Table of contents

Twenty-one centimeter tomography is emerging as a powerful tool to explore the reionization epoch and cosmological parameters, but it will only be as good as our ability to accurately model and remove astrophysical foreground contamination. Previous treatments of this problem have focused on the angular structure of the signal and foregrounds and what can be achieved with limited spectral resolution (channel widths in the 1 MHz range). In this paper we introduce and evaluate a "blind" method to extract the multifrequency 21 cm signal by taking advantage of the smooth frequency structure of the Galactic and extragalactic foregrounds. We find that 21 cm tomography is typically limited by foregrounds on scales of k ≪ 1 h Mpc^-1 and is limited by noise on scales of k ≫ 1 h Mpc^-1, provided that the experimental channel width can be made substantially smaller than 0.1 MHz. Our results show that this approach is quite promising even for scenarios with rather extreme contamination from point sources and diffuse Galactic emission, which bodes well for upcoming experiments such as LOFAR, MWA, PAST, and SKA.

We study the effects of galaxy formation on the Sunyaev-Zel'dovich effect (SZE) observable-mass relations using high-resolution cosmological simulations. The simulations of 11 individual clusters spanning a decade in mass are performed with the shock-capturing Eulerian adaptive mesh refinement N-body+gasdynamics ART code. To assess the impact of galaxy formation, we compare two sets of simulations performed in an adiabatic regime (without galaxy formation) with those with several physical processes critical to various aspects of galaxy formation: radiative cooling, star formation, stellar feedback, and metal enrichment. We show that a SZE signal integrated to a sufficiently large fraction of cluster volume correlates strongly with its enclosed mass, independent of details of gas physics and the dynamical state of the cluster. The slope and redshift evolution of the SZE flux-mass relation are also insensitive to processes of galaxy formation and are well characterized by a simple self-similar cluster model. Its normalization, on the other hand, is significantly affected by gas cooling and associated star formation. Our simulations show that inclusion of these processes suppresses the normalization by ≈30%-40%. The effect is due to a decrease in gas mass fraction, which is offset slightly by an increase in gas-mass-weighted temperature. Gas cooling and star formation also cause an increase in total mass and modify the normalization by a few percent. Finally, we compare the results of our simulations to recent observations of the SZE scaling relations obtained using 36 OVRO/BIMA SZE + Chandra X-ray observations. The comparison highlights the importance of galaxy formation in theoretical modeling of clusters and shows that the current generation of simulations produces clusters with gross properties quite similar to their observed counterparts.

We compute the locations of satellite galaxies with respect to their hosts using the ΛCDM GIF simulation. If the major axes of the hosts' images are perfectly aligned with the major axes of the projected mass, the satellites are located preferentially close to the hosts' major axes. In this case, the degree of anisotropy in the satellite locations is a good tracer of the flattening of the hosts' halos. If all hosts have luminous circular disks, the symmetry axes of the projected mass and light are not perfectly aligned, and the locations of the satellites depend on how the hosts' disks are placed within their halos. If the disk angular momentum vectors are aligned with the major axes of the halos, the satellites show a pronounced Holmberg effect. If the disk angular momentum vectors are aligned with the intermediate axes of the local large-scale structure, the distribution of the satellite locations is essentially isotropic. If the disk angular momentum vectors are aligned with either the minor axes or with the net angular momentum vectors of the halos, the satellites are distributed anisotropically about their hosts, with a preference for being found near the hosts' major axes. This agrees well with the observation that satellite galaxies in the Sloan Digital Sky Survey tend to be found near the major axes of their hosts and suggests that the mass and light of SDSS host galaxies must be fairly well aligned in projection on the sky.

Numerical simulations of the intergalactic medium have shown that at the present epoch, a significant fraction (40%-50%) of the baryonic component should be found in the (T ~ 10⁶ K) warm-hot intergalactic medium (WHIM)—with several recent observational lines of evidence indicating the validity of the prediction. We here recompute the evolution of the WHIM with the following major improvements: (1) galactic superwind feedback processes from galaxy and star formation are explicitly included; (2) major metal species (O V to O IX) are computed explicitly in a nonequilibrium way; and (3) mass and spatial dynamic ranges are larger by factors of 8 and 2, respectively, than in our previous simulations. Here are the major findings: (1) Galactic superwinds have dramatic effects, increasing the WHIM mass fraction by about 20%, primarily through heating of warm gas near galaxies with density 10^1.5-10⁴ times the mean density. (2) The fraction of baryons in the WHIM is increased modestly from the earlier work but is still ~40%-50%. (3) The gas density of the WHIM is broadly peaked at a density 10-20 times the mean density, ranging from underdense regions to regions that are overdense by 10³-10⁴. (4) The median metallicity of the WHIM is 0.18 Z_☉ for oxygen, with 50% and 90% intervals being (0.040, 0.38) and (0.0017, 0.83).

A significant fraction (40%-50%) of baryons at the present epoch are predicted to be shock-heated to the warm-hot intergalactic medium (WHIM) by our previous numerical simulations. Here we recompute the evolution of the WHIM with several major improvements: (1) galactic superwind feedback processes from galaxy and star formation are explicitly included; (2) major metal species (O V to O IX) are computed explicitly in a nonequilibrium way; and (3) mass and spatial dynamic ranges are larger by factors of 8 and 2, respectively, than in our previous simulations. We find the following: (1) Nonequilibrium calculations produce significantly different results than do ionization equilibrium calculations. (2) The abundance of O VI absorption lines based on nonequilibrium simulations with galactic superwinds is in remarkably good agreement with the latest observations, strongly validating our model, while the predicted abundances for O VII and O VIII absorption lines appear to be lower than the still very uncertain observations. The expected abundances for O VI (as well as Lyα), O VII, and O VIII absorption systems are in the range 50-100 per unit redshift at equivalent width EW = 1 km s^-1, decreasing to 10-20 per unit redshift at EW = 10 km s^-1, to one to three lines for O VII and O VIII and negligible for O VI at EW > 100 km s^-1. (3) Emission lines, primarily O VI and Lyα in the UV and O VII and O VIII in soft X-rays, are potentially observable by future missions, and different lines provide complementary probes of the WHIM in the temperature-density-metallicity phase space. The number of emission lines per unit redshift that may be detectable by planned UV and soft X-ray missions are of order 0.1-1.

We present 350 μm observations of 15 Chapman et al. submillimeter galaxies (SMGs) with radio counterparts and optical redshifts. We detect 12 and obtain sensitive upper limits for three, providing direct, precise measurements of their far-infrared luminosities and characteristic dust temperatures. With these, we verify the linear radio-far-infrared correlation at redshifts of z ~ 1-3 and luminosities of 10¹¹-10¹³L_☉, with a power-law index of 1.02 ± 0.12 and rms scatter of 0.12 dex. However, either the correlation constant q or the dust emissivity index β is lower than measured locally. The best-fitting q ≃ 2.14 is consistent with SMGs being predominantly starbust galaxies, without significant AGN contribution, at far-infrared wavelengths. Gas-to-dust mass ratios are estimated at 54 , depending on the absoption efficiency κ_ν, with intrinsic dispersion ≃40% around the mean value. Dust temperatures consistent with 34.6 ± 3 K (1.5/β)^0.71, at z ~ 1.5-3.5, suggest that far-infrared photometric redshifts may be viable, and perhaps accurate to 10% ≲ dz/(1 + z), for up to 80% of the SMG population in this range, if the above temperature characterizes the full range of SMGs. However, observed temperature evolution of T_d ∝ (1 + z) is also plausible and could result from selection effects. From the observed luminosity-temperature (L-T) relation, L ∝ T, we derive scaling relations for dust mass versus dust temperature, and we identify expressions to interrelate the observed quantities. These suggest that measurements at a single wavelength, in the far-infrared, submillimeter, or radio wave bands, might constrain dust temperatures and far-infrared luminosities for most SMGs with redshifts at z ~ 0.5-4.

We present ¹²CO(J = 1 → 0) observations of the high-redshift quasi-stellar objects (QSOs) BR 1202-0725 (z = 4.69), PSS J2322+1944 (z = 4.12), and APM 08279+5255 (z = 3.91) using the NRAO Green Bank Telescope (GBT) and the MPIfR Effelsberg 100 m telescope. We detect, for the first time, the CO ground-level transition in BR 1202-0725. For PSS J2322+1944 and APM 08279+5255, our observations result in line fluxes that are consistent with previous NRAO Very Large Array (VLA) observations, but they reveal the full line profiles. We report a typical lensing-corrected velocity-integrated intrinsic ¹²CO(J = 1 → 0) line luminosity of L = 5 × 10¹⁰ K km s^-1 pc² and a typical total H₂ mass of M(H₂) = 4 × 10¹⁰M_☉ for the sources in our sample. The CO/FIR luminosity ratios of these high-z sources follow the same trend as seen for low-z galaxies, leading to a combined solution of log L_FIR = (1.39 ± 0.05) log L_CO - 1.76. It has previously been suggested that the molecular gas reservoirs in some quasar host galaxies may exhibit luminous, extended ¹² CO(J = 1 → 0) components that are not observed in the higher J CO transitions. Using the line profiles and the total intensities of our observations and large velocity gradient (LVG) models based on previous results for higher J CO transitions, we derive that emission from all CO transitions is described well by a single gas component in which all molecular gas is concentrated in a compact nuclear region. Thus, our observations and models show no indication of a luminous extended, low surface brightness molecular gas component in any of the high-redshift QSOs in our sample. If such extended components exist, their contribution to the overall luminosity is limited to at most 30%.

We report the first detection of CO (1→0) emission from a submillimeter-selected galaxy, using the Green Bank Telescope. We identify the line in the spectrum of SMM J13120+4242 as a broad emission feature at z = 3.408, with ΔV_FWHM = 1040 ± 190 km s^-1. If the observed CO (1→0) line profile arises from a single object and not several merging objects, then the CO (4 → 3)/CO (1 → 0) brightness temperature ratio of ~0.26 suggests n(H₂) > (3-10) × 10² cm^-3 and the presence of subthermally excited gas. The integrated line flux implies a cold molecular gas mass M(H₂) = 1.6 × 10¹¹M_☉, comparable to the dynamical mass estimate and 4 times larger than the H₂ mass predicted from the CO (4→3) line, assuming a brightness temperature ratio of 1.0. While our observations confirm that this submillimeter galaxy is massive and gas-rich, they also suggest that extrapolating gas masses from J_upper ≥ 3 transitions of CO leads to considerable uncertainties. We also report an upper limit to the mass of cold molecular gas in a second submillimeter galaxy, SMM J09431+4700, of M(H₂) ≲ 4 × 10¹⁰M_☉.

We present the evolution of the volume-averaged properties of the rest-frame optically luminous (L_V > 3 × 10¹⁰hL_☉) galaxy population to z ~ 3, determined from four disjoint deep fields. We characterize their rest-frame UV through optical properties via the mean SED. To measure evolution, we apply the selection criteria to a sample of galaxies from the SDSS and COMBO-17 survey. The mean rest-frame 2200 Å through V-band SED becomes steadily bluer with increasing redshift, but at all redshifts z < 3 the mean SED falls within the range defined by ``normal'' galaxies in the nearby universe. We measure the mean stellar mass-to-light ratios (_⋆/L) and stellar mass densities (ρ_⋆) by fitting models to the mean rest-frame UV-optical SEDs. The ρ_⋆ in galaxies selected at a fixed luminosity has increased by a factor of 3.5-7.9 from z = 3 to 0.1. If we instead use our observed _⋆/L_V evolution to select galaxies at a fixed mass, ρ_⋆ evolves by a factor of 5.3-16.7. After correcting to total, the measured ρ_⋆ at z < 2 lie below the integral of the star formation rate density as a function of redshift as derived from UV-selected samples after a standard correction for extinction. We find large discrepancies between recent model predictions for the evolution of ρ_⋆ and our results, even when our observational selection is applied to the models. Finally, we determine that distant red galaxies (selected to have J_s - K_s > 2.3) in our L-selected samples contribute 30% and 64% of the stellar mass budget at z ~ 2 and z ~ 2.8, respectively. These galaxies are largely absent from UV surveys, and this result highlights the need for mass selection of high-redshift galaxies.

The vertical profiles of chain and spiral galaxies in the Hubble Space Telescope Ultra Deep Field (UDF) are fit to sech² functions convolved with stellar profiles in order to measure the disk scale heights z₀ in four passbands. The bulge regions of the spiral galaxies are avoided. Photometric redshifts give absolute scales. The rms heights of the giant clumps in these galaxies are also measured. The results indicate that UDF disks are thick, with an average z₀ = 1.0 ± 0.4 kpc. The ratio of radial exponential scale length to z₀ is ~3 ± 1.5. The scale heights are only 20% larger than the radii of the giant star-forming clumps and a factor of ~10 larger than the rms clump deviations around the midplanes. This suggests that the clumps formed from midplane gas and dissolved to make the thick disks. Redshifted stellar population models suggest ages of ~1 Gyr and mass column densities from 4 to 40 M_☉ pc^-2. The UDF disks look like young versions of modern thick disks. This resemblance is difficult to understand if galaxies grow over time or if subsequent accretion of thin disks gravitationally shrinks the observed thick disks. More likely, high-redshift disks are thick because their mass column densities are low; a velocity dispersion of only 14 km s^-2 reproduces the observed thickness. Modern thick disks require more heating at high redshift. This is possible if the gas that eventually makes the thin disk is in place before the youngest age of a modern thick disk, and if the existing stars are heated during the delivery of this gas.

Using the near-infrared integral field spectrograph SPIFFI on the VLT, we have studied the spatially resolved dynamics in the z = 3.2 strongly lensed galaxy 1E 0657-56 arc+core by observing the rest-frame optical emission lines [O III] λ5007 and Hβ. The lensing configuration suggests that the high surface brightness core is the ~ 20 magnified central ~1 kpc of the galaxy, whereas the fainter arc is the more strongly magnified peripheral region of the same galaxy at about a half-light radius, which otherwise appears to be a typical z ~ 3 Lyman break galaxy. The overall shape of the position-velocity diagram resembles the rotation curves of the inner few kpc of nearby ~^⋆ spiral galaxies. For = 20, our data have a spatial resolution of ~200 pc in the source plane. The projected velocities v_rot rise rapidly to ~75 km s^-1 within radii ~0.5 kpc from the center and asymptotically reach a velocity of ~190 km s^-1 within the arc, at a projected radius of a few kpc radius. The rotation curve implies a dynamical mass of log M_dyn/M_☉ ~ 9.3 within the central kpc and suggests that in this system the equivalent of the mass of a present-day ~^⋆ bulge at the same radius was already in place by z ≳ 3. Approximating the circular velocity of the halo by the measured asymptotic velocity of the rotation curve, we estimate a dark matter halo mass of log M_halo/M_☉ ~ 11.7 ± 0.3, in good agreement with large-scale clustering studies of Lyman break galaxies. The baryonic collapse fraction is low compared to z ~ 2 actively star-forming BX and low-redshift galaxies, perhaps implying comparatively less gas infall to small radii or efficient feedback. Even more speculatively, the high central mass density might indicate highly dissipative gas collapse in very early stages of galaxy evolution, in approximate agreement with what is expected for "inside-out" galaxy formation models.

The Sloan Digital Sky Survey has detected luminous quasars at very high redshift, z > 6. Follow-up observations indicate that at least some of these quasars are powered by supermassive black holes (SMBHs), with masses in excess of 10⁹M_☉. SMBHs, therefore, seem to have already existed when the universe was less than 1 Gyr old and the bulk of galaxy formation had yet to take place. Here we investigate the extent to which accretion and dynamical processes influence the early growth of SMBHs. We assess the impact of (1) black hole mergers, (2) the influence of the merger efficiency, and (3) the negative contribution due to dynamical effects, which can kick black holes out of their host halos (gravitational recoil). We find that if accretion is always limited by the Eddington rate via a thin disk, the maximum allowed radiative efficiency (or spin) to reproduce the luminosity function at z = 6 is = 0.12 (or = 0.8), against the adverse effect of the gravitational recoil. Dynamical effects unquestionably cannot be neglected in studies of high-redshift SMBHs. If black holes can accrete at a supercritical rate during an early phase, reproducing the observed SMBH mass values is not an issue, even in the case that the recoil velocity is in the upper limit range, as the mass ratios of merging binaries are skewed toward low values, where the gravitational recoil effect is very mild. We propose that SMBH growth at early times is very selective, and efficient only for black holes hosted in high density peak halos.

A newly identified kiloparsec-scale X-ray jet in the high-redshift z = 3.89 quasar 1745+624 is studied with multifrequency radio, HST, and Chandra X-ray imaging data. This is only the third large-scale X-ray jet beyond z > 3 known and is further distinguished as being the most luminous relativistic jet observed at any redshift, exceeding 10⁴⁵ ergs s^-1 in both the radio and X-ray bands. Apart from the jet's extreme redshift and luminosity, its basic properties, such as X-ray/radio morphology, radio polarization, and the convex broadband spectral energy distributions of three distinct knots are also similar to lower z examples. Relativistically beamed inverse Compton and "nonstandard" synchrotron models have been considered to account for such excess X-ray emission in other jets; both models are applicable here, but with differing requirements for the underlying jet physical properties, such as velocity, energetics, and electron acceleration processes. One potentially very important distinguishing characteristic between the two models is their strongly diverging predictions for the X-ray/radio emission with increasing redshift. This is considered, although with the limited sample of three z > 3 jets it is apparent that future studies targeted at very high-redshift jets are required for further elucidation. Finally, from the broadband jet emission we estimate the jet kinetic power to be no less than 10⁴⁶ ergs s^-1, which is about 10% of the Eddington luminosity corresponding to this galaxy's central supermassive black hole mass M_BH ≳ 10⁹M_☉ estimated here via the virial relation. The optical luminosity of the quasar core is about 10 times over Eddington, hence the inferred jet power seems to be much less than that available from mass accretion. The apparent super-Eddington accretion rate may, however, suggest contribution of an unresolved jet in the observed optical nucleus.

To explain the properties of the most massive low-redshift galaxies and the shape of their mass function, recent models of galaxy evolution include strong AGN feedback to complement starburst-driven feedback in massive galaxies. Using the near-infrared integral-field spectrograph SPIFFI on the VLT, we searched for direct evidence for such feedback in the optical emission line gas around the z = 2.16 powerful radio galaxy MRC 1138-262, likely a massive galaxy in formation. The kiloparsec-scale kinematics, with FWHMs and relative velocities ≲2400 km s^-1 and nearly spherical spatial distribution, do not resemble large-scale gravitational motion or starburst-driven winds. Order-of-magnitude timescale and energy arguments favor the AGN as the only plausible candidate to accelerate the gas, with a total energy injection of a few ×10⁶⁰ ergs or more, necessary to power the outflow, and relatively efficient coupling between radio jet and ISM. Observed outflow properties are in gross agreement with the models and suggest that AGN winds might have a cosmological significance that is similar to, or perhaps larger than, starburst-driven winds if MRC 1138-262 is indeed archetypal. Moreover, the outflow has the potential to remove significant gas fractions (≲50%) from a >L* galaxy within a few tens to 100 Myr, fast enough to preserve the observed [α/Fe] overabundance in massive galaxies at low redshift. Using simple arguments, it appears that feedback like that observed in MRC 1138-262 may have sufficient energy to inhibit material from infalling into the dark matter halo and thus regulate galaxy growth as required in some recent models of hierarchical structure formation.

We present an investigation of the properties and environments of bright extremely red objects (EROs) found in the fields of the quasars TXS 0145+386 and 4C 15.55, both at z ~ 1.4. There is marginal evidence from Chandra Advanced CCD Imaging Spectrometer (ACIS) imaging for hot cluster gas with a luminosity of a few 10⁴⁴ ergs s^-1 in the field of 4C 15.55. The TXS 0145+386 field has an upper limit at a similar value, but it also clearly shows an overdensity of faint galaxies. None of the EROs are detected as X-ray sources. For two of the EROs that have spectral energy distributions and rest-frame near-UV spectra that show that they are strongly dominated by old stellar populations, we determine radial surface brightness profiles from adaptive optics images. Both of these galaxies are best fit by profiles close to exponentials, plus a compact nucleus comprising ~30% of the total light in one case and 8% in the other. Neither is well fit by an r^1/4-law profile. This apparent evidence for the formation of massive ~2 × 10¹¹ disks of old stars in the early universe indicates that at least some galaxies formed essentially monolithically, with high star formation rates sustained over a few 10⁸ yr and without the aid of major mergers.

Analysis of the frequency and physical properties of galaxies with star formation and active galactic nucleus (AGN) activity in different environments in the local universe is a cornerstone for understanding structure formation and galaxy evolution. We have built a new multiwavelength catalog for galaxies in a complete redshift survey (the 15R Survey), gathering information on their Hα, R-band, radio, far-infrared, and X-ray emission, as well as their radio and optical morphologies, and developed a classification scheme to compare different selection methods and accurately select samples of radio-emitting galaxies with AGN and star-forming activity. While alternative classification schemes do not lead to major differences for star-forming galaxies, we show that spectroscopic and photometric classifications of AGNs lead to incomplete samples. In particular, a large population of AGN-containing galaxies with absorption-line spectra, and in many cases extended radio structures (jets, lobes), is missed in the standard Baldwin-Phillips-Terlevich emission-line classification of active galaxies. This missed class of objects accounts for roughly half of the radio AGN population. Similarly, for X-ray-selected AGNs in our sample, we find that absorption-line AGNs account for half of the sample. Spectroscopically unremarkable, passive galaxies with AGN activity are not an exception, but the norm, and we show that although they exist in all environments, these systems preferentially reside in higher density regions. Because of the existence of this population, the fractional abundance of AGNs increases with increasing density, in contrast to the results based on emission-line AGNs extracted from the 15R, SDSS, and 2dF redshift surveys. Since emission-line radio AGNs are mostly associated with late-type galaxies and absorption-line radio AGNs with early-type galaxies, the trends found are connected to the well-known but poorly understood density-morphology relation.

Based on the Sloan Digital Sky Survey (SDSS) DR2 sample, we present a multiparameter analysis of the spatial clustering of nearby active galactic nuclei (AGNs). Estimates of the redshift-space two-point correlation function reveal that Seyferts are less clustered than normal galaxies, while LINERs' clustering amplitude (s₀) is consistent with that of the parent galaxy population. This difference in clustering is not driven by the morphology-density relation, as colors and concentration indices follow similar distributions. The fact that objects of given spectral types are clustered differently seems correlated with a variety of their physical properties, including L, L, the emitting gas density n_e, and the obscuration level. LINERs, which exhibit high s₀, show the lowest luminosities and obscuration levels, and relatively low n_e, suggesting that these objects harbor relatively massive black holes that are weakly active or inefficient in their accretion, probably due to the insufficiency of their fuel supply. Seyfert galaxies, which have low s₀, are very luminous and show large n_e, suggesting that their black holes are less massive but accrete quickly and efficiently enough to clearly dominate the ionization. Star-forming galaxies, the H IIs, are weakly clustered; this trend can be understood as a consequence of both the morphology-density and star formation rate-density relations. The spectral properties of the H II galaxies suggest, however, that they hide in their centers, amid large amounts of obscuring material, black holes of generally low mass whose activity remains relatively feeble. Our own Milky Way may be such a case.

Periodic outbursts are observed in many active galactic nuclei (AGNs) and are usually explained with a supermassive black hole binary (SMBHB) scenario. However, multiple periods are observed in some AGNs and cannot be explained in this way. Here we analyze the periodicity of the radio light curves of AO 0235+164 at multiple frequencies and report the discovery of six quasi-periodic oscillations (QPOs) in the integer frequency ratio 1 : 2 : 3 : 4 : 5 : 6, of which the second, with period P₂ = 5.46 ± 0.47 yr, is the strongest. We fit the radio light curves and show that the initial phases of the six QPOs have differences of 0 or π relative to each other. We suggest a harmonic relationship among the QPOs. The centroid frequency, relative strength, harmonic relationship, and relative initial phases of the QPOs are independent of radio frequency. The harmonic QPOs are likely due to quasi-periodic injection of plasma from an oscillating accretion disk into the jet. We estimate a supermassive black hole mass of M_BH ≃ (4.72 ± 2.04) × 10⁸M_☉ and an accretion rate ≃ 0.007. With knowledge of the accretion disk, this implies that the inner region of the AO 0235+164 accretion disk is a radiatively inefficient accretion flow. The oscillatory accretion is due to p-mode oscillations of the thick disk, probably excited by a SMBHB. Theoretical predictions for the fundamental oscillation frequency and the harmonics show good consistency with the observations. Harmonic QPOs would disappear if the thick disk became geometrically thin as the result of an increase in accretion rate. We discuss the observations of AO 0235+164 in the context of the SMBHB-thick disk oscillation scenario.

There are good observational reasons to believe that the progenitors of these red galaxies have undergone starbursts, followed by a poststarburst phase. Poststarburst ("K+A" or "E+A") galaxies appear in the SDSS visible spectroscopic data by showing an excess of A-star light but relatively little Hα emission. We investigate the environments of these galaxies by measuring (1) number densities in 8 h^-1 Mpc radius comoving spheres, (2) transverse distances to nearest Virgo-like galaxy clusters, and (3) transverse distances to nearest luminous-galaxy neighbors. We compare the poststarburst galaxies to currently star-forming galaxies identified solely by A-star excess or Hα emission. We find that poststarburst galaxies are in the same kinds of environments as star-forming galaxies; this is our "null hypothesis." More importantly, we find that at each value of the A-star excess, the star-forming and poststarburst galaxies lie in very similar distributions of environment. Other studies finding similar results have argued that galaxy transformations occur slowly (timescales >1 Gyr), but this is at odds with the evidence that red galaxies are formed via starbursts. The only deviations from our null hypothesis are barely significant: a slight deficit of poststarburst galaxies (relative to the star-forming population) in very low-density regions, a small excess inside the virial radii of clusters, and a slight excess with nearby neighbors. None of these effects is strong enough to make the poststarburst galaxies a high-density phenomenon, or to argue that the starburst events are primarily triggered by external tidal impulses from close passages. The small excess inside cluster virial radii suggests that some poststarbursts are triggered by interactions with the intracluster medium, but this represents a very small fraction of all poststarburst galaxies.

We estimate the average group morphological and dynamical characteristics of the Percolation-Inferred Galaxy Group (2PIGG) catalog within z ≤ 0.08, for which the group space density is roughly constant. We quantify the different biases that enter in the determination of these characteristics, and we devise statistical correction procedures to recover their bias-free values. We find that the only acceptable morphological model is that of prolate, or triaxial with pronounced prolateness, group shapes having a roughly Gaussian intrinsic axial ratio distribution with mean ~0.46 and dispersion ~0.16. After correcting for various biases, the most important of which is a redshift-dependent bias, the median values of the virial mass and virial radius of groups with 4-30 galaxy members are _v ~ 6 × 10¹²hM_☉, _v ~ 0.4 h Mpc, which are significantly smaller than recent literature values that do not take into account the previously mentioned biases. The group mean crossing time is ~1.5 Gyr, independent of the group galaxy membership. We also find that there is a correlation of the group size, velocity dispersion, and virial mass with the number of group member galaxies, a manifestation of the hierarchy of cosmic structures.

We present an X-ray analysis of the radial mass profile of the radio-quiet galaxy cluster A2589 between 0.015 and 0.25r_vir, using an XMM-Newton observation. Except for a ≈16 kpc shift of the X-ray center of the R = 45-60 kpc annulus, A2589 possesses a remarkably symmetrical X-ray image and is therefore an exceptional candidate for precision studies of its mass profile by applying hydrostatic equilibrium. The total gravitating matter profile is well described by the NFW model with c_vir = 6.1 ± 0.3 and M_vir = 3.3 ± 0.3 × 10¹⁴M_☉ (r_vir = 1.74 ± 0.05 Mpc), in excellent agreement with ΛCDM. When the mass of the hot intracluster medium is subtracted from the gravitating matter profile, the NFW model fitted to the resulting dark matter (DM) profile produces essentially the same result. However, when accounting for the stellar mass (M_*) of the cD galaxy, the NFW fit to the DM profile substantially degrades in the central r ~ 50 kpc for reasonable values of M_*/L_V. Modifying the NFW DM halo by adiabatic contraction arising from the early condensation of stellar baryons in the cD galaxy further degrades the fit. The fit is improved substantially with a Sérsic-like model recently suggested by high-resolution N-body simulations but with an inverse Sérsic index, α ~ 0.5, that is a factor of ~3 higher than predicted. We argue that neither random turbulent motions nor magnetic fields can provide sufficient nonthermal pressure support to reconcile the XMM-Newton mass profile with adiabatic contraction of a CDM halo, assuming reasonable values of M_*/L_V. Our results support the scenario in which, at least for galaxy clusters, processes during halo formation counteract adiabatic contraction so that the total gravitating mass in the core approximately follows the NFW profile.

We use numerical simulations to study the kinematic structure of remnants formed from mergers of equal-mass disk galaxies. In particular, we show that remnants of dissipational mergers, which include the radiative cooling of gas, star formation, feedback from supernovae, and the growth of supermassive black holes, are smaller, rounder, have, on average, a larger central velocity dispersion, and show significant rotation compared to remnants of dissipationless mergers. The increased rotation speed of dissipational remnants owes its origin to star formation that occurs in the central regions during the galaxy merger. We have further quantified the anisotropy, three-dimensional shape, minor-axis rotation, and isophotal shape of each merger remnant, finding that dissipational remnants are more isotropic, closer to oblate, have the majority of their rotation along their major axis, and are more disky than dissipationless remnants. Individual remnants display a wide variety of kinematic properties. A large fraction of the dissipational remnants are oblate isotropic rotators. Many dissipational remnants, and all of the dissipationless ones, are slowly rotating and anisotropic. The remnants of gas-rich major mergers can well reproduce the observed distribution of projected ellipticities, rotation parameter (V/σ)*, kinematic misalignments, Ψ, and isophotal shapes. The dissipationless remnants are a poor match to this data. We also investigate the properties of merger remnants as a function of initial disk gas fraction, orbital angular momentum, and the mass of the progenitor galaxies. Our results support the merger hypothesis for the origin of low-luminosity elliptical galaxies provided that the progenitor disks are sufficiently gas-rich, however our remnants are a poor match to the bright ellipticals that are slowly rotating and uniformly boxy.

Optical and near-IR observations of the halos of disk galaxies and blue compact galaxies have revealed a very red spectral energy distribution that cannot easily be reconciled with a normal, metal-poor stellar population such as that in the stellar halo of the Milky Way. Here spectral evolutionary models are used to explore the consequences of these observations. We demonstrate that a stellar population of low to intermediate metallicity but with an extremely bottom-heavy initial mass function can explain the red halos around both types of objects. Other previously suggested explanations, such as nebular emission or very metal-rich stars, are shown to fail in this respect. This indicates that if the reported halo colors are correct, halo populations dominated by low-mass stars may be a phenomenon common to galaxies of very different Hubble types. Potential tests of this hypothesis are discussed, along with its implications for the baryonic dark matter content of galaxies.

We present a survey of spiral arm extinction substructures referred to as feathers in 223 spiral galaxies using Hubble Space Telescope WFPC2 images. The sample includes all galaxies in the RC3 catalog with cz < 5000 km s^-1, B_T < 15, i < 60^°, and types Sa-Sd with well-exposed broadband WFPC2 images. The detection frequency of delineated, periodic feathers in this sample is 20% (45 of 223). This work is consistent with Lynds, who concluded that feathers are common in prototypical Sc galaxies; we find that feathers are equally common in Sb galaxies. Sb-Sc galaxies without clear evidence for feathers either had poorer quality images, or flocculent or complex structure. We did not find clearly defined feathers in any Scd-Sd galaxy. The probability of detecting feathers was highest (83%) for spiral galaxies with well-defined primary dust lanes (the lanes that line the inner edge of an arm); well-defined primary dust lanes were only noted in Sab-Sc galaxies. Consistent with earlier work, we find that neighboring feathers tend to have similar shapes and pitch angles. OB associations are often found lining feathers, and many feathers transition to the stellar substructures known as spurs (Elmegreen). We find that feathers are coincident with interarm filaments strikingly revealed in Spitzer 8 μm images. Comparison with CO (1-0) maps of NGC 0628 and NGC 5194 from BIMA SONG shows that feathers originate at the primary dust lane coincident with gas surface density peaks. Contrary to the appearance at 8 μm, the CO maps show that gas surface density in feathers decreases rapidly with distance from the primary dust lane. We also find that the spacing between feathers decreases with increasing gas surface density, consistent with formation via a gravitational instability.

We use Hubble Space Telescope (HST) NICMOS continuum and Paα observations to study the near-infrared and star formation properties of a representative sample of 30 local (d ~ 35-75 Mpc) luminous infrared galaxies (LIRGs, infrared [8-1000 μm] luminosities of log L_IR = 11-11.9 L_☉). The data provide spatial resolutions of 25-50 pc and cover the central ~3.3-7.1 kpc regions of these galaxies. About half of the LIRGs show compact (~1-2 kpc) Paα emission with a high surface brightness in the form of nuclear emission, rings, and minispirals. The rest of the sample show Paα emission along the disk and the spiral arms extending over scales of 3-7 kpc and larger. About half of the sample contains H II regions with Hα luminosities significantly higher than those observed in normal galaxies. There is a linear empirical relationship between the mid-IR 24 μm and hydrogen recombination (extinction-corrected Paα) luminosity for these LIRGs, and the H II regions in the central part of M51. This relation holds over more than four decades in luminosity, suggesting that the mid-IR emission is a good tracer of the star formation rate (SFR). Analogous to the widely used relation between the SFR and total IR luminosity of R. Kennicutt, we derive an empirical calibration of the SFR in terms of the monochromatic 24 μm luminosity that can be used for luminous, dusty galaxies.

The luminous infrared galaxy Arp 299 (IC 694 + NGC 3690) is studied using optical integral field spectroscopy obtained with the INTEGRAL system, together with archival Hubble Space Telescope WFPC2 and NICMOS images. The stellar and ionized gas morphology shows λ-dependent variations due to the combined effects of the dust internal extinction and the nature and spatial distribution of the different ionizing sources. The two-dimensional ionization maps have revealed an off-nuclear conical structure of about 4 kpc in length, characterized by high-excitation conditions and a radial gradient in the gas electron density. The apex of this structure coincides with B1 region of NGC 3690, which in turn presents Seyfert-like ionization, high extinction, and a high velocity dispersion. These results strongly support the hypothesis that B1 is the true nucleus of NGC 3690, where an AGN is located. In the circumnuclear regions H II-like ionization dominates, while LINER-like ionization is found elsewhere. The Hα-emitting sources with ages from 3.3 to 7.2 × 10⁶ yr, have masses of between 6 and 680 × 10⁶M_☉ and contribute about 45% to the bolometric luminosity (extinction corrected). The ionized (Hα) and neutral (Na D) gas velocity fields show similar structure on scales of several hundred to about 1 kpc, indicating that these gas components are kinematically coupled. The kinematic structure is complex and on scales of about 0.2 kpc does not appear to be dominated by the presence of ordered, rotational motions. The large velocity dispersion measured in NGC 3690 indicates that this galaxy is the most massive of the system. The low velocity amplitude and dispersion of the interface suggest that the ionized gas is slowly rotating or in a close to quiescent phase.

We report the discovery of an eclipsing X-ray binary with a 3.62 hr period within 24'' of the center of the dwarf starburst galaxy NGC 4214. The orbital period places interesting constraints on the nature of the binary and allows for a few very different interpretations. The most likely possibility is that the source lies within NGC 4214 and has an X-ray luminosity, L_X, of up to 7 × 10³⁸ ergs s^-1. In this case, the binary may well be comprised of a naked He-burning donor star with a neutron star accretor, although a stellar-mass black hole accretor cannot be completely excluded. There is no obvious evidence for a strong stellar wind in the X-ray orbital light curve that would be expected from a massive He star; thus, the mass of the He star should be ≲3-4 M_☉. If correct, this would represent a new class of very luminous X-ray binary—perhaps related to Cyg X-3. Other, less likely possibilities include a conventional low-mass X-ray binary that somehow manages to produce such a high X-ray luminosity and is apparently persistent over an interval of years; or a foreground AM Her binary of much lower luminosity that fortuitously lies in the direction of NGC 4214. Any model for this system must accommodate the lack of an optical counterpart down to a limiting magnitude of 22.6 in the visible.

We report recent Chandra observations of the ULX in the elliptical galaxy NGC 3379 that clearly detect two flux variability cycles. Comparing these data with the Chandra observation of ~5 years ago, we measure a flux modulation with a period of ~12.6 hr. Moreover, we find that the emission undergoes a correlated spectral modulation, becoming softer at low flux. We argue that our results establish this source as a ULX binary in NGC 3379. Given the old stellar population of this galaxy, the ULX is likely to be a soft transient; however, historical X-ray sampling suggests that the current "on" phase has lasted ~10 yr. We discuss our results in terms of ADC and wind-feedback models. If the flux modulation is orbital, we can constrain the donor mass and orbital period at the onset of mass transfer within 1.15-1.4 M_☉ and 12.5-17 hr, respectively. The duration of the mass transfer phase so far is probably ~1 Gyr, and the binary has been a soft X-ray transient throughout this time. These constraints are insensitive to the mass of the accretor.

We present the luminosity function to very faint magnitudes for the globular clusters in M87, based on a 30 orbit Hubble Space Telescope (HST) WFPC2 imaging program. The very deep images and corresponding improved false source rejection allow us to probe the mass function further beyond the turnover than has been done before. We compare our luminosity function to those that have been observed in the past, and confirm the similarity of the turnover luminosity between M87 and the Milky Way. We also find with high statistical significance that the M87 luminosity function is broader than that of the Milky Way. We discuss how determining the mass function of the cluster system to low masses can constrain theoretical models of the dynamical evolution of globular cluster systems. Our mass function is consistent with the dependence of mass loss on the initial cluster mass given by classical evaporation, and somewhat inconsistent with newer proposals that have a shallower mass dependence. In addition, the rate of mass loss is consistent with standard evaporation models, and not with the much higher rates proposed by some recent studies of very young cluster systems. We also find that the mass-size relation has very little slope, indicating that there is almost no increase in the size of a cluster with increasing mass.

We present a new Spitzer Infrared Spectrograph (IRS) spectrum of the carbon star IRAS 04496-6958 in the Large Magellanic Cloud, which exhibits a fairly broad absorption feature at ~11 μm. This feature is consistent with SiC absorption, as seen in a few Galactic sources. Furthermore, the C₂H₂ (and other molecular) absorption bands are the deepest ever observed, indicative of a very high column density. While the Galactic sources with SiC absorption have cool colors (continuum temperature ≈300 K), IRAS 04496-6958 is much bluer, with a continuum temperature of ≈600 K. Based on the Galactic sample, SiC dust at this temperature should still display an emission feature at ~11 μm. If SiC is the cause of the absorption feature, it suggests a subtly different evolutionary path and a change to a different condensation sequence than assumed for Galactic carbon stars. An alternative explanation for this feature is molecular line absorption; however, currently available line lists are not sufficient to properly assess this hypothesis.

We present results of N-body simulations aimed at understanding the dynamics of young stars near the Galactic center. Specifically, we model the inspiral of a cluster core containing an intermediate-mass black hole and N ~ 50 cluster stars in the gravitational potential of a supermassive black hole. We first study the elliptic three-body problem to isolate issues of tidal stripping and subsequent scattering, followed by full N-body simulations to treat the internal dynamics consistently. We find that our simulations reproduce several dynamical features of the observed population. These include the observed inner edge of the claimed clockwise disk, as well as the thickness of said disk. We find that high-density clumps, such as that claimed for IRS 13E, also result generically from our simulations. However, not all features of the observations are reproduced. In particular, the surface density profile of the simulated disk scales as Σ ∝ r^-0.75, which is considerably shallower than that observed. Further, at no point is any significant counterrotating population formed.

We present a population synthesis calculation to derive the total number of planetary nebulae (PNs) in the Galaxy that descend from single stars and stars in binary systems. Using the most recent literature results on Galactic and stellar formation and stellar evolution, we predict the total number of Galactic PNs with radii <0.9 pc to be (4.6 ± 1.3) × 10⁴. We do not claim this to be the complete population, since there can be visible PNs with radii larger than this limit. However, by taking this limit, we make our predicted population inherently comparable to the observationally based value of Peimbert and Jacoby (8000 ± 2000 objects). Our prediction is discrepant with the observations at the 2.9 σ level, a disagreement that we argue is meaningful in view of our specific treatment of the uncertainty. We conclude that it is likely that only a subset of the stars thought to be capable of making a visible PN actually do. In the second paper in this series, an argument will be presented that the bulk of the Galactic PN population might be better explained if only binaries produce PNs. The predicted local PN formation rate density from single stars and binaries is (1.1 ± 0.5) × 10^-12 PNs yr^-1 pc^-3, lower than recent estimates (2.1 × 10^-12 PNs yr^-1 pc^-3), which are based on local PN counts and the PN distance scale, but more in line with the white dwarf (WD) birthrate densities [(1.0 ± 0.25) × 10^-12 WDs yr^-1 pc^-3]. The predicted PN birthrate density will be revised down if we assume that only binaries make PNs, implying that the PN distance scale has to be revised to larger values.

We show that the ratio of molecular to atomic gas in galaxies is determined by hydrostatic pressure and that the relation between the two is nearly linear. The pressure relation is shown to be good over 3 orders of magnitude for 14 galaxies, including dwarfs, H I-rich, and H₂-rich galaxies, as well as the Milky Way. The sample spans a factor of 5 in mean metallicity. The rms scatter of individual points of the relation is only about a factor of 2 for all the galaxies, although some show much more scatter than others. Using these results, we propose a modified star formation prescription based on pressure determining the degree to which the ISM is molecular. The formulation is different in high- and low-pressure regimes, defined by whether the gas is primarily atomic or primarily molecular. This formulation can be implemented in simulations and provides a more appropriate treatment of the outer regions of spiral galaxies and molecule-poor systems, such as dwarf irregulars and damped Lyα systems.

New visible polarization data combined with existing IR and FIR polarization data are used to study how the magnetic field threading the filamentary molecular cloud GF 9 connects to larger structures in its general environment. When visible and NIR polarization data are combined, no evidence is found for a plateau in the polarization above extinction A_V ≈ 1.3, as seen in dark clouds in Taurus. This lack of saturation effect suggests that even in the denser parts of GF 9 magnetic fields can be probed. The visible polarization is smooth and has a well-defined orientation. In the core region, the IR and FIR data are also well defined, but each with a different direction. A multiscale analysis of the magnetic field shows that on the scale of a few times the mean radial dimension of the molecular cloud, it is as if the magnetic field were "blind" to the spatial distribution of the filaments, while on smaller scales in the core region, multiwavelength polarimetry shows a rotation of the magnetic field lines in these denser phases. Finally, the Chandrasekhar and Fermi method is used to evaluate the magnetic field strength, indicating that the core region is approximately magnetically critical. A global interpretation suggests that in the core region an original poloidal field could have been twisted by a rotating elongated (core+envelope) structure. There is no evidence for turbulence, and ambipolar diffusion does not seem to be effective at the present time.

We present new numerical simulations in the thin disk approximation that characterize the burst mode of protostellar accretion. The burst mode begins upon the formation of a centrifugally balanced disk around a newly formed protostar. It comprises prolonged quiescent periods of low accretion rate (typically ≲10^-7M_☉ yr^-1) that are punctuated by intense bursts of accretion (typically ≳10^-4M_☉ yr^-1, with duration ≲100 yr) during which most of the protostellar mass is accumulated. The accretion bursts are associated with the formation of dense protostellar/protoplanetary embryos, which are later driven onto the protostar by the gravitational torques that develop in the disk. Gravitational instability in the disk, driven by continuing infall from the envelope, is shown to be an effective means of transporting angular momentum outward and mass inward to the protostar. We show that the disk mass always remains significantly less than the central protostar's mass throughout this process. The burst phenomenon is robust enough to occur for a variety of initial values of rotation rate and frozen-in (supercritical) magnetic field and a variety of density-temperature relations. Even in cases where the bursts are nearly entirely suppressed, a moderate increase in cloud size or rotation rate can lead to vigorous burst activity. We conclude that most (if not all) protostars undergo a burst mode of evolution during their early accretion history, as inferred empirically from observations of FU Orionis variables.

We investigate the mass function of cold, dusty clumps in 11 low- and high-mass star-forming regions. Using a homogeneous fitting technique, we analyze the shape of each region's clump mass function and examine the commonalities among them. We find that the submillimeter continuum clump mass function in low-mass star-forming regions is typically best fitted by a lognormal distribution, while that in high-mass star-forming regions is better fitted by a double power law. A single power-law clump mass distribution is ruled out in all cases. Fitting all of the regions with a double power law, we find that the mean power-law exponent at the high-mass end of each mass function is α_high = -2.4 ± 0.1, consistent with the Salpeter result of α = -2.35. We find no region-to-region trend in α_high with the mass scale of the clumps in a given region, as characterized by their median mass. Similarly, nonparametric tests show that the shape of the clump mass function does not change much from region to region, despite the obvious changes in the intrinsic mass scale. This result is consistent with the hypothesis that the clump mass distribution is determined by a highly stochastic process, such as turbulent fragmentation. It may also suggest that the data reduction and analysis techniques strongly affect the shape of the derived mass function.

We present an observational picture of the HH 409 bipolar outflow including the detection of six previously unreported Herbig-Haro knots from the Herbig Ae star HD 163296. This study combines seven years of data from ground-based Fabry-Pérot and HST coronagraphic imagery, as well as HST long-slit spectral imagery. The redshifted counterjet includes a chain of six Herbig-Haro knots spanning >27'' to the northeast (P.A. ≈ 42°) of the source and has been active for >80 yr. The brightest knot in the counterjet is HH 409 C, a low-excitation bow shock with a shock velocity V_s ~ 50 km s^-1 and total space motion V_jet ≈ 260 km s^-1. The presence of additional knots in the counterjet beyond the bow shock may indicate precession of the jet axis. The blueshifted jet includes two closely spaced knots within 10'' and a distant bow shock (≈21'') southwest (P.A. ≈ 223°) of the source. The brightest knot in the jet is HH 409 A, a higher excitation more bullet-like shock with V_s ~ 90 km s^-1 and V_jet ≈ 360 km s^-1. The average opening angles for both the jet and counterjet are similar, α_a ~ 2°, and consistent with opening angles of lower mass T Tauri stars. The mass-loss rates in both lobes of the flow, despite the asymmetry of the knots, are also comparable, _out ~ 1.0 × 10^-8M_☉ yr^-1. This suggests that variations in the mass outflow rate are not more than a factor of ~2.

Recent discovery of the afterglow emission from short gamma-ray bursts suggests that binary neutron star or black hole-neutron star binary mergers are the likely progenitors of these short bursts. The accretion of neutron star material and its subsequent ejection by the central engine implies a neutron-rich outflow. We consider here a neutron-rich relativistic jet model of short bursts and investigate the high-energy neutrino and photon emission as neutrons and protons decouple from each other. We find that upcoming neutrino telescopes are unlikely to detect the 50 GeV neutrinos expected in this model. For bursts at z ~ 0.1, we find that the Gamma-Ray Large Area Space Telescope (GLAST) and ground-based Cerenkov telescopes should be able to detect prompt 100 MeV and 100 GeV photon signatures, respectively, which may help test the neutron star merger progenitor identification.

The spectra obtained above 100 MeV by the EGRET experiment aboard the Compton Gamma Ray Observatory for a handful of gamma-ray bursts (GRBs) have given no indication of any spectral attenuation that might preclude detection of bursts at higher energies. With the discovery of optical afterglows and counterparts to bursts in the last few years, enabling the determination of significant redshifts for these sources, it is anticipated that profound spectral attenuation will arise in the Gamma-Ray Large Area Space Telescope (GLAST) energy band of 30 MeV to 300 GeV for many, if not most, bursts. This paper explores time-dependent expectations for burst spectral properties in the EGRET/GLAST band, focusing on how attenuation of photons by pair creation internal to the source generates distinctive spectral signatures. The energy of spectral breaks and the associated spectral indices provide valuable information that constrains the bulk Lorentz factor of the GRB outflow at a given time. Moreover, the distinct temporal behavior that is present for internal attenuation is easily distinguished from extrinsic absorption due to intervening cosmic background fields. These characteristics define palpable observational goals for both spaced-based hard gamma-ray experiments such as GLAST, and ground-based Cerenkov telescopes, and strongly impact the observability of bursts above 300 MeV.

The present paper is the last of a series studying the first-order Fermi acceleration processes at relativistic shock waves with the method of Monte Carlo simulations applied to shocks propagating in realistically modeled turbulent magnetic fields. The model of the background magnetic field structure of Niemiec & Ostrowski has been augmented here by a large-amplitude short-wave downstream component, imitating that generated by plasma instabilities at the shock front. Following the recent work of Niemiec & Ostrowski, we have considered ultrarelativistic shocks with the mean magnetic field oriented both oblique and parallel to the shock normal. For both cases, simulations have been performed for different choices of magnetic field perturbations, represented by various wave power spectra within a wide wavevector range. The results show that the introduction of the short-wave component downstream of the shock is not sufficient to produce power-law particle spectra with the "universal" spectral index 4.2. On the contrary, concave spectra with cutoffs are preferentially formed, the curvature and cutoff energy being dependent on the properties of turbulence. Our results suggest that the electromagnetic emission observed from astrophysical sites with relativistic jets, e.g., active galactic nuclei and gamma-ray bursts, is likely generated by particles accelerated in processes other than the widely invoked first-order Fermi mechanism.

The collapsar engine behind long-duration gamma-ray bursts extracts the energy released from the rapid accretion of a collapsing star onto a stellar mass black hole. In a collapsing star, this black hole can form in two ways: the direct collapse of the stellar core into a black hole and the delayed collapse of a black hole caused by fallback in a weak supernova explosion. In the case of a delayed-collapse black hole, the strong collapsar-driven explosion overtakes the weak supernova explosion before shock breakout, and it is very difficult to distinguish this black hole formation scenario from the direct-collapse scenario. However, the delayed-collapse mechanism, with its double explosion, produces explosive nucleosynthetic yields that are very different from those in the direct-collapse scenario. We present one-dimensional studies of the nucleosynthetic yields from both black hole formation scenarios, deriving differences and trends in their nucleosynthetic yields.

The subpulse drifting phenomenon in pulsar radio emission is considered within the partially screened inner gap model, in which the sub-Goldreich-Julian thermionic flow of iron ions or electrons coexists with the spark-associated electron-positron plasma flow. We derive a simple formula that relates the thermal X-ray luminosity L_X from the spark-heated polar cap and the × subpulse periodicity ₃ (polar cap carousel time). For PSRs B0943+10 and B1133+16, the only two pulsars for which both ₃ and L_X are known observationally, this formula holds well. For a few other pulsars, for which only one quantity is measured observationally, we predict the value of the other quantity and propose relevant observations that can confirm or discard the model. Then we further study the detailed physical conditions that allow such partially screened inner gap to form. By means of the condition T_c/T_s > 1 (where T_c is the critical temperature above which the surface delivers a thermal flow to adequately supply the corotation charge density, and T_s is the actual surface temperature), it is found that a partially screened gap (PSG) can be formed given that the near surface magnetic fields are very strong and curved. We consider both curvature radiation (CR) and resonant inverse Compton scattering (ICS) to produce seed photons for pair production, and find that the former is the main agency to produce gamma rays to discharge the PSG.

With the excellent angular resolution of the Chandra X-Ray Observatory, it is possible to geometrically determine the distance to variable Galactic sources, based on the phenomenon that scattered radiation appearing in the X-ray halo has to travel along a slightly longer path than the direct, unscattered radiation. By measuring the delayed variability, constraints on the source distance can be obtained if the halo brightness is large enough to dominate the point-spread function (PSF) and to provide sufficient statistics. The distance to Cyg X-3, which has a quasi-sinusoidal light curve, has been obtained with this approach by Predehl et al. Here we examine the feasibility of using the delayed signature of type I X-ray bursts as distance indicators. We use simulations of delayed X-ray burst light curves in the halo to find that the optimal annular region and energy band for a distance measurement with a grating observation are roughly 10''-50'' and 1-5 keV, respectively, assuming Chandra's effective area and PSF, uniformly distributed dust, the input spectrum and optical depth to GX 13+1, and the Weingartner & Draine interstellar grain model. We find that the statistics are dominated by Poisson noise rather than systematic uncertainties, such as the PSF contribution to the halo. Using Chandra, a distance measurement to such a source at 4 (8) kpc could be made to about 23% (30%) accuracy with a single burst with 68% confidence. By stacking many bursts, a reasonable estimate of systematic errors limits the distance measurement to about 10% accuracy.

We identify "red clump stars"—core helium-burning giants—among 2MASS stars and use them to measure the run of reddening with distance in the direction of each of the Galactic anomalous X-ray pulsars (AXPs). We combine this with extinction estimates from X-ray spectroscopy to infer distances and find that the locations of all AXPs are consistent with being in Galactic spiral arms. We also find that the 2-10 keV luminosities implied by our distances are remarkably similar for all AXPs, being all around ~1.3 × 10³⁵ ergs s^-1. Furthermore, using our distances to estimate effective blackbody-emitting radii, we find that the radii are tightly anticorrelated with pulsed fraction and somewhat less tightly anticorrelated with blackbody temperature. We find no obvious relationship of any property with the dipole magnetic field strength inferred from the spin-down rate.

The X-ray spectra of anomalous X-ray pulsars have long been fit by smooth, empirical models such as the sum of a blackbody plus a power law. These reproduce the ~0.5-10 keV range well, but fail at lower and higher energies, grossly overpredicting the optical and underpredicting the hard X-ray emission. A poorly constrained source of uncertainty in determining the true, intrinsic spectra, in particular at lower energies, is the amount of interstellar extinction. In previous studies, extinction column densities with small statistical errors were derived as part of the fits of the spectra to simple continuum models. Different choices of model, however, each produced statistically acceptable fits but a wide range of columns. Here we attempt to measure the interstellar extinction in a model-independent way, using individual absorption edges of the elements O, Fe, Ne, Mg, and Si in X-ray grating spectra taken with XMM-Newton. We find that our inferred equivalent hydrogen column density N_H for 4U 0142+61 is a factor of 1.4 lower than the typically quoted value from blackbody plus power-law fits, and is now consistent with estimates based on the dust scattering halo and visual extinction. For three other sources, we find column densities consistent with earlier estimates. We use our measurements to recover the intrinsic spectra of the AXPs empirically, without making assumptions on what the intrinsic spectral shapes ought to be. We find that the power-law components that dominate at higher energies do not extend below the thermal peak.

High-ionization forbidden lines from Ca VII, Fe VII, Mg V, Mg VI, Mg VII, and Si VII are found in recent Hubble Space Telescope STIS ultraviolet spectra of the symbiotic star AG Draconis. These species have ionization potentials between 99 and 205 eV, which are unexpected due to the high density (≈10¹⁰ cm^-3) of the AG Dra nebula. The identification of the Mg VII λλ2510, 2629 lines is the first in astrophysical or laboratory spectra, and revised rest wavelengths are suggested from the STIS spectra. Plasma diagnostics from Mg V-VII are applied, but do not provide a consistent constraint on temperature or density. A density ≥10⁸ cm^-3 is confirmed, however. The lines show double-peaked profiles with widths ≈100-160 km s^-1, suggestive of an origin in an accretion disk. However, the line widths, if identified with motion in a Keplerian disk, indicate radii much smaller than sizes inferred from the line fluxes themselves. The source of these high-ionization forbidden lines remains unidentified.

We discuss new methods of measuring and interpreting the forbidden-to-intercombination line ratios of helium-like triplets in the X-ray spectra of O-type stars, including accounting for the spatial distribution of the X-ray-emitting plasma and using the detailed photospheric UV spectrum. Measurements are made for four O stars using archival Chandra HETGS data. We assume an X-ray-emitting plasma spatially distributed in the wind above some minimum radius R₀. We find minimum radii of formation typically in the range of 1.25 < R₀/R_* < 1.67, which is consistent with results obtained independently from line profile fits. We find no evidence for anomalously low f/i ratios, and we do not require the existence of X-ray-emitting plasmas at radii that are too small to generate sufficiently strong shocks.

The Microvariability and Oscillations of Stars (MOST) satellite observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and detected 48 frequencies ≲2.8 cycles day^-1 with amplitudes of a few millimagnitudes (mmag) and less. The frequency range embraces g- and p-mode pulsations. It was generally thought that no g-modes are excited in less luminous B supergiants because strong radiative damping is expected in the core. Our theoretical models, however, show that such g-modes are excited in massive post-main-sequence stars, in accordance with these observations. The nonradial pulsations excited in models between 20 M_☉ at log T_eff ≈ 4.41 and 15 M_☉ at log T_eff ≈ 4.36 are roughly consistent with the observed frequency range. Excitation by the Fe bump in opacity is possible because g-modes can be partially reflected at a convective zone associated with the hydrogen-burning shell, which significantly reduces radiative damping in the core. The MOST light curve of HD 163899 shows that such a reflection of g-modes actually occurs and reveals the existence of a previously unrecognized type of variable, slowly pulsating B supergiants (SPBsg) distinct from α Cyg variables. Such g-modes have great potential for asteroseismology.

Short-period binaries represent extreme cases in the generation of stellar coronae via a rotational dynamo. Such stars are important for probing the origin and nature of coronae in the regimes of rapid rotation and activity saturation. VW Cep (P = 0.28 days) is a relatively bright, partially eclipsing, very active object. Light curves made from Chandra HETGS data show flaring and rotational modulation but no eclipses. Velocity modulation of emission lines indicates that one component dominates the X-ray emission. The emission measure is highly structured, having three peaks. Helium-like triplet lines give electron densities of about (3-18) × 10¹⁰ cm^-3. We conclude that the corona is predominantly on the polar regions of the primary star and is compact.

We present Spitzer IRAC and MIPS observations for a sample of eight M dwarfs: six dMe, one dM, and one sdMe star. All of our targets are found to have SEDs that are fitted within the error bars by a purely photospheric spectrum out to 24 μm. We find no evidence for IR excess. None of our targets are detected in the MIPS 70 and 160 μm bands. The estimated ages for all are >10 Myr, suggesting that enough disk dissipation has occurred within the inner several AU of the star. For four of these, Mullan et al. had reported IRAS detections at 12 μm, although the reported fluxes were below the 5 σ IRAS detection limit (~0.2 Jy). Mullan et al. also pointed out that V-K colors in dMe stars are larger than those in dM stars, possibly because of the presence of a chromosphere. Here we suggest that metallicity effects provide a better explanation of the V-K data. For reasons of observational selection, our targets are not the most active flare stars known, but being dMe stars indicates the presence of a chromosphere. Scaling from Houdebine's model of the AU Mic chromosphere, we have computed the free-free IR excesses for a range of densities. Our Spitzer 24 μm data show that the chromospheres in two of our targets are less dense than in AU Mic by a factor of 10 or more. This is consistent with the fact that our sample includes the less active flare stars. Our models also indicate that the chromospheric contribution to the observed AU Mic emission at submillimeter wavelengths is only about 2%.

We have created a general methodology for calculating the wavelength-dependent light curves of close-in extrasolar giant planets (EGPs) as they traverse their orbits. Focusing on the transiting EGPs HD 189733b, TrES-1, and HD 209458b, we calculate planet/star flux ratios during secondary eclipse and compare them with the Spitzer data points obtained so far in the mid-infrared. We introduce a simple parameterization for the redistribution of heat to the planet's night side, derive constraints on this parameter (P_n), and provide a general set of predictions for planet/star contrast ratios as a function of wavelength, model, and phase. Moreover, we calculate average dayside and nightside atmospheric temperature/pressure profiles for each transiting planet/P_n pair with which existing and anticipated Spitzer data can be used to probe the atmospheric thermal structure of severely irradiated EGPs. We find that the baseline models do a good job of fitting the current secondary eclipse data set, but that the Spitzer error bars are not yet small enough to discriminate cleanly among all the various possibilities.

The high density of the recently discovered close-in extrasolar planet HD 149026b suggests the presence of a huge core in its interior, which challenges planet formation theory. We first derive constraints on the total mass of heavy elements in the planet and find its preferred value is 50-80 M_⊕ . We then explore the possibility of the formation of HD 149026b through subcritical core accretion as envisioned for Uranus and Neptune, and find the subcritical accretion scenario is very unlikely in the case of HD 149026b for at least two reasons: (1) subcritical planets are such that the ratio of their core mass to their total mass is above ~0.7, in contradiction with constraints for all but the most extreme interior models of HD 149026b and (2) high accretion rates and large isolation mass required for the formation of a subcritical >50 M_⊕ core are possible only at specific orbital distances in a disk with a surface density of dust equal to at least 30 times that of the minimum-mass solar nebula. These facts point toward two main routes for the formation of HD 149026b: (i) gas accretion limited by a slow viscous inflow in an evaporating disk or (ii) a significant modification of the planetary composition after gas accretion ended. Illustrating the second route, we show that collision between two gas giants leads to a substantial loss of the gas component and thus may make the planet highly enriched in heavy elements. Alternatively, the planet may be supplied with heavy elements by planetesimals through secular perturbations. In both the giant impact and the secular perturbation scenarios, we expect an outer giant planet to be present. Observational studies by imaging, astrometry, and long-term interferometry of this system are needed to better narrow down the ensemble of possibilities.

We report on the BVRI multiband follow-up photometry of the transiting extrasolar planet HD 189733b. We revise the transit parameters and find a planetary radius of R_P = 1.154 ± 0.033R_J and an inclination of i_P = 8579 ± 024. The new density (~1 g cm^-3) is significantly higher than the former estimate (~0.75 g cm^-3); this shows that from the current sample of nine transiting planets, only HD 209458 (and possibly OGLE-10b) have anomalously large radii and low densities. We note that due to the proximity of its parent star, HD 189733b currently has one of the most precise radius determinations among extrasolar planets. We calculate new ephemerides, P = 2.218573 ± 0.000020 days and T₀ = 2453629.39420 ± 0.00024 (HJD), and estimate the timing offsets of the 11 distinct transits with respect to the predictions of a constant orbital period, which can be used to reveal the presence of additional planets in the system.

In previous studies employing the Large Angle and Spectrometric Coronagraph (LASCO), we identified a class of white-light ejections that separate into incoming and outgoing components at distances of ~3-5 R_☉ from Sun center. These events, of which up to several per month are observed during high solar activity, are generally preceded by a gradual outward expansion of faint loops over a period of a day or more. The expansion terminates when the streamer material, in the form of an elongated stalk or a sheetlike structure, suddenly tears apart. The collapsing material is sometimes recognizable as a collection of loops, while the ejected component is usually poorly resolved. Here we describe a streamer detachment observed on 2005 December 11, in which the outgoing component can be clearly identified as a cylindrical flux rope with its ends anchored in the Sun. Based on simple three-dimensional white-light reconstructions, we conclude that in/out pairs in general represent the pinching off of streamer loop arcades to form flux ropes, as seen from different viewing angles.

The X17 flare on 2003 October 28 was observed by high-resolution imaging or spectroscopic instruments on CORONAS, GOES, INTEGRAL, RHESSI, SOHO, and TRACE. These spacecraft observed the temporal evolution of the γ-ray positron-annihilation and nuclear de-excitation line spectra, imaged the hard X-ray bremsstrahlung and EUV and UV emission, and measured the surface magnetic field and subphotospheric pressure perturbations. In the usual pattern, the onset of the flare is dominated by particle acceleration and interaction, and by the filling of coronal magnetic structures with hot plasma. The associated positron-annihilation signatures early in the impulsive phase from 11:06 to 11:16 UT have a line-broadening temperature characteristic of a few hundred thousand kelvins. The most intense precipitation sites within the extended flare ribbons are very compact, with diameters of less than 1400 km, and a 195 Å TRACE intensity that can exceed 7500 times the quiescent active-region value. These regions appear to move at speeds of up to 60 km s^-1. The associated rapidly evolving, compact perturbations of the photosphere below these sites excite acoustic pulses that propagate into the solar interior. Less intense precipitation sites typically persist for several minutes behind the advancing flare ribbons. After ~1 ks, the flare enters a second phase, dominated by coronal plasma cooling and downflows and by annihilation-line radiation characteristic of a photospheric environment. We point out (1) that these detailed observations underscore that flare models need to explicitly incorporate the multitude of successively excited environments whose evolving signals differ at least in their temporal offsets and energy budgets, if not also in the exciting particle populations and penetration depths, and (2) that the spectral signatures of the positron annihilation do not fit conventional model assumptions.

The results of the analysis of the first spectropolarimetric observations of the 3p¹P₁-4d¹D₂ Mg I line at 5528.4 Å made during a solar flare are presented in this paper. The line is found to be polarized with a polarization degree at the line center that reaches up to 3% and a direction of polarization nearly parallel to the local transverse magnetic field. After eliminating scattering, the Zeeman effect, and the intensity gradient as possible origins of the observed polarization, this polarization is interpreted as due either to a low-energy proton beam or to the return current associated with electron beams.

For a number of impulsive solar particle events we examine variations of maximum intensities and times to maximum intensity as a function of longitude, using observations from the two Helios spacecraft and near the Earth. We find that electrons in the MeV range can be detected more than 80° from the flare longitude, corresponding to a considerably wider "well connected" region than that (~20° half-width) reported for ³He-rich impulsive solar events. This wide range and the decrease of peak intensities with increasing connection angle revive the concept of some diffusive propagation process in the low corona. Delays to intensity maxima are not systematically correlated with connection angles. We argue that interplanetary scattering parallel to the average interplanetary magnetic field, which varies with position in space, plays an important role in flare particle events. In a specific case variations of the time profiles with radial distance and with particle rigidity are used to quantitatively confirm spatial diffusion. For a few cases near the edges of the well-connected region, the very long times to maximum intensity might result from interplanetary lateral transport.

Angular momentum transport must have occurred in the Sun's radiative zone, to explain its current solid body rotation. We survey the stability of the early Sun's radiative zone with respect to diffusive rotational instabilities for a variety of plausible past configurations. We find that the (faster rotating) early Sun was prone to rotational instabilities even if only weak levels of radial differential rotation were present, while the current Sun is not. Stability domains are determined by approximate balance between dynamical and diffusive timescales, allowing generalizations to other stellar contexts. Depending on the strength and geometry of the weak magnetic field present, the fastest growing unstable mode can be hydrodynamic or magnetohydrodynamic (MHD) in nature. Our results suggest that diffusive MHD modes may be more efficient at transporting angular momentum than their hydrodynamic (Goldreich-Schubert-Fricke) counterparts because the minimum spatial scale required for magnetic tension to be destabilizing limits the otherwise very small scales favored by double-diffusive instabilities. Diffusive magnetorotational instabilities are thus attractive candidates for angular momentum transport in the early Sun's radiative zone.

We claim that the discrepancy found between the theoretical predictions for the cosmic mass function and those found in numerical simulations is due to the fact that in deriving the former, all mass elements are assumed to be at the center of the object they belong to (the all-mass-at-center problem). By an appropriate treatment of this problem, using the spherical collapse model (which is not a bad approximation in the high-mass limit), we obtain both the high-mass behavior found in simulations and the true asymptotic behavior of the mass function (for arbitrarily high masses). Therefore, we conclude that, by combining ellipsoidal dynamics with a suitable treatment of the all-mass-at-center problem (which we will show in follow-up work), a theoretical prediction for the cosmic mass function in full agreement with simulations may be obtained.

We study the evolution of two fundamental properties of galaxy clusters: the luminosity function (LF) and the scaling relations between the total galaxy number N (or luminosity) and the cluster mass M. Using a sample of 27 clusters (0 ≤ z ≤ 0.9) with new near-IR observations and mass estimates derived from X-ray temperatures, in conjunction with data from the literature, we construct the largest sample for such studies to date. The evolution of the characteristic luminosity of the LF can be described by a passively evolving population formed in a single burst at z = 1.5-2. Under the assumption that the mass-temperature relation evolves self-similarly, and after the passive evolution is accounted for, the N-M scaling shows no signs of evolution out to z = 0.9. Our data provide direct constraints on halo occupation distribution models and suggest that the way galaxies populate cluster-scale dark matter halos has not changed in the past 7 Gyr, in line with previous investigations.

The interstellar medium (ISM) at the centers of active galaxies is exposed to a combination of cosmic-ray, far-ultraviolet (FUV), and X-ray radiation. We apply photodissociation region (PDR) models to this ISM with both "normal" and highly elevated (5 × 10^-15 s^-1) cosmic-ray (CR) rates and compare the results to those obtained for X-ray dissociation regions (XDRs). Our existing PDR-XDR code is used to construct models over a 10³-10⁵ cm^-3 density range and for 0.16-160 ergs s^-1 cm^-2 impingent fluxes. We obtain larger high-J (J > 10) CO ratios in PDRs when we use the highly elevated CR rate, but these are always exceeded by the corresponding XDR ratios. The [C I] 609 μm/¹³CO (2-1) line ratio is boosted by a factor of a few in PDRs with n ~ 10³ cm^-3 exposed to a high CR rate. At higher densities, ratios become identical irrespective of CR flux, while XDRs always show elevated [C I] emission per CO column. The HCN/CO and HCN/HCO⁺ line ratios, combined with high-J CO emission lines, are good diagnostics to distinguish between PDRs, under either low or high CR irradiation conditions, and XDRs. Hence, the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory, which can detect these CO lines, will be crucial in the study of active galaxies.

We present deep F606W, F814W ACS photometry of the recently discovered globular cluster B514, the outermost known globular in the M31 galaxy. The cluster appears quite extended, and member stars are unequivocally identified out to ~200 pc from the center. The color-magnitude diagram reveals a steep red giant branch (RGB), and a horizontal branch extending blueward of the instability strip, indicating that B514 is a classical old metal-poor globular cluster. The RGB locus and the position of the RGB bump are both consistent with a metallicity [Fe/H] ~ -1.8, in excellent agreement with spectroscopic estimates. A preliminary estimate of the integrated absolute V magnitude (M_V ≲ -9.1) suggests that B514 is among the brightest globulars of M31.

We present the age distributions for star clusters and individual stars in the Small Magellanic Cloud (SMC) based on data from the Magellanic Clouds Photometric Survey by Zaritsky and collaborators. The age distribution of the SMC clusters shows a steep decline, dN_cluster/dτ ∝ τ^-0.85±0.15, over the period 10⁷ yr ≲ τ ≲ 10⁹ yr. This decline is essentially identical to that observed previously for more massive clusters in the merging Antennae galaxies and also for lower mass embedded clusters in the solar neighborhood. The SMC cluster age distribution therefore provides additional evidence for the rapid disruption of star clusters ("infant mortality"). These disrupted clusters deliver their stars to the general field population, implying that the field star age distribution, dN_{field star}/dτ, should have an inverse relation to dN_cluster/dτ if most stars form initially in clusters. We make specific predictions for dN_{field star}/dτ based on our cluster disruption models and compare them with current data available for stars in the SMC. While these data do not extend to sufficiently young ages for a definitive test, they are consistent with a scenario wherein most SMC stars formed initially in clusters. Future analyses of dN_{field star}/dτ that extend down to ages of ~few million years are needed to verify the age relationship between stars residing in clusters and in the field.

Cl I is the atomic species most directly coupled to molecular hydrogen due to its chemistry. Its weakest lines are thereby probably the best tracer of optically thick H₂ components in diffuse clouds. We report on the empirical determination of the oscillator strengths for four Cl I absorption lines predicted to be weak and often detected toward moderately reddened sight lines observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). We compared our oscillator strength estimates with the oscillator strength calculations listed in Morton. We find that our empirical oscillator strength values for the Cl I 1004, 1079, 1090, and 1094 Å lines differ from the theoretical predictions by factors of ~3.1, 1.2, 2.4, and 0.42, respectively. We briefly discuss the value of Cl I as tracer of molecular gas for our star sample.

We calculate the thermal and dynamical evolution of the surface layers of an accreting neutron star during the rise of a superburst. For the first few hours following unstable ¹²C ignition, the nuclear energy release is transported by convection. However, as the base temperature rises, the heating time becomes shorter than the eddy turnover time and convection becomes inefficient. This results in a hydrodynamic nuclear runaway, in which the heating time becomes shorter than the local dynamical time. Such hydrodynamic burning can drive shock waves into the surrounding layers and may be the trigger for the normal X-ray burst found to immediately precede the onset of the superburst in both cases in which the Rossi X-Ray Timing Explorer was observing.

The Galactic high-mass X-ray binary and jet source (microquasar) LS I +61 303 has recently been detected at TeV γ-ray energies by the MAGIC telescope. We have applied a time-dependent leptonic jet model to the broadband spectral energy distribution (SED), and we have suggested (although not unambiguously detected) an orbital modulation of the very high energy (VHE) γ-ray emission of this source. Our model takes into account the time-dependent electron injection and acceleration, and the adiabatic and radiative cooling of nonthermal electrons. It includes synchrotron, synchrotron self-Compton (SSC), and external inverse Compton (EC; with seed photons from the companion star) emission as well as γγ absorption of γ-rays by starlight photons. The model can successfully reproduce the available multiwavelength observational data. Our best fit to the SED indicates that a magnetic field of B₀ ~ 5 × 10³ G at ~10³R_g is required and that electrons need to be accelerated out to TeV energies (γ₂ = 10⁶) with a nonthermal injection spectrum with a spectral index of q = 1.7, indicating the operation of acceleration mechanisms beyond the standard first-order Fermi mechanism at relativistic or nonrelativistic shocks. The orbital modulation of the VHE γ-ray emission can be explained solely by the geometrical effect of changes in the relative orientation of the stellar companion with respect to the compact object and jet as it impacts the position and depth of the γγ absorption trough. Such a scenario predicts a trend of spectral hardening during VHE γ-ray low orbital phases.

We have investigated the abundances of heavy neutron-capture elements, including osmium (Os) and iridium (Ir), in the two carbon-enhanced metal-poor (CEMP) subgiants CS 31062-050 and LP 625-44. CS 31062-050 is known to be a so-called CEMP-r/s star, which exhibits large excesses of s-process elements such as barium (Ba) and lead (Pb), as well as a significant enhancement of europium (Eu) that cannot be explained by conventional s-process production in asymptotic giant branch star models. Our analysis of the high-resolution spectrum for this object has determined, for the first time, the abundances of Ir and Os, elements in the third peak of the r-process nucleosynthesis. They also exhibit significant excesses relative to the predictions of standard s-process calculations. These two elements are not detected in a similar quality spectrum of LP 625-44; the derived upper limits on their abundances are lower than the abundances in CS 31062-050. We compare the observed abundance patterns of neutron-capture elements, including Os and Ir, in these two stars with recent model calculations of the s-process, and we discuss possible interpretations.

We have used the Palomar 200'' adaptive optics (AO) system to directly detect the astrometric brown dwarf GJ 802B reported by Pravdo et al. This observation is achieved with a novel combination of aperture masking interferometry and AO. The dynamical masses are 0.175 ± 0.021 and 0.064 ± 0.032 M_☉ for the primary and secondary, respectively. The inferred absolute H-band magnitude of GJ 802B is M_H = 12.8 resulting in a model-dependent T_eff of 1850 ± 50 K and mass range of 0.057-0.074 M_☉.

Magnetic precursors of C-type shocks accelerate, compress, and heat molecular ions, modifying the kinematics and the physical conditions of the ion fluid with respect to the neutral one. Electron densities are also expected to be significantly enhanced in shock precursors. In this Letter, we present observations of strongly polar ion and neutral molecules such as SiO, H¹³CO⁺, HN¹³C, and H¹³CN, which reveal the electron density enhancements associated with the precursor of the young L1448-mm outflow. While in the ambient gas the excitation of the ions and neutrals is explained by collisional excitation by H₂ with a single density of ~10⁵ cm^-3, H¹³CO⁺ shows an overexcitation in the shock precursor component that requires H₂ densities a factor of ≥10 larger than those derived from the neutral species. This overexcitation in H¹³CO⁺ can be explained if we consider an additional excitation by collisions with electrons and an electron density enhancement in the precursor stage by a factor of ~500, i.e., a fractional ionization of 5 × 10^-5. These results show that multiline observations can be used to study the evolution of the ion and electron fluids at the first stages of the C-type shock interaction.

We develop a semianalytic model for planet formation during the pre-main-sequence contraction phase of a low-mass star. During this evolution, the stellar magnetosphere maintains a fixed ratio between the inner disk radius and the stellar radius. As the star contracts at constant effective temperature, the "snow line," which separates regions of rocky planet formation from regions of icy planet formation, moves inward. This process enables rapid formation of icy protoplanets that collide and merge into super-Earths before the star reaches the main sequence. The masses and orbits of these super-Earths are consistent with super-Earths detected in recent microlensing experiments.

We investigated the frequency distributions of flares with and without coronal mass ejections (CMEs) as a function of flare parameters (peak flux, fluence, and duration of soft X-ray flares). We used CMEs observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) mission and soft X-ray flares (C3.2 and above) observed by the Geostationary Operational Environmental Satellite (GOES) during 1996-2005. We found that the distributions obey a power law of the form dN/dX ∝ X^-α, where X is a flare parameter and dN is the number of events recorded within the interval [X, X + dX]. For the flares with (without) CMEs, we obtained the power-law index α = 1.98 ± 0.05 (α = 2.52 ± 0.03) for the peak flux, α = 1.79 ± 0.05 (α = 2.47 ± 0.11) for the fluence, and α = 2.49 ± 0.11 (α = 3.22 ± 0.15) for the duration. The power-law indices for flares without CMEs are steeper than those for flares with CMEs. The larger power-law index for flares without CMEs supports the possibility that nanoflares contribute to coronal heating.