Table of contents

We point out that the usual self-similarity in cold dark matter models is broken by encounters with individual normal galactic stars on a subparsec scale. Tidal heating and stripping must have redefined the density and velocity structures of the population of the Earth-mass dark matter halos, which are likely to have been the first bound structures to form in the universe. The disruption rate depends strongly on galaxy types and the orbital distribution of the microhalos; in the Milky Way, stochastic radial orbits are destroyed first by stars in the triaxial bulge, and microhalos on nonplanar retrograde orbits with large pericenters and/or apocenters survive the longest. The final microhalo distribution in the solar neighborhood is better described as a superposition of filamentary microstreams rather than as a set of discrete spherical clumps in an otherwise homogeneous medium. This has important consequences to our detections of microhalos by direct recoil signal and indirect annihilation signal.

We examine the clustering properties of H I-selected galaxies through an analysis of the H I Parkes All-Sky Survey Catalogue (HICAT) two-point correlation function. Various subsamples are extracted from this catalog to study the overall clustering of H I-rich galaxies and its dependence on luminosity, H I gas mass, and rotational velocity. These samples cover the entire southern sky δ < 0°, containing up to 4174 galaxies over the radial velocity range 300-12,700 km s^-1. A scale length of r₀ = 3.45 ± 0.25 h^-1 Mpc and slope of γ = 1.47 ± 0.08 is obtained for the H I-rich galaxy real-space correlation function, making gas-rich galaxies among the most weakly clustered objects known. H I-selected galaxies also exhibit weaker clustering than optically selected galaxies of comparable luminosities. Good agreement is found between our results and those of synthetic H I-rich galaxy catalogs generated from the Millennium Run cold dark matter (CDM) simulation. Bisecting HICAT using different parameter cuts, clustering is found to depend most strongly on rotational velocity and luminosity, while the dependency on H I mass is marginal. Splitting the sample around v_rot = 108 km s^-1, a scale length of r₀ = 2.86 ± 0.46 h^-1 Mpc is found for galaxies with low rotational velocities compared to r₀ = 3.96 ± 0.33 h^-1 Mpc for the high rotational velocity sample.

We present a detailed investigation into which properties of CDM halos make them effective strong gravitational lenses. Strong-lensing cross sections of 878 clusters from an N-body simulation are measured by ray-tracing through 13,594 unique projections. We measure concentrations, axis ratios, orientations, and the substructure of each cluster, and compare the lensing-weighted distribution of each quantity to that of the cluster population as a whole. The concentrations of lensing clusters are on average 34% larger than the typical cluster in the universe. Despite this bias, the anomalously high concentrations (c > 14) recently measured by several groups appear to be inconsistent with the concentration distribution in our simulations, which predict that <2% of lensing clusters should have concentrations this high. No correlation is found between lensing cross section and substructure. We introduce several types of simplified dark matter halos and use them to isolate which properties of CDM clusters make them effective lenses. The abundance of giant arcs is primarily determined by the mass distribution within an average overdensity of ~10,000. A multiple-lens-plane ray-tracing algorithm is used to show that projections of large-scale structure increase the giant arc abundance modestly, by <7%. We revisit the claimed excess of giant arcs behind high redshift clusters in the RCS survey and find that the number of high-redshift (z > 0.6) lenses is in good agreement with ΛCDM, although our simulations predict more low-redshift (z < 0.6) lenses than observed.

We combine a large set of quasar luminosity function (QLF) measurements from the rest-frame optical, soft and hard X-ray, and near- and mid-IR bands to determine the bolometric QLF in the redshift interval z = 0-6. Accounting for the observed distributions of quasar column densities and variation of SED shapes, as well as their dependence on luminosity, makes it possible to integrate the observations in a reliable manner and provides a baseline in redshift and luminosity larger than that of any individual survey. We infer the QLF break luminosity and faint-end slope out to z ~ 4.5 and confirm at high significance (≳10 σ) previous claims of a flattening in both the faint- and bright-end slopes with redshift. With the best-fit estimates of the column density distribution and quasar SED, which both depend on luminosity, a single bolometric QLF self-consistently reproduces the observed QLFs in all bands and at all redshifts for which we compile measurements. Ignoring this luminosity dependence does not yield a self-consistent bolometric QLF and there is no evidence for any additional dependence on redshift. We calculate the expected relic black hole mass function and mass density, cosmic X-ray background, and ionization rate as a function of redshift and find that they are consistent with existing measurements. The peak in the total quasar luminosity density is well constrained at z = 2.15 ± 0.05. We provide a number of fitting functions to the bolometric QLF and its manifestations in various bands, as well as a script to return the QLF at arbitrary frequency and redshift from these fits.

We present line and continuum measurements for 9818 SDSS type 1 active galactic nuclei (AGNs) with z ≤ 0.75. The data are used to study the four-dimensional space of black hole mass, normalized accretion rate (L/L_Edd), metallicity, and redshift. The main results are (1) L/L_Edd is smaller for larger mass black holes at all redshifts. (2) For a given black hole mass, L/L_Edd ∝ z^γ or (1 + z)^δ, where the slope γ increases with black hole mass. The mean slope is similar to the star formation rate slope over the same redshift interval. (3) The Fe II/Hβ line ratio is significantly correlated with L/L_Edd. It also shows a weaker negative dependence on redshift. Combined with the known dependence of metallicity on accretion rate, we suggest that the Fe II/Hβ line ratio is a metallicity indicator. (4) Given the measured accretion rates, the growth times of most AGNs exceed the age of the universe. This suggests past episodes of faster growth for all those sources. Combined with the Fe II/Hβ result, we conclude that the broad emission line metallicity goes through cycles and is not a monotonously decreasing function of redshift. (5) FWHM([O III] λ5007) is a poor proxy for σ_*, especially for high L/L_Edd. (6) We define a group of narrow-line type 1 AGNs (NLAGN1s) by their luminosity- (or mass-) dependent Hβ line width. Such objects have L/L_Edd ≥ 0.25, and they comprise 8% of the type 1 population. Other interesting results include negative Baldwin relationships for EW(Hβ) and EW(Fe II) and a relative increase of the red part of the Hβ line with luminosity.

We use highly spectroscopically complete observations of the radio sources from the VLA 1.4 GHz survey of the Hubble Deep Field-North region to study the faint radio galaxy population and its evolution. The fraction that can be optically spectroscopically identified is fairly independent of radio flux, with about 60%-80% identified at all fluxes. We spectrally classify the sources as absorbers, star formers, Seyfert galaxies, and broad-line AGNs, and we analyze their properties by type. We supplement the spectroscopic redshifts with photometric redshifts measured from the rest-frame ultraviolet to mid-infrared spectral energy distributions. Using deep X-ray observations of the field, we do not confirm the existence of an X-ray-radio correlation for star-forming galaxies. We also do not observe any correlations between 1.4 GHz flux and R magnitude or redshift. We find that the radio powers of the host galaxies rise dramatically with increasing redshift, while the optical properties show at most small changes. Assuming that the locally determined far-infrared-radio correlation holds at high redshifts, we estimate total FIR luminosities for the radio sources. We note that the FIR luminosity estimates for any radio-loud AGNs, which we conservatively do not try to remove from the sample, will be overestimates. Considering only the radio sources with quasar-like bolometric luminosities, we find a maximum ratio of candidate highly obscured AGNs to X-ray-luminous (L_{0.5-2 keV} or L_{2-8 keV} ≥ 10⁴² ergs s^-1) sources of about 1.9. Finally, we use source-stacking analyses to measure the X-ray surface brightnesses of various populations. We find the contributions to the 4-8 keV light from our candidate highly obscured AGNs to be very small, and hence these sources are unable to account for the light that has been suggested may be missing at these energies.

We have used the Submillimeter Array for the first interferometric CO imaging toward the starburst-Seyfert nucleus of the southern barred spiral galaxy NGC 1365, which is one of the four galaxies within 30 Mpc that have L_{8-1000 μm} ≥ 10¹¹L_☉. Our mosaic maps of ¹²CO, ¹³CO, and C¹⁸O (J = 2-1) emission at up to 2'' (200 pc) resolutions have revealed a circumnuclear gas ring and several CO clumps in the central 3 kpc. The molecular ring shows morphological and kinematical signs of bar-driven gas dynamics, and the region as a whole is found to follow the star formation laws of Kennicutt. We have found that some of the gas clumps and peaks in CO brightness temperature, which we collectively call CO hot spots, coincide with the radio and mid-infrared sources previously identified as dust-enshrouded super star clusters. This hot spot-cluster association suggests that either the formation of the most massive clusters took place in large molecular gas concentrations (of Σ_mol ~ 10³M_☉ pc^-2 in 200 pc scales) or the clusters have heated their ambient gas to cause or enhance the CO hot spots. The active nucleus is in the region of weak CO emission and is not associated with distinctive molecular gas properties.

It has been suggested that narrow-line Seyfert 1 galaxies (NLS1s) are evolutionarily young objects, powered by accretion onto central black holes that are significantly lower in mass than those found in typical broad-line Seyfert galaxies. We explore this hypothesis through the analysis of high spatial resolution, near-IR imaging data. By employing the correlation between black hole mass and host galaxy bulge luminosity, we determine the mean black hole mass for our sample to be, in solar units, ⟨log(M_BH)⟩ = 7.9. Using the correlation between the size of the broad-line region and the monochromatic continuum luminosity, we obtain black hole mass estimates under the assumption that the emission-line gas is in virial equilibrium. The mean black hole mass derived from this relation is ⟨log(M_BH)⟩ = 6.4. We explore possible causes for this discrepancy in M_BH estimates and the ramifications for our understanding of the role played by NLS1s in AGN evolution. Because numerical simulations constrain the start of the AGN duty cycle to a time shortly after a gravitational interaction, we examine the morphology and near-IR bulge colors of the NLS1 sample for evidence of recent encounters. The mean bulge color is found to be redder than that of both a matched sample of nonactive galaxies and published estimates for broad-line Seyfert galaxies. The source of the unusual bulge colors may be an excess of flux, peaking at 2.2 μm, that has been detected near the centers of some NLS1s. No evidence is found for light asymmetries or an extra stellar component that would indicate NLS1s are young objects. Finally, we postulate that there may be some interesting lines of circumstantial evidence suggesting that secular processes may be relevant in NLS1s.

It has long been realized that the dark matter halos that form in cosmological N-body simulations are characterized by density profiles ρ(r) that, when suitably scaled, have similar shapes. In addition, combining the density and velocity dispersion profiles σ(r), both of which have decidedly non-power-law shapes, leads to a quantity ρ/σ³ that is a power law in radius over 3 orders of magnitude in radius. Halos' velocity anisotropy profiles β(r) vary from isotropic near their centers to quite radially anisotropic near the virial radius. Finally, there appears to be a nearly linear correlation between β and the logarithmic density slope γ for a wide variety of halos. This work is part of a continuing investigation of the above interrelationships and their origins using analytical and semianalytical techniques. Our findings suggest that the nearly linear β-γ relationship is not just another expression of scale-free ρ/σ³ behavior. We also note that simultaneously reproducing density and anisotropy profiles like those found in simulations requires β(r) and γ(r) to have similar shapes, leading to nearly linear β-γ correlations. This work suggests that the β-γ and power-law ρ/σ³ relations have distinct physical origins.

We have observed the cluster RX J0152.7-1357 (z ~ 0.83) at 24 μm with the Multiband Imaging Photometer for Spitzer (MIPS). We detected 22 sources associated with spectroscopically confirmed cluster members, while 10 more have photometric redshifts compatible with membership. Two of the 32 likely cluster members contain obvious active nuclei, while the others are associated with dusty star formation. The median IR-determined star formation rate among the remaining galaxies is estimated at 22 M_☉ yr^-1, significantly higher than in previous estimates from optical data. Most of the mid-infrared (MIR) emitting galaxies also have optical emission lines, but a few do not and hence have completely hidden bursts of star formation or AGN activity. An excess of MIR-emitting galaxies is seen in the cluster, compared to the field at the same redshift. The MIR cluster members are more associated with previously detected infalling late-type galaxies rather than triggered by the ongoing merging of bigger X-ray clumps. Rough estimates also show that ram pressure may not be capable of stripping the gas away from cluster outskirt galaxies, but it may disturb the gas enough to trigger the star formation activity. Harassment can also play a role if, for example, these galaxies belong to poor galaxy groups. Thus, bursts of star formation occur in the cluster environment and could also help consume the galaxy gas content, in addition to ram pressure, harassment, or galaxy-galaxy strong interactions.

We report the discovery of four candidate intracluster globular clusters (IGCs) in a single deep HST ACS field of the Virgo Cluster. We show that each cluster is roughly spherical, has a magnitude near the peak of the Virgo globular cluster luminosity function, has a radial profile that is best fitted by a King model, and is surrounded by an excess of point sources that have the colors and magnitudes of cluster red giant stars. Despite the fact that two of our IGC candidates have integrated colors redder than the mean of the M87 globular cluster system, we propose that all of the objects are metal-poor, with [M/H] < -1. We show that the tidal radii of our intracluster globular clusters are all larger than the mean for Milky Way clusters and suggest that the clusters have undergone less tidal stress than their Galactic counterparts. Finally, we normalize our globular cluster observations to the luminosity of intracluster stars and derive a value of S_N ~ 6 for the specific frequency of Virgo intracluster globular clusters. We use these data to constrain the origins of Virgo's intracluster population and suggest that globular clusters in our intracluster field have a different origin than globular clusters in the vicinity of M87. In particular, we argue that dwarf elliptical galaxies may be an important source of intracluster stars.

Mid-infrared (MIR; 12-60 μm) diffuse emission in the Galactic plane and the Carina Nebula is investigated relative to the far-infrared (FIR; ≥100 μm) emission. Observations show that the ratio of the 12 μm emission to the total FIR intensity is more or less constant in the Galactic plane but exhibits a slight decrease followed by an increase as the FIR color becomes bluer in the Carina Nebula. The constancy is compatible with predictions from models of stochastic heating of very small grains or infrared fluorescence of polycyclic aromatic hydrocarbons. The decrease can be attributed to a weakening of the unidentified infrared band emission in strong radiation fields. Contrarily, the ratio of the 25 and 60 μm emission to FIR intensity increases linearly with field strength, which is incompatible with the model predictions. The Carina Nebula data show a much bluer FIR color than the Galactic plane, whereas the ratio of MIR to FIR emission is in a range similar to that of the plane. We interpret these characteristics in terms of a superposition of emission from dust grains in various radiation field strengths. The linear increase can be accounted for by an increasing contribution from emission by dust grains in strong radiation environments, whereas the shift in FIR color between the Galactic plane and the Carina Nebula can be attributed to different contributions from grains in weak radiation fields. Other possibilities, such as the effect of multiphoton processes, variations in the incident radiation spectrum, and possible contributions from iron grains, have also been examined, but none can account for the observations consistently. Applications of the present model to external galaxies are also discussed.

We trace the assembly history of red galaxies since z = 1 by measuring their evolving space density with the B-band luminosity function. Our sample of 39,599 red galaxies, selected from 6.96 deg² of imaging from the NOAO Deep Wide-Field Survey and the Spitzer IRAC Shallow Survey, is an order of magnitude larger, in size and volume, than comparable samples in the literature. We measure a higher space density of z ~ 0.9 red galaxies than some of the recent literature, in part because we account for the faint yet significant galaxy flux that falls outside of our photometric aperture. The B-band luminosity density of red galaxies, which effectively measures the evolution of ~L* galaxies, increases by only 36% ± 13% from z = 0 to z = 1. If red galaxy stellar populations have faded by ≃1.24 B-band magnitudes since z = 1, the stellar mass contained within the red galaxy population has roughly doubled over the past 8 Gyr. This is consistent with star-forming galaxies being transformed into ≲L* red galaxies after a decline in their star formation rates. In contrast, the evolution of ≃4L* red galaxies differs only slightly from a model with negligible z < 1 star formation and no galaxy mergers. If this model approximates the luminosity evolution of red galaxy stellar populations, then ≃80% of the stellar mass contained within today's 4L* red galaxies was already in place at z = 0.7. While red galaxy mergers have been observed, such mergers do not produce rapid growth of 4L* red galaxy stellar masses between z = 1 and the present day.

Starting with the first detection of an afterglow from a short-duration hard-spectrum γ-ray burst (SHB) by Swift last year, a growing body of evidence has suggested that SHBs are associated with an older and lower redshift galactic population than long-soft GRBs and, in a few cases, with large (≳10 kpc) projected offsets from the centers of their putative host galaxies. Here we present observations of the field of GRB 060502B, a SHB detected by Swift and localized by the X-Ray Telescope (XRT). We find a massive red galaxy at a redshift of z = 0.287 at an angular distance of 17.1'' from our revised XRT position. Using associative and probabilistic arguments, we suggest that this galaxy hosted the progenitor of GRB 060502B. If true, this offset would correspond to a physical displacement of 73 ± 19 kpc in projection, about twice the largest offset inferred for any SHB to date and almost an order of magnitude larger than a typical long-soft burst offset. Spectra and modeling of the star formation history of this possible host show it to have undergone a large ancient starburst. If the progenitor of GRB 060502B was formed in this starburst episode, the time of the GRB explosion since birth is τ ≈ 1.3 ± 0.2 Gyr and the minimum kick velocity of the SHB progenitor is v_kick,min = 55 ± 15 km s^-1.

We propose a modified version of the X-ray spectral index and an intrinsic cutoff frequency of inverse Compton radiation from the brightest knot of the M87 jet, in conjunction with an application of the new conceptions of injection and diffusive shock acceleration (DSA) of electrons in magnetized filamentary plasma to the specified source. The drop of the X-ray flux density in a transitive frequency region is associated with the interplay of ordinary synchrotron cooling and weaker magnetic fields concomitant with the smaller scale filaments that allow the electron injection, while the radio-optical synchrotron continuum is dominantly established by the major electrons that are quasi-secularly bound to larger filaments. With reference to, particularly, the updated external Compton model, we demonstrate that in the Klein-Nishina regime fading inverse Comptonization, the injected electrons can be stochastically energized up to a Lorentz factor as high as 5 × 10¹⁰ in the temporal competition with diffuse synchrotron cooling; this value is larger than that attainable for a simple DSA scenario based on the resonant scattering diffusion of the gyrating electrons bound to a supposed magnetic field homogeneously pervading the entire knot. The upper limits of the photon frequency boosted via conceivable inverse Compton processes are predicted to be of the common order of ~10³⁰ Hz. The variability of the broadband spectrum is also discussed in comparison to the features of a blazar light curve. The present scenario of a peta-eV (PeV; 10¹⁵ eV) electron accelerator, the "Pevatron," might provide some guidance for exploring untrod hard X-ray and gamma-ray bands in forthcoming observations.

We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed up with either the Subaru or the Isaac Newton Telescopes. They include four probable new dwarf galaxies—one each in the constellations of Coma Berenices, Canes Venatici, Leo, and Hercules—together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii, and color-magnitude diagrams for all five satellites. The morphological features of the color-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the past two years, a total of 10 new Milky Way satellites with effective surface brightness μ_v ≳ 28 mag arcsec^-2 have been discovered in SDSS data. They are less luminous, more irregular, and apparently more metal-poor than the previously known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between ~40 and ~100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.

We consider the dynamical evolution of a disk of stars orbiting a central black hole. In particular, we focus on the effect of the stellar mass function on the evolution of the disk, using both analytic arguments and numerical simulations. We apply our model to the ring of massive stars at ≃0.1 pc from the Galactic center, assuming that the stars formed in a cold, circular disk, and find that our model requires the presence of a significant population of massive (>100 M_☉) stars in order to explain the observed eccentricities of 0.2-0.3. Moreover, in order to limit the damping of the heavier stars' eccentricities, we also require fewer low-mass stars than expected from a Salpeter mass function, giving strong evidence for a significantly "top-heavy" mass function in the rings of stars seen near to the Galactic center. We also note that the maximum possible eccentricities attainable from circular initial conditions at ages of <10 Myr are around 0.4-0.5, and we suggest that any rings of stars found with higher eccentricities were probably not formed from circular disks.

We present the results of a deep photometric survey performed with the VLT FORS1 aimed at investigating the complex main-sequence structure of the stellar system ω Centauri. We confirm the presence of a double main sequence and identify its blue component (bMS) over a large field of view up to 26' from the cluster center. We found that bMS stars are significantly more concentrated toward the cluster center than the other "normal" MS stars. The bMS morphology and its position in the CMD have been used to constrain the helium overabundance required to explain the observed MS morphology.

We present a new formalism to describe the ratios and profiles of emission lines from hydrogen in Balmer-dominated shocks. We use this model to interpret the measured widths and ratios of broad and narrow Hα, Hβ, and Lyα emission lines in supernova remnants (SNRs). Our model results agree fairly well with those obtained previously by Chevalier, Kirshner, & Raymond and are consistent with observations of several SNRs. The same model fails to account for the ratio of broad to narrow line emission from the reverse shock in SNR 1987A as observed by Heng and coworkers. We suggest that this discrepancy between theory and observation results from a faulty assumption that Balmer-dominated shocks can be treated as sharp discontinuities. If the spatial structure of the shock transition zone is taken into account, the predicted ratios of broad to narrow line emission in most SNRs will change by modest factors, but the ratio in SNR 1987A will increase substantially. Significantly greater shock velocities will be required to account for the observed full widths at half-maximum of the broad emission lines in most SNRs.

We present near-infrared narrowband images of the supernova remnant W49 B, taken with the WIRC instrument on the Hale 200 inch (5 m) telescope on Mount Palomar. The 1.64 μm [Fe II] image reveals a barrel-shaped structure with coaxial rings, which is suggestive of bipolar wind structures surrounding massive stars. The 2.12 μm shocked molecular hydrogen image extends 1.9 pc outside of the [Fe II] emission to the southeast. We also present archival Chandra data, which show an X-ray jetlike structure along the axis of the [Fe II] barrel, flaring at each end. Fitting single-temperature X-ray emission models reveals an enhancement of heavy elements, with particularly high abundances of hot Fe and Ni, and relatively metal-rich core and jet regions. We interpret these findings as evidence that W49 B originated inside a wind-blown bubble (R ~ 5 pc) inside a dense molecular cloud. This suggests that W49 B's progenitor was a supermassive star that could significantly shape its surrounding environment. We also suggest two interpretations for the jet morphology, abundance variations, and molecular hydrogen emission: (1) the explosion may have been jet driven, interacting with the molecular cavity (i.e., a gamma-ray burst); or (2) the explosion could have been a traditional supernova, with the jet structure being the result of interactions between the shock and an enriched interstellar cloud.

The thermal instability with a piecewise power law cooling function is investigated using one- and three-dimensional simulations with periodic and shearing-periodic boundary conditions in the presence of constant thermal diffusion and kinematic viscosity coefficients. Consistent with earlier findings, the flow behavior depends on the average density, ⟨ρ⟩. When ⟨ρ⟩ is in the range (1-5) × 10^-24 g cm^-3, the system is unstable and segregates into cool and warm phases with temperatures of roughly 100 and 10⁴ K, respectively. However, in all cases the resulting average pressure ⟨p⟩ is independent of ⟨ρ⟩ and just a little above the minimum value. For a constant heating rate of 0.015 ergs g^-1 s^-1, the mean pressure is around 24 × 10^-14 dyn (corresponding to p/k_B ≈ 1750 K cm^-3). Cool patches tend to coalesce into bigger ones. In all cases investigated, there is no sustained turbulence, which is in agreement with earlier results. Simulations in which turbulence is driven by a body force show that when rms velocities of between 10 and 30 km s^-1 are obtained, the resulting dissipation rates are comparable to the thermal energy input rate. The resulting mean pressures are then about 30 × 10^-14 dyn, corresponding to p/k_B ≈ 2170 K cm^-3. This is comparable to the value expected for the Galaxy. Differential rotation tends to make the flow two-dimensional, that is, uniform in the streamwise direction, but this does not lead to instability.

We report on the abundance of interstellar neutral nitrogen (N I) for 30 sight lines, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Hubble Space Telescope (HST). N I column densities are derived by measuring the equivalent widths of several ultraviolet absorption lines and subsequently fitting those to a curve of growth. We find a mean interstellar N/H of 51 ± 4 ppm. This is below the mean found by Meyer et al. of 62 ppm (adjusted for a difference in f-values). Our mean N/H is similar, however, to the (f-value adjusted) mean of 51 ± 3 ppm found by Knauth et al. for a larger sample of sight lines with larger hydrogen column densities comparable to those in this study. We discuss the question of whether or not nitrogen shows increased gas-phase depletion in lines of sight with column densities log N(H_tot) ≳ 21, as claimed by Knauth et al. The nitrogen abundance in the line of sight toward HD 152236 is particularly interesting. We derive very small N/H and N/O ratios for this line of sight that may support a previous suggestion that members of the Sco OB1 association formed from an N-deficient region.

Observations of two of the formaldehyde (H₂CO) masers (A and D) in Sgr B2 using the VLBA+Y27 (resolution ≈0.01'') and the VLA (resolution ≈9'') are presented. The VLBA observations show compact sources (≲10 mas, ≲80 AU) with brightness temperatures >10⁸ K. The maser sources are partially resolved in the VLBA observations. The flux densities in the VLBA observations are about 1/2 those of the VLA, and the line widths are about 2/3 of the VLA values. The applicability of a core-halo model for the emission distribution is demonstrated. Comparison with earlier H₂CO absorption observations and with ammonia (NH₃) observations suggests that H₂CO masers form in shocked gas. Comparison of the integrated flux densities in current VLA observations with those in previous observations indicates that (1) most of the masers have varied in the past 20 years, and (2) intensity variations are typically less than a factor of 2 compared to the 20 yr mean. No significant linear or circular polarization is detected with either instrument.

In previous works we have modeled the different gas regions of the proto-planetary nebula CRL 618 by studying the large number of lines from the cyanopolyynes detected in a millimeter-wave line survey of this object. In this work we retrieve the rotational temperatures (T_r) and abundance ratios R with respect to HC₃N (used as reference) for all molecular species detected in the survey by running grids of models in the (R,T_r) space to find the minimum of a weighted χ² defined for this analysis. This provides the best knowledge to date of the (polar) molecular composition of CRL 618 thanks to the uniform calibration of the whole survey and the large number of lines available from each species, allowing comparisons with predictions made by chemical models of C-rich post-AGB objects. A significantly lower value of the ¹²C/¹³C ratio has been revealed in the gas closest to central star with respect to the colder and outer envelope. It can be due to ¹³C-rich material, produced in a late CNO cycling occurred in the central star, being currently injected into this inner gas envelope.

We suggest that the Orion A cloud is gravitationally collapsing on large scales, and is producing the Orion Nebula Cluster through the focusing effects of gravity acting within a finite cloud geometry. In support of this suggestion, we show how an elliptical rotating sheet of gas with a modest density gradient along the major axis can collapse to produce a structure qualitatively resembling Orion A, with a fan-shaped structure at one end, ridges or filaments along the fan, and a narrow curved filament at the other end reminiscent of the famous integral-shaped filament. The model produces a local concentration of mass within the narrow filament, which in principle could form a dense cluster of stars like that of the Orion Nebula. We suggest that global gravitational contraction might be a more common feature of molecular clouds than previously recognized, and that the formation of star clusters is a dynamic process resulting from the focusing effects of gravity acting on the geometry of finite clouds.

Recent observations from the Swift gamma-ray burst mission indicate that a fraction of gamma-ray bursts are characterized by a canonical behavior of the X-ray afterglows. We present an effective theory that allows us to account for X-ray light curves of both (short/long) gamma-ray bursts and X-ray-rich flashes. We propose that gamma-ray bursts originate from massive magnetic-powered pulsars.

We analyze the linear stability of a stalled accretion shock in a perfect gas with a parameterized cooling function ∝ ρ^β-αP^α. The instability is dominated by the l = 1 mode if the shock radius exceeds 2-3 times the accretor radius, depending on the parameters of the cooling function. The growth rate and oscillation period are comparable to those observed in the numerical simulations of Blondin & Mezzacappa. The instability mechanism is analyzed by separately measuring the efficiencies of the purely acoustic cycle and the advective-acoustic cycle. These efficiencies are estimated directly from the eigenspectrum and also through a WKB analysis in the high-frequency limit. Both methods prove that the advective-acoustic cycle is unstable and that the purely acoustic cycle is stable. Extrapolating these results to low frequency leads us to interpret the dominant mode as an advective-acoustic instability, different from the purely acoustic interpretation of Blondin & Mezzacappa. A simplified characterization of the instability is proposed, based on an advective-acoustic cycle between the shock and the radius r_∇ where the velocity gradients of the stationary flow are strongest. The importance of the coupling region in this mechanism calls for a better understanding of the conditions for an efficient advective-acoustic coupling in a decelerated, nonadiabatic flow, in order to extend these results to core-collapse supernovae.

Neutron stars, with their strong surface gravity, have interestingly short timescales for the sedimentation of heavy elements. Motivated by observations of Type I X-ray bursts from sources with extremely low persistent accretion luminosities, L_X < 10³⁶ ergs s^-1 (≃0.01L_Edd), we study how sedimentation affects the distribution of isotopes and the ignition of H and He in the envelope of an accreting neutron star. For local mass accretion rates ≲ 10^-2_Edd (for which the ignition of H is unstable), where _Edd = 8.8 × 10⁴ g cm^-2 s^-1, the helium and CNO elements sediment out of the accreted fuel before reaching a temperature at which H would ignite. Using one-zone calculations of the thermonuclear burning, we find a range of accretion rates for which the unstable H ignition does not trigger unstable He burning. This range depends on the emergent flux from reactions in the deep neutron star crust; for F = (0.1 MeV) (/m_u), the range is 3 × 10^-3_Edd ≲ ≲ 10^-2_Edd. We speculate that sources accreting in this range would build up a massive He layer that would later produce an energetic and long X-ray burst. At mass accretion rates lower than this range, we find that the H flash leads to a strong mixed H/He flash. Surprisingly, even at accretion rates ≳ 0.1_Edd, although the H and He do not completely segregate, the H abundance at the base of the accumulated layer is still reduced. While following the evolution of the X-ray burst is beyond the scope of this introductory paper, we note that the reduced proton-to-seed ratio favors the production of ¹²C—an important ingredient for subsequent superbursts.

FUSE, HST, and SDSS spectra of the cataclysmic variable SDSS J080908.39+381406.2 provide a spectral flux distribution from 900 to 9200 Å. This data set is used to illustrate procedures for calculating and testing system models. The spectra are not contemporaneous; it is necessary to assume that the combined spectra are representative of the system. The illustrations are based on a system with a 1.0 M_☉ white dwarf, a 0.30 M_☉, 3500 K, Roche lobe-filling secondary star, and an accretion disk extending to the tidal cutoff radius. Assuming a similar accretion state for the nonsimultaneous spectra, the best standard model fit is with a mass transfer rate of 3.0 × 10^-9M_☉ yr^-1. Extensive simulations demonstrate that the accretion disk must be truncated at its inner edge if the temperature profile follows the standard model, but truncated models face severe objections, which we address. Following additional simulation tests, we obtain a model accretion disk with a temperature profile comparable to the profile for SW Sex as determined from tomographic image reconstruction. This model fits the discovery SDSS spectrum well but has a flux deficit in the UV and FUV. Emission from a white dwarf is a plausible source of additional flux. Adding this source to the disk synthetic spectrum produces FUV flux that can explain the observed flux. An additional (archival) SDSS spectrum is fainter by about 0.3 mag in the optical. Additional analysis showed that UV residuals from a model fitting the archival optical wavelength spectrum are unacceptably large. Contemporaneous spectra from all wavelength regions would be necessary for a reliable system model. Our discussion illustrates how this conclusion follows from the system models.

We report on an X-ray flare detected on the active binary system II Pegasi with the Swift telescope. The event triggered the Burst Alert Telescope (BAT) in the hard X-ray band on 2005 December 16 at 11:21:52 UT with a 10-200 keV luminosity of 2.2 × 10³² ergs s^-1—a superflare, by comparison with energies of typical stellar flares on active binary systems. The trigger spectrum indicates a hot thermal plasma with T ~ 180 × 10⁶ K. X-ray spectral analysis from 0.8 to 200 keV with the X-Ray Telescope and BAT in the next two orbits reveals evidence for a thermal component (T > 80 × 10⁶ K) and Fe K 6.4 keV emission. A tail of emission out to 200 keV can be fit with either an extremely high temperature thermal plasma (T ~ 3 × 10⁸ K) or power-law emission. Based on analogies with solar flares, we attribute the excess continuum emission to nonthermal thick-target bremsstrahlung emission from a population of accelerated electrons. We estimate the radiated energy from 0.01 to 200 keV to be ~6 × 10³⁶ ergs, the total radiated energy over all wavelengths ~10 ³⁸ ergs, the energy in nonthermal electrons above 20 keV ~3 × 10⁴⁰ ergs, and conducted energy <5 × 10⁴³ ergs. The nonthermal interpretation gives a reasonable value for the total energy in electrons >20 keV when compared to the upper and lower bounds on the thermal energy content of the flare. This marks the first occasion in which evidence exists for nonthermal hard X-ray emission from a stellar flare. We investigate the emission mechanism responsible for producing the 6.4 keV feature, and find that collisional ionization from nonthermal electrons appears to be more plausible than the photoionization mechanism usually invoked on the Sun and pre-main-sequence stars.

We present far-UV to optical analyses of four hydrogen-deficient central stars of planetary nebulae: BD +30 3639, NGC 40, NGC 5315, and NGC 6905. Using the radiative transfer code CMFGEN, we determined new physical parameters and chemical abundances for these stars. The results were analyzed in the context of the [WR] → PG 1159 evolution via the transformed radius-temperature (R_T × T_*) and H-R diagrams. NGC 5315 showed itself as an odd object among the previously analyzed central stars. Its temperature (~76 kK) is considerably lower than other early-type [WR] stars (~120-150 kK). From our models for NGC 5315 and NGC 6905, it is unclear if early-type [WR] stars have smaller C/He mass ratios than other spectral classes, as claimed in the literature. A ratio of ~0.8 is found for NGC 6905. We analyzed FUSE spectra of these stars for the first time and identified phosphorus in the spectra of BD +30 3639, NGC 40, and NGC 5315 through the doublet transition P V λλ1118, 1128 (3p²P^o-3s²S). The Fe, Si, P, S, and Ne abundances were analyzed in the context of the nucleosynthesis occurring in previous evolutionary phases. We found evidence for Fe deficiency in BD +30 3639 and NGC 5315, and we determined a solar Si abundance for BD +30 3639 and NGC 40. Regarding P, an oversolar abundance in the NGC 5315 model was preferred. Upper limits for the S abundance were estimated. We found that Ne is overabundant in BD +30 3639. In the other stars, Ne is weak or undetectable. Our results are in agreement with theoretical predictions and show the usefulness of [WR] stars as test beds for nucleosynthesis calculations in the AGB and post-AGB phases.

We examine in this paper the nature of the intriguing small-frequency spacings that have been reported in the pulsation spectra of rapidly pulsating hot B subdwarf stars. It has been said that these mysterious and puzzling spacings should not be present if current models of these stars and their pulsations are any guide to reality. They lead to highly accurate fits to observed frequency data and have been suggested to contain valuable (but still encoded) information about the inner structure of sdB stars. The empirical relation that leads to such high-accuracy fits involves a zero-point frequency, two small-frequency spacings (a factor of 15-30 smaller than the so-called large-frequency spacing for acoustic modes in the asymptotic regime), and two sets of integers, all optimized to best match the observed frequencies. After investigating its true nature, we have to report that this empirical relation contains, in fact, no physical information and is a purely numerical artifact. In particular, we show that the two best-fit frequency spacings used in the proposed relation correspond basically to the largest common denominators between the real frequency spacings in the observed sequence: one acting on a coarse scale and the other on a finer scale. As common denominators, the numerical values of these spacings depend sensitively on the signal-to-noise ratio (S/N) of the observations and cannot therefore represent intrinsic properties of a pulsating star. We suggest that, for the time being, progress on the front of the asteroseismology of sdB stars might still best be done with the help of physical models such as those pioneered by Brassard et al.

The bright, K1 III-IV star γ Cep has been reported previously to have a possibly substellar companion in a ~2.5 yr orbit, as well as an unseen stellar companion at a larger separation. We determine for the first time the three-dimensional orbit of the latter, accounting also for the perturbation from the closer object. We combine new and existing radial velocity measurements with intermediate astrometric data from the Hipparcos mission (abscissa residuals), as well as ground-based positional observations going back more than a century. The orbit of the secondary star is eccentric (e = 0.4085 ± 0.0065) and has a period P = 66.8 ± 1.4 yr. We establish the primary star to be on the first ascent of the giant branch and to have a mass of 1.18 ± 0.11 M_☉, an effective temperature of 4800 ± 100 K, and an age around 6.6 Gyr (for an assumed metallicity [Fe/H] = +0.01 ± 0.05). The unseen secondary star is found to be an M4 dwarf with a mass of 0.362 ± 0.022 M_☉ and is expected to be ~6.4 mag fainter than the primary in K. The minimum mass of the putative planetary companion is M_p sin i = 1.43 ± 0.13 M_Jup. Based on high-precision Hipparcos observations, we are able to place a dynamical upper limit on this mass of 13.3M_Jup at the 95% confidence level, and 16.9M_Jup at the 99.73% (3 σ) confidence level, thus confirming that it is indeed substellar in nature. The orbit of this object is only 9.8 times smaller than the orbit of the secondary star (the smallest ratio among exoplanet host stars in multiple systems), but it is stable if coplanar with the binary.

The existence of extrasolar planets with short orbital periods suggests that planetary migration induced by tidal interaction with the protoplanetary disk is important. Cores and terrestrial planets may undergo migration as they form. In this paper we investigate the evolution of a population of cores with initial masses in the range 0.1-1 M_⊕ embedded in a disk. Mutual interactions lead to orbit crossing and mergers, so that the cores grow during their evolution. Interaction with the disk leads to orbital migration, which results in the cores capturing each other in mean motion resonances. As the cores migrate inside the disk inner edge, scatterings and mergers of planets on unstable orbits, together with orbital circularization, causes strict commensurability to be lost. Near commensurability however is usually maintained. All the simulations end with a population typically between two and five planets, with masses depending on the initial mass. These results indicate that if hot super-Earths or Neptunes form by mergers of inwardly migrating cores, then such planets are most likely not isolated. We would expect to always find at least one, more likely a few, companions on close and often near-commensurable orbits. To test this hypothesis, it would be of interest to look for planets of a few to about 10 M_⊕ in systems where hot super-Earths or Neptunes have already been found.

We applied a numerical modeling of the solar energetic particle (SEP) event inside a magnetic cloud in the solar wind to analyze the 17-22 MeV proton flux anisotropy observed on 1998 May 2 with the Energetic and Relativistic Nuclei and Electron (ERNE) instrument on the Solar and Heliospheric Observatory (SOHO), when SOHO was inside a magnetic cloud associated with a previous coronal mass ejection (CME). The analysis revealed a strong intermittency of the SEP transport parameters when different magnetic tubes were convected past the spacecraft. The estimated mean free path value varies over 1 order of magnitude, from ~2 to ~20 AU. The SEP event has been modeled with a prolonged injection of particles from a new CME into the previous ejecta comprising a set of magnetic loops. Both the prompt, direct proton flux and the delayed, counterstreaming flux were observed in the beginning and maximum phase of the event, but then the counterstreaming flux waned. The lack of counterstreaming protons can be explained either by the very fast escape of high-energy protons from the magnetic cloud, before they could complete one bounce in a narrow loop, or by the proton injection predominantly into one leg of a wide loop. An imprint of the magnetic compression at the leading part of the CME can be also found in the proton flux anisotropy data of SOHO/ERNE. These findings illustrate how high-precision anisotropy measurements and a numerical modeling can provide a kind of probe for the CME structure.

We study the dynamical interaction of the solar chromosphere with the transition region in mossy and nonmossy active-region plage. We carefully align image sequences taken with the Transition Region And Coronal Explorer (TRACE) in the ultraviolet passbands around 1550, 1600, and 1700 Å and the extreme ultraviolet passbands at 171 and 195 Å. We compute Fourier phase-difference spectra that are spatially averaged separately over mossy and nonmossy plage to study temporal modulations as a function of temporal frequency. The 1550 versus 171 Å comparison shows zero phase difference in nonmossy plage. In mossy plage, the phase differences between all UV and EUV passbands show pronounced upward trends with increasing frequency, which abruptly changes into zero phase difference beyond 4-6 mHz. The phase difference between the 171 and 195 Å sequences exhibits a shallow dip below 3 mHz and then also turns to zero phase difference beyond this value. We attribute the various similarities between the UV and EUV diagnostics that are evident in the phase-difference diagrams to the contribution of the C IV resonance lines in the 1550 and 1600 Å passbands. The strong upward trend at the lower frequencies indicates the presence of upward-traveling disturbances. It points to correspondence between the lower chromosphere and the upper transition region, perhaps by slow-mode magnetosonic disturbances, or by a connection between chromospheric and coronal heating mechanisms. The transition from this upward trend to zero phase difference at higher frequencies is due to the intermittent obscuration by fibrils that occult the footpoints of hot loops, which are bright in the EUV and C IV lines, in an oscillatory manner.

The resonant peaks of solar p modes show small amounts of asymmetry in frequency. Here, we use five independent sets of contemporaneous data, collected over a ≃8 yr period, to investigate whether peak asymmetry in low angular degree p modes changes over the solar activity cycle. Three of the data sets are from instruments on board the ESA/NASA SOHO spacecraft (GOLF, MDI, and VIRGO/SPM); and two are from ground-based networks (BiSON and GONG). Evidence for variation in asymmetry, well correlated with the activity cycle, is uncovered in the GOLF and BiSON Doppler velocity data. Suggestions of a similar trend are present in the GONG Doppler velocity data. Apparent changes in the MDI Doppler velocity data are somewhat less significant. Meanwhile, analysis of the SPM intensity data failed to uncover any evidence for significant change of the asymmetry parameter.

We present results on the structure of the near-surface layers of the Sun obtained by inverting frequencies of high-degree solar modes from "ring diagrams." We have results for eight epochs between 1996 June and 2003 October. The frequencies for each epoch were obtained from ring diagrams constructed from Michelson Doppler Imager (MDI) Dopplergrams spanning complete Carrington rotations. We find that there is a substantial latitudinal variation of both sound speed and the adiabatic index Γ₁ in the outer 2% of the Sun. We find that both the sound-speed and Γ₁ profiles change with changes in the level of solar activity. In addition, we also study differences between the northern and southern hemispheres of the Sun and find a small asymmetry that appears to reflect the difference in magnetic activity between the two hemispheres.

To understand magnetic diffusion, momentum transport, and mixing in the interior of the Sun, we consider an idealized model of the tachocline, namely, magnetohydrodynamic (MHD) turbulence on a β-plane subject to a large-scale shear (provided by the latitudinal differential rotation). This model enables us to self-consistently derive the influence of shear, Rossby, and Alfvén waves on the transport properties of turbulence. In the strong magnetic field regime, we find that the turbulent viscosity and diffusivity are reduced by magnetic fields only, as in the two-dimensional MHD case (without Rossby waves). In the weak magnetic field regime, we find a crossover scale (L_R) from a Alfvén-dominated regime (on small scales) to a Rossby-dominated regime (on large scales). For parameter values typical of the tachocline, L_R is larger than the solar radius so that Rossby waves are unlikely to play an important role in the transport of magnetic field and angular momentum. This is mainly due to the enhancement of magnetic back-reaction by shearing, which efficiently generates small scales, and thus strong currents.

Using accurate interaction potentials, we perform refined calculations of rate coefficients for the fine-structure excitations in collisions of O(³P) and C(³P) with atomic hydrogen. The results are presented in the form of analytical functions approximating the rate coefficients over a wide range of temperatures. We examine the sensitivity of the collision dynamics to variations of the interaction potentials and the couplings to electronically excited states.

We use the deep NIR imaging of the FIRES survey to investigate trends with redshift of the properties of galaxies selected to have strong Balmer/4000 Å breaks at 2 < z < 4.5. Analogous to the J - K > 1.3 (AB) color criterion designed to select red galaxies at z > 2, we propose two color criteria, J - H > 0.9 and H - K > 0.9, to select red galaxies in two redshift bins at 2 < z < 3 and 3 < z < 4.5, respectively. From the FIRES catalogs of the HDF-S (4.7 arcmin²) and MS 1054-03 (26.3 arcmin²) fields, we find 18 galaxies with ⟨z_phot⟩ = 2.4 that satisfy J_s - H > 0.9, H < 23.4 and 23 galaxies with ⟨z_phot⟩ = 3.7 that satisfy H - K_s > 0.9, K_s < 24.6, where the flux limits are chosen to match the limiting rest-frame luminosities at the different median redshifts of the two samples. The space densities of the J_s - H and H - K_s samples are (1.5 ± 0.5) × 10^-4 and (1.2 ± 0.4) × 10^-4 Mpc^-3, respectively. The rest-frame U - B colors of galaxies in both samples are similarly red (as expected from the definition of the color criteria), but the rest-frame UV properties are different: galaxies in the higher redshift H - K_s selected sample have blue NUV-optical colors and UV slopes similar to those of Lyman break galaxies, while the J_s - H galaxies are generally red over the entire wavelength range observed. Synthetic template fits indicate that the distinct rest-NUV properties of the two samples are primarily a result of dust: we find ⟨A_V⟩_JH = 1 mag and ⟨A_V⟩_HK = 0.2 mag. The median stellar mass determined from the template fits decreases by a factor of ~5 from z = 2.4 to 3.7, which, coupled with the fact that the space density of such galaxies remains roughly constant, may imply that the stellar mass density in red galaxies decreases by a similar factor over this redshift range.

Intervening H I 21 cm absorption systems at z ≥ 1.0 are very rare, and only four confirmed detections have been reported in the literature. Despite their scarcity, they provide interesting and unique insights into the physical conditions in the interstellar medium of high-z galaxies. Moreover, they can provide independent constraints on the variation of fundamental constants. We report three new detections based on our ongoing Giant Metrewave Radio Telescope (GMRT) survey for 21 cm absorbers at 1.10 ≤ z_abs ≤ 1.45 from candidate damped Lyα systems. The 21 cm lines are narrow for the z_abs = 1.3710 system toward SDSS J0108-0037 and z_abs = 1.1726 system toward SDSS J2358-1020. Based on line FWHM, the kinetic temperatures are ≤5200 and ≤800 K, respectively. The 21 cm absorption profile of the third system, z_abs = 1.1908 system toward SDSS J0804+3012, is shallow, broad, and complex, extending up to 100 km s^-1. The centroids of the 21 cm lines are found to be shifted with respect to the corresponding centroids of the metal lines derived from SDSS spectra. This may mean that the 21 cm absorption is not associated with the strongest metal-line component.

We combine optical and X-ray data for eight low-redshift (z ~ 0.06) poor groups of galaxies from the XI (XMM-IMACS) Groups Project to study the AGN population in the group environment. Among ~140 group members, we identify five AGNs based on their optical emission lines. None of these optically selected AGNs are detected by XMM-Newton. One additional AGN is discovered in the XMM-Newton observations. This X-ray-detected AGN, which has no obvious AGN-emission-line signatures in its optical spectrum, is a member of the only X-ray-luminous group in our sample. The lack of a significant population of X-ray-bright but optically dull AGNs among less dynamically evolved groups is in stark contrast to the large fraction of such objects in rich clusters of galaxies. We suggest this result can be explained by a physical scenario for AGN accretion evolution: AGN activity is initially triggered by galaxy merging, leading to a high accretion rate and an optically dominant phase (via thin-disk accretion). As the accretion rate drops in time, the AGN gradually enters an X-ray-dominant low-accretion phase (via a radiative inefficient accretion flow). In this picture, optically and X-ray selected AGN are the same population of supermassive black holes observed at different epochs. Within the context of this scenario, the majority of AGNs in poor groups are in the high-accretion optically dominant phase, while the AGN population in rich clusters is mostly in the low-accretion X-ray-dominant phase.

In 2005 and 2006, the MAGIC telescope observed very high energy gamma-ray emission from the distant BL Lac object PG 1553+113. The overall significance of the signal was 8.8 σ for 18.8 hr of observation time. The light curve shows no significant flux variations on a daily timescale; the flux level during 2005 was, however, significantly higher compared to 2006. The differential energy spectrum between ~90 and 500 GeV is well described by a power law with photon index Γ = 4.2 ± 0.3. The combined 2005 and 2006 energy spectrum provides an upper limit of z = 0.74 on the redshift of the object.

The use of Type Ia supernovae (SNe Ia) as cosmological standard candles is fundamental in modern observational cosmology. In this Letter, we derive a simple empirical photometric redshift estimator for SNe Ia using a training set of SNe Ia with multiband (griz) light curves and spectroscopic redshifts obtained by the Supernova Legacy Survey (SNLS). This estimator is analytical and model-independent; it does not use spectral templates. We use all the available SNe Ia from SNLS with near-maximum photometry in griz (a total of 40 SNe Ia) to train and test our photometric redshift estimator. The difference between the estimated redshifts z_phot and the spectroscopic redshifts z_spec, (z_phot - z_spec)/(1 + z_spec), has rms dispersions of 0.031 for 20 SNe Ia used in the training set, and 0.050 for 20 SNe Ia not used in the training set. The dispersion is of the same order of magnitude as the flux uncertainties at peak brightness for the SNe Ia. There are no outliers. This photometric redshift estimator should significantly enhance the ability of observers to accurately target high-redshift SNe Ia for spectroscopy in ongoing surveys. It will also dramatically boost the cosmological impact of very large future supernova surveys, such as those planned for the Advanced Liquid-mirror Probe for Astrophysics, Cosmology, and Asteroids (ALPACA) and the Large Synoptic Survey Telescope (LSST).

A large fraction of barred galaxies host secondary bars that are embedded in their large-scale primary counterparts. The evolution of such double-barred galaxies is still not well understood, partly because of a lack of realistic N-body models with which to study them. Here we report a new mechanism for generating such systems, namely, the presence of rotating pseudobulges. We demonstrate with high mass and force resolution collisionless N-body simulations that long-lived secondary bars can form spontaneously without requiring gas, contrary to previous claims. We find that secondary bars rotate faster than primary ones. The rotation is not rigid: the secondary bars pulsate, with their amplitude and pattern speed oscillating as they rotate through the primary bars. This self-consistent study supports previous work based on orbital analysis in the potential of two rigidly rotating bars. The pulsating nature of secondary bars may have important implications for understanding the central region of double-barred galaxies.

Bright emission nebulae, or H II regions, around hot stars are readily seen in Hα light. However, the all-pervasive faint Hα emission has only recently been detected and mapped over the whole sky. Mostly the Hα emission observed along a line of sight is produced by ionized gas in situ. There are, however, cases where all or most of the Hα radiation is due to scattering by electrons or dust particles that are illuminated by an Hα-emitting source off the line of sight. Here we demonstrate that diffuse, translucent, and dark dust clouds at high Galactic latitudes are in many cases observed to have an excess of diffuse Hα surface brightness; i.e., they are brighter than the surrounding sky. We show that the majority of this excess surface brightness can be understood as light scattered off the interstellar dust grains. The source of incident photons is the general Galactic Hα background radiation impinging on the dust clouds from all over the sky.

TeV J2032+4130 is the first extended very high energy gamma-ray source and has remained enigmatic since its discovery because of the lack of identification. We report here deep radio observations covering the TeV J2032+4130 field and revealing for the first time an extended and diffuse radio emission, as well as a remarkable population of compact radio sources. Some of these radio sources are in positional coincidence with X-ray and optical/IR sources. Future follow-up studies of these new radio sources will likely contribute to solving the mystery of this extended unidentified TeV source.

The origin of a double main sequence (MS) in ω Centauri is explored. We have shown from theoretical calculations of stellar evolution that the colors of MS stars are shifted to those of the observed blue MS if the surface layers are polluted by He-rich materials with a mass of ~0.1 M_☉. Stars are thought to be polluted through numerous encounters with the ejecta descended from massive asymptotic giant branch (AGB) stars. Two populations of stars with different kinematics observed especially in ω Cen indicate that kinematically cooler stars are more polluted through encounters with AGB ejecta than kinematically hotter ones because the accretion rate is inversely proportional to the cube of the relative velocity. We propose that both of these factors split the MS in ω Cen. This theoretical scheme explains why only ω Cen exhibits a double MS and matches the amount of He necessary to produce the blue MS to that supplied from massive AGB stars. Furthermore, we predict that even if globular clusters (GCs) possess only one generation of stars, the velocity dispersion of stars broadens the MS in the color-magnitude diagram as long as the GCs are massive enough to retain the AGB ejecta after the burst of star formation. This view explains the broad MS recently found in the GC NGC 2808, which exhibits no scatter in [Fe/H] and thus is likely to consist of a single generation of stars, unlike the case of ω Cen.

We present sensitive, high angular resolution 1.3 cm and 7 mm observations of the massive core NGC 6334 I(N), a region known to be undergoing massive star formation. At 1.3 cm we detect three sources, two of which had previously been detected at centimeter or millimeter wavelengths. At 7 mm we detect four sources. We suggest that three of these sources are subcomponents of the millimeter source SMA 1, which at these wavelengths is the dominant source in the region. The fourth 7 mm source appears to be associated with the relatively isolated source SMA 6. In all four 7 mm sources, the continuum emission is arising from structures of dimensions of order 1000 AU for which we estimate masses of order a few solar masses. We interpret these 7 mm sources as massive circumstellar disks that, however, surround stars or compact small stellar groups that at present have masses comparable to those of the disks but that may be accreting on their way to becoming massive stars.

The mid-infrared emission at the center of IRAS 21026+4104 is not that of a simple compact source, as one would expect from an isolated high-mass protostellar object. Furthermore, the central thermal infrared emission does not appear to be coming directly from a circumstellar disk as has been recently hypothesized from near-infrared observations. The mid-infrared structure is complex, but with the help of multiple-wavelength information two plausible scenarios to explain the emission in the region are advanced. The first is that there is a tight cluster of young stellar objects here. The second is that the mid-infrared emission and masers are delineating the walls of the outflow cavities of a massive stellar source located in the center of the near- and mid-infrared dark lane.

We have reanalyzed archival HST and Subaru data of the recently discovered planetary mass companion (PMC) GQ Lup B. With these we produce the first R- and I-band photometry of the companion and fit a radius and effective temperature using detailed model atmospheres. We find an effective temperature of 2335 ± 100 K, a radius of 0.38 ± 0.05 R_☉, and a luminosity of log(L/L_☉) = -2.42 ± 0.07 (at 140 pc). Since we fit wavelengths that span most of the emitted radiation from GQ Lup, this luminosity estimate is robust, with uncertainty dominated by the distance uncertainty (±50 pc). The radius obtained for 140 pc (0.38 R_☉) is significantly larger than the one originally derived and larger than model predictions. The mass of the object is much more model-dependent than the radiative properties, but for the Gaia dusty models we find a mass between 10M_Jup and 20M_Jup, in the range of the brown dwarf and PMC deuterium-burning boundary. Assuming a distance of 140 pc, observations fit to 1 σ the Baraffe evolution model for an ~15M_Jup brown dwarf. Additionally, the F606W photometric band is significantly overluminous compared to model predictions and other brown dwarfs. Such overluminosity could be explained by a bright Hα emission from chromospheric activity, interaction with another undetected companion, or accretion. Assuming that GQ Lup B has a bright Hα emission line, its Hα emission strength is 10^-1.71±0.10L_bol, significantly larger than field late-type dwarfs. GQ Lup B might be strongly accreting and might still be in its formation phase.

We present three-dimensional hydrodynamic simulations of a gravitationally unstable protoplanetary disk model under the condition of local isothermality. Ordinarily, local isothermality precludes the need for an artificial viscosity (AV) scheme to mediate shocks. Without AV, the disk evolves violently, shredding into dense (although short-lived) clumps. When we introduce our AV treatment in the momentum equation, but without heating due to irreversible compression, our grid-based simulations begin to resemble smoothed particle hydrodynamics (SPH) calculations, where clumps are more likely to survive many orbits. In fact, the standard SPH viscosity appears comparable in strength to the AV that leads to clump longevity in our code. This sensitivity to one numerical parameter suggests extreme caution in interpreting simulations by any code in which long-lived gaseous protoplanetary bodies appear.

Previous models of dust growth in protoplanetary disks considered either uniformly laminar or turbulent disks. This Letter explores how dust growth occurs in a layered protoplanetary disk in which the magnetorotational instability (MRI) generates turbulence only on the surface layers of a disk. Two cases are considered: a completely laminar dead zone and a dead zone in which turbulence is "stirred up" from the MRI acting above. It is found that dust is depleted from high altitudes in layered disks faster than in uniformly laminar or turbulent disks. This is a result of the accelerated growth of particles in the turbulent regions and their storage in the laminar lower levels where they escape energetic collisions that would result in disruption. Thus, the regions of a protoplanetary disk above a dead zone would become rapidly depleted in small dust grains, whereas the outer regions will maintain a small dust population at all heights due to the disruptive collisions and vertical mixing from turbulence. This structure is similar to that inferred for disks around TW Hydra, GM Auriga, DM Tau, and CoKu Tau/4, which are depleted in dust close to the star but are optically thick at larger heliocentric distances.

We present a new MHD model for simulating the large-scale structure of the solar corona and solar wind under "steady state" conditions stemming from the Wang-Sheeley-Arge empirical model. The processes of turbulent heating in the solar wind are parameterized using a phenomenological, thermodynamical model with a varied polytropic index. We employ the Bernoulli integral to bridge the asymptotic solar wind speed with the assumed distribution of the polytropic index on the solar surface. We successfully reproduce the mass flux from Sun to Earth, the temperature structure, and the large-scale structure of the magnetic field. We reproduce the solar wind speed bimodal structure in the inner heliosphere. However, the solar wind speed is in a quantitative agreement with observations at 1 AU for solar maximum conditions only. The magnetic field comparison demonstrates that the input magnetogram needs to be multiplied by a scaling factor in order to obtain the correct magnitude at 1 AU.

Previous studies of coronal magnetic flux rope systems indicated that these systems exhibit a catastrophic behavior for background fields with a specific photospheric flux distribution, and that the catastrophic energy threshold exceeds the associated open field energy. In this Letter, we take axisymmetrical bipolar fields in spherical geometry with different flux distributions on the photosphere as the background, and then examine how the flux distribution affects the catastrophic behavior and the energy threshold of a flux rope system. It is found that when the photospheric flux is concentrated too much toward the equator, the flux rope system loses its catastrophic behavior, i.e., that it evolves smoothly with the change of the rope properties. On the other hand, if the flux shifts poleward more than a split monopole field, a catastrophe behavior is recovered, and the catastrophic energy threshold increases in magnitude and percentage relative to the associated open field energy with increasing poleward shift of the flux. As a result, the flux rope erupts upward after catastrophe with a faster acceleration and a larger final speed when the flux is concentrated more toward the solar poles.

In this study, we seek a correlation between the speed of active region-related halo coronal mass ejections (CMEs) and the configuration of the ambient magnetic fields. Having studied 99 halo CMEs in the period from 2000 to 2004, we find that CMEs under the heliospheric current sheet are significantly slower than CMEs situated under unidirectional open field structures. The average speed of the former is 883 km s^-1, while the latter is 1388 km s^-1. The effect is not biased by the flare importance. This implies that the ambient magnetic field structure plays a role in determining the speed of the halo CMEs.

We show that the use of Doppler shifts of Zeeman-sensitive spectral lines to observe waves in sunspots is subject to measurement-specific phase shifts arising from (1) the altered height range of spectral line formation and the propagating character of p-mode waves in penumbrae and (2) Zeeman broadening and splitting. We also show that these phase shifts depend on wave frequencies, strengths and line-of-sight inclinations of magnetic fields, and the polarization state used for Doppler measurements. We discuss how these phase shifts could contribute to local helioseismic measurements of "surface effects" in sunspot seismology.