iso ~ 3 × 10⁵⁰ ergs s^-1. Spectrally harder bursts will arise in outflows which encounter no dense stellar envelope. The duration of spikes in the inverse-Compton emission is narrower at higher frequencies, as observed. The transition from prompt gamma-ray emission to afterglow can be explained by the termination of the thermal X-ray seed and the onset of synchrotron-self-Compton emission.

We discuss Monte Carlo techniques for addressing the three-dimensional time-dependent radiative transfer problem in rapidly expanding supernova atmospheres. The transfer code SEDONA has been developed to calculate the light curves, spectra, and polarization of aspherical supernova models. From the onset of free expansion in the supernova ejecta, SEDONA solves the radiative transfer problem self-consistently, including a detailed treatment of gamma-ray transfer from radioactive decay and with a radiative equilibrium solution of the temperature structure. Line fluorescence processes can also be treated directly. No free parameters need be adjusted in the radiative transfer calculation, providing a direct link between multidimensional hydrodynamic explosion models and observations. We describe the computational techniques applied in SEDONA and verify the code by comparison to existing calculations. We find that convergence of the Monte Carlo method is rapid and stable even for complicated multidimensional configurations. We also investigate the accuracy of a few commonly applied approximations in supernova transfer, namely, the stationarity approximation and the two-level atom expansion opacity formalism.

The presumed Wolf-Rayet star progenitors of Type Ib/c supernovae have fast, low-density winds, and the shock waves generated by the supernova interaction with the wind are not expected to be radiative at typical times of observation. The injected energy spectrum of radio-emitting electrons typically has an observed index p = 3, which is suggestive of acceleration in cosmic-ray-dominated shocks. The early, absorbed part of the radio light curves can be attributed to synchrotron self-absorption, which leads to constraints on the magnetic field in the emitting region and on the circumstellar density. The range of circumstellar densities inferred from the radio emission is somewhat broader than that for Galactic Wolf-Rayet stars, if similar efficiencies of synchrotron emission are assumed in the extragalactic supernovae. For the observed and expected ranges of circumstellar densities to roughly overlap, a high efficiency of magnetic field production in the shocked region is required (_B ≈ 0.1). For the expected densities around a Wolf-Rayet star, a nonthermal mechanism is generally required to explain the observed X-ray luminosities of Type Ib/c supernovae. Inverse Compton emission is a candidate for the emission, if the observations are near optical maximum. In other cases we suggest that the mechanism is X-ray synchrotron emission in a situation in which the shock wave is cosmic-ray-dominated so that the electron energy spectrum flattens at high energy. More comprehensive X-ray observations of a Type Ib/c supernova are needed to determine whether this suggestion is correct.

We use full three-body scattering experiments to study the ejection of hypervelocity stars (HVSs) by massive black hole binaries (MBHBs) at the centers of galaxies. Ambient stars drawn from a Maxwellian distribution unbound to the binary are expelled by the gravitational slingshot. Accurate measurements of thermally averaged hardening, mass ejection, and eccentricity growth rates (H, J, and K) for MBHBs in a fixed stellar background are obtained by numerical orbit integration from initial conditions determined by Monte Carlo techniques. Three-body interactions create a subpopulation of HVSs on nearly radial orbits, with a spatial distribution that is initially highly flattened in the inspiral plane of the MBHB, but becomes more isotropic with decreasing binary separation. The degree of anisotropy is smaller for unequal mass binaries and larger for stars with higher kick velocities. Eccentric MBHBs produce a more prominent tail of high-velocity stars and break planar symmetry, ejecting HVSs along a broad jet perpendicular to the semimajor axis. The jet two-sidedness decreases with increasing binary mass ratio, while the jet opening angle increases with decreasing kick velocity and orbital separation. The detection of a numerous population of HVSs in the halo of the Milky Way by the next generation of large astrometric surveys such as Gaia may provide a unique signature of the history, nature, and environment of the MBH at the Galactic center.

We present simultaneous multicolor infrared and optical photometry of the black hole X-ray transient XTE J1118+480 during its short 2005 January outburst, supported by simultaneous X-ray observations. The variability is dominated by short timescales, ~10 s, although a weak superhump also appears to be present in the optical. The optical rapid variations, at least, are well correlated with those in X-rays. Infrared JHK_s photometry, as in the previous outburst, exhibits especially large-amplitude variability. The spectral energy distribution (SED) of the variable infrared component can be fitted with a power law of slope α = -0.78 ± 0.07, where F_ν ∝ ν^α. There is no compelling evidence for evolution in the slope over five nights, during which time the source brightness decayed along almost the same track as seen in variations within the nights. We conclude that both short-term variability and longer timescale fading are dominated by a single component of constant spectral shape. We cannot fit the SED of the IR variability with a credible thermal component, either optically thick or thin. This IR SED is, however, approximately consistent with optically thin synchrotron emission from a jet. These observations therefore provide indirect evidence to support jet-dominated models for XTE J1118+480 and also provide a direct measurement of the slope of the optically thin emission, which is impossible, based on the average spectral energy distribution alone.

We present first results of a campaign to find and identify new compact objects in the Galactic center. Selecting candidates from a combination of Chandra and 2MASS survey data, we search for accretion disk signatures via infrared spectroscopy. We have found the infrared counterpart to the Chandra source CXO J174536.1-285638, the spectrum of which has strong Brγ and He I emission. The presence of C III, N III, and He II indicate a binary system. We suspect that the system is some form of high-mass binary system, either a high-mass X-ray binary or a colliding-wind binary.

We report on the spectral analysis of the neutron star (atoll-type) low-mass X-ray binary 4U 1636-53 as observed by the INTEGRAL and BeppoSAX satellites. The spectral behavior in three different epochs, corresponding to three different spectral states, has been investigated in depth. Spectra from two data sets show a continuum that is well described by one or two soft blackbodies plus a Comptonized component, with changes in the Comptonizing electrons and blackbody temperature and accretion rate that are typical of the spectral transition from high to low state. On one occasion the INTEGRAL spectrum shows, for first time in this source, a hard tail dominating the emission above 30 keV. The total spectrum is fitted as the sum of a Comptonized component similar to the soft state and a power-law component (Γ = 2.76), indicating the presence of a nonthermal distribution of electron velocities. In this case, a comparison with the hard tails detected in soft states from neutron star systems and some black hole binaries suggests that a similar mechanism could give rise to these components in both cases.

We present results from quantitative modeling and spectral analysis of the high-mass X-ray binary system Vela X-1 obtained with the Chandra HETGS. The spectra exhibit emission lines from H- and He-like ions driven by photoionization, as well as fluorescent emission lines from several elements in lower charge states. The properties of these X-ray lines are measured with the highest accuracy to date. In order to interpret and make full use of the data, we have developed a simulator, which calculates the ionization structure of a stellar wind and performs Monte Carlo simulations of X-ray photons propagating through the wind. From comparisons of the observed spectra with results from the simulator, we are able to find the ionization structure and the geometrical distribution of material in the stellar wind that can reproduce the observed spectral line intensities and continuum shapes remarkably well. We find that the stellar wind profile can be represented by a CAK model with a star mass-loss rate of (1.5-2.0) × 10^-6M_☉ yr^-1, assuming a terminal velocity of 1100 km s^-1. It is found that a large fraction of emission lines from highly ionized ions are formed in the region between the neutron star and the companion star. We also find that the fluorescent lines must be produced in at least three distinct regions: the extended stellar wind, reflection off the stellar photosphere, and in a distribution of dense material partially covering and possibly trailing the neutron star, which may be associated with an accretion wake. Finally, from detailed analysis of the emission-line profiles, we demonstrate that the stellar wind dynamics is affected by X-ray photoionization.

We show that enhanced extra mixing in low-mass red giants can result in a fluorine abundance that is correlated with abundance variations of other elements participating in H burning, such as C, N, O, and Na. This finding is used to explain the fluorine abundance variations recently found in bright red giants of the globular cluster M4.

We present the results of a spectroscopic survey of the Ca II H and K core strengths in a sample of 60 solar-type stars that are members of the solar-age and solar-metallicity open cluster M67. We adopt the HK index, defined as the summed H+K core strengths in 1 Å bandpasses centered on the H and K lines, respectively, as a measure of the chromospheric activity that is present. We compare the distribution of mean HK index values for the M67 solar-type stars with the variation of this index as measured for the Sun during the contemporary solar cycle. We find that the stellar distribution in our HK index is broader than that for the solar cycle. Approximately 17% of the M67 Sun-like stars exhibit average HK indices that are less than solar minimum. About 7%-12% are characterized by relatively high activity in excess of solar maximum values, while 72%-80% of the solar analogs exhibit Ca II H+K strengths within the range of the modern solar cycle. The ranges given reflect uncertainties in the most representative value of the maximum in the HK index to adopt for the solar cycle variations observed during the period AD 1976-2004. Thus, ~20%-30% of our homogeneous sample of Sun-like stars have mean chromospheric H+K strengths that are outside the range of the contemporary solar cycle. Any cycle-like variability that is present in the M67 solar-type stars appears to be characterized by periods greater than ~6 yr. Finally, we estimate a mean chromospheric age for M67 in the range of 3.8-4.3 Gyr.

We have investigated the "universality" of the twisted-torus equilibrium structures of the magnetic fields in deformed stars. The Tomimura-Eriguchi scheme for equilibrium structures of uniformly rotating magnetized stars is extended to treat equilibrium configurations of differentially rotating and meridional circulation-free magnetized polytropes with infinite conductivity. Characteristics of the magnetic fields obtained are summarized as follows: The exterior magnetic fields of the stars behave like dipolar poloidal fields, which decrease as r^-3 when r → ∞. The interior magnetic fields are a mixture of poloidal and toroidal fields. Their geometric structures are tori of twisted field lines around the symmetry axis and of untwisted poloidal fields that penetrate the surface of the stars to continue to the exterior fields. It has been found that these structures of magnetic fields in stationary magnetic stars are robust and universal in the sense that they are nearly independent of the compressibility of the matter, rotation laws, and degrees of differential rotation. This universality is a consequence of the natural assumption that the electric current should be confined inside the star and of the integrability condition of the basic equation, i.e., that the electric current in the meridional plane is a function of the flux function, whose behavior is governed by a partial differential equation of the elliptic type.

We have studied linear polarization in the RS CVn binary IM Pegasi in order to check the possibility that this star has variable polarization. For this purpose, accurate polarization measurements in the B, V, and R passbands with the new CCD polarimeter on the remotely operated KVA telescope at La Palma were carried out in 2005 August-November. Our observations have yielded accurate determination of the polarization in IM Peg: P_B = 0.064% ± 0.004%, θ_B = 97° ± 2°; P_V = 0.102% ± 0.006%, θ_V = 91° ± 3°; and P_R = 0.078% ± 0.005%, θ_R = 95° ± 4°. Analysis of the data indicates no significant polarization variability in IM Peg. The upper limit for the amplitude of possible variability is ≤0.03% in all passbands. In contrast to the previously published results, the linear polarization in this star is most likely nonvariable and interstellar in origin.

Although the most abundant type of presolar grain found in meteorites is nanometer-sized diamond ("nanodiamond"), the dimensions make study of individual crystallites rather uninformative; instead, laboratory astrophysicists usually work with an ensemble of grains (generally in concentrated form, produced by chemical treatments). This, of course, produces results that are just a measure of average properties, which makes assessing the origin of diamonds quite difficult. An apparently uniform distribution of chemical and physical properties of the grains has been interpreted as their having a single origin. In this paper, however, we demonstrate that slight variations in average grain size can be exploited, using differential centrifugation (followed by analysis using electron microscopy and isotope-ratio mass spectrometry), to produce separates that reflect contributions from specific sources. In this way we identify a contribution from carbon stars, at the asymptotic giant branch stage of evolution, in addition to components already ascribed to supernovae and solar nebular processing. The astrophysical significance of the new discovery is discussed.

We present chemical abundances in K and M red giant members of the Galactic bulge derived from high-resolution infrared spectra obtained with the Phoenix spectrograph on Gemini-South. The elements studied are carbon, nitrogen, oxygen, sodium, titanium, and iron. The evolution of C and N abundances in the studied red giants shows that their oxygen abundances represent the original values with which the stars were born. Oxygen is a superior element for probing the timescale of bulge chemical enrichment via [O/Fe] versus [Fe/H]. The [O/Fe]-[Fe/H] relation in the bulge does not follow the disk relation, with [O/Fe] values falling above those of the disk. Titanium also behaves similarly to oxygen with respect to iron. Based on these elevated values of [O/Fe] and [Ti/Fe] extending to large Fe abundances, it is suggested that the bulge underwent a more rapid chemical enrichment than the halo. In addition, there are declines in both [O/Fe] and [Ti/Fe] in those bulge targets with the largest Fe abundances, signifying another source affecting chemical evolution: perhaps supernovae of Type Ia. Sodium abundances increase dramatically in the bulge with increasing metallicity, possibly reflecting the metallicity-dependent yields from supernovae of Type II, although Na contamination from H-burning in intermediate-mass stars cannot be ruled out.

We present the results of a program to acquire photometry for 86 late M, L, and T dwarfs using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We examine the behavior of these cool dwarfs in various color-color and color-magnitude diagrams composed of near-IR and IRAC data. The T dwarfs exhibit the most distinctive positions in these diagrams. In M_5.8 versus [5.8]-[8.0], the IRAC data for T dwarfs are not monotonic in either magnitude or color, giving the clearest indication yet that the T dwarfs are not a one-parameter family in T_eff. Because metallicity does not vary enough in the solar neighborhood to act as the second parameter, the most likely candidate then is gravity, which in turn translates to mass. Among objects with similar spectral type, the range of mass suggested by our sample is about a factor of 5 (~70M_J to ~15M_J), with the less massive objects making up the younger members of the sample. We also find the IRAC 4.5 μm fluxes to be lower than expected, from which we infer a stronger CO fundamental band at ~4.67 μm. This suggests that equilibrium CH₄/CO chemistry underestimates the abundance of CO in T dwarf atmospheres, confirming earlier results based on M-band observations from the ground. In combining IRAC photometry with near-IR JHK photometry and parallax data, we find the combination of K_s, IRAC 3.6 μm, and 4.5 μm bands to provide the best color-color discrimination for a wide range of M, L, and T dwarfs. Also noteworthy is the M versus K_s-[4.5] relation, which shows a smooth progression over spectral type, and splits the M, L, and T types cleanly.

This paper presents a fully three-dimensional radiative hydrodymanics simulation with realistic opacities for a gravitationally unstable 0.07 M_☉ disk around a 0.5 M_☉ star. We address the following aspects of disk evolution: the strength of gravitational instabilities under realistic cooling, mass transport in the disk that arises from GIs, comparisons between the gravitational and Reynolds stresses measured in the disk and those expected in an α-disk, and comparisons between the SED derived for the disk and SEDs derived from observationally determined parameters. The mass transport in this disk is dominated by global modes, and the cooling times are too long to permit fragmentation for all radii. Moreover, our results suggest a plausible explanation for the FU Ori outburst phenomenon.

Extrasolar planets as light as a few Earth masses are now being detected. Such planets are likely not gas or ice giants. Here, we present a study on the possible properties of the small and cold extrasolar planets, applied to the case of the recently discovered planet OGLE 2005-BLG-390Lb. This planet (5.5M_⊕) orbits 2.6 AU away from an old M-type star of the Galactic bulge. The planet should be entirely frozen given the low surface temperature (35-47 K). However, depending on the rock-to-ice mass ratio in the planet, the radiogenic heating could be sufficient to make the existence of liquid water within an icy crust possible. This possibility is estimated as a function of the planetary mass and the illumination received from the parent star, both being strongly related by the observational constraints. The results are presented for water-poor and water-rich planets. We find that no oceans can be present in any cases at 9-10 Gyr, a typical age for a star of the bulge. However, we find that in the past, when the planet was ≲5 Gyr old, liquid water was likely present below an icy surface. Nevertheless, the planet is now likely to be entirely frozen.

A series of missions will be launched over the next few decades that will be designed to detect and characterize extrasolar planets around nearby stars. These missions will search for habitable environments and signs of life (biosignatures) in planetary spectra. The vegetation's "red edge," an enhancement in the Earth's spectrum near 700 nm when sunlight is reflected from greenery, is often suggested as a tool in the search for life in terrestrial-like extrasolar planets. Here, through ground-based observations of the Earth's spectrum, satellite observations of clouds, and an advanced atmospheric radiative-transfer code, we determine the temporal evolution of the vegetation signature of Earth. We find a strong correlation between the evolution of the spectral intensity of the red edge and changes in the cloud-free vegetated area over the course of observations. This relative increase for our single day corresponds to an apparent reflectance change of about 0.0050 ± 0.0005 with respect to the mean albedo of 0.25 at 680 nm (2.0% ± 0.2%). The excellent agreement between models and observations motivated us to probe more deeply into the red-edge detectability using real cloud observations at longer timescales. Overall, we find the evolution of the red-edge signal in the globally averaged spectra to be weak, and only attributable to vegetation changes when the real land and cloud distributions for the day are known. However, it becomes prominent under certain Sun-Earth-Moon orbital geometries that are applicable to the search for life in extrasolar planets. Our results indicate that vegetation detection in Earth-like planets will require a considerable level of instrumental precision and will be a difficult task, but not as difficult as the normally weak earthshine signal might seem to suggest.

The effect of a self-induced electric field is investigated analytically and numerically on differential and mean electron spectra produced by beam electrons during their precipitation into a flaring atmosphere as well as on the emitted hard X-ray (HXR) photon spectra. The induced electric field is found to be a constant in upper atmospheric layers and to fall sharply in the deeper atmosphere from some "turning point" occurring either in the corona (for intense and softer beams) or in the chromosphere (for weaker and harder beams). The stronger and softer the beam, the higher the electric field before the turning point and the steeper its decrease after it. Analytical solutions are presented for the electric fields, which are constant or decreasing with depth, and the characteristic "electric" stopping depths are compared with the "collisional" ones. A constant electric field is found to decelerate precipitating electrons and to significantly reduce their number in the upper atmospheric depth, resulting in their differential spectra flattening at lower energies (<100 keV). While a decreasing electric field slows down the electron deceleration, allowing them to precipitate into deeper atmospheric layers than for a constant electric field, the joint effect of electric and collisional energy losses increases the energy losses by lower energy electrons compared to pure collisions and results in maxima at energies of 40-80 keV in the differential electron spectra. This, in turn, leads to the maxima in the mean source electron spectra and to the "double power law" HXR photon spectra (with flattening at lower energies) similar to those reported from the RHESSI observations. The more intense and soft the beams are, the stronger is the lower energy flattening and the higher is the "break" energy where the flattening occurs.

A three-dimensional reconstruction of the 2002 April 21 partial halo Coronal Mass Ejection (CME) has been made based on the O VI 1032 Å and [Fe XVIII] 974 Å lines observed by the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO). We use the Doppler velocities to derive the CME structure along the line of sight. UVCS observed the O VI line profiles split into strongly Doppler red- and blueshifted components, and the region of split profiles grew rapidly along the long spectrograph slit. The more localized [Fe XVIII] bright emission starts at the same time as the maximum Doppler redshift of O VI, indicating that it is inside the CME. In the view from the solar west, the O VI looks like halo CMEs seen by the Large Angle and Spectrometric Coronagraph (LASCO). The [Fe XVIII] bright emission appears as a barlike structure seen nearly end-on from the Earth, and side-on from the solar north and from solar west. The reconstructed [Fe XVIII] emission allows two interpretations, as ejection of preexisting hot plasma or as a current sheet. The evidence favors the current sheet interpretation, although we cannot rule out the alternatives.

The results of numerical simulations of acoustic wave propagation and dispersion in the nonmagnetic solar subphotosphere are presented. Initial equilibrium density and pressure stratifications are taken from a standard solar model but modified to suppress convective instabilities in fully compressible two-dimensional ideal hydrodynamical modeling. Acoustic waves are generated by sources located below the height corresponding to the visible solar surface. The dynamic response of the solar interior to two acoustic source types, namely a harmonic source and one representing downward-propagating photospheric plumes, is studied. A large number of randomly distributed localized cooling sources with random amplitudes is also introduced. The methods used to analyze the simulation data are similar to ones used in observational studies in local helioseismology. Time-distance diagrams of the pressure and vertical velocity perturbations at the level corresponding to the solar surface show the appearance of wave packets propagating with different speeds, which are reflected at different depths beneath the subphotosphere. The (ω, k_h) power spectra, derived from the vertical velocity data, show the existence of g-, f-, and p-modes; p-mode ridges are identifiable up to high radial orders of n ≈ 11; g-modes appear in the simulations, unlike in the real Sun, where they cannot propagate in the convectively unstable solar subphotosphere. Cross-correlation analysis of vertical velocity perturbations shows a good correspondence with the observed time-distance helioseismic data for quiet Sun. Thus, the ability of the implemented approach of forward modeling to investigate propagation of acoustic, internal, and surface gravity waves in a realistic solar interior model is shown.

Studies of solar-flare cosmic-ray particle transport in the interplanetary medium and data analysis of the fluctuating solar wind magnetic fields have revealed the existence of dominating, two-dimensional transverse magnetic fluctuations. Here it is demonstrated that the filamentation instability of counterstreaming magnetized plasmas provides a plausible mechanism for the origin of this two-dimensional turbulence component. Solar coronal mass ejections into the interplanetary medium, as well as overtaking solar wind streams in the appropriate center of plasma mass reference system, correspond to energetic collisions of plasma shells with different nonrelativistic velocities. By analyzing the dispersion relation, it is shown that these plasma shell collisions quickly lead to the onset of purely growing aperiodic plasma instabilities perpendicular to the flow direction if the flow velocity difference is larger than ^1/2 times the local Alfvén speed, where r_n denotes the density contrast of the colliding shells. For typical coronal mass ejections and parameters that allow overtaking the solar wind stream, the instability condition is well fulfilled, and the calculated growth rates of the fluctuations are short compared to the dynamical flare timescales.

Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of astrophysical plasmas, including accretion disks, the solar wind, and the interstellar and intracluster medium. On small scales, this turbulence is often expected to consist of highly anisotropic fluctuations with frequencies small compared to the ion cyclotron frequency. For a number of applications, the small scales are also collisionless, so a kinetic treatment of the turbulence is necessary. We show that this anisotropic turbulence is well described by a low-frequency expansion of the kinetic theory called gyrokinetics. This paper is the first in a series to examine turbulent astrophysical plasmas in the gyrokinetic limit. We derive and explain the nonlinear gyrokinetic equations and explore the linear properties of gyrokinetics as a prelude to nonlinear simulations. The linear dispersion relation for gyrokinetics is obtained, and its solutions are compared to those of hot-plasma kinetic theory. These results are used to validate the performance of the gyrokinetic simulation code GS2 in the parameter regimes relevant for astrophysical plasmas. New results on global energy conservation in gyrokinetics are also derived. We briefly outline several of the problems to be addressed by future nonlinear simulations, including particle heating by turbulence in hot accretion flows and in the solar wind, the magnetic and electric field power spectra in the solar wind, and the origin of small-scale density fluctuations in the interstellar medium.

Relativistic blast waves can be described using a mechanical model. In this model, the "blast"—the compressed gas between the forward and reverse shocks—is viewed as one hot body. Equations governing its dynamics are derived from the conservation of mass, energy, and momentum. Simple analytical solutions are obtained in the two limiting cases of an ultrarelativistic and a nonrelativistic reverse shock. Equations are derived for the general explosion problem.

The recently discovered gamma-ray burst GRB 060218/SN 2006aj is classified as an X-ray flash with very long duration driven possibly by a neutron star. Since GRB 060218 is very near, ~140 Mpc, and very dim, the 1 yr observation by Swift suggests that the rate of GRB 060218-like events might be very high so that such low-luminosity (LL) GRBs might form a different population from the cosmological high-luminosity (HL) GRBs. We found that the high-energy neutrino background from LL GRBs could be comparable with that from HL GRBs. If each neutrino event is detected by IceCube, later optical-infrared follow-up observations such as those by Subaru and HST can possibly identify a Type Ibc supernova associated with LL GRBs, even if gamma rays and X-rays are not observed by Swift. This is in a sense a new window from neutrino astronomy, which might enable us to confirm the existence of LL GRBs and to obtain information about their rate and origin. We also suggest that LL GRBs are high-energy gamma-ray and cosmic-ray sources.

The shock model of gamma-ray bursts (GRBs) contains two equipartition parameters: the magnetic energy density and the kinetic energy density of the electrons relative to the total energy density of the shock, _B and _e, respectively. These are free parameters within the model. Whereas the Weibel shock theory and numerical simulations fix _B at the level of a few times 10^-3 to 10^-4, no understanding of _e exists so far. Here we demonstrate that it inevitably follows from the theory that _e ≃ ^1/2. The GRB afterglow data fully agree with this theoretical prediction. Our result explains why the electrons are close to equipartition in GRBs. The _e-_B relation can potentially be used to reduce the number of free parameters in afterglow models.

We report the discovery of hard X-ray delays in the X-ray emission of the Seyfert 1 galaxy Mrk 110, based on a long XMM-Newton observation. Cross-correlation between the X-ray light curves of different energy bands reveals an energy-dependent delay ranging from a few minutes to an hour. We find that the energy spectrum can be modeled by Comptonization of disk blackbody photons. The energy-dependent delay can be modeled as due to the effect of Comptonization in a hot plasma confined within 10 Schwarzschild radii of the black hole. We discuss our results in the context of inverse Comptonization of the soft photons by highly energetic plasma.

3C 216 has a weak accretion flow luminosity, well below the Seyfert 1/QSO dividing line, weak broad emission lines (BELs), and powerful radio lobes. As a consequence of the extreme properties of 3C 216, it is the most convincing example known of an FR II radio source that is kinetically dominated: the jet kinetic luminosity, Q, is larger than the total thermal luminosity (IR to X-ray) of the accretion flow, L_bol. Using three independent estimators for the central black hole mass, we find that the jet in 3C 216 is very super-Eddington, 3.3L_Edd < < 10L_Edd, where is the long-term time-averaged Q(t), calculated at 151 MHz. It is argued that 3C 216 satisfies the contemporaneous kinetically dominated condition, R(t) ≡ Q(t)/L_bol(t) > 1, either presently or in the past, based on the rarity of L_bol > L_Edd quasars. The existence of an R(t) > 1 AGN is a strong constraint on the theory of the central engine of FR II radio sources.

The premise that ultraluminous X-ray sources (ULXs) exist beyond the optical extent of nearby galaxies is investigated. A published catalog containing 41 ULX candidates located between 1 and ~3 times the standard D₂₅ isophotal radius of their putative host galaxies is examined. Of these sources, 21 have spectroscopically confirmed distances. All 21 are background objects, giving a 95% probability that at least 37 of the 41 candidates are background sources. In addition, 39 of the 41 sources have X-ray-to-optical flux ratios in the range -1.6 < log(F_X/F_O) < +1.3, consistent with those of background active galactic nuclei. [The remaining two are not detected in the optical to a weak limit of m_B ~ 21.5 mag, corresponding to log(F_X/F_O) ≳ 1.6.] The uniform spatial distribution of the sample is also consistent with a background population. This evidence suggests that ULXs rarely, if at all, exist beyond the distribution of luminous matter in nearby galaxies and, as a consequence, there is no correlation between the population of ULXs and halo objects, such as old globular clusters or Population III remnants.

We present results from a study of the globular cluster luminosity function (GCLF) in a sample of 89 early-type galaxies observed as part of the ACS Virgo Cluster Survey. Using a Gaussian parameterization of the GCLF, we find a highly significant correlation between the GCLF dispersion, σ, and the galaxy luminosity, M_{B, gal}, in the sense that the GC systems in fainter galaxies have narrower luminosity functions. The GCLF dispersions in the Milky Way and M31 are fully consistent with this trend, implying that the correlation between σ and galaxy luminosity is more fundamental than older suggestions that GCLF shape is a function of galaxy Hubble type. We show that the σ-M_{B, gal} relation results from a bona fide narrowing of the distribution of (logarithmic) cluster masses in fainter galaxies. We further show that this behavior is mirrored by a steepening of the GC mass function for relatively high masses, ≳ 3 × 10⁵ _☉, a mass regime in which the shape of the GCLF is not strongly affected by dynamical evolution over a Hubble time. We argue that this trend arises from variations in initial conditions and requires explanation by theories of cluster formation. Finally, we confirm that in bright galaxies the GCLF "turns over" at the canonical mass scale of _TO ≃ 2 × 10⁵ _☉. However, we find that _TO scatters to lower values [≈(1-2) × 10⁵ _☉] in galaxies fainter than M_{B, gal} ≳ -18.5, an important consideration if the GCLF is to be used as a distance indicator for dwarf ellipticals.

We report on the detection in Sloan Digital Sky Survey data of at least three, roughly parallel components in a 65°-long, stellar stream complex previously identified with the Monoceros Ring. The three-stream complex varies in width from 4° to 6° along its length and appears to be made up of two or more narrow substreams as well as a broader, diffuse component. The width and complexity of the stream indicate that the progenitor was likely a dwarf galaxy of significant size and mass. The stream is 8.9 kpc distant and is oriented almost perpendicularly to our line of sight. The visible portion of the stream does not pass near any known dwarf galaxies, and a preliminary orbit does not point to any viable progenitor candidates. Orbits for the narrower substreams can be modeled with velocity offsets from the broad component of ≈8 km s^-1. We suggest that the broad component is likely to be the remains of a dwarf galaxy, while the narrower streams constitute the remnants of dynamically distinct components that may have included a native population of globular clusters. While the color of the main-sequence turnoff is not unlike that for the Monoceros Ring, neither the visible stream nor any reasonable projection of its orbit passes through Monoceros or Canis Major, and we conclude that this stream is probably unrelated to the overdensities found in these regions.

Here we report the discovery of a 6° tidal stream related to NGC 5053 using the Sloan Digital Sky Survey data. A matched-filter analysis has been employed to optimize the contrast between the cluster and background noise. The stream of tidal debris associated with NGC 5053 corresponds to a projected length of approximately 1.7 kpc at the cluster's distance. The tidal detection is also seen in photometric data obtained at McDonald Observatory. Our result adds further observational evidence that low-concentration clusters are more likely to exhibit stellar tidal extensions.

The nature of the local interstellar cloud (LIC) is highly constrained by the combination of in situ heliospheric and line-of-sight data toward nearby stars. We present a new interpretation of the LIC components of the absorption-line data toward CMa, based on recent atomic data that include new rates for the Mg⁺ to Mg⁰ dielectronic recombination rate, and using in situ measurements of the temperature and density of neutral helium inside of the heliosphere. With these data we are able to place interesting limits on the gas-phase abundance of carbon in the LIC. If the C/S abundance ratio is solar, ~20, then no simultaneous solution exists for the N(Mg I), N(Mg II), N(C II), and N(C II*) data. The combined column density and in situ data favor an abundance ratio A_C/A_S = 47. We find that the most probable gas-phase C abundance is in the range 400-800 ppm with a lower limit of ~330. We speculate that such a supersolar abundance could have come to be present in the LIC via destruction of decoupled dust grains. Similar enhanced C/H ratios are seen in very low column density material, N(H) < 10¹⁹ cm^-2, toward several nearby stars.

We report observations of gaseous methanol in an edge-on torus surrounding the young stellar object L1551 IRS 5. The peaks in the torus are separated by ~10,000 AU from L1551 IRS 5 and contain ~0.03 M_⊕ of cold CH₃OH. We infer that the CH₃OH abundance increases in the outer part of the torus, probably as a result of methanol evaporation from dust grain surfaces heated by the shock luminosity associated with the shocks associated with the jets of an externally located X-ray source. Any methanol released in such a cold environment will rapidly freeze again, spreading CH₃OH throughout the circumbinary torus to nascent dust grains, planetesimals, and primitive bodies. These observations probe the initial chemical conditions of matter infalling onto the disk.

We present observations with the INTEGRAL IBIS telescope of the wind nebula powered by the young pulsar B1509-58, and we discuss the spatial and spectral properties of the unpulsed emission in the 20-200 keV energy band. The source extension and orientation along the northwest-southeast axis correspond to the jet emission seen at keV and TeV energies. The hard X-ray spectrum is consistent with the earlier BeppoSAX measurements. It follows a power law with a photon index α = -2.12 ± 0.05 up to 160 keV. A possible break at this energy is found at the 2.9 σ confidence level. The 0.1-100 keV data are consistent with synchrotron aging of pairs in the jet and yield a magnetic field strength of 22-33 μG for a bulk velocity of 0.3c-0.5c. The synchrotron cutoff energy thus corresponds to a maximum electron energy of 400-730 TeV.

We present 4.5, 8, and 16 μm photometry from the Spitzer Space Telescope for 204 stars in the Upper Scorpius OB association. The data are used to investigate the frequency and properties of circumstellar disks around stars with masses between ~0.1 and 20 M_☉ at an age of ~5 Myr. We identify 35 stars that have emission at 8 or 16 μm in excess of the stellar photosphere. The lower mass stars (~0.1-1.2 M_☉) appear surrounded by primordial optically thick disks based on the excess emission characteristics. Stars more massive than ~1.8 M_☉ have lower fractional excess luminosities suggesting that the inner ~10 AU of the disk has been largely depleted of primordial material. None of the G and F stars (~1.2-1.8 M_☉) in our sample have an infrared excess at wavelengths ≤16 μm. These results indicate that the mechanisms for dispersing primordial optically thick disks operate less efficiently, on average, for low-mass stars, and that longer timescales are available for the buildup of planetary systems in the terrestrial zone for stars with masses ≲1 M_☉.

We present numerical models of the circumstellar disks of Be stars, and we describe the resulting synthetic Hα emission lines and maps of the wavelength-integrated emission flux projected onto the sky. We demonstrate that there are monotonic relationships between the emission-line equivalent width and the ratio of the angular half-width at half-maximum of the projected disk major axis to the radius of the star. These relationships depend mainly on the temperatures of the disk and star, the inclination of the disk normal to the line of sight, and the adopted outer boundary for the disk radius. We show that the predicted Hα disk radii are consistent with those observed directly through long baseline interferometry of nearby Be stars (especially once allowance is made for disk truncation in binaries and for dilution of the observed Hα equivalent width by continuum disk flux in the V band).

We report the discovery of a bright (H = 12.77) brown dwarf designated SIMP J013656.5+093347. The discovery was made as part of a near-infrared proper-motion survey, SIMP (Sondage Infrarouge de Mouvement Propre), which uses proper motion and near-infrared/optical photometry to identify brown dwarf candidates. A low-resolution (λ/Δλ ~ 40) spectrum of this brown dwarf covering the 0.88-2.35 μm wavelength interval is presented. Analysis of the spectrum indicates a spectral type of T2.5 ± 0.5. A photometric distance of 6.4 ± 0.3 pc is estimated, assuming it is a single object. Current observations rule out a binary of mass ratio ~1 and separation ≳5 AU. SIMP J013656.5+093347 is the brightest T dwarf in the northern hemisphere and is surpassed only by ε Indi Bab over the whole sky. It is thus an excellent candidate for detailed studies and should become a benchmark object for the early T spectral class.

We announce the discovery of the second transiting hot Jupiter discovered by the Trans-atlantic Exoplanet Survey. The planet, which we dub TrES-2, orbits the nearby star GSC 03549-02811 every 2.47063 days. From high-resolution spectra, we determine that the star has T_eff = 5960 ± 100 K and log g = 4.4 ± 0.2, implying a spectral type of G0 V and a mass of 1.08M_☉. High-precision radial velocity measurements confirm a sinusoidal variation with the period and phase predicted by the photometry, and rule out the presence of line bisector variations that would indicate that the spectroscopic orbit is spurious. We estimate a planetary mass of 1.28M_Jup. We model B, r, R, and I photometric time series of the 1.4% deep transits and find a planetary radius of 1.24R_Jup. This planet lies within the field of view of the NASA Kepler mission, ensuring that hundreds of upcoming transits will be monitored with exquisite precision and permitting a host of unprecedented investigations.

We describe a model that accounts for the complex morphology of spiral density waves raised in Saturn's rings by the co-orbital satellites, Janus and Epimetheus. Our model may be corroborated by future Cassini observations of these time-variable wave patterns.

Strong signals of neutral emissions were detected in association with a solar flare that occurred on 2005 September 7. They were produced by both relativistic ions and electrons. In particular, relativistic neutrons were observed with the solar neutron telescopes (SNTs) located at Mount Chacaltaya in Bolivia and Mount Sierra Negra in Mexico and with neutron monitors (NMs) at Chacaltaya and Mexico City with high statistical significances. At the same time, hard X-rays and γ-rays, which were predominantly emitted by high-energy electrons, were detected by the Geotail and the INTEGRAL satellites. We found that a model of the impulsive neutron emission at the time of the X-ray/γ-ray peak can explain the main peaks of all the detected neutron signals, but failed to explain the long tailed decaying phase. An alternative model, in which the neutron emission follows the X-ray/γ-ray profile, also failed to explain the long tail. These results indicate that the acceleration of ions began at the same time as the electrons but that ions were continuously accelerated or trapped longer than the electrons in the emission site. We also demonstrate that the neutron data observed by multienergy channels of SNTs put constraints on the neutron spectrum.

We have carried out multiconfiguration Breit-Pauli AUTOSTRUCTURE calculations for the dielectronic recombination (DR) of Fe⁸⁺-Fe¹²⁺ ions. We obtain total DR rate coefficients for the initial ground level that are an order of magnitude larger than those corresponding to radiative recombination (RR), at temperatures where Fe 3p^q (q = 2-6) ions are abundant in photoionized plasmas. The resultant total (DR+RR) rate coefficients are then an order of magnitude larger than those currently in use by photoionized plasma modeling codes such as CLOUDY, ION, and XSTAR. These rate coefficients, together with our previous results for q = 0 and 1, are critical for determining the ionization balance of the M-shell Fe ions that give rise to the prominent unresolved-transition-array X-ray absorption feature found in the spectrum of many active galactic nuclei. This feature is poorly described by CLOUDY and ION, necessitating an ad hoc modification to the low-temperature DR rate coefficients. Such modifications are no longer necessary, and a rigorous approach to such modeling can now take place using these data.