Showing papers in "Astrophysical Journal Supplement Series in 1999"

Starburst99: Synthesis Models for Galaxies with Active Star Formation

Abstract: Starburst99 is a comprehensive set of model predictions for spectrophotometric and related properties of galaxies with active star formation. The models are an improved and extended version of the data set previously published by Leitherer & Heckman. We have upgraded our code by implementing the latest set of stellar evolution models of the Geneva group and the model atmosphere grid compiled by Lejeune et al. Several predictions which were not included in the previous publication are shown here for the first time. The models are presented in a homogeneous way for five metallicities between Z = 0.040 and 0.001 and three choices of the initial mass function. The age coverage is 106—109 yr. We also show the spectral energy distributions which are used to compute colors and other quantities. The full data set is available for retrieval at a Web site, which allows users to run specific models with nonstandard parameters as well. We also make the source code available to the community.

Nucleosynthesis in Chandrasekhar Mass Models for Type Ia Supernovae and Constraints on Progenitor Systems and Burning-Front Propagation

Abstract: The major uncertainties involved in the Chandrasekhar mass models for Type Ia supernovae (SNe Ia) are related to the companion star of their accreting white dwarf progenitor (which determines the accretion rate and consequently the carbon ignition density) and the flame speed after the carbon ignition. We calculate explosive nucleosynthesis in relatively slow deflagrations with a variety of deflagration speeds and ignition densities to put new constraints on the above key quantities. The abundance of the Fe group, in particular of neutron-rich species like 48Ca,50Ti,54Cr,54,58Fe, and 58Ni, is highly sensitive to the electron captures taking place in the central layers. The yields obtained from such a slow central deflagration, and from a fast deflagration or delayed detonation in the outer layers, are combined and put to comparison with solar isotopic abundances. To avoid excessively large ratios of 54Cr/56Fe and 50Ti/56Fe, the central density of the average white dwarf progenitor at ignition should be as low as 2 ? 109 g cm-3. To avoid the overproduction of 58Ni and 54Fe, either the flame speed should not exceed a few percent of the sound speed in the central low Ye layers or the metallicity of the average progenitors has to be lower than solar. Such low central densities can be realized by a rapid accretion as fast as -->img1.gif 1 ? 10-7 M? yr-1. In order to reproduce the solar abundance of 48Ca, one also needs progenitor systems that undergo ignition at higher densities. Even the smallest laminar flame speeds after the low-density ignitions would not produce sufficient amount of this isotope. We also found that the total amount of 56Ni, the Si-Ca/Fe ratio, and the abundance of some elements like Mn and Cr (originating from incomplete Si burning), depend on the density of the deflagration-detonation transition in delayed detonations. Our nucleosynthesis results favor transition densities slightly below 2.2 ? 107 g cm-3.

A Spectroscopic catalog of 10 distant rich clusters of galaxies

Abstract: We present spectroscopic observations of galaxies in the fields of 10 distant clusters for which we have previously presented deep imaging with WFPC2 on board the Hubble Space Telescope. The clusters span the redshift range z=0.37-0.56 and are the subject of a detailed ground- and space-based study to investigate the evolution of galaxies as a function of environment and epoch. The data presented here include positions, photometry, redshifts, spectral line strengths, and classifications for 657 galaxies in the fields of the 10 clusters. The catalog is composed of 424 cluster members across the 10 clusters and 233 field galaxies, with detailed morphological information from our WFPC2 images for 204 of the cluster galaxies and 71 in the field. We illustrate some basic properties of the catalog, including correlations between the morphological and spectral properties of our large sample of cluster galaxies. A direct comparison of the spectral properties of the high-redshift cluster and field populations suggests that the phenomenon of strong Balmer lines in otherwise passive galaxies (commonly called E + A but renamed here as the k + a class) shows an order-of-magnitude increase in the rich cluster environment compared with a more modest increase in the field population. This suggests that the process or processes involved in producing k + a galaxies are either substantially more effective in the cluster environment or that this environment prolongs the visibility of this phase. A more detailed analysis and modeling of these data is presented in Poggianti et al.

Evolution of Nonradiative Supernova Remnants

Abstract: We conduct an analytic and numerical study of the dynamics of supernova remnant (SNR) evolution from the ejecta-dominated stage through the Sedov-Taylor (ST) stage, the stages that precede the onset of dynamically significant radiative losses and/or pressure confinement by the ambient medium. We assume spherical symmetry and focus on the evolution of ejecta described by a power-law density distribution expanding into a uniform ambient medium. We emphasize that all nonradiative remnants of a given power-law structure evolve according to a single unified solution, and we discuss this general property in detail. Use of dimensionless quantities constructed from the characteristic dimensional parameters of the problem—the ejecta energy, ejecta mass, and ambient density—makes the unified nature of the solution manifest. It is also possible to obtain a unified solution for the ST and radiative stages of evolution, and we place our work in the context of scaling laws for solutions for SNR evolution in those stages. We present numerical simulations of the flow and approximate analytic solutions for the motions of both the reverse shock and blast-wave shock. These solutions follow the shocks through the nonradiative stages of remnant evolution across periods of self-similar flow linked by non-self-similar behavior. We elucidate the dependence of the ejecta-dominated evolution on the ejecta power-law index n by developing a general trajectory for all n and explaining its relation to the solutions of Chevalier and Nadyozhin for n>5 and Hamilton & Sarazin for n=0. We demonstrate excellent agreement between our analytic solutions and numerical simulations. These solutions should be valuable in describing remnants such as SN 1006, Tycho, Kepler, Cassiopeia A, and other relatively young SNRs that are between the early ejecta-dominated stage and the late Sedov-Taylor stage. In appendices, we extend our results to power-law ambient media, and we describe an early period of the evolution in which the SNR is radiative and evolves according to a nonunified solution.

A Three-dimensional Diagnostic Diagram for Seyfert 2 Galaxies: Probing X-Ray Absorption and Compton Thickness

Abstract: We present and discuss a three-dimensional diagnostic diagram for Seyfert 2 galaxies obtained using X-ray and [O III] data on a large sample of objects (reported in the Appendix). The diagram shows the Kα iron line equivalent width as a function of both the column density derived from the photoelectric cutoff and the 2-10 keV flux normalized to the [O III] optical-line flux (the latter corrected for extinction and assumed to be a true indicator of the source intrinsic luminosity). We find that the hard X-ray properties of type 2 objects depend on a single parameter, the absorbing column density along the line of sight, in accordance with the unified model. The diagram can be used to identify Compton-thick sources and to isolate and study peculiar objects. From this analysis we have obtained a column density distribution of Seyfert 2 galaxies that is thought to be a good approximation of the real distribution. A large population of heavily absorbed objects is discovered, including many Compton-thick candidates. Our results indicate that the mean log NH cm-2 in type 2 Seyfert galaxies is 23.5 and that as many as 23%-30% of sources have NH ≥ 1024 cm-2.

The Fourth BATSE Gamma-Ray Burst Catalog (Revised)

Abstract: The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) has triggered on 1637 cosmic gamma-ray bursts between 1991 April 19 and 1996 August 29. These events constitute the Fourth BATSE burst catalog. The current version (4Br) has been revised from the version first circulated on CD-ROM in 1997 September (4B) to include improved locations for a subset of bursts that have been reprocessed using additional data. A significant difference from previous BATSE catalogs is the inclusion of bursts from periods when the trigger energy range differed from the nominal 50-300 keV. We present tables of the burst occurrence times, locations, peak fluxes, fluences, and durations. In general, results from previous BATSE catalogs are confirmed here with greater statistical significance.

Dense Cores Mapped in Ammonia: A Database

Abstract: We present a database of 264 cores mapped in the (J,K) = (1,1) and (2,2) lines of NH3. We list the core gas properties?peak positions, total ammonia column densities, intrinsic line widths, kinetic temperatures, volume densities, core sizes, aspect ratios, and velocity gradients, as well as the properties of associated young stellar objects (YSOs)?associated IRAS sources along with their luminosities and core-YSO distances, outflow velocities, and SIMBAD and cluster associations. We also present the results of our statistical analysis and enumerate important pairwise correlations among the various gas and YSO properties. The results indicate that the association of stellar clusters with star-forming cores has a greater impact on their properties than does the presence of associated YSOs within these cores, although the latter influence is also statistically significant. In other words, the difference in core properties (nonthermal line widths, kinetic temperatures, and core sizes) between cores with and without associated YSOs is less significant when compared with the difference in these properties between cores with and without cluster associations. Furthermore, core gas and YSO properties show a significant dependence on the star-forming region in which the core is located. For instance, cores in Orion have larger line widths, higher kinetic temperatures, and larger sizes compared with cores in Taurus. Similarly, YSOs in Orion are more luminous than those in Taurus. These cluster and regional dependencies seem important enough that they ought to be accounted for in any self-consistent theory of star formation. Finally, the ratio of starless to stellar cores is too small (8:12 in Taurus, 2:41 in Orion A) to be consistent with ambipolar diffusion timescales that predict ratios as high as 3-30. This result is true even for regions that are known to be well surveyed and not to suffer from significant sample biases.

A Comprehensive Spectral and Variability Study of Narrow-Line Seyfert 1 Galaxies Observed by ASCA. II. Spectral Analysis and Correlations

Abstract: I present a comprehensive and uniform analysis of 25 ASCA observations from 23 narrow-line Seyfert 1 galaxies (NLS1s). The spectral analysis and correlations are presented in this paper; the reduction and time series analysis are presented in the companion paper. A maximum likelihood analysis confirms that the hard X-ray photon index is significantly steeper at more than 90% confidence in this sample of NLS1s compared with a random sample of Seyfert galaxies with broad optical lines. Soft excess emission was detected in 17 of the 19 objects that had no significant absorption, a result that demonstrates that soft excesses appear considerably more frequently in NLS1s than in Seyfert 1 galaxies with broad optical lines. The strength of the soft excess, parameterized using a flux ratio obtained from the blackbody plus power-law model, has a wide range of values in these objects (a factor of 50). The photon index was found to be correlated with the Hβ FWHM, despite the small range of the latter parameter; however, neither parameter is correlated with the strength of the soft excess or ROSAT slope. Therefore, assuming that an excess of soft photons causes the steep photon index and narrow Hβ FWHM, that excess may lie primarily in the unobservable EUV. The strength of the soft excess is correlated with the variability parameters, so that objects with strong soft excesses show higher amplitude variability; this potentially important result is not easily explained. While a range of 2 orders of magnitude in luminosity is represented, the temperature of the soft excess is approximately consistent throughout the sample, in contrast with expectations of simple accretion disk models. The presence of ionized absorption was sought using a two-edge model. It was found that this component appears to be typically less common in NLS1s and some evidence was found that the typical ionization state is lower compared with broad-line Seyfert galaxies. This fact, plus evidence for a correlation between the presence of the warm absorber and significant optical polarization, may imply that the inner warm absorber is missing or is too highly ionized to be detected, and only the outer, dusty, less ionized warm absorber is present in many cases. The iron line equivalent width appears to be similar among narrow and broad-line Seyfert galaxies. This could mean that reprocessing occurs with similar geometry in both classes of objects; however, the detection of ionized iron lines in a few objects, implying possibly altered fluorescence yields, and poor statistics, makes this conclusion tentative. Constraints on physical processes and models based on extreme values of orientation and accretion rate for NLS1s are examined in light of the observational results.

A Comprehensive Spectral and Variability Study of Narrow-Line Seyfert 1 Galaxies Observed by ASCA. I. Observations and Time Series Analysis

Abstract: I present a comprehensive and uniform analysis of 25 ASCA observations from 23 narrow-line Seyfert 1 galaxies. The time series analysis is presented in this paper, Part 1, and the spectral analysis and correlations are presented in the companion paper, Part 2. Time series analysis shows that the excess variance from the NLS1 light curves is inversely correlated with their X-ray luminosity. However, with a logarithmic slope of ~-0.3, the dependence of the excess variance on luminosity is flat compared with broad-line objects and the expected value of -1 from simple models. At a particular X-ray luminosity, the excess variance is typically an order of magnitude larger for NLS1s than for Seyfert 1 with broad optical lines. There is, however, a large scatter, and a few objects show an even larger excess variance. The excess variance can be interpreted as a timescale if the shape of the variability power spectrum, the length of the observation, and the window function are the same for all observations, and the properties of the sample objects are shown to be roughly consistent with this requirement. In particular, no strong evidence for changes during an observation in the shape or normalization of the power spectrum was found once the systematic errors due to the 1/f nature of the power spectrum was accounted for properly. Some of the more variable light curves are shown to be inconsistent with a linear, Gaussian process, implying that the process is non-Gaussian. It is possible that the process is nonlinear, but while the distinction between these possibilities is very important for differentiating between models, such a distinction cannot be made using these data. The enhanced excess variance exhibited by NLS1s can be interpreted as evidence that they are scaled-down versions of broad-line objects, having black hole masses roughly an order of magnitude smaller and requiring an accretion rate an order of magnitude higher. Alternatively, NLS1s may exhibit an inherently different type of variability, characterized by high-amplitude flares, in which case a smaller black hole mass would not be required.

Integration of Nuclear Reaction Networks for Stellar Hydrodynamics

Abstract: Methods for solving the stiff system of ordinary differential equations that constitute nuclear reaction networks are surveyed. Three semi-implicit time integration algorithms are examined; a traditional first-order-accurate Euler method, a fourth-order-accurate Kaps-Rentrop method, and a variable-order Bader-Deuflhard method. These three integration methods are coupled to eight different linear algebra packages. Four of the linear algebra packages operate on dense matrices (LAPACK, LUDCMP, LEQS, GIFT), three of them are designed for the direct solution of sparse matrices (MA28, UMFPACK, Y12M), and one uses an iterative method for sparse matrices (BiCG). The scaling properties and behavior of the 24 combinations (3 time integration methods times 8 linear algebra packages) are analyzed by running each combination on seven different nuclear reaction networks. These reaction networks range from a hardwired 13 isotope α-chain and heavy-ion reaction network, which is suitable for most multidimensional simulations of stellar phenomena, to a 489 isotope reaction network, which is suitable for determining the yields of isotopes lighter than technetium in spherically symmetric models of Type II supernovae. Each of the time integration methods and linear algebra packages are capable of generating accurate results, but the efficiency of the various methods—evaluated across several different machine architectures and compiler options—differ dramatically. If the execution speed of reaction networks that contain less than about 50 isotopes is an overriding concern, then the variable-order Bader-Deuflhard time integration method coupled with routines generated from the GIFT matrix package or LAPACK with vendor-optimized BLAS routines is a good choice. If the amount of storage needed for any reaction network is a concern, then any of the sparse matrix packages will reduce the storage costs by 70%-90%. When a balance between accuracy, overall efficiency, and ease of use is desirable, then the variable-order Bader-Deuflhard time integration method coupled with the MA28 sparse matrix package is a strong choice.

A Catalog of Optically Selected Cores

Abstract: We present a new catalog of 406 dense cores optically selected by using the STScI Digitized Sky Survey (DSS). In this catalog 306 cores have neither an embedded young stellar object (EYSO) nor a pre-main-sequence (PMS) star, 94 cores have EYSOs (one core has both an EYSO and a PMS star), and six cores have PMS stars only. Our sample of dense cores in the catalog is fairly complete within a category of northern Lynds class 5 and 6 clouds and southern Hartley et al. class A clouds, providing a database useful for the systematic study of dense cores. Most of the cores listed in the catalog have diameters between 0.05 and 0.36 pc with a mean of ~0.24 pc. The sizes (~0.33 pc in the mean) of cores with EYSOs are found to be usually larger than the sizes (~0.22 pc in the mean) of starless cores. The typical mean gas density of the cores is ~7 ? 103 cm-3. Most of the cores are more likely elongated than spherical (mean aspect ratio: ~2.4). The ratio of the number of cores with EYSOs to the number of starless cores for our sample is about 0.3, suggesting that the typical lifetime of starless cores is 0.3-1.6 Myr, about 3 times longer than the duration of the class 0 and class I phases. This lifetime is shorter than expected from models of ambipolar diffusion, by factors of 2-44.

The CfA Redshift Survey: Data for the South Galactic Cap*

Abstract: We present redshifts for a sample of 4391 galaxies with magnitude mB(0) ≤ 15.5 covering 20h ≤ α ≤ 4h in right ascension and -25 ≤ δ ≤ 90° in declination. This sample is complete for all galaxies in the merge of the Zwicky et al. and Nilson catalogs in the south Galactic cap. Redshifts for 2964 of these were measured as part of the second CfA Redshift Survey. The data reveal large voids in the foreground and background of the Perseus-Pisces Supercluster. The largest of these voids lies at a mean velocity ~8000 km s-1, has diameter ~5000 km s-1, and is enclosed by a complex of dense structures. The large structure known as the Perseus-Pisces Supercluster forms the near side of this complex. On the far side of this large void, at a mean velocity ~12,000 km s-1, there is another coherent dense wall. The structures in this survey support the view that galaxies generally lie on surfaces surrounding or nearly surrounding low-density regions or voids.

Dense Molecular Clouds in the Galactic Center Region. I. Observations and Data

Abstract: We have observed a 160' × 40' (l × b) region around the Galactic center in the CS J=1-0 line with a high spatial resolution and high sensitivity using the 45 m telescope at Nobeyama Radio Observatory. The observed area includes the Sgr A radio arc complex, Sgr B, Sgr C, and Sgr D. About 24,000 spectra of CS J = 1-0 emission, which is a good tracer of high-density (n 104 cm-3) molecular clouds, were obtained. The observed data are presented in l-b, l-v, and b-v maps. The total CS line flux in this area is -->∫ S dv = 1.1 × 106 Jy km s-1. Typical optical thickness of C32S J = 1-0 emission is equal to or lower than τ=2-3. The total molecular mass in the Galactic center region is estimated to be M(H2) = (3-8) × 107 M☉ from the assumption of LTE. One-third of the molecular clouds in the Galactic center region are involved in several continuous curved ridges extended along the Galactic longitude. The most prominent ridge has a bow-like structure with a length of 300 pc.

The Accuracy, Consistency, and Speed of Five Equations of State for Stellar Hydrodynamics

Abstract: We compare the thermodynamic properties and execution speed of five independent equations of state. A wide range of temperatures, densities, and compositions are considered—conditions appropriate for modeling the collapse of a cloud of hydrogen gas (or an exploding supernova) to the outer layers of a neutron star. The pressures and specific thermal energies calculated by each equation-of-state routine are reasonably accurate (typically 0.1% error or less) and agree remarkably well with each other, despite the different approaches and approximations used in each routine. The derivatives of the pressure and specific thermal energies with respect to the temperature and density generally show less accuracy (typically 1% error or less) and more disagreement with one another. Thermodynamic consistency, as measured by deviations from the appropriate Maxwell relations, shows that the Timmes equation of state and the Nadyozhin equation of state achieve thermodynamic consistency to a high degree of precision. The execution speed of the five equation-of-state routines—evaluated across several different machine architectures, compiler options, and modes of operation—differ dramatically. The Arnett equation of state is the fastest of the five routines, with the Nadyozhin equation of state close behind.

Electron Energy Deposition in a Gas Mixture of Atomic and Molecular Hydrogen and Helium

Abstract: A detailed discussion is presented of the slowing down of fast electrons in a partly ionized gas mixture of H, H2, and He with a varying ratio of H to H2 and a fixed fraction of He. The mean energies and yields for ionization, electronic excitation, dissociation, and vibrational excitation and the heating efficiencies are calculated for fractional ionizations from 0 to 0.1. Analytical formulas are obtained for many of the parameters.

Chemical Abundances of the Damped Lyα Systems at z > 1.5

Abstract: We present chemical abundance measurements for 19 damped Lyα systems observed with the high-resolution echelle spectrograph (HIRES) on the 10 m W.M. Keck Telescope. We perform a detailed analysis of every system, deriving ionic column densities for all unblended metal-line transitions. Our principal goal is to investigate the abundance patterns of the damped systems and thereby determine the underlying physical processes that dominate their chemical evolution. We place particular emphasis on gauging the relative importance of two complementary effects often invoked to explain the damped Lyα abundances (1) nucleosynthetic enrichment from Type II supernovae and (2) an interstellar medium-like (ISM-like) dust-depletion pattern. Similar to the principal results of Lu et al., our observations lend support both for dust depletion and Type II supernova (SN) enrichment. Specifically, the observed overabundance of Zn/Fe and underabundance of Ni/Fe relative to solar abundances suggest significant dust depletion within the damped Lyα systems. Meanwhile, the relative abundances of Al, Si, and Cr versus Fe are consistent with both dust depletion and Type II supernova enrichment. Our measurements of Ti/Fe and the Mn/Fe measurements from Lu et al., however, cannot be explained by dust depletion and indicate an underlying Type II SN pattern. Finally, the observed values of [S/Fe] are inconsistent with the combined effects of dust depletion and the nucleosynthetic yields expected for Type II supernovae. This last result emphasizes the need for another physical process to explain the damped Lyα abundance patterns. We also examine the metallicity of the damped Lyα systems both with respect to Zn/H and Fe/H. Our results confirm previous surveys by Pettini and collaborators, i.e., [ Zn/H ] = -1.15±0.15 dex. In contrast with other damped Lyα surveys at z > 1.5, we do not formally observe an evolution of metallicity with redshift, although we stress that this result is based on the statistics from a small sample of high-z damped systems.

CO Images of the Central Regions of 20 Nearby Spiral Galaxies

Abstract: We report a CO(J = 1-0) aperture-synthesis survey of the central regions of 20 nearby spiral galaxies. The sample is selected on the basis of inclination, single-dish CO flux, and lack of significant dynamical perturbation. No selection is made on the basis of their nuclear activity, starburst, or infrared luminosity. The observations have been made using the Nobeyama and Owens Valley millimeter arrays, with typical resolutions of 4'' (300 pc) and 20 km s-1, and sensitivities of ~30 mJy beam-1 for a 20 km s-1 channel. In this paper, we present integrated intensity and mean velocity maps, azimuthally averaged radial distributions, and position-velocity plots. Most galaxies in the sample have scale lengths for the radial CO distribution that are much smaller in the nuclear regions than in the outer disks. Typically, the CO emission falls to 1/e of the central peak value at radii of ~500 pc. This is significantly smaller than the scale lengths for the global CO distributions in the galactic disks. CO position-velocity diagrams exhibit a steep rise in rotation velocity, dV/dr ~ 1 km s-1 pc-1, in the central regions of most galaxies. The CO line width typically reaches ~95% of the overall H I line width within a radius of 1 kpc. This steep rise in the rotation velocity mainly reflects the highly concentrated mass distributions in the galactic centers, though it may well be partly due to noncircular motions in barred galaxies. In a number of galaxies, the position-velocity diagrams show a small central hole (d 100 pc) in CO emission. In many galaxies, we detect CO clumps of subkiloparsec size which may be giant molecular associations (GMAs). In the Appendix, we demonstrate that small holes at the galactic center can be readily detected in position-velocity diagrams even when they are smaller than the spatial resolution of the data.

The Diffuse Interstellar Clouds toward 23 Orionis

Abstract: Spectra obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph are combined with high-resolution optical spectra and UV spectra from Copernicus to study the abundances and physical conditions in the diffuse interstellar clouds seen along the line of sight to the star 23 Ori. Multiple absorption components are present for each of several distinct types of gas, which are characterized by different relative abundance and depletion patterns and physical conditions.?????Strong low-velocity (SLV) absorption, due to cool, moderately dense neutral gas and representing about 92% of the total N(H I), is seen for various neutral and singly ionized species at +20 km s-1 v? +26 km s-1. Most typically severely depleted species are less depleted by factors of 2-4, compared to the cold, dense cloud pattern found, for example, in the main components toward ? Oph.For the two strongest SLV components, T ~ 100 K and the thermal pressure log (nHT) ~ 3.1 cm-3 K; we thus have nH ~ 10-15 cm-3 and a total thickness of 12-16 pc. The adopted average SLV electron density, ne = 0.15 ? 0.05 cm-3, implies a relatively large ne/nH ~ 0.01 and thus some ionization of hydrogen in these predominantly neutral components.?????Weaker low-velocity (WLV) absorption, probably largely due to warmer neutral gas, is seen primarily for various singly ionized species at 0 km s-1 v? +30 km s-1. The depletions in the WLV gas are typically less severe by a factor of 2-3 than in the SLV gas and are somewhat similar to the warm cloud pattern seen in lines of sight with low reddening, low mean density, and/or low molecular fraction. If T ~ 3000 K for the WLV components, then we have log(nHT) ~ 4.7-4.8 cm-3 K, nH ~ 15-20 cm-3, ne ~ 0.2 cm-3, ne/nH ~ 0.01, and a total thickness of 0.7-0.9 pc.?????Absorption from a number of singly and doubly ionized species, perhaps due to a radiative shock, is seen at -108 km s-1 v? -83 km s-1. While the depletions in these ionized components are uncertain owing to unobserved ionization stages, aluminum (typically severely depleted) is probably depleted there by only a factor ~3, even at cloud velocities in excess of 100 km s-1. The individual high-velocity components typically have T ~ 8000 ? 2000 K, ne = nH ~ 0.4-0.5 cm-3, thermal pressure log(2neT) ~ 3.7-4.0 cm-3 K, and thicknesses of order 0.1 pc.?????Weak absorption components from ionized (H II) gas are seen in C II, Mg II, and Si III at intermediate velocities (-43 km s-1 v? -4 km s-1). Broad, weak absorption from the higher ions S III, C IV, Si IV, and N V is centered at -5 km s-1 v? +6 km s-1. No obvious absorption is discerned from a circumstellar H II region around 23 Ori itself.?????The large range in ne (from 0.04 to 0.95 cm-3) derived independently from nine pairs of neutral and singly ionized species in the SLV gas suggests that additional processes besides simple photoionization and radiative recombination affect the ionization balance. Charge exchange with protons may reduce the abundances of S I, Mn I, and Fe I; dissociative recombination of CH+ may help to enhance C I. The large apparent fractional ionization in the SLV and WLV gas may be due to an enhanced flux of X-rays in the Orion region, to mixing of neutral and ionized gas at the boundary of the Orion-Eridanus bubble, or perhaps (in part) to charge exchange between singly ionized atomic species and large molecules (in which case the true ne would be somewhat smaller). Comparisons of the SLV depletions and nH with those found for the strong component B (v? ~ -14 km s-1) blend toward ? Oph hint at a possible relationship between depletion and local density for relatively cold interstellar clouds. Calcium appears to be more severely depleted in warm, low density gas than has generally been assumed.?????An appendix summarizes the most reliable oscillator strengths currently available for a number of the interstellar absorption lines analyzed in this work.

The Type Ia Supernova 1998bu in M96 and the Hubble Constant

Abstract: We present optical and near-infrared photometry and spectroscopy of the Type Ia SN 1998bu in the Leo I Group galaxy M96 (NGC 3368). The data set consists of 356 photometric measurements and 29 spectra of SN 1998bu between UT 1998 May 11 and July 15. The well-sampled light curve indicates the supernova reached maximum light in B on UT 1998 May 19.3 (JD 2450952.8 ± 0.8) with B = 12.22 ± 0.03 and V = 11.88 ± 0.02. Application of a revised version of the Multicolor Light Curve Shape (MLCS) method yields an extinction toward the supernova of AV = 0.94 ± 0.15 mag, and indicates the supernova was of average luminosity compared to other normal Type Ia supernovae. Using the HST Cepheid distance modulus to M96 and the MLCS fitted parameters for the supernova, we derive an extinction-corrected absolute magnitude for SN 1998bu at maximum, MV = -19.42 ± 0.22. Our independent results for this supernova are consistent with those of Suntzeff et al. Combining SN 1998bu with three other well-observed local calibrators and 42 supernovae in the Hubble flow yields a Hubble constant, H0 = 64 -->img1.gif km s-1 Mpc-1, where the error estimate incorporates possible sources of systematic uncertainty including the calibration of the Cepheid period-luminosity relation, the metallicity dependence of the Cepheid distance scale, and the distance to the LMC.

Synthetic Spectra of H Balmer and He I Absorption Lines. II. Evolutionary Synthesis Models for Starburst and Poststarburst Galaxies

Abstract: We present evolutionary stellar population synthesis models to predict the spectrum of a single-metallicity stellar population, with a spectral sampling of 0.3 A in five spectral regions between 3700 and 5000 A. The models, which are optimized for galaxies with active star formation, synthesize the profiles of the hydrogen Balmer series (Hβ, Hγ, Hδ, H8, H9, H10, H11, H12, and H13) and the neutral helium absorption lines (He I λ4922, He I λ4471, He I λ4388, He I λ4144, He I λ4121, He I λ4026, He I λ4009, and He I λ3819) for a burst with an age ranging from 106 to 109 yr, and different assumptions about the stellar initial mass function (IMF). Continuous star formation models lasting for 1 Gyr are also presented. The input stellar library includes non-LTE absorption profiles for stars hotter than 25,000 K and LTE profiles for lower temperatures. The temperature and gravity coverage is 4000 K ≤ Teff ≤ 50,000 K and 0.0 ≤ log g ≤ 5.0, respectively. The metallicity is solar. It is found that the Balmer and He I line profiles are sensitive to the age, except during the first 4 Myr of the evolution, when the equivalent widths of these lines are constant. At these early stages of the evolution, the profiles of the lines are also sensitive to the IMF. However, strong H Balmer and He I lines are predicted even when the low-mass cutoff of the IMF is as high as 10 M☉. The equivalent widths of the Balmer lines range from 2 to 16 A and those of the He I lines from 0.2 to 1.2 A. During the nebular phase (cluster younger than about 10 Myr), Hβ ranges from 2 to 5 A and He I λ4471 ranges between 0.5 and 1.2 A. The strength of the lines is maximum when the cluster is a few hundred (for the Balmer lines) and a few tens (for the He I lines) of Myr old. In the continuous star formation scenario, the strength of the Balmer and He I lines increases monotonically with time until 500 and 100 Myr, respectively. However, the lines are weaker than in the burst models owing to the dilution of the Balmer and He I lines by the contribution from very massive stars. The high spectral resolution of the profiles is useful to reproduce the absorption wings observed in regions of recent star formation and to estimate the effect of the underlying absorption on the nebular emission lines. The strength of the nebular emission Balmer and He I lines compared with the stellar absorption components indicates that Hδ and the higher order terms of the Balmer series and He I are dominated by the stellar absorption component if an instantaneous burst is older than 5 Myr. Some of the He I lines (e.g., He I λ3819 and He I λ4922) are more favorable than others (e.g., He I λ4471) for the detection of stellar features in the presence of nebular emission. We estimate that the correction to the He I λ4471 nebular emission line due to the stellar absorption is between 5% and 25%, if the nebular emission has equivalent width between 10 and 2.5 A (corresponding to a burst age between 1 and 3 Myr). The models can be used to date starburst and poststarburst galaxies until 1 Gyr. They have been tested on data for clusters in the LMC, the super-star cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical NGC 205 and a luminous "E+A" galaxy. The full data set is available for retrieval at our websites or on request from the authors.

Radio Structures of Seyfert Galaxies. VIII. A Distance- and Magnitude-Limited Sample of Early-Type Galaxies

Abstract: The VLA has been used at 3.6 and 20 cm to image a sample of about 50 early-type Seyfert galaxies with recessional velocities less than 7000 km s-1 and total visual magnitude less than 14.5. Emission-line ([O III] and Hα+[N II]) and continuum (green and red) imaging of this sample has been presented in a previous paper. In this paper, we present the radio results, discuss statistical relationships between the radio and other properties, and investigate these relationships within the context of unified models of Seyferts. The mean radio luminosities of early-type Seyfert 1's (i.e., Seyfert 1.0's, 1.2's, and 1.5's) and 2.0's are found to be similar (consistent with the unified scheme), and the radio luminosity is independent of morphological type within this sample. The fraction of resolved radio sources is larger in the Seyfert 2.0's (93%) than in the Seyfert 1's (64%). However, the mean radio extents of Seyfert 2.0's and 1's are not significantly different, although this result is limited by the small number of resolved Seyfert 1's. The nuclear radio structures of Seyfert 2.0's in the early-type sample tend to be aligned with the [O III] and Hα+[N II] structures, even though the radio extents are smaller than the [O III] and Hα+[N II] extents by a factor of ~2 to more than 5. This alignment, previously known for individual Seyferts with "linear" radio sources, is here shown to be characteristic of early-type Seyfert galaxies as a class. Seyfert 2.0's in the early-type sample also show a significant alignment between the emission-line ([O III] and Hα+[N II]) axes and the major axis of the host galaxy. These alignments are consistent with a picture in which the ionized gas represents ambient gas predominantly coplanar with the galaxy stellar disk. This ambient gas is ionized by nuclear radiation that may escape preferentially along and around the radio axis, and is compressed in shocks driven by the radio ejecta. We use this alignment to constrain the product of the velocity of the radio ejecta and the period of any large-angle precession of the inner accretion disk and jet: Vejecta × P ≥ 2 kpc. An investigation of a larger sample of Seyferts reveals the unexpected result that the Seyfert 1's with the largest radio extent (≥1.5 kpc) are all of type Seyfert 1.2. It appears that classification as this type of intermediate Seyfert depends on some factor other than the relative orientation of the nuclear-obscuring torus to the line of sight. Among all the other Seyferts, the distribution of radio extent with Seyfert intermediate type is consistent with the expectations of the unified scheme.

GENESIS: A High-Resolution Code for Three-dimensional Relativistic Hydrodynamics

Abstract: The main features of a three-dimensional, high-resolution special relativistic hydro code based on relativistic Riemann solvers are described. The capabilities and performance of the code are discussed. In particular, we present the results of extensive test calculations that demonstrate that the code can accurately and efficiently handle strong shocks in three spatial dimensions. Results of the performance of the code on single and multiprocessor machines are given. Simulations (in double precision) with ≤7×106 computational cells require less than 1 Gbyte of RAM memory and ≈ 7×10-5 CPU s per zone and time step (on a SCI Cray-Origin 2000 with a R10000 processor). Currently, a version of the numerical code is under development, which is suited for massively parallel computers with distributed memory architecture (such as, e.g., Cray T3E).

Iron Line Diagnostics of the Postshock Hot Plasma in MagneticCataclysmic Variables Observed with ASCA

Abstract: The ASCA spectra of ~20 magnetic cataclysmic variables are presented. Owing to the high spectral resolution of the solid-state imaging spectrometer, we successfully resolved the iron Kα emission line into the fluorescent (6.4 keV) and plasma (6.7 and 7.0 keV) components. By comparing the ionization temperature, which is obtained from the intensity ratio of the plasma line components of iron, with the continuum temperature, we have obtained the evidence that the postshock plasma has a temperature distribution. Detailed analysis indicates that the observed temperature distribution is consistent with that expected from the postshock plasma model in the bremsstrahlung cooling domain. In the framework of this postshock plasma model, we have constrained the mass of the white dwarf in totally nine intermediate polars and obtained the iron abundances. The obtained masses are generally consistent with the previous X-ray work. We have found that the iron abundance is generally subsolar, and its distribution probably peaks at 0.2-0.6 solar. We have also shown that the reflection from the white dwarf surface makes significant contribution to the observed fluorescent iron Kα emission line.

Anelastic Magnetohydrodynamic Equations for Modeling Solar and Stellar Convection Zones

Abstract: The anelastic approximation has strong advantages for numerical simulations of stellar and solar convection zones. The chief and generally known one is that it suppresses acoustic modes, permitting larger simulated time steps to be taken than would be possible in a fully compressible model. This paper clarifies and extends previous work on the anelastic approximation by presenting a new vorticity-based formulation that can be used for two- and three-dimensional MHD simulations. In the new formulation, all fluctuating thermodynamic variables except the entropy are eliminated from the equations. This shows in the plainest way how the anelastic approximation generalizes the Boussinesq approximation, which appears as a special limit. The roots of both models are traced to the mixing-length theory of convection, which establishes the scaling parameters for "deep" (weakly superadiabatic) convection at low Mach numbers. The Ogura & Phillips and the Glatzmaier derivations of the anelastic model are broadened to include a possible depth dependence in the thermodynamic properties of constituent gases. This permits a variable state of gas ionization, for example, which is important for stars like the Sun, in which the convecting regions coincide with the ionization zones of hydrogen and helium. Tests with the new model are presented, in which it is shown that the new model is capable of reproducing earlier results in the linear and nonlinear stages of convection.

The Population of Weak Mg II Absorbers. I. A Survey of 26 QSO HIRES/Keck Spectra* **

Abstract: We present a search for weak Mg II absorbers [those with Wr(2796) (2796)=0.02 A and that this is primarily a high-metallicity selection effect ([Z/Z☉] ≥ -1). This implies that Mg II absorbing structures figure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of Wr(2796) ≥ 0.02 A absorbers with that of both high and low surface brightness galaxies and find a fiducial absorber size of 35 h-1-63 h-1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing clouds have Wr(2796) ≤ 0.15 A, and their narrow (often unresolved) line widths imply temperatures of ~25,000 K. We measured Wr(1548) from C IV in Faint Object Spectrograph/Hubble Space Telescope archival spectra and, based upon comparisons with Fe II, found a range of ionization conditions (low, high, and multiphase) in absorbers selected by weak Mg II.

Polarization of broad absorption line qsos I. a spectropolarimetric atlas

Abstract: We present a spectropolarimetric survey of 36 broad absorption line quasi-stellar objects (BAL QSOs). The continuum, absorption trough, and emission-line polarization of BAL QSOs yield clues about their structure. We confirm that BAL QSOs are in general more highly polarized than non-BAL QSOs, consistent with a more equatorial viewing direction for the former than the latter. We have identified two new highly polarized QSOs in our sample (1232+1325 and 1333+2840). The polarization rises weakly to the blue in most objects, perhaps owing to scattering and absorption by dust particles. We find that a polarization increase in the BAL troughs is a general property of polarized BAL QSOs, indicating an excess of scattered light relative to direct light, and consistent with the unification of BAL QSOs and non-BAL QSOs. We have also discovered evidence of resonantly scattered photons in the red wing of the C IV broad emission lines of a few objects. In most cases, the broad emission lines have lower polarization and a different position angle than the continuum. The polarization characteristics of low-ionization BAL QSOs are similar to those of high-ionization BAL QSOs, suggesting a similar BAL wind geometry.

Numerical Simulations of Fe II Emission Spectra

Abstract: This paper describes the techniques that we have used to incorporate a large-scale model of the Fe+ ion and resulting Fe II emission into CLOUDY, a spectral synthesis code designed to simulate conditions within a plasma and model the resulting spectrum. We describe the numerical methods we use to determine the level populations, mutual line overlap fluorescence, collisional effects, and the heating-cooling effects of the atom on its environment. As currently implemented, the atom includes the lowest 371 levels (up to 11.6 eV) and predicts intensities of 68,635 lines. We describe our data sources, which include the most recent transition probabilities and collision strengths. Although we use detailed fits to temperature-dependent collision strengths where possible, in many cases the uncertain approximation is the only source for collision data. The atom is designed to be readily expanded to include more levels and to incorporate more accurate sets of collision and radiative data as computers grow faster and the atomic databases expand. We present several test cases showing that the atom goes to LTE in the limits of high particle and radiation densities. We give an overview of general features of the Fe II spectra as their dependencies on the basic parameters of our models (density, flux, microturbulent velocity, the Fe abundance, and Lyα pumping). Finally, we discuss several applications to active galactic nuclei to illustrate the diagnostic power of the Fe II spectrum and make some predictions for UV observations.

Classical Cepheid Pulsation Models. I. Physical Structure

Abstract: The pulsation properties and modal stability of Cepheid models are investigated in both linear and nonlinear regimes. The linear survey is based on nonadiabatic, radiative models, whereas the nonlinear one relies on full-amplitude models that include a nonlocal and time-dependent treatment of stellar convection. To account for Cepheid pulsation characteristics over a substantial portion of the region in which they are expected to be pulsationally unstable, a wide range of stellar masses (5?M/M${s}$ -->?11) and effective temperatures (4000?T -->e?7000 K) was adopted. The luminosity of each model was fixed according to the mass-luminosity (ML) relations predicted by evolutionary models that either neglect or take into account a mild convective core overshooting. Moreover, in order to estimate the effects of the helium and metal content on the limiting amplitude behavior of both Magellanic Clouds and Galactic Cepheids we adopted three different chemical compositions, namely, Y=0.25, Z=0.004; Y=0.25, Z=0.008; and Y=0.28, Z=0.02. For each set of input parameters we investigated the modal stability of both fundamental and first-overtone modes. The results of recent linear investigations are confirmed by our finding that linear observables such as periods and blue edges of the instability strip are only marginally affected by the chemical composition and that either an increase in metallicity or an increase in both the helium and metal content causes a mild shift of these edges toward lower effective temperatures. The approach to the nonlinear limit cycle stability, the physical structure, and the mechanisms that govern the pulsation instability are described in detail. The main results of this analysis are as follows: (1) At fixed chemical composition the width of the instability strip changes going from low- to high-mass Cepheids. (2) At fixed mass and luminosity an increase in metallicity shifts the instability strip toward lower effective temperatures. A thorough analysis of the total nonlinear work inside the instability strip points out that this effect is due to a decrease in the pulsation destabilization caused by the H ionization region. Therefore, the current theoretical scenario suggests that, at fixed period, metal-poor pulsators are brighter than metal-rich pulsators. (3) The dynamical structure of full-amplitude, first-overtone models supports the evidence that their nonlinear limit cycle behavior has been properly identified. The variations over a full pulsation cycle of the convective structure of fundamental and first-overtone pulsators located close to the blue and the red edges of the instability strip are discussed by taking into account the changes of the convective quantities across the convectively unstable region. As expected, we find that the main effect of convection on the limit cycle behavior is either to reduce the local radiative driving of the destabilizing regions, thus reducing the final amplitudes, or to damp the oscillations toward lower effective temperatures. We also find that the limiting amplitude behavior of high-mass, high-amplitude fundamental pulsators, and in particular the appearance of secondary features along their light and velocity curves, is tightly connected with the convection/pulsation interaction. By comparing the convective velocity perturbations close to the surface layers with the turbulent velocities obtained by spectroscopic measurements we find that toward lower effective temperatures both the absolute values of the convective velocity and its variation over the cycle agree reasonably well with observational data. However, the time behavior of the convective velocity in blue models and the strong decrease of this velocity predicted at low optical depths out of the maximum compression phases are presently not confirmed by observations. Both theoretical and observational shortcomings that could explain such a discrepancy are briefly discussed. The comparison between the linear periods currently adopted in the literature and the nonlinear periods obtained in this investigation shows a very good agreement in the mass range from 5 to 9 M?, whereas at 11 M? we find that linear, nonadiabatic, convective periods are systematically shorter than the nonlinear ones. Finally, the drawbacks of adopting linear observables for constraining the actual properties of Cepheids are discussed.

Axisymmetric Three-Integral Models for Galaxies

Abstract: We describe an improved, practical method for constructing galaxy models that match an arbitrary set of observational constraints, without prior assumptions about the phase-space distribution function (DF). Our method is an extension of Schwarzschild's orbit superposition technique. As in Schwarzschild's original implementation, we compute a representative library of orbits in a given potential. We then project each orbit onto the space of observables, consisting of position on the sky and line-of-sight velocity, while properly taking into account seeing convolution and pixel binning. We find the combination of orbits that produces a dynamical model that best fits the observed photometry and kinematics of the galaxy. A new element of this work is the ability to predict and match to the data the full line-of-sight velocity profile shapes. A dark component (such as a black hole and/or a dark halo) can easily be included in the models. In an earlier paper (Rix et al.) we described the basic principles and implemented them for the simplest case of spherical geometry. Here we focus on the axisymmetric case. We first show how to build galaxy models from individual orbits. This provides a method to build models with fully general DFs, without the need for analytic integrals of motion. We then discuss a set of alternative building blocks, the two-integral and the isotropic components, for which the observable properties can be computed analytically. Models built entirely from the two-integral components yield DFs of the form f(E, Lz), which depend only on the energy E and angular momentum Lz. This provides a new method to construct such models. The smoothness of the two-integral and isotropic components also makes them convenient to use in conjunction with the regular orbits. We have tested our method by using it to reconstruct the properties of a two-integral model built with independent software. The test model is reproduced satisfactorily, either with the regular orbits, or with the two-integral components. This paper mainly deals with the technical aspects of the method, while applications to the galaxies M32 and NGC 4342 are described elsewhere (van der Marel et al.; Cretton & van den Bosch).