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We present a fast, accurate, robust, and flexible method of accelerating parameter estimation. This algorithm, called Pico, can compute the CMB power spectrum and matter transfer function, as well as any computationally expensive likelihoods, in a few milliseconds. By removing these bottlenecks from parameter estimation codes, Pico decreases their computational time by 1 or 2 orders of magnitude. Pico has several important properties. First, it is extremely fast and accurate over a large volume of parameter space. Furthermore, its accuracy can continue to be improved by using a larger training set. This method is generalizable to an arbitrary number of cosmological parameters and to any range of l-values in multipole space. Pico is approximately 3000 times faster than CAMB for flat models, and approximately 2000 times faster than the WMAP 3 yr likelihood code. In this paper, we demonstrate that using Pico to compute power spectra and likelihoods produces parameter posteriors that are very similar to those using CAMB and the official WMAP3 code, but in only a fraction of the time. Pico and an interface to CosmoMC are made publicly available on the authors' Web site at http://www.astro.uiuc.edu/~bwandelt/pico/.

We present results from a large volume simulation of hydrogen reionization. We combine 3D radiative transfer calculations and an N-body simulation, describing structure formation in the intergalactic medium, to detail the growth of H II regions around high-redshift galaxies. Our simulation tracks 1024³ dark matter particles, in a box of comoving side length 65.6 Mpc h^-1. This large volume allows us to accurately characterize the size distribution of H II regions throughout most of the reionization process. At the same time, our simulation resolves many of the small galaxies likely responsible for reionization. It confirms a picture anticipated by analytic models: H II regions grow collectively around highly clustered sources and have a well-defined characteristic size, which evolves from a sub-Mpc scale at the beginning of reionization to R > 10 Mpc toward the end. We present a detailed statistical description of our results and compare them with a numerical scheme based on the analytic model by Furlanetto and coworkers. We find that the analytic calculation reproduces the size distribution of H II regions and the 21 cm power spectrum of the radiative transfer simulation remarkably well. The ionization field from the simulation, however, has more small-scale structure than the analytic calculation, owing to Poisson scatter in the simulated abundance of galaxies on small scales. We propose and validate a simple scheme to incorporate this scatter into our calculations. Our results suggest that analytic calculations are sufficiently accurate to aid in predicting and interpreting the results of future 21 cm surveys. In particular, our fast numerical scheme is useful for forecasting constraints from future 21 cm surveys and in constructing mock surveys to test data analysis procedures.

We use observed rotation velocity-luminosity (VL) and size-luminosity (RL) relations to single out a specific scenario for disk galaxy formation in the ΛCDM cosmology. Our model involves four independent lognormal random variables: dark halo concentration c, disk spin λ_gal, disk mass fraction m_gal, and stellar mass-to-light ratio ϒ_I. A simultaneous match of the VL and RL zero points with adiabatic contraction requires low-c halos, but this model has V_2.2 ~ 1.8V_vir (where V_2.2 and V_vir are the circular velocity at 2.2 disk scale lengths and the virial radius, respectively), which will be unable to match the luminosity function (LF). Similarly models without adiabatic contraction but standard c also predict high values of V_2.2/V_vir. Models in which disk formation induces an expansion rather than the commonly assumed contraction of the dark matter halos have V_2.2 ~ 1.2V_vir, which allows a simultaneous fit of the LF. This may result from nonspherical, clumpy gas accretion, where dynamical friction transfers energy from the gas to the dark matter. This model requires low λ_gal and m_gal values, contrary to naive expectations. However, the low λ_gal is consistent with the notion that disk galaxies predominantly survive in halos with a quiet merger history, while a low m_gal is also indicated by galaxy-galaxy lensing. The smaller than expected scatter in the RL relation and the lack of correlation between the residuals of the VL and RL relations, respectively, imply that the scatter in λ_gal and in c needs to be smaller than predicted for ΛCDM halos, again consistent with the idea that disk galaxies preferentially reside in halos with a quiet merger history.

We show by means of a high-resolution N-body simulation how the mass assembly histories of galaxy-sized cold dark matter (CDM) halos depend on environment. Halos in high-density environments form earlier than those in low-density environments, and a higher fraction of their mass is assembled in major mergers. The distribution of the present-day specific mass aggregation rate is strongly dependent on environment. While in low-density environments only ~20% of the halos are not accreting mass at the present epoch, this fraction rises to ~80% at high densities. At z = 1 the median of the specific aggregation rate is ~4 times larger than at z = 0 and almost independent of environment. All the dependences on environment found here are critically enhanced by local processes associated with subhalos because the fraction of subhalos increases as the environment gets denser. The distribution of the halo specific mass aggregation rate and its dependence on environment resemble the relations for the specific star formation rate distribution of galaxies. An analog of the morphology-density relation is also present at the level of CDM halos, being driven by the halo major-merging history. Nevertheless, baryonic processes are necessary in order to explain further details and the evolution of the relations of star formation rate, color, and morphology to environment.

We perform 12 extremely high resolution adaptive mesh refinement cosmological simulations of Population III star formation in a ΛCDM universe, varying the box size and large-scale structure, to understand systematic effects in the formation of primordial protostellar cores. We find results that are qualitatively similar to those of previous groups. We observe that in the absence of a photodissociating ultraviolet background, the threshold halo mass for formation of a Population III protostar does not evolve significantly with time in the redshift range studied (33 > z > 19) but exhibits substantial scatter (1.5 < M_vir/10⁵M_☉ < 7) due to different halo assembly histories: halos that assembled more slowly develop cooling cores at lower mass than those that assemble more rapidly, in agreement with previous work. We do, however, observe significant evolution in the accretion rates of Population III protostars with redshift, with objects that form later having higher maximum accretion rates ( ≃ 10^-4M_☉ yr^-1 at z = 33 and ≃10^-2M_☉ yr^-1 at z = 20). This can be explained by considering the evolving virial properties of the halos with redshift and the physics of molecular hydrogen formation at low densities. Our result implies that the inferred mass distribution of Population III stars is broader than previously thought and may evolve with redshift. Finally, we observe that our collapsing protostellar cloud cores do not fragment, consistent with previous results, which suggests that Population III stars that form in halos of mass 10⁵-10⁶M_☉ always form in isolation.

We compute the expected number of quasars multiply imaged by cluster-size dark halos for current wide-field quasar surveys by carrying out a large ensemble of ray-tracing simulations through clusters from a cosmological N-body simulation of the ΛCDM cosmology, with power spectrum normalization σ₈ = 0.95. Our calculation predicts ~4 quasar lenses with splittings θ > 10'' in the SDSS spectroscopic quasar sample, consistent with the recent discovery of the wide separation lens SDSS J1004+4112, which has θ = 14.6''. The SDSS faint photometric quasar survey will contain ~12 multiply imaged quasars with splittings θ > 10''. Of these, ~2 will be lenses with θ > 30'', and ~2 will be at high redshift (z_s ~ 4).

We present a technique to explore the radio sky into the nanojansky regime by employing image stacking using the FIRST survey. We first discuss the nonintuitive relationship between the mean and median values of a non-Gaussian distribution that is dominated by noise, followed by an analysis of the systematic effects present in FIRST's 20 cm VLA snapshot images. Image stacking allows us to recover the properties of source populations with flux densities a factor of 30 or more below the rms noise level. Mean estimates of radio flux density, luminosity, etc. are derivable for any source class having arcsecond positional accuracy. We use this technique to compute the mean radio properties for 41,295 quasars from the SDSS DR3 catalog. There is a tight correlation between optical and radio luminosity, with the radio luminosity increasing as the 0.85 power of optical luminosity. This implies declining radio loudness with optical luminosity: the most luminous objects (M_UV = -28.5) have average radio-to-optical ratios 3 times lower than the least luminous objects (M_UV = -20). There is also a striking correlation between optical color and radio loudness: quasars that are either redder or bluer than the norm are brighter radio sources, with objects 0.8 mag redder than the SDSS composite spectrum having radio loudness ratios that are higher by a factor of 10. We explore the long-standing question of whether a radio-loud/radio-quiet dichotomy exists in quasars, finding that optical selection effects probably dominate the distribution function of radio loudness, which has at most a modest (~20%) inflection between the radio-loud and radio-quiet ends of the distribution. We also find, surprisingly, that broad absorption line quasars have higher mean radio flux densities, with the greatest disparity arising in the rare low-ionization BAL subclass.

We present the clustering of Deep Extragalactic Evolutionary Probe 2 (DEEP2) galaxies at 0.7 < z < 1.4 around quasars identified using both the Sloan Digital Sky Survey (SDSS) and DEEP2 surveys. We measure the two-point cross-correlation of a sample of 36 optically selected, spectroscopically identified quasars from the SDSS and 16 more found in the DEEP2 survey with the full DEEP2 galaxy sample over scales 0.1 h^-1 Mpc < r_p < 10 h^-1 Mpc. The clustering amplitude is found to be similar to the autocorrelation function of DEEP2 galaxies, with a relative bias of b = 0.89 ± 0.24 between quasars and DEEP2 galaxies at z ~ 1. No significant dependence is found on scale, quasar luminosity, or redshift over the ranges we probe here. The clustering amplitude errors are comparable to those from significantly larger quasar samples, such as the 2dF (Two Degree Field) QSO Redshift Survey. This results from the statistical power of cross-correlation techniques, which exploit the fact that galaxies are much more numerous than quasars. We also measure the local environments of quasars using the third-nearest-neighbor surface density of surrounding DEEP2 galaxies. Quasars are found in regions of similar mean overdensity to blue DEEP2 galaxies; they differ in environment from the red DEEP2 galaxy population at 2 σ significance. Our results imply that quasars do not reside in particularly massive dark matter halos at these redshifts, with a mean dark matter halo mass of M₂₀₀ ~ 3 × 10¹²M_☉ in a concordance ΛCDM cosmology.

We present results from a program of optical spectroscopy for 23 nearby galaxies with emission-line nuclei. This investigation takes advantage of the spatial resolution of the Hubble Space Telescope to study the structure and energetics of the central ~10-20 pc, and the resulting data have value for quantifying central black hole masses, star formation histories, and nebular properties. This paper provides a description of the experimental design, and new findings from the study of emission lines. The sample targets span a range of nebular spectroscopic class, from H II to Seyfert nuclei. This data set and the resulting measurements are unique in terms of the sample size, the range of nebular class, and the investigation of physical scales extending down to parsecs. The line ratios indicative of nebular ionization show only modest variations over order-of-magnitude differences in radius, and demonstrate in a systematic way that geometrical dilution of the radiation field from a central source cannot be assumed as a primary driver of ionization structure. Comparisons between large- and small-aperture measurements for the H II/LINER transition objects provide a new test that challenges conventional wisdom concerning the composite nature of these systems. We also list a number of other quantitative results that are of interest for understanding galaxy nuclei, including (1) the spatial distribution/degree of concentration of Hα emission as a function of nebular type; (2) the radial variation in electron density as a function of nebular type; and (3) quantitative broad Hα estimates obtained at a second epoch for these low-luminosity nuclei. The resulting measurements provide a new basis for comparing the nuclei of other galaxies with that of the Milky Way. We find that the Galactic center is representative across a wide span of properties as a low-luminosity emission-line nucleus.

We study the clustering properties of K-selected galaxies at 2 < z < 3.5 using deep multiwavelength imaging in three fields from the MUSYC survey. These are the first measurements to probe the spatial correlation function of K-selected galaxies in this redshift range on large scales, allowing for robust conclusions about the dark matter halos that host these galaxies. The K-selected galaxies with K < 21 have a correlation length r₀ ~ 6 h^-1 Mpc, larger than typical values found for optically selected galaxies. The correlation length does not depend on K-band magnitude in our sample but does increase strongly with color; the J - K > 2.3 distant red galaxies (DRGs) have r₀ ~ 11 h^-1 Mpc. Furthermore, contrary to findings for optically selected galaxies, K-selected galaxies that are faint in the R band cluster more strongly than brighter galaxies. These results suggest that a color-density relation was in place at z > 2; it will be interesting to see whether this relation is driven by galaxies with old stellar populations or by dusty star-forming galaxies. Irrespective of the cause, our results indicate that K-bright blue galaxies and K-bright red galaxies are fundamentally different, having different clustering properties. Using a simple model of one galaxy per halo, we infer halo masses ~5 × 10¹²M_☉ for K < 21 galaxies and ~2 × 10¹³M_☉ for DRGs. A comparison of the observed space density of DRGs to that of their host halos suggests large halo occupation numbers; however, this result conflicts with the lack of a strong small-scale excess in the angular correlation function. Using the predicted evolution of halo mass to investigate relationships between galaxy populations at different redshifts, we find that the z = 0 descendants of the galaxies considered here reside primarily in groups and clusters.

We measure the evolution in the virial mass-to-light ratio (M₂₀₀/L_B) and virial-to-stellar mass ratio (M₂₀₀/M_*) for isolated ~L* galaxies between z ~ 1 and z ~ 0 by combining data from the DEEP2 Galaxy Redshift Survey and the Sloan Digital Sky Survey. Utilizing the motions of satellite galaxies around isolated galaxies, we measure line-of-sight velocity dispersions and derive dark matter halo virial masses for these host galaxies. At both epochs the velocity dispersion of satellites correlates with host galaxy stellar mass, σ ∝ M, while the relation between satellite velocity dispersion and host galaxy B-band luminosity may grow somewhat shallower from σ ∝ L at z ~ 1 to σ ∝ L at z ~ 0. The evolution in M₂₀₀/M_* from z ~ 1 to z ~ 0 displays a bimodality insofar as host galaxies with stellar mass below M_* ~ 10¹¹h^-1M_☉ maintain a constant ratio (the intrinsic increase is constrained to a factor of 1.1 ± 0.5) while host galaxies above M_* ~ 10¹¹h^-1M_☉ experience a factor of 3.3 ± 2.2 increase in their virial-to-stellar mass ratio. This result can be easily understood if galaxies below this stellar mass scale continue to form stars while star formation in galaxies above this scale is quenched and the dark matter halos of galaxies both above and below this scale grow in accordance with ΛCDM cosmological simulations. Host galaxies that are red in U - B color have larger satellite dispersions and hence reside on average in more massive halos than blue galaxies at both z ~ 1 and z ~ 0. The satellite population of host galaxies varies little between these epochs. The redshift and host galaxy stellar mass dependence of M₂₀₀/M_* agrees qualitatively with the Millennium Run semianalytic model of galaxy formation.

Using the multiwavelength photometric and spectroscopic data covering the Chandra Deep Field South obtained within the Great Observatories Origins Deep Survey, we investigate the rest-frame UV properties of galaxies to z ~ 2.2, including the evolution of the luminosity function, the luminosity density, star formation rate (SFR), and galaxy morphology. We find a significant brightening (~1 mag) in the rest-frame 2800 Å characteristic magnitude (M*) over the redshift range 0.3 ≲ z ≲ 1.7 and no evolution at higher redshifts. The rest-frame 2800 Å luminosity density shows an increase by a factor of ~4 over the redshift range investigated. We estimate the SFR density to z ~ 2.2 from the 1500 and 2800 Å luminosities. When no correction for extinction is made, we find that the SFR derived from the 2800 Å luminosity density is almost a factor of 2 higher than that derived from the 1500 Å luminosities. Attributing this difference to differential dust extinction, we find that E(B - V) = 0.20 results in the same extinction-corrected SFR from both 1500 and 2800 Å luminosities. The extinction-corrected SFR is a factor of ~6.5 (~3.7) higher than the uncorrected SFR derived from 1500 Å (2800 Å) luminosity. We investigate the morphological composition of our sample by fitting Sérsic profiles to the HST ACS galaxy images at a fixed rest-frame wavelength of 2800 Å at 0.5 ≲ z ≲ 2.2. We find that the fraction of apparently bulge-dominated galaxies (Sérsic index n > 2.5) increases from ~10% at z ~ 0.5 to ~30% at z ~ 2.2. At the same time, we note that galaxies get bluer at increasing redshift. This suggests a scenario where an increased fraction of the star formation takes place in bulge-dominated systems at high redshift. This could be evidence that the present-day elliptical galaxies are a result of assembly (i.e., mergers) of galaxies at z ≳ 1. Finally, we find that galaxy size for a luminosity-selected sample evolves as r_h ∝ (1 + z)^-1.1 between redshifts z = 2.2 and 1.1. This is consistent with previous measurements and suggests a similar evolution over the redshift range 0 ≲ z ≲ 6.

We discuss a new distance to NGC 5128 (Centaurus A) based on Cepheid variables observed with the Hubble Space Telescope. Twelve F555W (V) and six F814W (I) epochs of cosmic-ray-split WFPC2 observations were obtained. A total of 56 bona fide Cepheids were discovered, with periods ranging from 5 to ~50 days; five of these are likely Population II Cepheids of the W Virginis class, associated with the bulge or halo of NGC 5128. Based on the period and V- and I-band luminosities of a subsample of 42 classical (Population I) Cepheids, and adopting an LMC distance modulus and extinction of 18.50 ± 0.10 mag and E(B - V) = 0.10 mag, respectively, the true reddening-corrected distance modulus to NGC 5128 is μ₀ = 27.67 ± 0.12 (random) ± 0.16 (systematic) mag, corresponding to a distance of 3.42 ± 0.18 (random) ± 0.25 (systematic) Mpc. The random uncertainty in the distance is dominated by the error on the assumed value for the ratio of total to selective absorption, R_V, in NGC 5128, and by the possible metallicity dependence of the Cepheid period-luminosity relation at V and I. This represents the first determination of a Cepheid distance to an early-type galaxy.

Cosmic-ray protons interacting with gas at the mean density of the interstellar medium (ISM) in starburst galaxies lose energy rapidly via inelastic collisions with ambient nuclei. The resulting pions produce secondary electrons and positrons, high-energy neutrinos, and γ-ray photons. We estimate the cumulative γ-ray emission from starburst galaxies. We find a total integrated background above 100 MeV of F_γ ≈ 10^-6 GeV cm^-2 s^-1 sr^-1 and a corresponding specific intensity at GeV energies of νI_ν ≈ 10^-7 GeV cm^-2 s^-1 sr^-1. Starbursts may thus account for a significant fraction of the extragalactic γ-ray background. We show that the FIR-radio correlation provides a strong constraint on the γ-ray emission from starburst galaxies because pions decay into both γ-rays and radio-emitting electron/positron pairs. We identify several nearby systems where the potential for observing γ-ray emission is the most favorable (M82, NGC 253, and IC 342), predict their fluxes, and predict a linear FIR-γ-ray correlation for the densest starbursts. If established, the FIR-γ-ray correlation would provide strong evidence for the "calorimeter" theory of the FIR-radio correlation and would imply that cosmic rays in starburst galaxies interact with gas at approximately the mean density of the ISM, thereby providing an important constraint on the physics of the ISM in starbursts.

We studied the properties of very young massive star-forming regions in H II galaxies, with the aim of detecting signs of evolution where the first supernovae (SNe) start to appear. Our sample consists of 31 H II galaxies, characterized by strong hydrogen emission lines, for which low-resolution VLA 3.5 and 6 cm observations were obtained. We found that the radio spectral energy distribution (SED) has a range of behaviors: galaxies where the SED is characterized by a synchrotron-type slope; galaxies with a thermal slope; and galaxies with possible free-free absorption at long wavelengths. The latter represent a signature of heavily embedded massive star clusters. Comparing the different star formation rates (SFRs), we find that SFR(Hα) is on average significantly higher than SFR(1.4 GHz). We confirm this tendency by comparing the ratio of the observed flux at 20 cm to the expected one, calculated based on the SFR Hα, both for the galaxies in our sample and for normal ones. We show that this ratio is a factor of 2 smaller in our galaxies than in normal ones, indicating that they do not follow the FIR/radio correlation (q-parameter). These results suggest that the emission of these galaxies is dominated by a recent star-forming event in which the first SNe started to explode, consistent with the radio emission being dominated by free-free continuum. We propose an evolutionary scenario to explain the observed trends and conclude that the systematic lack of synchrotron emission in those systems with the largest equivalent width of Hβ can only be explained if those are young starbursts of less than 3.5 Myr of age, i.e., before the first Type II SNe start to explode.

We present Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometer observations of giant molecular clouds (GMCs) along a spiral arm in M31. The observations consist of a survey using the compact configuration of the interferometer and follow-up, higher resolution observations on a subset of the detections in the survey. The data are processed using an analysis algorithm designed to extract GMCs and correct their derived properties for observational biases, thereby facilitating comparison with Milky Way data. The algorithm identifies 67 GMCs, of which 19 have a sufficient signal-to-noise ratio to accurately measure their properties. The GMCs in this portion of M31 are indistinguishable from those found in the Milky Way, having a similar size-line width relationship and distribution of virial parameters, confirming the results of previous, smaller studies. The velocity gradients and angular momenta of the GMCs are comparable to the values measured in M33 and the Milky Way, and in all cases are below expected values based on the local galactic shear. The studied region of M31 has an interstellar radiation field, metallicity, Toomre Q parameter, and midplane volume density similar to those of the inner Milky Way, so the similarity of GMC populations between the two systems is not surprising.

We present a theory for the generation of mesoscale (kr_g ≪ 1, where r_g is the cosmic-ray gyroradius) magnetic fields during diffusive shock acceleration. The decay or modulational instability of resonantly excited Alfvén waves scattering off ambient density perturbations in the shock environment naturally generates larger scale fields. For a broad spectrum of perturbations, the physical mechanism of energy transfer is random refraction, represented by the diffusion of Alfvén wave packets in k-space. The scattering field can be produced directly by the decay instability or by the Drury instability, a hydrodynamic instability driven by the cosmic-ray pressure gradient. This process is of interest to acceleration since it generates waves of longer wavelength, and so enables the confinement and acceleration of higher energy particles. This process also limits the intensity of resonantly generated turbulent magnetic fields on r_g scales.

Deep X-ray imaging spectroscopy of the bright pulsar wind nebula 3C 58 confirms the existence of an embedded thermal X-ray shell surrounding the pulsar PSR J0205+6449. Radially resolved spectra obtained with the XMM-Newton telescope are well characterized by a power-law model with the addition of a soft thermal emission component in varying proportions. These fits reproduce the well-studied increase in the spectral index with radius attributed to synchrotron burn off of high energy electrons. Most interestingly, a radially resolved thermal component is shown to map out a shell-like structure ≈6' in diameter. The presence of a strong emission line corresponding to the Ne IX He-like transition requires an overabundance of ~3 × (Ne/Ne_☉) in the Raymond-Smith plasma model. The best-fit temperature kT ~ 0.23 keV is essentially independent of radius for the derived column density of N_H = (4.2 ± 0.1) × 10²¹ cm^-2. Our result suggests that thermal shells can be obscured in the early evolution of a supernova remnant by nonthermal pulsar wind nebulae emission; the luminosity of the 3C 58 shell is more than an order of magnitude below the upper limit on a similar shell in the Crab Nebula. We find the shell centroid to be offset from the pulsar location. If this neutron star has a velocity similar to that of the Crab pulsar, we derive an age of 3700 yr and a velocity vector aligned with the long axis of the PWN. The shell parameters and pulsar offset add to the accumulating evidence that 3C 58 is not the remnant of the supernova of CE 1181.

We have investigated the timescale for formation of molecular clouds by examining the conversion of H I to H₂ using a time-dependent model with H₂ photodissociation including self-shielding. H₂ formation on dust grains and cosmic-ray destruction are also included in one-dimensional model slab clouds that incorporate time-independent density and temperature distributions. We calculate 21 cm spectral line profiles seen in absorption against a background provided by general Galactic H I emission and compare the model spectra with H I narrow self-absorption (HINSA) profiles absorbed in a number of nearby molecular clouds. The time evolution of the H I and H₂ densities is dramatic, with the atomic hydrogen disappearing in a wave propagating from the central, denser regions, which have a shorter H₂ formation timescales, to the edges, where the density is lower and the timescales for H₂ formation longer. The model 21 cm spectra are characterized by very strong absorption at early times. Emission at early times produced by the warm edges of the cloud is difficult to separate from variations in the background spectrum, when the background temperature is low. The minimum time for cloud evolution based on the model spectra is set by the requirement that most of the H I in the outer portions of the cloud be removed. The characteristic time that has elapsed since cloud compression and initiation of the H I → H₂ conversion is a few ×10¹⁴ s, or ≃10⁷ yr. This sets a minimum time for the age of these molecular clouds and thus for star formation that may take place within them.

We analyze the processes relevant for star formation in a model with dark matter in the form of sterile neutrinos. Sterile neutrino decays produce an X-ray background radiation that has a twofold effect on the collapsing clouds of hydrogen. First, the X-rays ionize the gas and cause an increase in the fraction of molecular hydrogen, which makes it easier for the gas to cool and to form stars. Second, the same X-rays deposit a certain amount of heat, which could, in principle, thwart the cooling of gas. We find that in all the cases we have examined the overall effect of sterile dark matter is to facilitate the cooling of gas. Hence, we conclude that dark matter in the form of sterile neutrinos can help the early collapse of gas clouds and the subsequent star formation.

It has been known for more than 30 years that star formation in giant molecular clouds (GMCs) is slow, in the sense that only ~1% of the gas forms stars every free-fall time. This result is entirely independent of any particular model of molecular cloud lifetime or evolution. Here we survey observational data on higher density objects in the interstellar medium, including infrared dark clouds and dense molecular clumps, to determine whether these objects form stars slowly like GMCs, or rapidly, converting a significant fraction of their mass into stars in one free-fall time. We find no evidence for a transition from slow to rapid star formation in structures covering 3 orders of magnitude in density. This has important implications for models of star formation, since competing models make differing predictions for the characteristic density at which star formation should transition from slow to rapid. The data are inconsistent with models that predict that star clusters form rapidly and in free-fall collapse. Magnetic- and turbulence-regulated star formation models can reproduce the observations qualitatively, and the turbulence-regulated star formation model of Krumholz & McKee quantitatively reproduces the infrared-HCN luminosity correlation recently reported by Gao & Solomon Slow star formation also implies that the process of star cluster formation cannot be one of global collapse, but must instead proceed over many free-fall times. This suggests that turbulence in star-forming clumps must be driven, and that the competitive accretion mechanism does not operate in typical cluster-forming molecular clumps.

The IC 1396N cometary globule (CG) within the large nearby H II region IC 1396 has been observed with the ACIS detector on board the Chandra X-Ray Observatory. We detect 117 X-ray sources, of which ~50-60 are likely members of the young open cluster Trumpler 37 dispersed throughout the H II region, and 25 are associated with young stars formed within the globule. Infrared photometry (2MASS and Spitzer) shows that the X-ray population is very young: 3 older Class III stars, 16 classical T Tauri stars, and 6 protostars including a Class 0/I system. We infer a total T Tauri population of ~30 stars in the globule, including the undetected population, with a star formation efficiency of 1%-4%. An elongated source spatial distribution with an age gradient oriented toward the exciting star is discovered in the X-ray population of IC 1396N, supporting similar findings in other cometary globules. The geometric and age distribution is consistent with the radiation-driven implosion (RDI) model for triggered star formation in CGs by H II region shocks. The inferred velocity of the shock front propagating into the globule is ~0.6 km s^-1. The large number of X-ray-luminous protostars in the globule suggests either an unusually high ratio of Class I/0 to Class II/III stars or a nonstandard initial mass function favoring higher mass stars by the triggering process. We find that the Chandra source associated with the luminous Class 0/I protostar IRAS 21391+5802 is one of the youngest stars ever detected in the X-ray band. We also establish for the first time that the X-ray absorption in protostars arises from the local infalling envelopes rather than from ambient molecular cloud material.

We present new images of the giant molecular cloud W3 obtained with the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer for Spitzer (MIPS) on board the Spitzer Space Telescope. The images encompass the star forming regions W3 Main, W3(OH), and a region that we refer to as the Central Cluster, which encloses the emission nebula IC 1795. We present a star count analysis of the point sources detected in W3. The star count analysis shows that the stellar population of the Central Cluster, when compared to that in the background, contains an over density of sources. The Central Cluster also contains an excess of sources with colors consistent with Class II young stellar objects (YSOs). An analysis of the color-color diagrams also reveals a large number of Class II YSOs in the Central Cluster. Our results suggest that an earlier epoch of star formation created the Central Cluster, created a cavity, and triggered the active star formation in the W3 Main and W3(OH) regions. We also detect a new outflow and its candidate exciting star.

We present and analyze the first high-resolution X-ray images ever obtained of the Eagle Nebula star-forming region. On 2001 July 30 the Chandra X-Ray Observatory obtained a 78 ks image of the Eagle Nebula (M16) that includes the core of the young galactic cluster NGC 6611 and the dark columns of dust and cold molecular gas in M16 known as the "Pillars of Creation." We find a total of 1101 X-ray sources in the 17' × 17' ACIS-I field of view. Most of the X-ray sources are low-mass pre-main-sequence or high-mass main-sequence stars in this young cluster. A handful of hard X-ray sources in the pillars are spatially coincident with deeply embedded young stellar objects seen in high-resolution near-infrared images recently obtained with the VLT (McCaughrean & Andersen). In this paper, we focus on the 40 X-ray sources in and around pillars 1-4 at the heart of the Eagle Nebula. None of the X-ray sources are associated with the evaporating gaseous globules (EGGs) first observed by Hester and coworkers) in HST WFPC2 images of M16, implying either that the EGGs do not contain protostars or that the protostars have not yet become X-ray active. Eight X-ray counts are coincident with the Herbig-Haro object HH 216, implying log L_X ≈ 30.0.

We present high angular resolution observations toward two massive star-forming regions, IRAS 18264-1152 and IRAS 23151+5912, with the Plateau de Bure Interferometer (PdBI) in the SiO(J = 2-1) and H¹³CO⁺(J = 1-0) lines and at 1.3 and 3.4 mm continuum, and with the Very Large Array (VLA) in the NH₃(J,K) = (1,1), (2, 2) lines. The NH₃(1, 1) and (2, 2) emission is detected toward IRAS 18264-1152 only. For IRAS 18264-1152, the SiO observations reveal at least two quasi-perpendicular outflows with high collimation factors, and the most dominant feature is a redshifted jetlike outflow with very high velocities up to about Δv = 60 km s^-1 with respect to the systemic velocity. The very high velocity component (Δv = 22-60 km s^-1) of this outflow is spatially offset from its high-velocity (Δv = 3-21 km s^-1) component. The SiO line profiles and position-velocity characteristics of these two components suggest that this outflow could be driven by an underlying precessing jet. For IRAS 23151+5912, the bipolar but mainly blueshifted SiO outflow coincides with the inner parts of the single-dish CO outflow. In particular, the quasi-parabolic shape of the blueshifted outflow coincides with the near-infrared nebulosity and is consistent with entrainment of the gas by an underlying wide-angle wind. The analysis of the molecular outflow data of the two luminous sources further supports high-mass stars forming via a disk-mediated accretion process similar to that in low-mass stars.

Gamma-ray bursts (GRBs) directed at Earth from within a few kiloparsecs may have damaged the biosphere, primarily through changes in atmospheric chemistry that admit greatly increased solar UV. However, GRBs are highly variable in spectrum and duration. Recent observations indicate that short (~0.1 s) burst GRBs, which have harder spectra, may be sufficiently abundant at low redshift that they may offer an additional significant effect. A much longer timescale is associated with shock breakout luminosity observed in the soft X-ray (~10³ s) and UV (~10⁵ s) emission and radioactive decay gamma-ray line radiation emitted during the light-curve phase of supernovae (~10⁷ s). Here, we generalize our atmospheric computations to include a broad range of peak photon energies and investigate the effect of burst duration while holding total fluence and other parameters constant. The results can be used to estimate the probable impact of various kinds of ionizing events (such as short GRBs, X-ray flashes, and supernovae) on the Earth's atmosphere. We find that the ultimate intensity of atmospheric effects varies only slightly with burst duration from 10^-1 to 10⁸ s. Therefore, the effect of many astrophysical events causing atmospheric ionization can be approximated without including time development. Detailed modeling requires specification of the season and latitude of the event. Harder photon spectra produce greater atmospheric effects for spectra with peaks up to about 20 MeV because of greater penetration into the stratosphere.

We present a detailed spectral analysis of the prompt and afterglow emission of four nearby long-soft gamma-ray bursts (GRBs 980425, 030329, 031203, and 060218) that were spectroscopically found to be associated with Type Ic supernovae and compare them to the general GRB population. For each event, we investigate the spectral and luminosity evolution and estimate the total energy budget based on broadband observations. The observational inventory for these events has become rich enough to allow estimates of their energy content in relativistic and subrelativistic form. The result is a global portrait of the effects of the physical processes responsible for producing long-soft GRBs. In particular, we find that the values of the energy released in mildly relativistic outflows appears to have a significantly smaller scatter than those found in highly relativistic ejecta. This is consistent with a picture in which the energy released inside the progenitor star is roughly standard, while the fraction of that energy that ends up in highly relativistic ejecta outside the star can vary dramatically between different events.

Swift discovered XRF 050416A with the Burst Alert Telescope and began observing it with its narrow-field instruments only 64.5 s after the burst onset. Its very soft spectrum classifies this event as an X-ray flash. The afterglow X-ray emission was monitored up to 74 days after the burst. The X-ray light curve initially decays very fast (decay slope α ~ 2.4), subsequently flattens (α ~ 0.44), and eventually steepens again (α ~ 0.88), similar to many X-ray afterglows. The first and second phases end ~172 and ~1450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index Γ ~ 3.0 during the initial steep decay, to a harder emission with Γ ~ 2.0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy with column density of ~6.8 × 10²¹ cm^-2. The consistency of the initial photon index with the high-energy BAT photon index suggests that the initial fast decaying phase of the X-ray light curve may be the low-energy tail of the prompt emission. The lack of jet break signatures in the X-ray afterglow light curve is not consistent with empirical relations between the source rest-frame peak energy and the collimation-corrected energy of the burst. The standard uniform jet model can give a possible description of the XRF 050416A X-ray afterglow for an opening angle larger than a few tens of degrees, although numerical simulations show that the late-time decay is slightly flatter than expected from on-axis viewing of a uniform jet. A structured Gaussian-type jet model with uniform Lorentz factor distribution and viewing angle outside the Gaussian core is another possibility, although a full agreement with data is not achieved with the numerical models explored.

Swift discovered GRB 050713A and slewed promptly to begin observing with its narrow-field instruments 72.6 s after the burst onset, while the prompt gamma-ray emission was still detectable in the BAT. Simultaneous emission from two flares is detected in the BAT and XRT. This burst marks just the second time that the BAT and XRT have simultaneously detected emission from a burst and the first time that both instruments have produced a well-sampled, simultaneous data set covering multiple X-ray flares. The temporal rise and decay parameters of the flares are consistent with the internal-shock mechanism. In addition to the Swift coverage of GRB 050713A, we report on the Konus-Wind (K-W) detection of the prompt emission, an upper limiting GeV measurement of the prompt emission made by the MAGIC imaging atmospheric Cerenkov telescope, and XMM-Newton observations of the afterglow. Simultaneous observations with Swift XRT and XMM-Newton produce consistent results, showing a break in the light curve at T₀+ ~ 15 ks. Together, these four observatories provide unusually broad spectral coverage of the prompt emission and detailed X-ray follow-up of the afterglow for 2 weeks after the burst trigger. Simultaneous spectral fits of K-W with BAT and BAT with XRT data indicate that an absorbed broken power law is often a better fit to GRB flares than a simple absorbed power law. These spectral results together with the rapid temporal rise and decay of the flares suggest that flares are produced in internal shocks due to late-time central-engine activity.

The timescale of deleptonization by neutrino loss and associated contraction of a proto-neutron star is short compared to the time it takes to propagate a shock through the helium core of a massive star, and so the deleptonization phase does not occur in the vacuum of space, but within the supernova ambiance, whether or not there has been a successful explosion. Dynamical nonaxisymmetric instabilities (NAXI) are predicted for sufficiently strongly differentially rotating proto-neutron stars. Some modes are unstable for small values of the ratio of rotational kinetic energy to binding energy, T/|W| ≳ 0.01. The NAXI are likely to drive magnetoacoustic waves into the surrounding time-dependent density structure. These waves represent a mechanism of the dissipation of the free energy of differential rotation of the proto-neutron star, and the outward deposition of this energy may play a role in the supernova explosion process. We estimate the power produced by this process and the associated timescale and discuss the possible systematics of the deleptonization phase in this context. A likely possibility is that the proto-neutron star will spin down through these effects before deleptonization and produce substantial but not excessive energy input.

We develop a new theoretical model for the spectral formation process in accretion-powered X-ray pulsars based on a detailed treatment of the bulk and thermal Comptonization occurring in the accreting, shocked gas. A rigorous eigenfunction expansion method is employed to obtain the analytical solution for the Green's function describing the scattering of radiation injected into the column from a monochromatic source located at an arbitrary height above the stellar surface. The emergent spectrum is calculated by convolving the Green's function with source terms corresponding to bremsstrahlung, cyclotron, and blackbody emission. The energization of the photons in the shock, combined with cyclotron absorption, naturally produces an X-ray spectrum with a relatively flat continuum shape and a high-energy quasi-exponential cutoff. We demonstrate that the new theory successfully reproduces the phase-averaged spectra of the bright pulsars Her X-1, LMC X-4, and Cen X-3. In these luminous sources, it is shown that the emergent spectra are dominated by Comptonized bremsstrahlung emission.

We present ray-tracing computations for light emitted from the surface of a rapidly rotating neutron star in order to construct light curves for X-ray pulsars and bursters. These calculations are for realistic models of rapidly rotating neutron stars that take into account both the correct exterior metric and the oblate shape of the star. We find that the most important effect arising from rotation comes from the oblate shape of the rotating star. Approximating a rotating neutron star as a sphere introduces serious errors in fitted values of the star's radius and mass if the rotation rate is very large. However, in most cases acceptable fits to the ratio M/R can be obtained with the spherical approximation.

We report on the evolution of key spectral and temporal parameters of SGR 1806-20 prior to and following the highly energetic giant flare of 2004 December 27. Using RXTE, we track the pulse frequency of the SGR and find that the spin-down rate varied erratically in the months before and after the flare. Contrary to the giant flare in SGR 1900+14, we find no evidence for a discrete jump in spin frequency at the time of the December 27th flare (|Δν/ν| < 5 × 10^-6). In the months surrounding the flare, we find a strong correlation between pulsed flux and torque consistent with the model for magnetar magnetosphere electrodynamics proposed by Thompson et al. As with the flare in SGR 1900+14, the pulse morphology of SGR 1806-20 changes drastically following the flare. Using Chandra and other publicly available imaging X-ray detector observations, we construct a spectral history of SGR 1806-20 from 1993 to 2005. The usual magnetar persistent emission spectral model of a power law plus a blackbody provides an excellent fit to the data. We confirm the earlier finding by Mereghetti et al. of increasing spectral hardness of SGR 1806-20 between 1993 and 2004. However, our results indicate significant differences in the temporal evolution of the spectral hardening. Rather than a direct correlation between torque and spectral hardness, we find evidence for a sudden torque change that preceded a gradual hardening of the energy spectrum on a timescale of years. Interestingly, the spectral hardness, spin-down rate, phase-averaged flux, and pulsed flux of SGR 1806-20 all peak months before the flare epoch.

We report on Very Long Baseline Array (VLBA) astrometry and Chandra imaging of PSR J0538+2817 in the supernova remnant S147. We measure a parallax distance of 1.47 kpc along with a high-precision proper motion, giving a transverse velocity V_⊥ = 400 km s^-1. A small extended wind nebula is detected around the pulsar; the symmetry axis of this structure suggests that the spin axis lies 12° ± 4° from the velocity vector (two-dimensional), but the emission is too faint for robust model-independent statements. The neutron star is hot, consistent with the young ~40 kyr kinematic age. The pulsar progenitor is likely a runaway from a nearby cluster, with NGC 1960 (M36) a leading candidate.

We study the 4-200 keV spectral and temporal behavior of the low-mass X-ray binary 4U 1820-30 with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high-energy tail, above 50 keV, in the hard state has been observed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power-law component, with the photon index of ≃2.4, on top of thermal Comptonization emission from a plasma with electron temperature kT_e ≃ 6 keV and optical depth τ ≃ 4. Alternatively, but at a poorer goodness of fit, the hard-state broadband spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During this monitoring the source spent most of the time in the soft state, usual for this source, and the ≳4 keV spectra are described by thermal Comptonization with kT_e ≃ 3 keV and τ ≃ 6-7.

We have conducted a survey of 61 southern white dwarfs searching for magnetic fields using Zeeman spectropolarimetry. Our objective is to obtain a magnetic field distribution for these objects and, in particular, to find white dwarfs with weak fields. We found one possible candidate (WD 0310-688) that may have a weak magnetic field of -6.1 ± 2.2 kG. Next, we determine the fraction and distribution of magnetic white dwarfs in the solar neighborhood and investigate the probability of finding more of these objects based on the current incidence of magnetism in white dwarfs within 20 pc of the Sun. We have also analyzed the spectra of the white dwarfs to obtain effective temperatures and surface gravities.

Two new magnetic white dwarf accretion binaries with extremely low mass transfer rates have been discovered in the course of the Sloan Digital Sky Survey. Measured magnetic fields are 42 and 57 MG, and one system orbits with a period of just 82 minutes. The new systems therefore significantly expand the range in properties exhibited by the small class. The measured accretion rates are very low, 0.6-5 × 10^-13M_☉ yr^-1, and multiple visits spanning more than a year confirm that this is not a short-lived characteristic. It is becoming increasingly clear that the low- magnetic white dwarf binaries accrete by nearly complete magnetic capture of the stellar wind from the secondary star rather than by Roche lobe overflow. The accretion rates therefore provide some of the first realistic estimates of the total wind loss rates from M dwarfs. Although one or more of the eight systems known to date may be interrupted or possibly even extinct Polars, several lines of evidence suggest that most are pre-Polars whose evolution has not yet brought the secondaries into contact with their Roche surfaces. Considering the difficulties of identifying binaries over a wide range in field strength and accretion rate, it is quite possible that the space density of wind-accreting magnetic binaries exceeds that of the classical X-ray-emitting, Roche lobe overflow Polars.

We present the first K'-band, long-baseline interferometric observations of the northern Be stars γ Cas, ϕ Per, ζ Tau, and κ Dra. The measurements were made with multiple telescope pairs of the CHARA Array interferometer and in every case the observations indicate that the circumstellar disks of the targets are resolved. We fit the interferometric visibilities with predictions from a simple disk model that assumes an isothermal gas in Keplerian rotation. We derive fits of the four model parameters (disk base density, radial density exponent, disk normal inclination, and position angle) for each of the targets. The resulting densities are in broad agreement with prior studies of the IR excess flux, and the resulting orientations generally agree with those from interferometric Hα and continuum polarimetric observations. We find that the angular size of the K' disk emission is smaller than that determined for the Hα emission, and we argue that the difference is the result of a larger Hα opacity and the relatively larger neutral hydrogen fraction with increasing disk radius. All the targets are known binaries with faint companions, and we find that companions appear to influence the interferometric visibilities in the cases of ϕ Per and κ Dra. We also present contemporaneous observations of the Hα, Hγ, and Brγ emission lines. Synthetic model profiles of these lines that are based on the same disk inclination and radial density exponent as derived from the CHARA Array observations match the observed emission line strength if the disk base density is reduced by ≈1.7 dex.

The Microvariability and Oscillations of Stars (MOST) satellite has detected low-amplitude light variations (Δm ~ 1 mmag) in the Be star β CMi (B8 Ve). The observations lasted 41 days and the variations have typical periods ~0.3 days. We demonstrate that the dominant frequencies are consistent with prograde high-order g-modes of m = -1 excited by the Fe bump of opacity in an intermediate-mass (≈3.5 M_☉) star with a nearly critical rotation period of 0.38 days. This is the first detection of nonradial g-mode pulsations in a Be star later than B6 leading to the possibility that pulsations are excited in all classical Be stars.

Stellar oscillations, which can be extracted from observed time series of the star's brightness or radial velocity, can provide a wealth of information about a star. In this paper we address the question of how to extract as much information as possible from such a data set. We have developed a Markov chain Monte Carlo (MCMC) code that is able to infer the number of oscillation frequencies present in the signal and their values (with corresponding uncertainties), without having to fit the amplitudes and phases. Gaps in the data do not have any serious consequences for this method; in cases where severe aliasing exists, any ambiguity in the frequency determinations will be reflected in the results. It also allows us to infer parameters of the frequency pattern, such as the large separation Δν. We have previously applied this method to the star ν Indi, and here we describe the method fully and apply it to simulated data sets, showing that the code is able to give correct results even when some of the model assumptions are violated. In particular, the nonsinusoidal nature of the individual oscillation modes due to stochastic excitation and damping has no major impact on the usefulness of our approach.

We present results of a high-resolution, near-infrared survey of 41 nearby, young (≲300 Myr) M0-M5.0 dwarfs using the Altair natural guide star adaptive optics system at the Gemini North telescope. Twelve of the objects appear to be binaries, seven of which are reported here for the first time. One triple system was discovered. Statistical properties are studied and compared with earlier (F to K) and later (≥M6 very low mass [VLM]) populations. We find that the separation distribution of the binaries in this sample peaks at 13 AU, which is consistent with previous measurements of early M binaries. Hence, early M binaries seem to occur in—on average—tighter systems than G binaries. At the same time they are significantly wider than field VLM binary stars. The distribution of mass ratios q of primary and secondary stars was found to show an intermediate distribution between the strongly q → 1 peaked distribution of field VLM systems and the almost flat distribution of earlier type stars. Consequently, we show evidence for relatively young, early M binaries representing a transition between the well-known earlier star distributions and the recently examined field VLM population characteristics. Despite the fact that this survey was dedicated to the search for faint brown dwarf and planetary mass companions, all planetary mass candidates were background objects. We exclude the existence of physical companions with masses greater than 10 Jupiter masses (M_J) at separations of ≳40 AU and masses greater than 24M_J for separations ≳10 AU around 37 of the 41 observed objects.

We report the discovery of T dwarf companions to the nearby stars HN Peg (G0 V, 18.4 pc, τ ~ 0.3 Gyr) and HD 3651 (K0 V, 11.1 pc, τ ~ 7 Gyr). During an ongoing survey of 5' × 5' fields surrounding stars in the solar neighborhood with the Infrared Array Camera aboard the Spitzer Space Telescope, we identified these companions as candidate T dwarfs based on their mid-infrared colors. Using near-infrared spectra obtained with SpeX at the NASA Infrared Telescope Facility, we confirm the presence of methane absorption that characterizes T dwarfs and measure spectral types of T2.5 ± 0.5 and T7.5 ± 0.5 for HN Peg B and HD 3651B, respectively. By comparing our Spitzer data to images from the Two Micron All Sky Survey obtained several years earlier, we find that the proper motions of HN Peg B and HD 3651B are consistent with those of the primaries, confirming their companionship. A comparison of their luminosities to the values predicted by theoretical evolutionary models implies masses of 0.021 ± 0.009 and 0.051 ± 0.014 M_☉ for HN Peg B and HD 3651B, respectively. In addition, the models imply an effective temperature for HN Peg B that is significantly lower than the values derived for other T dwarfs at similar spectral types, which is the same behavior reported by Metchev & Hillenbrand for the young late L dwarf HD 203030B. Thus, the temperature of the L/T transition appears to depend on surface gravity. Meanwhile, HD 3651B is the first substellar companion directly imaged around a star that is known to harbor a close-in planet from radial velocity surveys. The discovery of this companion supports the notion that the high eccentricities of close-in planets like that near HD 3651 may be the result of perturbations by low-mass companions at wide separations.

We present 24 μm Spitzer MIPS photometric observations of the ~50 Myr open cluster IC 2391. Thirty-four cluster members ranging in spectral type from B3 to M5 were observed in the central square degree of the cluster. Excesses indicative of debris disks were discovered around one A star, six FGK stars, and possibly one M dwarf. For the cluster members observed to their photospheric limit, we find a debris disk frequency of 10% for B-A stars and 31% for FGK stars using a 15% relative excess threshold. Relative to a model of decaying excess frequency, the frequency of debris disks around A-type stars appears marginally low for the cluster's age while that of FGK stars appears consistent. Scenarios that may qualitatively explain this result are examined. We conclude that planetesimal activity in the terrestrial region of FGK stars is common in the first ~50 Myr and decays on timescales of ~100 Myr. Despite luminosity differences, debris disk evolution does not appear to depend strongly on stellar mass.

We have used the Hubble Space Telescope Advanced Camera for Surveys coronagraph to make the first polarization maps of the AU Microscopii debris disk. The polarization rises from 5% at 20 AU to 40% at 80 AU. The polarization is perpendicular to the disk, indicating that the scattered light originates from micron-sized grains in an optically thin disk. Disk models show that interior to the "birth ring" (40-50 AU) there is a hole in the dust distribution where micron-sized dust is depleted by a factor of more than 300. The disk is collision dominated, and grains that fall inward due to drag forces undergo a destructive collision. The presence of this hole implies that the localized enhancements in surface brightness that occur at projected radii interior to the birth ring are caused by nonaxisymmetric structures in the outer disk. The grains exhibit strong forward scattering and high polarization. Spherical grains composed of conventional materials cannot reproduce these optical properties. A Mie/Maxwell-Garnett analysis demands highly porous (91%-94%) particles. In the inner solar system, porous particles form in cometary dust, where the sublimation of ices leaves a "bird's nest" of refractory material. In AU Mic, the grain porosity may be primordial, because the dust birth ring lies beyond the ice sublimation point. The observed porosities span the range of values implied by laboratory studies of particle coagulation by ballistic cluster-cluster aggregation. To avoid compactification, the upper size limit for the parent bodies is in the decimeter range, in agreement with theoretical predictions based on collisional lifetime arguments. Consequently, AU Mic may exhibit the signature of the primordial agglomeration process whereby interstellar grains first assembled to form macroscopic objects.

In theory of accretion disks, angular momentum and mass transfer are associated with the generation of energy through viscous dissipation. In the construction of SED models of protostellar disks, the stellar irradiation is usually assumed to be the dominant heating source. Here we construct a new set of self-consistent analytical disk models by taking into account both sources of thermal energy and the thermal structure of the disk across the midplane. We deduce a set of general formulae for the relationship between the mass accretion rate and the surface density profile. We apply it to determine the structure of protostellar disks under a state of steady accretion and derive the radial distribution of surface density and midplane temperature. The incorporation of the viscous heating in our model reduces the disk flaring angle and leads to lower photospheric temperatures than previously thought. Around T Tauri stars, the snow line can evolve from outside 10 AU during FU Orionis outbursts, to 2 AU during the quasi-steady accretion phase, to 0.7 AU when the accretion rate falls to about 10^-9M_☉ yr^-1, and finally reexpand beyond 2.2 AU during the protostellar-to-debris disk transition. The nonmonotonous evolution of the snow line may lead to the observed isotopic composition of water on both Venus and Earth. We also infer the presence of a marginally opaque, isothermal region with a surface density distribution similar to that of the MSN model. With a 40% higher temperature than that in the region immediately within, this transition may lead to an upturn in the SEDs in the MIR (24-70 μm) wavelength range. The optically thin, outermost regions of the disk have a shallow surface density profile of the dust that is consistent with millimeter observations of spatially resolved disks.

The possible existence of additional long-period planetary-mass objects in the extrasolar planetary systems 47 UMa and 14 Her is investigated. We combine all available radial velocity data on these stars, spanning up to 18 yr. For the 47 UMa system, we show that while a second planet improves the fit to all available data, there is still substantial ambiguity as to the orbital parameters of the proposed planetary companion 47 UMa c. We also present new observations that clearly support a long-period companion in the 14 Her system. With a period of 6906 ± 70 days, 14 Her c may be in a 4 : 1 resonance with the inner planet. We also present revised orbital solutions for seven previously known planets, incorporating recent additional data obtained with the 2.7 m Harlan J. Smith Telescope at McDonald Observatory.

The major obstacle to the direct detection of companions to nearby stars is the overwhelming brightness of the host star. Current instruments employing the combination of adaptive optics (AO) and coronagraphy can typically detect objects within 2'' of the star that are ~10⁴-10⁵ times fainter. Correlated speckle noise is one of the biggest obstacles limiting such high-contrast imaging. We have obtained a series of 284 8 s, AO-corrected, coronagraphically occulted H-band images of the star Vega at the 3.63 m AEOS telescope located on Haleakala, Hawaii. This data set is unique for studying the temporal behavior of speckle noise and represents the first time such a study on highly corrected coronagraphic AO images has been carried out in a quantitative way. We find the speckle pattern to be highly stable in both position and time in our data. This is due to the fact that the AO system corrects disturbances to the stellar wave front at the level where the instrumental wave front errors dominate. Because of this, we find that our detection limit is not significantly improved simply with increased exposure time alone. However, we are able to improve our dynamic range by 1.5-2 mag through subtraction of static/quasi-static speckles in two rotating frames: the telescope pupil frame and the deformable mirror frame. The highly stable nature of speckles will exist for any program using coronagraphy and high-order AO. Furthermore, from our data, we are able to constrain the mass of any purported companion to Vega to be less than ~45M_J at 8 AU and less than ~30M_J at 16 AU, radii not previously probed at these sensitivities.

The claimed discovery of a Jupiter-mass planet in the close triple-star system HD 188753 poses a problem for planet formation theory. A circumstellar disk around the planet's parent star would be truncated close to the star, leaving little material available for planet formation. In this paper we attempt to model a protoplanetary disk around HD 188753A using a fairly simple α-disk model, exploring a range of parameters constrained by observations of T Tauri-type stars. The disk is truncated to within 1.5-2.7 AU, depending on model parameters. We find that the in situ formation of the planet around HD 188753A is implausible.

Identifying the two physical mechanisms behind the production and sustenance of the quiescent solar corona and solar wind poses two of the outstanding problems in solar physics today. We present analysis of spectroscopic observations from the Solar and Heliospheric Observatory that are consistent with a single physical mechanism being responsible for a significant portion of the heat supplied to the lower solar corona and the initial acceleration of the solar wind; the ubiquitous action of magnetoconvection-driven reprocessing and exchange reconnection of the Sun's magnetic field on the supergranular scale. We deduce that while the net magnetic flux on the scale of a supergranule controls the injection rate of mass and energy into the transition region plasma, it is the global magnetic topology of the plasma that dictates whether the released ejecta provides thermal input to the quiet solar corona or becomes a tributary that feeds the solar wind.

Local reconnection and energy release rates for an X3.8 flare that occurred on 2005 January 17 are derived. In particular, we distinguish between Hα flare ribbon segments that were accompanied by RHESSI hard X-ray (HXR) footpoints and those without HXRs. We find that the reconnection and energy release rates are not uniform along the flare ribbons but much larger at the locations where the HXR footpoints are observed. The difference is about 2 orders of magnitude in the case of the energy release rates and 1 order of magnitude for the reconnection rates (with peak values up to 8 kV m^-1). These differences are enough to explain the different flare morphologies typically observed in HXRs (compact footpoints) and Hα/UV (extended ribbons) by the limited dynamic range of present HXR instruments. Our results are consistent with a scenario in which the electrons are accelerated primarily along a certain subsystem of magnetic loops as outlined by the HXR footpoints, and only a minor fraction (for the 2005 January 17 flare estimated to be about 1/15) go into the large flare arcade outlined by the Hα ribbons and EUV postflare loops.

A well-known problem in solar physics is that solutions for the transverse magnetic field direction are ambiguous with respect to a 180° reversal in the field direction. In this paper we focus on three methods for the removal of the 180° ambiguity applied to three MHD models. These methods are (1) the reference field method, (2) the method of magnetic pressure gradient, and (3) the magnetic field divergence-free method. All three methods are noniterative, and methods 2 and 3 are analytical and fast. We apply these methods to three MHD equilibrium model fields: (1) an analytical solution of a nonlinear force-free magnetic field equilibrium from Low, (2) a simulation of an emerging twisted flux tube from Fan & Gibson, and (3) a pre-eruptive twisted magnetic flux rope equilibrium reached by relaxation from Amari et al. We measure the success of methods within "inverse horizontal field" regions in the boundary, which are mathematically defined by B_⊥ ∇_⊥B_z > 0. When such regions overlap with the magnetic field neutral lines, they are known as "bald patches" (BPs) or inverse topology. Our most important conclusion is that the magnetic divergence-free method is far more successful than the other two methods within BPs. This method requires a second level of measurements of the vertical magnetic field. As high-quality multilevel magnetograms will come online in the near future, our work shows that multilayer magnetic field measurements will be highly desirable to objectively and successfully tackle the 180° ambiguity problem.

Germanium-bearing molecules of the formula Ge₂H_x (x = 2-6) are plausible candidates for organometallic molecules present in the atmospheres of Jupiter and Saturn. Combining low-temperature spectroscopy with computed vibrational spectra of these molecules, we have observed strong fundamentals of seven germanium-bearing molecules GeH₃ (germyl radical), Ge₂H₆ (digermane), Ge₂H₅ (digermyl radical), H₂GeGeH₂ (digermene), HGeGeH₃ (digermanylidene), HGeGeH₂ (digermenyl radical), and GeH₂Ge (di-μ-hydrodigermanium) via infrared spectroscopy in the laboratory. These bands were confirmed in D4-germane matrices. Our data can guide the search for hydrogen-deficient germanium-carrying molecules in the atmospheres of Jupiter and Saturn, where the germane (GeH₄) precursor molecule is present.

We develop a series expansion of the plasma screening length away from the classical limit in powers of ℏ². It is shown that the leading-order quantum correction decreases the fusion rate by approximately 2%. We also calculate the next higher order quantum correction, which turns out to be approximately an order of magnitude smaller.

We discuss the interpretation of the cosmic infrared background (CIB) anisotropies detected by us recently in the Spitzer/IRAC-based measurements. The fluctuations are approximately isotropic on the sky, which is consistent with their cosmological origin. They remain after the removal of fairly faint intervening sources and must arise from a population that has a strong CIB clustering component with only a small shot-noise level. We discuss the constraints the data place on the luminosities, epochs, and mass-to-light ratios of the individual sources producing them. Assuming the concordance ΛCDM cosmology, the measurements imply that the luminous sources producing them lie at cosmic times <1 Gyr and were individually much brighter per unit mass than the present stellar populations.

Cosmic infrared background fluctuations may contain a measurable contribution from objects inaccessible to current telescopic studies, such as the first stars and other luminous objects in the first gigayear of the universe's evolution. In an attempt to uncover this contribution, we have analyzed the GOODS data obtained with the Spitzer Infrared Array Camera (IRAC). These data are deeper and cover larger scales than the Spitzer data that we have previously analyzed. Here we report these new measurements of the cosmic infrared background fluctuations remaining after removing cosmic sources to fainter levels than before. The remaining anisotropies on scales ~0.5'-10' have a significant clustering component with a low shot-noise contribution. We show that these fluctuations cannot be accounted for by instrumental effects or by the solar system and Galactic foreground emissions and that they must arise from extragalactic sources.

The detection of a nonthermal excess in the Coma Cluster spectrum by two BeppoSAX observations analyzed with the XAS package (Fusco-Femiano et al.) has been disavowed by an analysis (Rossetti & Molendi) performed with a different software package (SAXDAS) for the extraction of the spectrum. To resolve this discrepancy we reanalyze the PDS data considering the same software used by Rossetti & Molendi. A correct selection of the data and the exclusion of contaminating sources in the background determination show that the SAXDAS analysis also reports a nonthermal excess with respect to the thermal emission at about the same confidence level of that obtained with the XAS package (~4.8 σ). In addition, we report the lack of the systematic errors investigated by Rossetti & Molendi and Nevalainen et al., taking into account the whole sample of the PDS observations off the Galactic plane, as already shown in our data analysis of Abell 2256 (Fusco-Femiano, Landi, & Orlandini). All this eliminates any ambiguity and confirms the presence of a hard tail in the spectrum of the Coma Cluster.

We develop a new method to predict the density associated with weak-lensing maps of (un)relaxed clusters in a range of theories interpolating between general relativity (GR) and modified Newtonian dynamics (MOND). We apply it to fit the lensing map of the Bullet merging cluster 1E 0657-56, in order to constrain more robustly the nature and amount of collisionless matter in clusters beyond the usual assumption of spherical equilibrium (Pointecouteau & Silk) and the validity of GR on cluster scales (Clowe et al.). Strengthening the proposal of previous authors, we show that the Bullet Cluster is dominated by a collisionless—most probably nonbaryonic—component in GR as well as in MOND, a result consistent with the dynamics of many X-ray clusters. Our findings add to the number of known pathologies for a purely baryonic MOND, including its inability to fit the latest data from the Wilkinson Microwave Anisotropy Probe. A plausible resolution of all these issues and standard issues of cold dark matter (CDM) with galaxy rotation curves is the "marriage" of MOND with ordinary hot neutrinos of 2 eV. This prediction is just within the GR-independent maximum of neutrino mass from current β-decay experiments and will be falsifiable by the Karlsruhe Tritium Neutrino (KATRIN) experiment by 2009. Issues of consistency with strong-lensing arcs and the large relative velocity of the two clusters comprising the Bullet Cluster are also addressed.

GRB 050904 is the gamma-ray burst with the highest measured redshift. We performed time-resolved X-ray spectroscopy of the late GRB and early afterglow emission. We find robust evidence for a decrease with time of the soft X-ray-absorbing column. We model the evolution of the column density due to the flash ionization of the GRB and early afterglow photons. This allows us to constrain the metallicity and geometry of the absorbing cloud. We conclude that the progenitor of GRB 050904 was a massive star embedded in a dense metal-enriched molecular cloud with Z ≳ 0.03 Z_☉. This is the first local measurement of metallicity in the close environment of a GRB and one of the highest redshift metallicity measurements. We also find that the dust associated with the cloud cannot be similar to that of our Galaxy but must be either sizably depleted or dominated by silicate grains. We discuss the implications of these results for GRB progenitors and high-redshift star formation.

We report on two z ~ 4 gamma-ray bursts, GRB 060206 and GRB 060210, for which we have obtained well-sampled optical light curves. Both light curves show unusual behavior. GRB 060206 experienced a slow early decay, followed by a rapid increase in brightness by factor 2.5 about 1 hr after the burst. Its afterglow then faded in a broken power-law fashion, with a smooth break at t_b = 0.6 days, but with additional, less dramatic "bumps and wiggles." The afterglow of GRB 060210 is also unusual: the light curve was more or less flat between 60 and 300 s after the burst, followed by a 70% increase at 600 s after the burst, after which the light curve declined as a ~t^-1.3 power law. We argue that "anomalous" optical afterglows are likely to be the norm and that such rapid optical variations should be seen in many bursts, given good enough sampling. Given that, some of the usual procedures, such as deriving the jet opening angle from fitting a smooth function to the optical light curve, might often have a poor statistical significance. We propose that the rapid rise at ~3000 s in the optical for GRB 060206 and the optical bump at ~700 s in GRB 060210 might be due to the turn-on of the external shock. The existence and timing of such features could provide us with valuable additional information about the bursts.

Multiwavelength observations of the optical afterglow of GRB 050319 were performed from 1.31 to 9.92 hr after the burst. Our R-band light curves, combined with other published data, can be described by the smooth broken power-law function, with α₁ = -0.84 ± 0.02 to α₂ = -0.48 ± 0.03, 0.04 days after the gamma-ray burst. The optical light curves are characterized by shallow decays—as was also observed in the X-rays—which may have a similar origin, related to energy injection. However, our observations indicate that there is still a puzzle concerning the chromatic breaks in the R-band light curve (at 0.04 days) and the X-ray light curve (at 0.004 days) that remains to be solved.

Motivated by a recent interpretation of z ~ 3 objects, we examine processes that control the fraction of primordial (Z = 0) gas, and so primordial stars, in high star formation rate Lyman break galaxies (LBGs). A primordial fraction different from 1 or 0 requires microscopic diffusion catalyzed by a velocity field with a timescale comparable to the duration of star formation. The only process we found that satisfies this requirement for LBGs without fine-tuning is turbulence-enhanced mixing induced by exponential stretching and compressing of metal-rich ejecta. The time dependence of the primordial fraction for this model is calculated. We show that conclusions for all the models discussed here are virtually independent of the initial mass function (IMF), including extremely top-heavy IMFs.

Using Hubble Space Telescope imaging and Keck spectroscopy, we report the discovery of a very bright, highly magnified (~30 times) Lyman break galaxy (LBG) at z = 3.07 in the field of the massive z = 0.33 cluster MACS J2135.2-0102. The system comprises two high surface brightness arcs with a maximum extent of 3'', bracketing a central object that we identify as a massive early-type galaxy at z = 0.73. We construct a lens model that reproduces the main features of the system using a combination of a galaxy-scale lens and the foreground cluster. We show that the morphological, spectral, and photometric properties of the arcs are consistent with them arising from the lensing of a single ~L LBG. The most important feature of this system is that the lensing magnification results in an apparent magnitude of r = 20.3, making this one of the brightest LBGs known. Such a high magnification provides the opportunity of obtaining very high signal-to-noise ratio (and potentially spatially resolved) spectroscopy of a high-redshift galaxy to study its physical properties. We present initial imaging and spectroscopy demonstrating the basic properties of the system and discuss the opportunities for future observations.

It is shown that if gas accretion via a disk onto the central supermassive black hole is efficient only for a surface density Σ ≥ 10 g cm^-2, the black hole mass-galactic bulge velocity dispersion relation (Tremaine et al.) is borne out and so may be the modest dispersion in that relation, in the context of hierarchical structure formation theory. The relation is not expected to evolve with redshift in this model.

We have made 13 positive identifications of near-ultraviolet (NUV) transient sources in the giant elliptical galaxy M87 using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). We give a representative sample of the light curves that we derive for these transients, and based on their characteristics we identify them as classical nova candidates. We obtain a hard lower limit for the nova rate in M87 of 64 novae per year. Our results suggest an enhancement on the frequency of nova events toward the nucleus of the galaxy. No correlation is found with either jet activity or the position of present-day globular clusters.

The silicate cross section peak near 10 μm produces emission and absorption features in the spectra of dusty galactic nuclei observed with the Spitzer Space Telescope. Especially in ultraluminous infrared galaxies, the observed absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A foreground screen of obscuration cannot reproduce this observed feature, even at a large optical depth. Instead, the deep absorption requires a nuclear source to be deeply embedded in a smooth distribution of material that is both geometrically and optically thick. In contrast, a clumpy medium can produce only shallow absorption or emission, which are characteristic of optically identified active galactic nuclei. In general, the geometry of the dusty region and the total optical depth, rather than the grain composition or heating spectrum, determine the silicate feature's observable properties. The apparent optical depth calculated from the ratio of line to continuum emission generally fails to accurately measure the true optical depth. The obscuring geometry, not the nature of the embedded source, also determines the far-IR spectral shape.

We present a new diagnostic diagram for mid-infrared spectra of infrared galaxies based on the equivalent width of the 6.2 μm PAH emission feature and the strength of the 9.7 μm silicate feature. Based on the positions in this diagram, we classify galaxies into nine classes ranging from continuum-dominated AGN hot dust spectra and PAH-dominated starburst spectra to absorption-dominated spectra of deeply obscured galactic nuclei. We find that galaxies are systematically distributed along two distinct branches: one of AGN and starburst-dominated spectra and one of deeply obscured nuclei and starburst-dominated spectra. The separation into two branches likely reflects a fundamental difference in the dust geometry in the two sets of sources: clumpy versus nonclumpy obscuration. Spectra of ULIRGs are found along the full length of both branches, reflecting the diverse nature of the ULIRG family.

We present four spectra of the Type Ia supernova (SN Ia) 2006D extending from -7 to +13 days with respect to B-band maximum. The spectra include the strongest signature of unburned material at photospheric velocities observed in a SN Ia to date. The earliest spectrum exhibits C II absorption features below 14,000 km s^-1, including a distinctive C II λ6580 absorption feature. The carbon signatures dissipate as the SN approaches peak brightness. In addition to discussing implications of photospheric-velocity carbon for white dwarf explosion models, we outline some factors that may influence the frequency of its detection before and around peak brightness. Two effects are explored in this regard, including depopulation of the C II optical levels by non-LTE effects, and line-of-sight effects resulting from a clumpy distribution of unburned material with low volume filling factor.

The millimeter/submillimeter wavelength polarization of Sgr A* is known to be variable in both magnitude and position angle on timescales down to a few hours. The unstable polarization has prevented measurements made at different frequencies and different epochs from yielding convincing measurements of Faraday rotation in this source. Here we present observations made with the Submillimeter Array polarimeter at 227 and 343 GHz with sufficient sensitivity to determine the rotation measure at each band without comparing position angles measured at separate epochs. We find the 10-epoch mean rotation measure to be (-5.6 ± 0.7) × 10⁵ rad m^-2; the measurements are consistent with a constant value. We conservatively assign a 3 σ upper limit of 2 × 10⁵ rad m^-2 to rotation measure changes, which limits accretion rate fluctuations to 25%. This rotation measure detection limits the accretion rate to less than 2 × 10^-7M_☉ yr^-1 if the magnetic field is near equipartition, ordered, and largely radial, while a lower limit of 2 × 10^-9M_☉ yr^-1 holds even for a subequipartition, disordered, or toroidal field. The mean intrinsic position angle is 167^° ± 7^° and we detect variations of 31 deg. These variations must originate in the submillimeter photosphere, rather than arising from rotation measure changes.

We present a reanalysis of our H- and K-band photometry and light curves for GCIRS 16SW, a regular periodic source near the Galactic center. These data include those presented by DePoy et al.; we correct a sign error in their reduction, finding GCIRS 16SW to be an eclipsing binary with no color variations. We find the system to be an equal-mass overcontact binary (both stars overfilling their Roche lobes) in a circular orbit with a period P = 19.4513 days and an inclination angle i = 71^°. This confirms and strengthens the findings of Martins et al. that GCIRS 16SW is an eclipsing binary composed of two ~50 M_☉ stars, further supporting evidence of recent star formation very close to the Galactic center. Finally, the calculated luminosity of each component is close to the Eddington luminosity, implying that the temperature of 24,400 K given by Najarro et al. might be overestimated for these evolved stars.

We describe a new method for robustly testing theoretical predictions of red giant evolution near the tip of the giant branch. When theoretical cumulative luminosity functions are shifted to align the tip in the I band and normalized at a luminosity level slightly brighter than the red giant bump, virtually all dependence on age and composition (heavy elements and helium abundance) is eliminated. While significant comparisons with observations require large samples of giant stars, such samples are available for some of the most massive Milky Way globular clusters. We present comparisons with the clusters NGC 2808 and M5 and find that NGC 2808 has a deficiency of bright giants (with a probability of less than about 3% that a more extreme distribution of giant stars would have happened by chance). We discuss the possibilities that underestimated neutrino losses or strong mass loss could be responsible for the deficit of giants. While we cannot rule out the neutrino hypothesis, it cannot explain the apparent agreement between the M5 observations and models. On the other hand, strong mass loss provides a potential link between the giant star observations and NGC 2808's unusually blue horizontal branch. If the mass loss hypothesis is true, there is likely a significant population of He white dwarfs that could be uncovered with slightly deeper UV observations of the cluster.

The analysis of Balmer-dominated optical spectra from nonradiative (adiabatic) SNRs has shown that the ratio of the electron to proton temperature at the blast wave is close to unity at v_S ≲ 400 km s^-1 but declines sharply down to the minimum value of m_e/m_p dictated by the jump conditions at shock speeds exceeding 2000 km s^-1. We propose a physical model for the heating of electrons and ions in non-cosmic-ray-dominated, strong shocks (v_S > 400 km s^-1) wherein the electrons are heated by lower hybrid waves immediately ahead of the shock front. These waves arise naturally from the cosmic ray pressure gradient upstream from the shock. Our model predicts a nearly constant level of electron heating over a wide range of shock speeds, producing a relationship (T_e/T_p)₀ ∝ v (∝M^-2) that is fully consistent with the observations.

We describe observations of the seventh accretion-powered millisecond pulsar, HETE J1900.1-2455, made with the Rossi X-Ray Timing Explorer during the year of activity that followed its discovery in 2005 June. We detected intermittent pulsations at a peak fractional amplitude of 3%, but only in the first two months of the outburst. On three occasions during this time we observed an abrupt increase in the pulse amplitude, approximately coincident with the time of a thermonuclear burst, followed by a steady decrease on a timescale of ≈10 days. HETE J1900.1-2455 has shown the longest active period by far for any transient accretion-powered millisecond pulsar, comparable instead to the outburst cycles for other transient X-ray binaries. Since the last detection of pulsations, HETE J1900.1-2455 has been indistinguishable from a low-accretion rate, nonpulsing low-mass X-ray binary (LMXB); we hypothesize that other, presently active LMXBs may have also been detectable initially as millisecond X-ray pulsars.

Using a new grism at the Keck Interferometer, we obtained spectrally dispersed (R ~ 230) interferometric measurements of the Mira star R Vir. These data show that the measured radius of the emission varies substantially from 2.0 to 2.4 μm. Simple models can reproduce these wavelength-dependent variations using extended molecular layers, which absorb stellar radiation and reemit it at longer wavelengths. Because we observe spectral regions with and without substantial molecular opacity, we determine the stellar photospheric radius, uncontaminated by molecular emission. We infer that most of the molecular opacity arises at approximately twice the radius of the stellar photosphere.

A robust unconstrained orbital solution is obtained for the G2 V star HIP 16853 = HD 22705 at 42 pc, which is a probable member of the 28-30 Myr old Tucana-Horologium stream of post-T Tauri stars. The solution yields an apparent semimajor axis of 5.1 ± 0.7 mas, a period of 201 ± 2 days, and an inclination of 80° ± 7°. Assuming a mass of 1 M_☉ for the primary, the close companion is only 0.4 M_☉, which implies a spectral type M0.5 at this age. The expected maximum separation (a) between the companions is 18 mas, which makes this system amenable for high-resolution observations. The wide companion HIP 16853 B at 14'' is investigated as a possible tertiary component but rejected on account of the near-infrared photometric data inconsistent with the well-defined H-R diagram of the Tucana-Horologium group.

High-precision radial velocity techniques, which enabled the detection of extrasolar planets, are now sensitive to relativistic effects in the data of spectroscopic binaries (SBs). We show how these effects can be used to derive the absolute masses of the components of eclipsing single-lined SBs and double-lined SBs from Doppler measurements alone. High-precision stellar spectroscopy can thus substantially increase the number of measured stellar masses, thereby improving the mass-radius and mass-luminosity calibrations.

We report a dual-band observation at 223 and 654 GHz (460 μm) toward an ultracompact (UC) H II region, G240.31+0.07, using the Submillimeter Array. With a beam size of 1.5'' × 0.8'', the dust continuum emission is resolved into two clumps, with clump A well coincident with an H₂O maser and the UC H II region. The newly discovered clump, B, about 1.3'' (≃8.3 × 10³ AU) to the southwest of clump A, is also associated with H₂O masers and may be a more recent star-forming site. The continuum flux densities imply an opacity spectral index of β = 1.5 ± 0.3, suggestive of a value lower than the canonical 2.0 found in the interstellar medium and in cold, massive cores. The presence of hot (≃100 K) molecular gas is derived by the brightness ratio of two H₂CO lines in the 223 GHz band. A radial velocity difference of 2.5 ± 0.4 km s^-1 is found between the two clumps in C¹⁸O(6-5) emission. The total (nebular and stellar) mass of roughly 58 M_☉ in the central region is close to, but not much larger than, the minimum mass required for the two clumps to be gravitationally bound for binary rotation. Our continuum data do not suggest a large amount of matter associated with the H₂ knots that were previously proposed to arise from a massive disk or envelope.

RY Tau is a rapidly rotating classical T Tauri star observed close to edge-on. The combination of new HST/STIS observations obtained in 2001 with HST/GHRS archive data from 1993 has allowed us to get, for the first time, information on the thermal structure and velocity of the wind. The repeated observations of the Si III] and C III] lines show a lack of changes with time on the blue side of the profile (dominated by the wind contribution). Very high temperature plasma (log T_e = 4.8) is detected at densities of 9.5 ≤ log n_e(cm^-3) ≤ 10.2 associated with the wind. The emitting volumes are ~(0.35 R_☉)³, suggesting a stellar origin. The wind kinematics derived from the profiles (Si III], C III], and [O II]) does not satisfy the theoretical predictions of MHD centrifugally driven disk winds. The profiles' asymmetry, large velocity dispersions, and small variability as well as the small emitting volumes are best explained if the wind is produced by the contributions of several outflows from atmospheric open-field structures like those observed in the Sun.

We report the discovery of a short-duration (less than 3 months) outburst of the H₂CO 6 cm maser in IRAS 18566+0408 (G37.55+0.20). During the flare, the peak flux density of the maser increased by a factor of 4; after less than a month, it decayed to the preflare value. This is the first detection of a short, burstlike variability of an H₂CO 6 cm maser. The maser shows an asymmetric line profile that is consistent with the superposition of two Gaussian components. We did not detect a change in the velocity or the line width of the Gaussian components during the flare. If the two Gaussian components trace two separate maser regions, then very likely an event outside the maser gas triggered simultaneous flares at two different locations.

Among the hot Jupiters known to date that transit their parent stars, the two best candidates to be observed with transmission spectroscopy in the mid-infrared (MIR) are HD 189733b and HD 209458b, due to their combined characteristics of planetary density, orbital parameters, and parent star distance and brightness. Here we simulate transmission spectra of these two planets during their primary transit in the MIR, and we present sensitivity studies of the spectra to the changes of atmospheric thermal properties, molecular abundances, and C/O ratios. Our model predicts that the dominant species absorbing in the MIR on hot Jupiters are water vapor and carbon monoxide, and their relative abundances are determined by the C/O ratio. Since the temperature profile plays a secondary role in the transmission spectra of hot Jupiters compared to molecular abundances, future primary transit observations in the MIR of those objects might offer insight on extrasolar giant planet atmospheric chemistry. We find here that the absorption features caused by water vapor and carbon monoxide in a cloud-free atmosphere are deep enough to be observable by the present and future generation of space-based observatories, such as Spitzer Space Telescope and James Webb Space Telescope. We discuss our results in light of the capabilities of these telescopes.

Multispacecraft measurements in the solar wind are used to determine the field-aligned anisotropy of magnetohydrodynamic inertial range turbulence. The ratio of the parallel to perpendicular correlation lengths is measured by using time-lagged two-point correlations to construct a spatial autocorrelation function. The mean ratio obtained, 1.79 ± 0.36, is significantly greater than unity and therefore consistent with solar wind fluctuations being anisotropic with energy predominantly in wavevectors perpendicular to the large-scale mean magnetic field. In analyzing eight 40-60 minute intervals of multipoint magnetic field data from the four Cluster spacecraft, the degree of variation in the ratio of the parallel to perpendicular correlation lengths about the mean was larger than expected. This variation does not appear to be correlated with the solar wind velocity or the plasma beta. The ratio of parallel to perpendicular correlation lengths was also uncorrelated between different field components.