Table of contents

We describe the general problem of estimating black hole masses of active galactic nuclei (AGNs) by calculating the conditional probability distribution of M_BH given some set of observables. Special attention is given to the case where one uses the AGN continuum luminosity and emission line widths to estimate M_BH, and we outline how to set up the conditional probability distribution of M_BH given the observed luminosity, line width, and redshift. We show how to combine the broad-line estimates of M_BH with information from an intrinsic correlation between M_BH and L, and from the intrinsic distribution of M_BH, in a manner that improves the estimates of M_BH. Simulation was used to assess how the distribution of M_BH inferred from the broad-line mass estimates differs from the intrinsic distribution, and we find that this can lead to an inferred distribution that is too broad. We use these results and a sample of 25 sources that have recent reverberation mapping estimates of AGN black hole masses to investigate the effectiveness of using the C IV emission line to estimate M_BH and to indirectly probe the C IV region size-luminosity (R-L) relationship. A linear regression of log L_λ (1549 Å) on log M_BH found that L₁₅₄₉ ∝ M. A linear regression also found that M_BH ∝ LFWHM, implying a C IV R-L relationship of the form R ∝ L. Including the C IV line FWHM resulted in a reduction of a factor of ~1/3 in the error in the estimates of M_BH over simply using the continuum luminosity, statistically justifying its use. We estimated M_BH from both C IV and Hβ for a sample of 100 sources, including new spectra of 29 quasars. We find that the two emission lines give consistent estimates if one assumes R ∝ L for both lines.

We present catalogs and images of optical counterparts to the Chandra-selected X-ray sources found in the fields of the five clusters RX J0152-1357, RX J0849+4452, RDCS J0910+5422, MS 1054-0321, and RDCS J1252-2927, which were imaged with the Advanced Camera for Surveys as part of the ACS Guaranteed Time Observer programs. A total of 98 X-ray sources fall within the ACS mosaics, and positive identifications are made for ~96% of them, including confirmed cluster members. We classify the sources as active galactic nuclei (AGNs) or QSOs depending on their X-ray output. The log N(> S)- log S test indicates a significant overdensity of X-ray sources in the RX J0152-1357, RDCS J0910+5422, and MS 1054-0321 fields with respect to the CDF-S, suggesting an association of some X-ray sources with the large-scale structure of the clusters. From the asymmetry and concentration indices, ~52% of the optical counterparts are early-type galaxies, ~35% late-type, and the remainder irregular. A blue core is found in approximately half of the early-type galaxies. From visual examination, approximately 40% of the counterparts possess an unresolved nucleus, a common signature of nuclear activity. A majority of these nuclei are found in near face-on late-type galaxies although selection effects might be important. The X-ray to optical flux ratio of the nuclei correlates with the inclination angle of the late-type galaxies but not of the early types, as expected if dust is significant in the circumnuclear regions. The AGNs possess a ~50% excess of nearby companions compared to the overall galaxy population in the same fields. The surface density of the X-ray sources is highest at projected radii of ≲1 Mpc from the cluster center and relatively flat at larger radii. We describe the morphology and environment of the cluster members and compare them with the other optical counterparts.

Using UV spectra obtained with FUSE, HST, and/or IUE together with higher resolution optical spectra, we determine interstellar column densities of ¹²CO, ¹³CO, and/or C₂ for 10 Galactic sight lines with E(B - V) ranging from 0.37 to 0.72. The N(CO)/N(H₂) ratio varies over a factor of 100 in this sample, due primarily to differences in N(CO). For a given N(H₂ ), published models of diffuse and translucent clouds predict less CO than is observed. The J = 1-3 rotational levels of ¹²CO are subthermally populated in these sight lines, with T_ex typically between 3 and 7 K. In general, there appears to be no significant difference between the excitation temperatures of ¹²CO and ¹³CO. Fits to the higher resolution CO line profiles suggest that CO (like CN) is concentrated in relatively cold, dense gas. We obtain C₂ column densities from the F-X (0-0) band at 1341 Å (three sight lines; J = 0-12), the F-X (1-0) band at 1314 Å (one sight line; J = 0-12), the D-X (0-0) band at 2313 Å (four sight lines; J = 0-18), and the A-X (3-0) and (2-0) bands at 7719 and 8757 Å (seven sight lines; J = 0-12). Comparisons among those column densities yield a set of mutually consistent band f-values for the UV and optical C₂ bands, but also reveal some apparent anomalies within the F-X (0-0) band. Both the kinetic temperature T_k inferred from the C₂ rotational populations and the excitation temperature T₀₂(C₂) are generally smaller than the corresponding T₀₁(H₂)—suggesting that C₂ is concentrated in colder, denser gas than H₂. Incorporating additional column density data for K I, HD, CH, C₂, C₃, CN, and CO from the literature (for a total sample of 74 sight lines), we find that (1) CO is most tightly correlated with CN; (2) the ratios ¹²CO/H₂ and ¹³CO/H₂ both are fairly tightly correlated with the density indicator CN/CH (but C₂/H ₂ is not); and (3) the ratio ¹²CO/¹³CO is somewhat anticorrelated with both CN/CH and N(CO). Sight lines with ¹²CO/¹³CO below the average local Galactic value of ¹²C/¹³C appear to sample colder, denser gas in which isotope exchange reactions have enhanced ¹³CO, relative to ¹²CO.

We present the first high spatial resolution X-ray study of the massive star-forming region NGC 6357, obtained in a 38 ks Chandra/ACIS observation. Inside the brightest constituent of this large H II region complex is the massive open cluster Pismis 24. It contains two of the brightest and bluest stars known, yet remains poorly studied; only a handful of optically bright stellar members have been identified. We investigate the cluster extent and initial mass function and detect ~800 X-ray sources with a limiting sensitivity of ~10³⁰ ergs s^-1; this provides the first reliable probe of the rich intermediate-mass and low-mass population of this massive cluster, increasing the number of known members from optical study by a factor of ~50. The high-luminosity end (log L_h[2-8 keV] ≥ 30.3 ergs s^-1) of the observed X-ray luminosity function in NGC 6357 is clearly consistent with a power-law relation as seen in the Orion Nebula Cluster and Cepheus B, yielding the first estimate of NGC 6357's total cluster population, a few times the known Orion population. We investigate the structure of the cluster, finding small-scale substructures superposed on a spherical cluster with 6 pc extent, and discuss its relationship to the nebular morphology. The long-standing L_X - 10^-7L_bol correlation for O stars is confirmed. Twenty-four candidate O stars and one possible new obscured massive YSO or Wolf-Rayet star are presented. Many cluster members are estimated to be intermediate-mass stars from available infrared photometry (assuming an age of ~1 Myr), but only a few exhibit K-band excess. We report the first detection of X-ray emission from an evaporating gaseous globule at the tip of a molecular pillar; this source is likely a B0-B2 protostar.

We report broadband UBV and/or BVR_CI_C CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/A, N = 576), or candidate low-metallicity stars (N = 1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/A candidates they are used to accurately separate the lower gravity FHB stars from various higher gravity A-type stars, a subsample that includes the so-called blue metal poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists.

We present a comprehensive description of the theory and practice of opacity calculations from the infrared to the ultraviolet needed to generate models of the atmospheres of brown dwarfs and extrasolar giant planets. Methods for using existing line lists and spectroscopic databases in disparate formats are presented, and plots of the resulting absorptive opacities versus wavelength for the most important molecules and atoms at representative temperature/pressure points are provided. Electronic, rovibrational, bound-free, bound-bound, free-free, and collision-induced transitions and monochromatic opacities are derived, discussed, and analyzed. The species addressed include the alkali metals, iron, heavy metal oxides, metal hydrides, H₂, H₂O, CH₄, CO, NH₃, H₂S, PH₃, and representative grains. Once monochromatic absorption cross sections for all constituents have been derived, chemical abundances have to be obtained before the resulting product can be summed to obtain total opacities. Hence, we include a review of the thermochemistry, techniques, and databases needed to derive equilibrium abundances and provide some sample results.

The measurable quantities associated with γ-ray and neutron observations of solar flares are nuclear-deexcitation line shapes, shifts, fluences, and time histories; neutron capture and annihilation line fluences and time histories; and energy-dependent escaping neutron fluence and time history. A comprehensive understanding of these quantities requires a model for ion acceleration, transport, and interaction. In this paper we address transport and interaction using a magnetic loop model that includes energy losses due to Coulomb collisions, removal by nuclear reactions, magnetic mirroring in the convergent flux tube, and MHD pitch-angle scattering in the corona. The accelerated ions are assumed to have a given kinetic energy spectrum and composition. Each measurable quantity depends to varying degree on the parameters of the loop model and of the accelerated ions. We explore these dependences in detail and construct a self-consistent approach to the analysis of high-energy flare data that provides an optimum set of parameters with meaningful uncertainties. To illustrate this approach, the calculations are applied in a comprehensive analysis of the γ-ray and neutron observations of the 1991 June 4 solar flare obtained with OSSE on CGRO. We find that the loop model can account for these observations with physically reasonable values for the parameters. In addition, our analysis of the neutron data shows that the accelerated ion spectrum for this flare was not an unbroken power law but had to steepen sharply above ~125 MeV nucleon^-1. The paper also provides yields and yield ratios calculated with assumed abundances and spectral forms currently considered appropriate for solar flares. They can be used by other researchers analyzing high-energy solar flare data.