Table of contents

On large angular scales, the polarization of the CMB contains information about the evolution of the average ionization during the epoch of reionization. Interpretation of the polarization spectrum usually requires the assumption of a fixed functional form for the evolution, e.g., instantaneous reionization. We develop a model-independent method where a small set of principal components completely encapsulates the effects of reionization on the large-angle E-mode polarization for any reionization history within an adjustable range in redshift. Using Markov chain Monte Carlo methods, we apply this approach to both the 3 year WMAP data and simulated future data. WMAP data constrain two principal components of the reionization history, approximately corresponding to the total optical depth and the difference between the contributions to the optical depth at high and low redshifts. The optical depth is consistent with the constraint found in previous analyses of WMAP data that assume instantaneous reionization, with only a slightly larger uncertainty due to the expanded set of models. Using the principal component approach, WMAP data also place a 95% confidence level upper limit of 0.08 on the contribution to the optical depth from redshifts z > 20. With improvements in polarization sensitivity and foreground modeling, approximately five of the principal components can ultimately be measured. Constraints on the principal components, which probe the entire reionization history, can test models of reionization, provide model-independent constraints on the optical depth, and detect signatures of high-redshift reionization.

In a hierarchical picture of galaxy formation virialization continually transforms gravitational potential energy into kinetic energies of the baryonic and dark matter. For the gaseous component the kinetic, turbulent energy is transformed eventually into internal thermal energy through shocks and viscous dissipation. Traditionally this virialization and shock heating has been assumed to occur instantaneously, allowing an estimate of the gas temperature to be derived from the virial temperature defined from the embedding dark matter halo velocity dispersion. As the mass grows the virial temperature of a halo grows. Mass accretion hence can be translated into a heating term. We derive this heating rate from the extended Press Schechter formalism and demonstrate its usefulness in semianalytical models of galaxy formation. Our method explicitly conserves energy, unlike the previous impulsive heating assumptions. Our formalism can trivially be applied in all current semianalytical models as the heating term can be computed directly from the underlying merger trees. Our analytic results for the first cooling halos and the transition from cold to hot accretion are in agreement with numerical simulations.

We report results from numerical simulations of star formation in the early universe that focus on gas at very high densities and very low metallicities. We argue that the gas in the central regions of protogalactic halos will fragment as long as it carries sufficient angular momentum. Rotation leads to the build-up of massive disklike structures which fragment to form protostars. At metallicities Z ≈ 10⁻⁵Z_☉, dust cooling becomes effective and leads to a sudden drop of temperature at densities above n = 10¹² cm⁻³. This induces vigorous fragmentation, leading to a very densely packed cluster of low-mass stars. This is the first stellar cluster. The mass function of stars peaks below 1 M_☉, similar to what is found in the solar neighborhood and comparable to the masses of the very low metallicity subgiant stars recently discovered in the halo of our Milky Way. We find that even purely primordial gas can fragment at densities 10¹⁴ cm ⁻³ ⩽ n⩽ 10¹⁶ cm ⁻³, although the resulting mass function contains only a few objects (at least a factor of 10 fewer than the Z = 10⁻⁵Z_☉ mass function) and is biased toward higher masses. A similar result is found for gas with Z = 10⁻⁶Z_☉. Gas with Z ⩽ 10⁻⁶Z_☉ behaves roughly isothermally at these densities (with polytropic exponent γ ≈ 1.06), and the massive disklike structures that form due to angular momentum conservation will be marginally unstable. As fragmentation is less efficient, we expect stars with Z ⩽ 10⁻⁶Z_☉ to be massive, with masses in excess of several tens of solar masses, consistent with the results from previous studies.

We model the escape of ionizing radiation from high-redshift galaxies using high-resolution adaptive mesh refinement N-body + hydrodynamics simulations. Our simulations include the time-dependent and spatially resolved transfer of ionizing radiation in three dimensions, including the effects of dust absorption. For galaxies of total mass M≳ 10¹¹M_☉ and star formation rates SFR ≈ 1–5 M_☉ yr⁻¹, we find angular averaged escape fractions of 1%-3% over the entire redshift interval studied (3 < z < 9). In addition, we find that the escape fraction varies by more than an order of magnitude along different lines of sight within individual galaxies, from the largest values near galactic poles to the smallest along the galactic disk. The escape fraction declines steeply at lower masses and SFR. We show that the low values of escape fractions are due to a small fraction of young stars located just outside the edge of the H I disk. This fraction, and hence the escape fraction, is progressively smaller in disks of smaller galaxies because their H I disks are thicker and more extended relative to the distribution of young stars compared to massive galaxies. We compare our predicted escape fraction of ionizing photons with previous results and find a general agreement with both other simulation results and available direct detection measurements at z ∼ 3. We also compare our simulations with a novel method to estimate the escape fraction in galaxies from the observed distribution of neutral hydrogen column densities along the lines of sight to long-duration γ-ray bursts (GRBs). Using this method we find escape fractions of the GRB host galaxies of 2%-3%, consistent with our theoretical predictions. Our results thus suggest that high-redshift galaxies are inefficient in releasing ionizing radiation produced by young stars into the intergalactic medium.

Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that there is only mild evolution in the relationship between radial size and stellar mass for galactic disks from z ∼ 1 to the present day. Using a sample of nearby disk-dominated galaxies from the Sloan Digital Sky Survey (SDSS) and high-redshift data from the GEMS (Galaxy Evolution from Morphology and SEDs) survey, we investigate whether this result is consistent with theoretical expectations within the hierarchical paradigm of structure formation. The relationship between virial radius and mass for dark matter halos in the ΛCDM model evolves by about a factor of 2 over this interval. However, N-body simulations have shown that halos of a given mass have less centrally concentrated mass profiles at high redshift. When we compute the expected disk size-stellar mass distribution, accounting for this evolution in the internal structure of dark matter halos and the adiabatic contraction of the dark matter by the self-gravity of the collapsing baryons, we find that the predicted evolution in the mean size at fixed stellar mass since z ∼ 1 is about 15%-20%, in good agreement with the observational constraints from GEMS. At redshift z ∼ 2, the model predicts that disks at fixed stellar mass were on average only 60% as large as they are today. Similarly, we predict that the rotation velocity at a given stellar mass (essentially the zero point of the Tully-Fisher relation) is only about 10% larger at z ∼ 1 (20% at z ∼ 2) than at the present day.

We present the results from a multiwavelength campaign conducted in 2006 August of the powerful γ-ray quasar PKS 1510–089 (z = 0.361). This campaign commenced with a deep Suzaku observation lasting 3 days for a total exposure time of 120 ks and continued with Swift monitoring over 18 days. Besides Swift observations, the campaign included ground-based optical and radio data and yielded a quasi-simultaneous broadband spectrum from 10⁹ to 10¹⁹ Hz. The Suzaku observation provided a high signal-to-noise ratio X-ray spectrum, which is well represented by an extremely hard power law with a photon index of Γ ≃ 1.2, augmented by a soft component apparent below 1 keV, which is well described by a blackbody model with a temperature of kT≃ 0.2 keV. Monitoring by Suzaku revealed temporal variability that differs between the low- and high-energy bands, again suggesting the presence of a second, variable component in addition to the primary power-law emission. We model the broadband spectrum, assuming that the high-energy spectral component results from Comptonization of infrared radiation produced by hot dust located in the surrounding molecular torus. The adopted internal shock scenario implies that the power of the jet is dominated by protons, but with a number of electrons and/or positrons that exceeds the number of protons by a factor of ~10. We also find that inhomogeneities responsible for the shock formation prior to the collision may produce bulk Compton radiation, which can explain the observed soft X-ray excess and possible excess at ~18 keV. We note, however, that the bulk Compton interpretation is not unique, as discussed briefly in the text.

We report measurements of centripetal accelerations of maser spectral components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second paper of a series, in which the goal is the determination of a new geometric maser distance to NGC 4258, accurate to possibly ~3%. We measure accelerations using a formal analysis method that involves simultaneous decomposition of maser spectra for all epochs into multiple, Gaussian components. Components are coupled between epochs by linear drifts (accelerations) from their centroid velocities at a reference epoch. For high-velocity emission, accelerations lie in the range –0.7 to +0.7 km s⁻¹ yr⁻¹, indicating an origin within 13° of the disk midline (the perpendicular to the line of sight [LOS] to the black hole). Comparison of the projected positions of high-velocity emission in VLBI images with those derived from acceleration data provides evidence that masers trace real gas dynamics. High-velocity emission accelerations do not support a model of trailing shocks associated with spiral arms in the disk. However, we find strengthened evidence for spatial periodicity in high-velocity emission, of wavelength 0.75 mas. This supports suggestions of spiral structure due to density waves in the nuclear accretion disk of an active galaxy. Accelerations of low-velocity (systemic) emission lie in the range 7.7 to 8.9 km s⁻¹ yr⁻¹, consistent with emission originating from a concavity where the thin, warped disk is tangent to the LOS. A trend in accelerations of low-velocity emission, as a function of Doppler velocity, may be associated with disk geometry and orientation or with the presence of spiral structure.

We present detailed fits of the spectral energy distributions (SEDs) of four submillimeter galaxies selected by the presence of a gamma-ray burst (GRB) event (GRBs 980703, 000210, 000418, and 010222). These faint ~3 mJy submillimeter emitters at redshift ~1 are characterized by an unusual combination of long- and short-wavelength properties, namely enhanced submillimeter and/or radio emission combined with optical faintness and blue colors. We exclude an active galactic nucleus as the source of long-wavelength emission. From the SED fits, we conclude that the four galaxies are young (ages <2 Gyr), highly star forming (star formation rates ~150 M_☉ yr⁻¹), low mass (stellar masses ~10¹⁰M_☉), and dusty (dust masses ~3 × 10⁸M_☉). Their high dust temperatures (T_d≳ 45 K) indicate that GRB host galaxies are hotter, younger, and less massive counterparts to the submillimeter-selected galaxies detected so far. Future facilities like Herschel, the James Clerk Maxwell Telescope SCUBA-2, and ALMA will test this hypothesis, enabling measurement of dust temperatures of fainter GRB-selected galaxies.

Observational evidence for the radial alignment of satellites with their dark matter host has been accumulating steadily over the past few years. The effect is seen over a wide range of scales, from massive clusters of galaxies down to galaxy-sized systems, yet the underlying physical mechanism has still not been established. To this end, we have carried out a detailed analysis of the shapes and orientations of dark matter substructures in high-resolution N-body cosmological simulations. We find a strong tendency for radial alignment of the substructure with its host halo: the distribution of halo major axes is very anisotropic, with the majority pointing toward the center of mass of the host. The alignment peaks once the subhalo has passed the virial radius of the host for the first time, but is not subsequently diluted, even after the halos have gone through as many as four pericentric passages. This evidence points to the existence of a very rapid dynamical mechanism acting on these systems, and we argue that tidal torquing throughout their orbits is the most likely candidate.

We derive probability density functions for the projected axial ratios of the real and mock 2PIGG galaxy groups, and use this data to investigate the intrinsic three-dimensional shape of the dark matter ellipsoids that they trace. As well as analyzing the raw data for groups of varying multiplicities, a convolution-corrected form of the data is also considered which weights the probability density function according to the results of multiple Monte Carlo realizations of discrete samples from the input spatial distributions. The important effect observed is that the best-fit distribution for all the raw data is a prolate ellipsoid with a Gaussian distribution of axial ratios with = 0.36 and σ = 0.14, while for the convolved data the best-fit solution is that of an oblate ellipsoid, = 0.22 and σ = 0.1. Previously, only prolate distributions were thought compatible with the data, this being interpreted as evidence of filamentary collapse at nodes. We also find that even after allowing for sampling effects, the corrected data are better fit using separate multiplicity bins that display a trend toward more spherical halos in higher multiplicity groups. Finally, we find that all results in the real data are in good agreement with the mock data from ΛCDM simulations, with K-S tests showing that all comparative data have been drawn from the same distributions within the 1 σ confidence limits.

We present new predictions for the galaxy three-point correlation function (3PCF) using high-resolution dissipationless cosmological simulations of a flat ΛCDM universe which resolve galaxy-size halos and subhalos. We create realistic mock galaxy catalogs by assigning luminosities and colors to dark matter halos and subhalos, and we measure the reduced 3PCF as a function of luminosity and color in both real and redshift space. As galaxy luminosity and color are varied, we find small differences in the amplitude and shape dependence of the reduced 3PCF, at a level qualitatively consistent with recent measurements from the SDSS and 2dFGRS. We confirm that discrepancies between previous 3PCF measurements can be explained in part by differences in binning choices. We explore the degree to which a simple local bias model can fit the simulated 3PCF. The agreement between the model predictions and galaxy 3PCF measurements lends further credence to the straightforward association of galaxies with CDM halos and subhalos.

We present the results of a deep near-infrared imaging survey of the Rosette complex made with FLAMINGOS at the 2.1 m telescope at Kitt Peak National Observatory. We studied the distribution of young embedded sources using a variation of the nearest neighbor method applied to a carefully selected sample of near-infrared excess (NIRX) stars that trace the latest episode of star formation in the complex. Our analysis confirmed the existence of seven clusters previously detected in the molecular cloud, and identified four more clusters across the complex. We determined that 60% of the young stars in the complex and 86% of the stars within the molecular cloud are contained in clusters, implying that the majority of stars in the Rosette formed in embedded clusters. Also, half of the young embedded population is contained in four clusters that coincide with the central core of the cloud, where the main interaction with the H II region is taking place. We compare the sizes, infrared excess fractions, and average extinction toward individual clusters to investigate their early evolution and expansion. In particular, the size and degree of central condensation within the clusters appear to be related to the degree of infrared excess and mean extinction in a way that suggests that the clusters form as compact entities and then quickly expand after formation. We found that the average infrared excess fraction of clusters increases as a function of distance from NGC 2244, implying a temporal sequence of star formation across the complex. This sequence appears to be primordial, possibly resulting from the formation and evolution of the molecular cloud and not from the interaction with the H II region. Instead, the main influence of the H II region could be to enhance or inhibit the underlying pattern of star formation in the cloud.

Nearby dwarf galaxies exhibit tight correlations between their global stellar and dynamical properties, such as circular velocity, mass-to-light ratio, stellar mass, surface brightness, and metallicity. Such correlations have often been attributed to gas or metal-rich outflows driven by supernova energy feedback to the interstellar medium. We use high-resolution cosmological simulations of high-redshift galaxies with and without energy feedback, as well as analytic modeling, to investigate whether the observed correlations can arise without supernova-driven outflows. We find that the simulated dwarf galaxies exhibit correlations similar to those observed as early as z ≈ 10, regardless of whether supernova feedback is included. We also show that the correlations can be well reproduced by our analytic model that accounts for realistic gas inflow but assumes no outflows, and a star formation rate obeying the Kennicutt-Schmidt law with a critical density threshold. We argue that correlations in simulated galaxies arise due to the increasingly inefficient conversion of gas into stars in low-mass dwarf galaxies, rather than supernova-driven outflows. We also show that the decrease of the observed effective yield in low-mass objects, often used as an indicator of gas and metal outflows, can be reasonably reproduced in our simulations without outflows. We show that this trend can arise if a significant fraction of metals in small galaxies is spread to the outer regions of the halo outside the stellar extent via mixing. In this case the effective yield can be significantly underestimated if only metals within the stellar radius are taken into account. Measurements of gas metallicity in the outskirts of gaseous disks of dwarfs would thus provide a key test of such an explanation.

We use N-body simulations to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. We adopt a cosmologically motivated model in which dSphs are approximated by a King model embedded in an NFW halo. We find that these NFW-embedded King models are extraordinarily resilient to tides; the stellar density profile still resembles a King model even after losing more than 99% of the stars. As tides strip the galaxy, the stellar luminosity, velocity dispersion, central surface brightness, and core radius decrease monotonically. Remarkably, we find that the evolution of these parameters is solely controlled by the total amount of mass lost from within the luminous radius. Of all parameters, the core radius is the least affected: after losing 99% of the stars, R_c decreases by just a factor of ~2. Interestingly, tides tend to make dSphs more dark matter dominated because the tightly bound central dark matter "cusp" is more resilient to disruption than the "cored" King profile. We examine whether the extremely large mass-to-light ratios of the newly discovered ultrafaint dSphs might have been caused by tidal stripping of once brighter systems. Although dSph tidal evolutionary tracks parallel the observed scaling relations in the luminosity-radius plane, they predict too steep a change in velocity dispersion compared with the observational estimates hitherto reported in the literature. The ultrafaint dwarfs are thus unlikely to be the tidal remnants of systems like Fornax, Draco, or Sagittarius. Despite spanning four decades in luminosity, dSphs appear to inhabit halos of comparable peak circular velocity, lending support to scenarios that envision dwarf spheroidals as able to form only in halos above a certain mass threshold.

In the third part of our photometric study of the star-forming region NGC 346/N66 and its surrounding field in the Small Magellanic Cloud (SMC), we focus on the large number of low-mass pre-main-sequence (PMS) stars revealed by the Hubble Space Telescope observations with the Advanced Camera for Surveys. We investigate the origin of the observed broadening of the PMS population in the V − I, V color-magnitude diagram. The most likely explanations are either the presence of differential reddening or an age spread among the young stars. Assuming the latter, simulations indicate that we cannot exclude the possibility that stars in NGC 346 might have formed in two distinct events occurring about 10 and 5 Myr ago, respectively. We find that the PMS stars are not homogeneously distributed across NGC 346, but instead are grouped in at least five different clusters. On spatial scales from 0.8'' to 8'' (0.24-2.4 pc at the distance of the SMC) the clustering of the PMS stars as computed by a two-point angular correlation function is self-similar with a power-law slope γ ≈ − 0.3. The clustering properties are quite similar to Milky Way star-forming regions like Orion OB or ρ Oph. Thus molecular cloud fragmentation in the SMC seems to proceed on the same spatial scales as in the Milky Way. This is remarkable given the differences in metallicity and hence dust content between SMC and Milky Way star-forming regions.

We combine optical and radio observations to trace the spiral structure in the third quadrant of the Milky Way. The optical observations consist of a large sample of young open clusters and associations, whereas the radio observations consist of a survey of nearby and distant clouds observed in CO. Both the optical and radio samples are the largest ones thus far presented in the literature. We use this unique material to analyze the behavior of interstellar extinction and to trace the detailed structure of the third Galactic quadrant (TGQ).We find that the outer (Cygnus) grand design spiral arm is traced by stellar and CO components, while the Perseus arm is traced solely by CO and is possibly being disrupted by the crossing of the Local (Orion) arm. The Local arm is traced by CO and young stars toward l = 240° and extends for over 8 kpc along the line of sight reaching the outer arm. Finally, we characterize the Galactic warp and compare the geometries implied by the young stellar and CO components.

A particle acceleration mechanism by radiation pressure of precursor waves in a relativistic shock is studied. For a relativistic, perpendicular shock with the upstream bulk Lorentz factor of γ₁≫ 1, large-amplitude electromagnetic (light) waves are known to be excited in the shock front due to the synchrotron maser instability, and those waves can propagate toward upstream as precursor waves. We find that nonthermal, high-energy electrons and ions can be quickly produced by an action of electrostatic wakefields generated by the ponderomotive force of the precursor waves. The particles can be quickly accelerated up to ε_max/γ₁m_ec² ∼ γ₁ in the upstream coherent wakefield region, and they can be further accelerated during the nonlinear stage of the wakefield evolution. The maximum attainable energy is estimated by ε_max/γ₁m_ec² ∼ L_sys/(c/ω_pe) , where L_sys and c/ω_pe are the size of an astrophysical object and the electron inertial length, respectively.

We report the serendipitous detection of the planetary nebula NGC 5315 by the Chandra X-Ray Observatory. The Chandra imaging spectroscopy results indicate that the X-rays from this PN, which harbors a Wolf-Rayet (W-R) central star, emanate from a T_X ∼ 2.5 × 10⁶ K plasma generated via the same wind-wind collisions that have cleared a compact (≲8000 AU radius) central cavity within the nebula. The inferred X-ray luminosity of NGC 5315 is ~2.5 × 10³² ergs s⁻¹ (0.3-2.0 keV), placing this object among the most luminous such "hot bubble" X-ray sources yet detected within PNe. With the X-ray detection of NGC 5315, objects with W-R-type central stars now constitute a clear majority of known examples of diffuse X-ray sources among PNe; all such "hot bubble" PN X-ray sources display well-defined, quasi-continuous optical rims. We therefore assert that X-ray-luminous hot bubbles are characteristic of young PNe with large central star wind kinetic energies and closed bubble morphologies. However, the evidence at hand also suggests that processes such as wind and bubble temporal evolution, as well as heat conduction and/or mixing of hot bubble and nebular gas, ultimately govern the luminosity and temperature of superheated plasma within PNe.

We present high-resolution Combined Array for Research in Millimeter-Wave Astronomy (CARMA) λ = 1 mm observations of several molecular species toward Orion-KL. These are the highest spatial and spectral resolution 1 mm observations of these molecules to date. Our observations show that ethyl cyanide [C₂H₅CN] and vinyl cyanide [C₂H₃CN] originate from multiple cores near the Orion hot core and IRc7. In addition we show that dimethyl ether [(CH₃)₂O] and methyl formate [HCOOCH₃] originate from IRc5 and IRc6 and that acetone [(CH₃)₂CO] originates only from areas where both N-bearing and O-bearing species are present.

We have discovered a bow shock shaped mid-infrared excess region in front of δ Velorum using 24 μm observations obtained with the Multiband Imaging Photometer for Spitzer (MIPS). Although the bow shock morphology was only detected in the 24 μm observations, its excess was also resolved at 70 μm. We show that the stellar heating of an ambient interstellar medium (ISM) cloud can produce the measured flux. Since δ Velorum was classified as a debris disk star previously, our discovery may call into question the same classification of other stars. We model the interaction of the star and ISM, producing images that show the same geometry and surface brightness as is observed. The modeled ISM is ~15 times overdense relative to the average Local Bubble value, which is surprising considering the close proximity (24 pc) of δ Velorum. The abundance anomalies of λ Boötis stars have been previously explained as arising from the same type of interaction of stars with the ISM. Low-resolution optical spectra of δ Velorum show that it does not belong to this stellar class. The star therefore is an interesting testbed for the ISM accretion theory of the λ Boötis phenomenon.

Interstellar dust grain models are not sufficiently constrained by UV extinction curves to allow researchers to distinguish between them. By testing grain models in the X-ray regime and applying elemental abundance constraints, we show to what extent the models can reproduce the observables in these regimes and whether they are capable of doing so while respecting the abundance limits. We tested the MRN and WD grain models using the UV extinction and X-ray halos along the line of sight toward X Per. Both models provide reasonable fits to the UV extinction and X-ray halos but cannot do so while respecting the elemental abundance constraints. Furthermore, the abundances and N_H required to reproduce the observables in these two regimes are not consistent with each other, reflecting the fact that X-ray regime constraints were not taken into account when the models' grain size distributions were constructed, and thus the models are incomplete. Both MRN and WD underestimate the hydrogen column density N_H when the standard model parameters for R_V = 3.1 are used. The problem is mitigated when model parameters derived from fitting the UV extinction are used, but this does not necessarily lead to agreement between the two regimes.

We have performed two-dimensional hydrodynamic simulations of a pulsed astrophysical jet propagating through a medium that is populated with circular inhomogeneities, or "clumps," which are smaller than the jet width. The clumps are seen to affect the jet in several ways, such as impeding jet propagation and deflecting the jet off-axis. While there has been some debate as to the prevalence of these types of condensations in the ISM or in molecular clouds, the exploration of this region of parameter space nonetheless both shows the potential for these clumps to disrupt astrophysical jets and yields results which recover aspects of recent observations of Herbig-Haro objects. We find that the propagation of the jet and the vorticity induced in the clump/ambient medium correlate well with a "dynamic filling function" f_d across all the simulations.

The observed increase in star formation efficiency with average cloud density, from several percent in whole giant molecular clouds to ~30% or more in cluster-forming cores, can be understood as the result of hierarchical cloud structure if there is a characteristic density at which individual stars become well defined. Also in this case, the efficiency of star formation increases with the dispersion of the density probability distribution function (PDF). Models with lognormal PDFs illustrate these effects. The difference between star formation in bound clusters and star formation in loose groupings is attributed to a difference in cloud pressure, with higher pressures forming more tightly bound clusters. This correlation accounts for the observed increase in the clustering fraction with star formation rate and with the observation of scaled OB associations in low-pressure environments. "Faint fuzzie" star clusters, which are bound but have low densities, can form in regions with high Mach numbers and low background tidal forces. The proposal by Burkert, Brodie & Larsen that faint fuzzies form at large radii in galactic collisional rings satisfies these constraints.

We present maps of 14.4 deg² of the Ophiuchus dark clouds observed by the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS). These high-quality maps depict both numerous point sources and extended dust emission within the star-forming and non-star-forming portions of these clouds. Using PSF-fitting photometry, we detect 5779 sources at 24 μm and 81 sources at 70 μm at the 10 σ level of significance. Three hundred twenty-three candidate young stellar objects (YSOs) were identified according to their positions on the MIPS/2MASS K versus K − [ 24] color-magnitude diagrams, as compared to 24 μm detections in the SWIRE extragalactic survey. We find that more than half of the YSO candidates, and almost all those with protostellar Class I spectral energy distributions, are confined to the known cluster and aggregates.

We calculate detailed non-LTE synthetic spectra of a pulsating reverse detonation (PRD) model, a novel explosion mechanism for Type Ia supernovae. While the hydro models are calculated in three dimensions, the spectra use an angle-averaged hydro model and thus some of the three-dimensional (3D) details are lost, but the overall average should be a good representation of the average observed spectra. We study the model at three epochs: maximum light, 7 days prior to maximum light, and 5 days after maximum light. At maximum the defining Si II feature is prominent, but there is also a prominent C II feature, not usually observed in normal SNe Ia near maximum. We compare to the early spectrum of SN 2006D, which did show a prominent C II feature, but the fit to the observations is not compelling. Finally, we compare to the postmaximum UV+optical spectrum of SN 1992A. With the broad spectral coverage it is clear that the iron-peak elements on the outside of the model push too much flux to the red and thus the particular PRD realizations studied would be intrinsically far redder than observed SNe Ia. We briefly discuss variations that could improve future PRD models.

We investigate the effects of neutrino-nucleus interactions (the ν-process) on the production of iron-peak elements in Population III core-collapse supernovae. The ν-process and the following proton and neutron capture reactions produce odd-Z iron-peak elements in the complete and incomplete Si-burning region. This reaction sequence enhances the abundances of Sc, Mn, and Co in the supernova ejecta. Supernova explosion models of 15 and 25 M_☉ stars with the ν-process reproduce well the average Mn/Fe ratio observed in extremely metal-poor halo stars. In order to reproduce the observed Mn/Fe ratio, the total neutrino energy in the supernovae should be (3–9) × 10⁵³ ergs. Stronger neutrino irradiation and other production sites are necessary to reproduce the observed Sc/Fe and Co/Fe ratios, although these ratios increase by the ν-process.

We consider the angular momentum exchange at the corotation resonance between a two-dimensional gaseous disk and a uniformly rotating external potential, assuming that the disk flow is adiabatic. We first consider the linear case for an isolated resonance, for which we give an expression of the corotation torque that involves the pressure perturbation and which reduces to the usual dependence on the vortensity gradient in the limit of a cold disk. Although this expression requires the solution of the hydrodynamic equations, it provides some insight into the dynamics of the corotation region. In the general case, we find an additional dependence on the entropy gradient at corotation. This dependence is associated with the advection of entropy perturbations. These are not associated with pressure perturbations. They remain confined to the corotation region, where they yield a singular contribution to the corotation torque. In a second part, we check our torque expression by means of customized two-dimensional hydrodynamical simulations. In a third part, we contemplate the case of a planet embedded in a Keplerian disk, assumed to be adiabatic. We find an excess of corotation torque that scales with the entropy gradient, and we check that the contribution of the entropy perturbation to the torque is in agreement with the expression obtained from the linear analysis. We finally discuss some implications of the corotation torque expression for the migration of low-mass planets in the regions of protoplanetary disks where the flow is radiatively inefficient on the timescale of the horseshoe U-turns.

Recent hydrodynamic studies of core-collapse supernovae imply that the neutrino-heated ejecta from a nascent neutron star develop to supersonic outflows. These supersonic winds are influenced by the reverse shock from the preceding supernova ejecta, forming the wind termination shock. We investigate the effects of the termination shock in neutrino-driven winds and its role in the r-process. Supersonic outflows are calculated with a semianalytic neutrino-driven wind model. Subsequent termination-shocked, subsonic outflows are obtained by applying the Rankine-Hugoniot relations. We find a couple of effects that can be relevant for the r-process. First is the sudden slowdown of the temperature decrease by the wind termination. Second is the entropy jump by termination-shock heating, up to several hundred N_Ak. Calculations of nucleosynthesis in the obtained winds are performed to examine these effects on the r-process. We find that the slowdown of the temperature decrease plays a decisive role in determining the r-process abundance curves. This is due to the strong dependences of the nucleosynthetic path on the temperature during the r-process freezeout phase. Our results suggest that only the termination-shocked winds with relatively small shock radii (~500 km) are relevant for the bulk of the solar r-process abundances (A ≈ 100-180). The heaviest part of the solar r-process curve (A ≈ 180-200), however, can be reproduced in both shocked and unshocked winds. These results may help to constrain the mass range of supernova progenitors relevant for the r-process. We find, on the other hand, a negligible role of the entropy jump in the r-process. This is because the sizable entropy increase takes place only at a large shock radius (≳10,000 km), where the r-process has already ceased.

We report on optical and near-infrared observations obtained during and after the 2004 December discovery outburst of the X-ray transient and accretion-powered millisecond pulsar IGR J00291+5934. Our observations monitored the evolution of the brightness and the spectral properties of IGR J00291+5934 during the outburst decay toward quiescence. We also present optical, near-infrared, and Chandra observations obtained during true quiescence. Photometry of the field during outburst reveals an optical and near-infrared counterpart that brightened from R≃ 23 to R≃ 17 and from K = 19 to K≃ 16. Spectral analysis of the RIJHK broadband photometry shows excess in the near-infrared bands that may be due to synchrotron emission. The Hα emission line profile suggests the orbital inclination is ≃22°-32°. The preferred range for the reddening toward the source is 0.7 ⩽ E(B − V) ⩽ 0.9, which is equivalent to 4.06 × 10²¹ cm ⁻² ⩽ N_H ⩽ 5.22 × 10²¹ cm⁻². The Chandra observations of the pulsar in its quiescent state gave an unabsorbed 0.5-10 keV flux for the best-fitting power-law model to the source spectrum of (7.0 ± 0.9) × 10⁻¹⁴ ergs cm⁻² s⁻¹ (adopting a hydrogen column of 4.6 × 10²¹ cm⁻²). The fit resulted in a power-law photon index of 2.4^{+ 0.5}_−0.4. The (R − K)₀ color observed during quiescence supports an irradiated donor star and accretion disk. We estimate a distance of 2-4 kpc toward IGR J00291+5934 by using the outburst X-ray light curve and the estimated critical X-ray luminosity necessary to keep the outer parts of the accretion disk ionized. Using the quiescent X-ray luminosity and the spin period, we constrain the magnetic field of the neutron star to be <3 × 10⁸ G.

We observed Circinus X-1 twice during a newly reached low-flux phase near zero orbital phase using the High-Energy Transmission Grating Spectrometer (HETGS) onboard Chandra. In both observations the source did not show the P Cygni lines we observed during the high-flux phases of the source in 2000 and 2001. During the prezero phase the source did not exhibit significant variability but did exhibit an emission-line spectrum rich in H- and He-like lines from high-Z elements such as Si, S, Ar, and Ca. The light curve in the postdip observation showed quiescent and flaring episodes. Only in these flaring episodes was the source luminosity significantly higher than observed during the prezero phase. We analyzed all high-resolution X-ray spectra by fitting photoionization and absorption models from the most recent version of the XSTAR code. The prezero-phase spectrum could be fully modeled with a very hot photoionized plasma with an ionization parameter of log ξ = 3.0, down from log ξ = 4.0 in the high-flux state. The ionization balances we measure from the spectra during the postzero-phase episodes are significantly different. Both episodes feature absorbers with variable high columns, ionization parameters, and luminosity. While cold absorption remains at levels quite similar to that observed in previous years, the new observations show unprecedented levels of variable warm absorption. The line emissivities also indicate that the observed low source luminosity is inconsistent with a static hot accretion disk corona (ADC), an effect that seems common to other near-edge-on ADC sources as well. We conclude that unless there exists some means of coronal heating other than X-rays, the true source luminosity is likely much higher, and we observe obscuration in analogy to the extragalactic Seyfert 2 sources. We discuss possible consequences and relate cold, lukewarm, warm, and hot absorbers to dynamic accretion scenarios.

We test statistically the hypothesis that radio pulsar glitches result from an avalanche process, in which angular momentum is transferred erratically from the flywheel-like superfluid in the star to the slowly decelerating, solid crust via spatially connected chains of local, impulsive, threshold-activated events, so that the system fluctuates around a self-organized critical state. Analysis of the glitch population (currently 285 events from 101 pulsars) demonstrates that the size distribution in individual pulsars is consistent with being scale invariant, as expected for an avalanche process. The measured power-law exponents fall in the range -0.13 ⩽ a⩽ 2.4, with a ≈ 1.2 for the youngest pulsars. The waiting-time distribution is consistent with being exponential in seven out of nine pulsars where it can be measured reliably, after adjusting for observational limits on the minimum waiting time, as for a constant-rate Poisson process. PSR J0537–6910 and PSR J0835–4510 are the exceptions; their waiting-time distributions show evidence of quasi-periodicity. In each object, stationarity requires that the rate λ equal −/⟨Δν⟩, where is the angular acceleration of the crust, ⟨ Δ ν ⟩ is the mean glitch size, and is the relative angular acceleration of the crust and superfluid. Measurements yield ⩽ 7 × 10⁻⁵ for PSR J0358+5413 and ⩽ 1 (trivially) for the other eight objects, which have a < 2. There is no evidence that λ changes monotonically with spin-down age. The rate distribution itself is fitted reasonably well by an exponential for λ ⩾ 0.25 yr⁻¹, with ⟨ λ ⟩ = 1.3^{+ 0.7}_−0.6 yr⁻¹. For λ < 0.25 yr⁻¹ the exact form is unknown; the exponential overestimates the number of glitching pulsars observed at low λ, where the limited total observation time exercises a selection bias. In order to reproduce the aggregate waiting-time distribution of the glitch population as a whole, the fraction of pulsars with λ > 0.25 yr⁻¹ must exceed ~70%.

SAX J1808.4–3658 has a 2.5 ms neutron star rotation period and exhibits X-ray pulsations due to its rotating hot spot. Here we present an analysis of the pulse shapes of SAX J1808.4–3658 during its 1998 outburst. The modeling of the pulse shape includes several effects, including gravitational light bending, Doppler effects, and two spectral components with different emissivity. In addition, we include the new effects of light travel time delays and the neutron star's oblate shape. We also consider two different data sets, with different selections in time period (1 vs. 19 days of data combined) and different energy binning and time resolution. We find that including time delays and oblateness results in a stronger restriction on allowed masses and radii. A second result is that the choice of data selection strongly affects the allowed masses and radii. Overall, the derived constraints on mass and radius favor compact stars and a soft equation of state.

We model cyclotron lines by calculating a superposition of the spectra of some cyclotron lines emerging from some individual line-forming regions with a uniform density, temperature, and magnetic field strength in each region. A whole line-forming region is composed of a series of a large number of individual different domains at the temperature, density, and magnetic field strength, which vary with height or location of each of the individual domains. We study the properties of the cyclotron lines formed by superposing some cyclotron lines emerging from different heights or locations of an accretion column or mound and present the influences of the superposition of the lines, a geometry of the line-forming region, and the variations in a density, temperature, and magnetic field on the shapes of cyclotron resonant scattering lines. Consequently, we found that broader and shallower cyclotron lines as commonly observed in accretion-powered pulsars are formed in such a line-forming region composed of a large number of domains. The resulting cyclotron lines also tend to have an asymmetric profile that is shallower toward lower energies due principally to the gradient in the widths of the original lines. We also find that the cyclotron absorption line can become broader, as it becomes deeper, regardless of the geometries, i.e., slabs or cylinders as a result of a superposition of individual lines.

We report the discovery of a compact object at high Galactic latitude. The object was initially identified as a ROSAT All-Sky Survey Bright Source Catalog X-ray source, 1RXS J141256.0+792204, statistically likely to possess a high X-ray to optical flux ratio. Further observations using Swift, Gemini-North, and the Chandra X-Ray Observatory refined the source position and confirmed the absence of any optical counterpart to an X-ray to optical flux ratio of F_X(0.1–2.4 keV)/F_V > 8700 (3 σ). Interpretation of 1RXS J141256.0+792204—which we have dubbed Calvera—as a typical X-ray-dim isolated neutron star would place it at z ≈ 5.1 kpc above the Galactic disk—in the Galactic halo—implying that it either has an extreme space velocity (v_z≳ 5100 km s⁻¹) or has failed to cool according to theoretical predictions. Interpretations as a persistent anomalous X-ray pulsar or a "compact central object" present conflicts with these classes' typical properties. We conclude that the properties of Calvera are most consistent with those of a nearby (80-260 pc) radio pulsar, similar to the radio millisecond pulsars of 47 Tucanae, with further observations required to confirm this classification. If it is a millisecond pulsar, it is has an X-ray flux equal to the X-ray brightest millisecond pulsar (and so is tied for highest flux); the closest northern hemisphere millisecond pulsar; and potentially the closest known millisecond pulsar in the sky, making it an interesting target for X-ray study, a radio pulsar timing array, and LIGO.

We determine the ratio of helium- to hydrogen-atmosphere white dwarf stars as a function of effective temperature from a model atmosphere analysis of the infrared photometric data from the Two Micron All Sky Survey combined with available visual magnitudes. Our study surpasses any previous analysis of this kind, both in terms of the accuracy of the T_eff determinations and the size of the sample. We observe that the ratio of helium- to hydrogen-atmosphere white dwarfs increases gradually from a constant value of ~0.25 between T_eff = 15,000 and 10,000 K to a value twice as large in the range 10,000 K > T_eff > 8000 K, suggesting that convective mixing, which occurs when the bottom of the hydrogen convection zone reaches the underlying convective helium envelope, is responsible for this gradual transition. The comparison of our results with an approximate model used to describe the outcome of this convective mixing process implies hydrogen mass layers in the range M_H/M_tot = 10⁻¹⁰ to 10⁻⁸ for about 15% of the DA stars that survived the DA-to-DB transition near T_eff ∼ 30,000 K, the remainder having presumably more massive layers above M_H/M_tot ∼ 10⁻⁶.

We present HST ACS observations of 19 nearby M subdwarfs in a search for binary systems. Other than the wide common proper-motion pair LHS 2140/2139, none of our sdM and esdM targets are found to be binaries. Our survey is sensitive to equal-luminosity companions at close (2-8 AU) separations, while substellar secondaries could have been detected at separations in the range of 6-30 AU. To check for wide binaries, we have compared the POSS I and II images in a field of view as large as 10' × 10', but could not detect a single comoving star for any of the targets. Combining our results with those from Gizis & Reid, we have a binary fraction of 3% (1/28). Detection of a small number of M subdwarf binaries reported in the literature suggests a higher fraction than the one obtained here, probably comparable to that found for the more massive solar-type stars in the halo (13%-15%). Comparison with the disk M dwarf fraction (~25%), however, suggests multiplicity to be rare among the lowest mass halo stars, implying the two populations formed under different initial conditions. The low binary fraction in our survey could be explained by selection biases. A decrease in multiplicity has been observed in the disk for masses below 0.1 M_☉, the peak in the disk mass function (MF). The globular cluster MF is found to peak at about 0.33 M_☉, with a decrease in the number of stars per unit mass below the peak mass. Our sample being composed of stars with masses between ~0.2 and 0.085 M_☉ suggests that a decrease in multiplicity similar to the disk may also be true for the halo stars, but perhaps below a mass of ~0.3 M_☉. A higher M subdwarf binary fraction may be obtained if the selected primaries have masses near or higher than the peak in the MF.

We present the first parallax and luminosity measurements for an L subdwarf, the sdL7 2MASS J05325346+8246465. Observations conducted over 3 years by the USNO infrared astrometry program yield an astrometric distance of 26.7 ± 1.2 pc and a proper motion of 2.6241'' ± 0.0018'' yr⁻¹. Combined with broadband spectral and photometric measurements, we determine a luminosity of log L_bol/L_☉ = − 4.24 ± 0.06 and T_eff = 1730 ± 90 K (the latter assuming an age of 5-10 Gyr), comparable to mid-type L field dwarfs. Comparison of the luminosity of 2MASS J05325346+8246465 to theoretical evolutionary models indicates that its mass is just below the sustained hydrogen-burning limit, and is therefore a brown dwarf. Its kinematics indicate a ~110 Myr, retrograde Galactic orbit that is both eccentric (3 kpc ≲ R≲ 8.5 kpc) and extends well away from the plane (Δ Z = ± 2 kpc), consistent with membership in the inner halo population. The relatively bright J-band magnitude of 2MASS J05325346+8246465 implies significantly reduced opacity in the 1.2 μ m region, consistent with inhibited condensate formation as previously proposed. Its as yet unknown subsolar metallicity remains the primary limitation in constraining its mass; determination of both parameters would provide a powerful test of interior and evolutionary models for low-mass stars and brown dwarfs.

We present ground-based SpectroCam-10 mid-infrared, MMT optical, and Spitzer Space Telescope IRS mid-infrared spectra taken 7.62, 18.75, and 19.38 yr, respectively, after the outburst of the old classical nova QU Vulpeculae (Nova Vul 1984#2). The spectra of the ejecta are dominated by forbidden line emission from neon and oxygen. Our analysis shows that neon was, at the first and last epochs respectively, more than 76 and 168 times overabundant by number with respect to hydrogen compared to the solar value. These high lower limits to the neon abundance confirm that QU Vul involved a thermonuclear runaway on an ONeMg white dwarf, and approach the yields predicted by models of the nucleosynthesis in such events.

The near-main-sequence B stars show a sharp dropoff in their X-ray-to-bolometric luminosity ratio in going from B1 to later spectral types. Here we focus attention on the subset of these stars that are also Oe/Be stars, to test the concept that the disks of these stars form by magnetic channeling of wind material toward the equator. Calculations are made of the X-rays expected from the magnetically torqued disk (MTD) model for Be stars discussed by Cassinelli et al., Maheswaran, and Brown et al. In this model, the wind outflow from Be stars is channeled and torqued by a magnetic field such that the flows from the upper and lower hemispheres of the star collide as they approach the equatorial zone. X-rays are produced by the material that enters the shocks above and below the disk region and radiatively cools and compresses while moving toward the MTD central plane. The model predictions are compared with ROSAT observations obtained for an O9.5 star, ζ Oph, by Berghöfer et al. and for seven Be stars from Cohen et al. Two types of fitting models are used to compare predictions with observations of X-ray luminosity versus spectral type. Extra consideration is also given here to the well-studied Oe star ζ Oph, for which we have Chandra observations of the X-ray line profiles of the triad of He-like lines from the ion Mg XI. Thus, the X-ray properties add to the list of observables that can be explained within the context of the MTD concept. This list already includes the Hα equivalent widths and white-light polarization of Be stars.

Protoplanetary disks are mainly heated by radiation from the central star. Since the incident stellar flux at any radius is sensitive to the disk structure near that location, destabilizing feedback may be present. Previous investigations have shown that the disk will be stable to finite-amplitude temperature perturbations if the vertical height of the optical surface is everywhere directly proportional to the gas scale height and if the intercepted fraction of stellar radiation is determined from the local grazing angle. We show that these assumptions may not be generally applicable. Instead, we calculate the quasi-static thermal evolution of irradiated disks by directly integrating the global optical depth to determine the optical surface and the total emitting area filling factor of surface dust. We show that in disks with modest mass accretion rates, thermal waves are spontaneously and continually excited in the outer disk, propagate inward through the planet-forming domains, and dissipate at small radii where viscous dissipation is dominant. This state is quasi-periodic over several thermal timescales, and its pattern does not depend on the details of the opacity law. The viscous dissipation resulting from higher mass accretion stabilizes this instability such that an approximately steady state is realized throughout the disk. In passive protostellar disks, especially transitional disks, these waves induce significant episodic changes in spectral energy distributions, on timescales of years to decades, because the midplane temperatures can vary by a factor of 2 between the exposed and shadowed regions. The transitory peaks and troughs in the potential vorticity distribution may also lead to baroclinic instability and excite turbulence in the planet-forming regions.

The commonality of collisionally replenished debris around main-sequence stars suggests that minor bodies are frequent around Sun-like stars. Whether or not debris disks in general are accompanied by planets is yet unknown, but debris disks with large inner cavities—perhaps dynamically cleared—are considered to be prime candidates for hosting large-separation massive giant planets. We present here a high-contrast VLT/NACO angular differential imaging survey for eight such cold debris disks. We investigated the presence of massive giant planets in the range of orbital radii where the inner edge of the dust debris is expected. Our observations are sensitive to planets and brown dwarfs with masses >3-7 Jupiter mass, depending on the age and distance of the target star. Our observations did not identify any planet candidates. We compare the derived planet mass upper limits to the minimum planet mass required to dynamically clear the inner disks. While we cannot exclude that single giant planets are responsible for clearing out the inner debris disks, our observations constrain the parameter space available for such planets. The nondetection of massive planets in these evacuated debris disks further reinforces the notion that the giant planet population is confined to the inner disk (<15 AU).

We report the discovery of a third planetary-mass companion to the G0 star HD 74156. High-precision radial velocity measurements made with the Hobby-Eberly Telescope aided the detection of this object. The best-fit triple-Keplerian model to all the available velocity data yields an orbital period of 347 days and a minimum mass of 0.4 M_Jup for the new planet. We determine revised orbital periods of 51.7 and 2477 days and minimum masses of 1.9 and 8.0 M_Jup, respectively, for the previously known planets. Preliminary calculations indicate that the derived orbits are stable, although all three planets have significant orbital eccentricities (e = 0.64, 0.43, and 0.25). With our detection, HD 74156 becomes the eighth normal star known to host three or more planets. Further study of this system's dynamical characteristics will likely give important insight into planet formation and evolutionary processes.

Electric currents are present in the coronae above solar active regions, producing nonpotential magnetic fields that can be approximated as nonlinear force-free fields (NLFFFs). In this paper NLFFF models for two active regions observed in 2002 June are presented. The models are based on magnetograms from SOHO MDI and are constrained by nonpotential structures seen in BBSO Hα images and TRACE EUV images. The models are constructed using the flux rope insertion method. We find that the axial fluxes of the flux ropes are well constrained by the observations. The flux ropes are only weakly twisted, and electric currents flow mainly at the interface between the flux rope and its surroundings. In one case, the flux rope is anchored with both ends in the active region; in the other case, the flux rope extends to the neighboring quiet Sun. We find that the magnetic fields in these active regions are close to an eruptive state: the axial flux in the flux ropes is close to the upper limit for eruption. We also derive estimates for magnetic free energy and helicity in these regions.

White-light observations of coronal mass ejections (CMEs) often show the classic "three-part" structure consisting of (1) a bright front; (2) a dark cavity; and (3) a bright, compact core. It has proven difficult to unambiguously associate these features with in situ measurements of interplanetary CMEs (ICMEs), in all but a few cases. In this study we use a global MHD model to simulate the eruption and evolution of a CME out to 0.25 AU, allowing us to continuously track these features from the Sun and through the solar wind. Our results support the generally held view that the interplanetary flux rope corresponds to the dark cavity. We find that the bright front merges with solar wind material swept up by the ICME. Thus, the sheath material found ahead of fast ejecta is in fact composed from both ambient solar wind material, as well the bright front. We also note that, in this simulation, the bright front is formed from the overlying streamer configuration from within which the CME erupted and is not itself coronal material swept up during the early phase of the eruption. The conclusions reached in this study are undoubtedly sensitive to the initial configuration and mechanism used to initiate the CME, and thus care should be taken when using them to interpret specific observations. On the other hand, they provide a unique, unbroken connection between remote solar and interplanetary observations. Ultimately, detailed comparisons between observations and simulation results may be able to constrain or even rule out some mechanisms of CME initiation.

Precise data on the uniformity of photospheric radiation over the solar disk seems not to exist. Such information is necessary to separate the radiative behavior of the quiet basal atmosphere from the active (magnetic) atmosphere. Is the latter the sole source of known irradiance variation? How uniform can a solar-like stellar disk be? To obtain this information we have made monochromatic scans along the central meridian of the quiet Sun using single element detectors which do not require flat fielding. The scans were in continua and in selected Fraunhofer lines ranging from 3134 to 46880 Å; the observational epoch was near solar minimum: 2006 October to 2007 February. The meridian was chosen to avoid rotational Doppler shifts. We extract the asymmetry between the north and south hemispheres and present it as our main product. In the near-infrared and visible continuum, averaging over granulation and avoiding sunspots, we found that such asymmetry was as low as 0.05% (at 34168 Å on 2007 February 8). In the violet and ultraviolet this asymmetry typically increases to 1%. Asymmetry is larger in the cores of the medium strong photospheric and chromospheric lines, which refer to higher levels in the atmosphere, and may reach 15%. The contrast of faculae increases in the blue (and with improved spatial resolution or seeing), and is the probable source for the measured asymmetries. We also find that line core scans are in general flatter than continuum scans.

Observations of very quiet Sun using the Solar Optical Telescope/Spectro-Polarimeter (SOT/SP) aboard the Hinode spacecraft reveal that the quiet internetwork regions are pervaded by horizontal magnetic flux. The spatial average horizontal apparent flux density derived from wavelength-integrated measures of Zeeman-induced linear polarization is B^T_app = 55 Mx cm ⁻², as compared to the corresponding average vertical apparent flux density of | B^L_app| = 11 Mx cm ⁻². Distributions of apparent flux density are presented. Magnetic fields are organized on mesogranular scales, with both horizontal and vertical fields showing "voids" of reduced flux density of a few granules spatial extent. The vertical fields are concentrated in the intergranular lanes, whereas the stronger horizontal fields are somewhat separated spatially from the vertical fields and occur most commonly at the edges of the bright granules. High-S/N observations from disk center to the limb help to constrain possible causes of the apparent imbalance between | B^L_app| and B^T_app, with unresolved structures of linear dimension on the surface smaller by at least a factor of 2 relative to the SOT/SP angular resolution being one likely cause of this discrepancy. Other scenarios for explaining this imbalance are discussed. The horizontal fields are likely the source of the "seething" fields of the quiet Sun discovered by Harvey et al. The horizontal fields may also contribute to the "hidden" turbulent flux suggested by studies involving Hanle effect depolarization of scattered radiation.

We study the temporal variation of subsurface flows associated with emerging active regions, focusing on four regions in detail. Two of them, AR 10314 and AR 10488, emerge near disk center and the other two, AR 10365 and AR 10375, are older regions where new flux emerges during their disk passage. We measure the horizontal subsurface flows from high-resolution Global Oscillation Network Group (GONG) data using ring-diagram analysis and derive the vertical flow component. Before flux emergence, we find upflows in AR 10314, while the other emerging region, AR 10488, shows mainly weak vertical flows. Both aging regions, AR 10365 and AR 10375, initially show downflows, as expected from already established regions. When new flux emerges, the weaker one of the two, AR 10365, shows upflows, while AR 10375 shows an even stronger downflow. In strong active regions, such as AR 10375 and AR 10488, strong downflows are present after the region has been established. In all four regions, the transition occurs on timescales of about one to two days. As a control experiment, we repeat the analysis for the same locations as those of the four active regions in 53 Carrington rotations and find that it is unlikely that the temporal variations of the vertical velocity are caused by systematics such as a projection effect. We then search our data set for emerging regions with similar characteristics to AR 10314 and AR 10488, i.e., emergence near disk center and large flux increase. From an analysis of 13 emerging regions, we conclude that there is a small preference for upflows before the emergence of new flux and for a transition toward downflows after flux emergence.

We report the discovery of a unique Fe- and O-bearing circumstellar grain from the Acfer 094 ungrouped carbonaceous chondrite. The grain has a close-to-solar ¹⁷O/¹⁶O ratio and an ¹⁸O/¹⁶O ratio that is 1.34 times the solar value. Iron isotopic compositions show depletions of 100‰-200‰ in both ⁵⁴Fe and ⁵⁷Fe, relative to ⁵⁶Fe. No evidence of excess ⁶⁰Ni from the decay of extinct ⁶⁰Fe was observed. Auger elemental spectra show that the grain is compositionally similar to wüstite (FeO), but may contain a small amount of Mg in addition to Fe and O. The solid solution series magnesiowüstite, (Mg,Fe)O, is predicted to form under nonequilibrium conditions in oxygen-rich asymptotic giant branch (AGB) stars with low mass-loss rates, and Fe-rich magnesiowüstite has been proposed as the carrier of the 19.5 μm feature observed in the spectra of certain low-mass-loss AGB stars that show little silicate emission. Although the isotopic data are ambiguous, these observations argue in favor of an AGB source for this Fe oxide grain.

We present a Bayesian Voronoi image reconstruction (VIR) technique for interferometric data. Bayesian analysis applied to the inverse problem allows us to derive the a posteriori probability of a novel parameterization of interferometric images. We use a variable Voronoi diagram as our model in place of the usual fixed-pixel grid. A quantization of the intensity field allows us to calculate the likelihood function and a priori probabilities. The Voronoi image is optimized including the number of polygons as free parameters. We apply our algorithm to deconvolve simulated interferometric data. Residuals, restored images, and χ² values are used to compare our reconstructions with fixed-grid models. VIR has the advantage of modeling the image with few parameters, obtaining a better image from a Bayesian point of view.

Data on Stark widths of spectral lines are of high interest for astrophysics and analytical techniques of stellar plasma diagnosis. Stark widths of 43 spectral lines of Sn I and 27 spectral lines of Sn II has been measured in a laser-induced plasma (LIP) at an electron temperature of 11,000 K and an electron density of 1.1 × 10¹⁶ cm⁻³. The LIP optical emission spectroscopy generated by a 10640 Å radiation, with a flux of 1.4 × 10¹⁰ W cm⁻² on several tin and lead targets in an atmosphere of argon was recorded at 2.5 μs and analyzed between 1890 and 7000 Å. The population level distribution and the corresponding temperatures were obtained using Boltzmann plots. The plasma electron densities were determined using well-known Stark broadening parameters of spectral lines. Special attention was dedicated to the possible self-absorption of the different transitions. The local thermodynamic equilibrium (LTE) conditions and plasma homogeneity have been checked. The experimental results obtained have been compared with the experimental and theoretical values given by other authors. The results obtained in this study will allow a substantial improvement in the interpretation of the data of the ultraviolet spectrum of the tin observed by the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. These atomic data are relevant to the analysis of the isotopic abundances of tin in stellar atmospheres.

Using a Gibbs sampling algorithm for joint CMB estimation and component separation, we compute the large-scale CMB and foreground posteriors of the 3 yr WMAP temperature data. Our parametric data model includes the cosmological CMB signal and instrumental noise, a single power law foreground component with free amplitude and spectral index for each pixel, a thermal dust template with a single free overall amplitude, and free monopoles and dipoles at each frequency. This simple model yields a good fit to the data over the full frequency range from 23 to 94 GHz. We obtain a new estimate of the CMB sky signal and power spectrum, and a new foreground model, including a measurement of the effective spectral index over the high-latitude sky. A noteworthy result in this respect is the detection of a common spurious offset in all frequency bands of ~–13 μK, as well as a dipole in the V-band data. Correcting for these is essential when determining the effective spectral index of the foregrounds. Fortunately, the CMB power spectrum is not significantly affected by these issues, as our new spectrum is in excellent agreement with that published by the WMAP team. The corresponding cosmological parameters are also virtually unchanged.

We provide preliminary quantitative evidence that a new solution to averaging the observed inhomogeneous structure of matter in the universe may lead to an observationally viable cosmology without exotic dark energy. We find parameters which simultaneously satisfy three independent tests: the match to the angular scale of the sound horizon detected in the cosmic microwave background anisotropy spectrum; the effective comoving baryon acoustic oscillation scale detected in galaxy clustering statistics; and Type Ia supernova luminosity distances. Independently of the supernova data, concordance is obtained for a value of the Hubble constant which agrees with the measurement of the Hubble Key team of Sandage and coworkers. Best-fit parameters include a global average Hubble constant H₀ = 61.7_−1.1^+1.2 km s⁻¹ Mpc⁻¹, a present epoch void volume fraction of f_{v 0} = 0.76_−0.09^+0.12, and an age of the universe of 14.7_−0.5^+0.7 billion years as measured by observers in galaxies. The mass ratio of nonbaryonic dark matter to baryonic matter is 3.1_−2.4^+2.5, computed with a baryon-to-photon ratio that is in concordance with primordial lithium abundances.

We present observations of a rare, rapid, high-amplitude extreme scattering event toward the compact BL Lac object AO 0235+164 at 6.65 GHz. The ESE cloud is compact; we estimate its diameter between 0.09 and 0.9 AU, with a distance of less than 3.6 kpc. Limits on the angular extent of the ESE cloud imply a minimum cloud electron density of ~4 × 10³ cm⁻³. Based on the amplitude and timescale of the ESE observed here, we suggest that at least one of the transients reported by Bower et al. may be attributed to ESEs.

We report our attempts to locate the progenitor of the peculiar Type Ic SN 2007gr in Hubble Space Telescope (HST) preexplosion images of the host galaxy, NGC 1058. Aligning adaptive optics Altair/NIRI imaging of SN 2007gr from the Gemini (North) Telescope with the preexplosion HST WFPC2 images, we identify the supernova (SN) position on the HST frames with an accuracy of 20 mas. Although nothing is detected at the SN position, we show that it lies on the edge of a bright source, 134 ± 23mas (6.9 pc) from its nominal center. On the basis of its luminosity, we suggest that this object is possibly an unresolved, compact, and coeval cluster and that the SN progenitor was a cluster member, although we note that model profile fitting favors a single bright star. We find two solutions for the age of this assumed cluster: 7∓ 0.5 Myr and 20-30 Myr, with turnoff masses of 28 ± 4 M_☉ and 12-9 M_☉, respectively. Preexplosion ground-based K-band images marginally favor the younger cluster age/higher turnoff mass. Assuming the SN progenitor was a cluster member, the turnoff mass provides the best estimate for its initial mass. More detailed observations, after the SN has faded, should determine whether the progenitor was indeed part of a cluster and, if so, allow an age estimate to within ~2 Myr, thereby favoring either a high-mass single star or lower-mass interacting binary progenitor.

We use a cosmological chemodynamical simulation to study how the group environment impacts the star formation (SF) properties of disk galaxies. The simulated group has a total mass of M ~ 8 × 10¹²M_☉ and a total X-ray luminosity of L_X ~ 10⁴¹ erg s⁻¹. Our simulation suggests that ram pressure is not sufficient in this group to remove the cold disk gas from a V_rot ~ 150 km s⁻¹ galaxy. However, the majority of the hot gas in the galaxy is stripped over a timescale of approximately 1 Gyr. Since the cooling of the hot-gas component provides a source for new cold gas, the stripping of the hot component effectively cuts off the supply of cold gas. This in turn leads to a quenching of SF. The galaxy maintains the disk component after the cold gas is consumed, which may lead to a galaxy similar to an S0. Our self-consistent simulation suggests that this strangulation mechanism works even in low-mass groups, providing an explanation for the lower SF rates in group galaxies relative to galaxies in the field.

We use a sample of 43,690 galaxies selected from the Sloan Digital Sky Survey Data Release 4 to study the systematic effects of specific star formation rate (SSFR) and galaxy size (as measured by the half-light radius, r_h) on the mass-metallicity relation. We find that galaxies with high SSFR or large r_h for their stellar mass have systematically lower gas-phase metallicities (by up to 0.2 dex) than galaxies with low SSFR or small r_h. We discuss possible origins for these dependencies, including galactic winds/outflows, abundance gradients, environment, and star formation rate efficiencies.

The globular cluster system of a typical spheroidal galaxy makes up about 0.25% of the total galaxy mass. This is roughly the same mass fraction as contained in the nuclear star cluster (or stellar nucleus) present in most nearby low-mass galaxies. Motivated by this "coincidence," this Letter discusses a scenario in which globular clusters of present-day galaxies are the surviving nuclei of the dwarf galaxies that—according to the hierarchical merging paradigm of galaxy formation—constitute the "building blocks" of present-day massive galaxies. This scenario, which was first suggested by Freeman, has become more attractive recently in light of studies that demonstrate a complex star formation history in a number of massive globular clusters.

We explore the possibility that the anomalous split in the subgiant branch (SGB) of the Galactic globular cluster NGC 1851 is due to the presence of two distinct stellar populations with very different initial metal mixtures: a normal α-enhanced component, and one characterized by strong anticorrelations among the CNONa abundances, with a total CNO abundance increased by a factor of 2. We test this hypothesis taking into account various empirical constraints, and conclude that the two populations should be approximately coeval, with the same initial He content. More high-resolution spectroscopic measurements of heavy elements—and in particular of the CNO sum—for this cluster are necessary to prove (or disprove) this scenario.

We analyze the properties of the star S2 orbiting the supermassive black hole at the center of the Galaxy. A high-quality SINFONI H- and K-band spectrum obtained from co-adding 23.5 hr of observation between 2004 and 2007 reveals that S2 is an early B dwarf (B0-B2.5 V). Using model atmospheres, we constrain its stellar and wind properties. We show that S2 is a genuine massive star and not the core of a stripped giant star, as sometimes speculated to resolve the problem of star formation so close to the supermassive black hole. We give an upper limit on its mass-loss rate and show that it is He enriched, possibly because of the presence of a magnetic field.

We show that TeV γ-ray emission produced via interactions of high-energy particles with the anisotropic radiation field of a massive star in binary systems should have a characteristic rotating hollow cone anisotropy pattern. The hollow cone, whose axis is directed away from the massive star, rotates with a period equal to the orbital period of the system. We note that the two-maxima pattern of the TeV energy band light curve of the γ-ray-loud binary LS 5039 can be interpreted in terms of this rotating hollow cone model. Adopting such an interpretation, we are able to constrain the geometry of the system—either the inclination angle of the binary orbit, or the elevation of the γ-ray emission region above the orbital plane.

The evolution of astrophysical disks is dominated by instabilities of gravity perturbations (e.g., those produced by a spontaneous disturbance). We develop a hydrodynamic theory of nonresonant Jeans instability in a dynamically cold subsystem (identified as the gaseous component) of a disk. We show analytically that gravitationally unstable systems, such as disks of rotationally supported galaxies, protoplanetary disks, and, finally, the solar nebula are efficient at transporting mass and angular momentum: already on a timescale of on the order of 2-3 rotational periods an unstable disk sees a large portion of its angular momentum transferred outward, and mass transferred both inward and outward.

In this Letter, we present radio diagnostics for the magnetic field and the density distribution of nonthermal electrons in the microwave source of the M1.1 flare of 2004 November 1. We calculate the nonthermal gyrosynchrotron radiation using observations made at the Nobeyama Radio Observatory to get the longitudinal magnetic field and the magnitude of the transverse magnetic field. The computed longitudinal magnetic field at two hard X-ray footpoints has opposite polarities, which basically agrees with SOHO MDI observations. The main finding is that the magnitude of the transverse magnetic field summed around the magnetic neutral line of the SOHO MDI magnetogram has a short-term impulsive increase during the rising phase of the flare. We propose that the increase of the transverse magnetic field may be considered evidence of the magnetic reconnection process in this flare. The computed mean angle between the magnetic field and line of sight decreases steadily during the flare, clearly indicating the reconfiguration of the magnetic field, presumably the relaxation of the sheared magnetic field. Moreover, the calculated density of nonthermal electrons increases from footpoints to looptop, while the spectral index calculated by the Nobeyama Radio Heliograph at 17 and 34 GHz decreases from footpoints (soft) to looptop (hard), which may confirm that the reconnection site is close to the looptop source.

All theoretical models of nanoflare heating in the solar corona predict loops with an unresolved multistrand structure, which is in discrepancy with the quasi-isothermal cross sections of the finest coronal loop structures observed with TRACE, which have spatially resolved widths of w ≈ 1000–2000 km. We suggest modifying the theoretical models by relocating the hypothetical nanoflare events from their coronal location down to the chromosphere/transition region.