Showing papers in "Astrophysical Journal Supplement Series in 2000"

Flash: An adaptive mesh hydrodynamics code for modeling astrophysical thermonuclear flashes

Abstract: We report on the completion of the first version of a new-generation simulation code, FLASH. The FLASH code solves the fully compressible, reactive hydrodynamic equations and allows for the use of adaptive mesh refinement. It also contains state-of-the-art modules for the equations of state and thermonuclear reaction networks. The FLASH code was developed to study the problems of nuclear flashes on the surfaces of neutron stars and white dwarfs, as well as in the interior of white dwarfs. We expect, however, that the FLASH code will be useful for solving a wide variety of other problems. This first version of the code has been subjected to a large variety of test cases and is currently being used for production simulations of X-ray bursts, Rayleigh-Taylor and Richtmyer-Meshkov instabilities, and thermonuclear flame fronts. The FLASH code is portable and already runs on a wide variety of massively parallel machines, including some of the largest machines now extant.

The Accuracy, Consistency, and Speed of an Electron-Positron Equation of State Based on Table Interpolation of the Helmholtz Free Energy

Abstract: An electron-positron equation of state based on table interpolation of the Helmholtz free energy is developed and analyzed. The interpolation scheme guarantees perfect thermodynamic consistency, independent of the interpolating function. The choice of a biquintic Hermite polynomial as the interpolating function results in accurately reproducing the underlying Helmholtz free energy data in the table, and yields derivatives of the pressure, specific entropy, and specific internal energy which are smooth and continuous. The execution speed—evaluated across several different machine architectures, compiler options, and modes of operation—suggests that the Helmholtz equation of state routine is faster than any of the five equation of state routines surveyed by Timmes & Arnett. When an optimal balance of accuracy, thermodynamic consistency, and speed is desirable then the tabular Helmholtz equation of state is an excellent choice, particularly for multidimensional models of stellar phenomena.

The BATSE Gamma-Ray Burst Spectral Catalog. I. High Time Resolution Spectroscopy of Bright Bursts Using High Energy Resolution Data

Abstract: This is the first in a series of gamma-ray burst spectroscopy catalogs from the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory, each covering a different aspect of burst phenomenology. In this paper, we present time sequences of spectral fit parameters for 156 bursts selected for either their high peak flux or fluence. All bursts have at least eight spectra in excess of 45 σ above background and span burst durations from 1.66 to 278 s. Individual spectral accumulations are typically 128 ms long at the peak of the brightest events but can be as short as 16 ms, depending on the type of data selected. We have used mostly high energy resolution data from the Large Area Detectors, covering an energy range of typically 28-1800 keV. The spectral model chosen is from a small empirically determined set of functions, such as the well-known "GRB" function, that best fits the time-averaged burst spectra. Thus, there are generally three spectral shape parameters available for each of the 5500 total spectra: a low-energy power-law index, a characteristic break energy, and possibly a high-energy power-law index. We present the distributions of the observed sets of these parameters and comment on their implications. The complete set of data that accompanies this paper is necessarily large and thus is archived in the electronic edition of the Astrophysical Journal.

Absorption-Line Probes of Gas and Dust in Galactic Superwinds

Abstract: We have obtained moderate resolution (R = few thousand) spectra of the Na I λλ5890, 5896 (Na D) absorption line in a sample of 32 far-IR-bright starburst galaxies. In 18 cases, the Na D line in the nucleus is produced primarily by interstellar gas, while cool stars contribute significantly in the others. In 12 of the 18 "interstellar-dominated" cases the Na D line is blueshifted by over 100 km s-1 relative to the galaxy systemic velocity (the "outflow sources"), while no case shows a net redshift of more than 100 km s-1. The absorption-line profiles in these outflow sources span the range from near the galaxy systemic velocity to a maximum blueshift of ~400-600 km s-1. The outflow sources are galaxies systematically viewed more nearly face-on than the others. We therefore argue that the absorbing material consists of ambient interstellar material that has been entrained and accelerated along the minor axis of the galaxy by a hot starburst-driven superwind. The Na D lines are optically thick, but indirect arguments imply total hydrogen column densities of NH ~ few × 1021 cm-2. This implies that the superwind is expelling matter at a rate comparable to the star formation rate. This outflowing material is evidently very dusty: we find a strong correlation between the depth of the Na D profile and the line-of-sight reddening. Typical implied values are E(B-V) = 0.3-1 over regions several-to-10 kpc in size. We briefly consider some of the potential implications of these observations. The estimated terminal velocities of superwinds inferred from the present data and extant X-ray data are typically 400-800 km-1, are independent of the galaxy rotation speed, and are comparable to (substantially exceed) the escape velocities for L* (dwarf) galaxies. The resulting selective loss of metals from shallower potential wells can establish the mass-metallicity relation in spheroids, produce the observed metallicity in the intracluster medium, and enrich a general IGM to of order 10-1 solar metallicity. If the outflowing dust grains can survive their journey into the IGM, their effect on observations of cosmologically distant objects would be significant.

The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey. I. X-Ray Properties of Clusters Detected as Extended X-Ray Sources*

Abstract: In the construction of an X-ray-selected sample of galaxy clusters for cosmological studies, we have assembled a sample of 495 X-ray sources found to show extended X-ray emission in the first processing of the ROSAT All-Sky Survey. The sample covers the celestial region with declination ? ? 0? and Galactic latitude |bII| ? 20? and comprises sources with a count rate ?0.06 counts s-1 and a source extent likelihood of L ? 7. In an optical follow-up identification program we find 378 (76%) of these sources to be clusters of galaxies.?????It was necessary to reanalyze the sources in this sample with a new X-ray source characterization technique to provide more precise values for the X-ray flux and source extent than obtained from the standard processing. This new method, termed growth curve analysis (GCA), has the advantage over previous methods in its ability to be robust, to be easy to model and to integrate into simulations, to provide diagnostic plots for visual inspection, and to make extensive use of the X-ray data. The source parameters obtained assist the source identification and provide more precise X-ray fluxes. This reanalysis is based on data from the more recent second processing of the ROSAT Survey. We present a catalog of the cluster sources with the X-ray properties obtained as well as a list of the previously flagged extended sources that are found to have a noncluster counterpart. We discuss the process of source identification from the combination of optical and X-ray data.?????To investigate the overall completeness of the cluster sample as a function of the X-ray flux limit, we extend the search for X-ray cluster sources to the data of the second processing of the ROSAT Survey for the northern sky region between 9h and 14h in right ascension. We include the search for X-ray emission of known clusters as well as a new investigation of extended X-ray sources. In the course of this search we find X-ray emission from 85 additional Abell clusters and 56 very probable cluster candidates among the newly found extended sources. A comparison of the X-ray cluster number counts of the NORAS sample with the ROSAT-ESO Flux-limited X-Ray (REFLEX) Cluster Survey results leads to an estimate of the completeness of the NORAS sample of ROSAT All-Sky Survey (RASS) I extended clusters of about 50% at an X-ray flux of FX(0.1-2.4 keV) = 3 ? 10-12 ergs s-1 cm-2. The estimated completeness achieved by adding the supplementary sample in the study area amounts to about 82% in comparison to REFLEX. The low completeness introduces an uncertainty in the use of the sample for cosmological statistical studies that will be cured with the completion of the continuing Northern ROSAT All-Sky (NORAS) Cluster Survey project.

Type Ia Supernova Explosions in Binary Systems: The Impact on the Secondary Star and Its Consequences

Abstract: One method of discriminating between the many Type Ia progenitor scenarios is by searching for contaminating hydrogen and helium stripped from the companion star. However, this requires an understanding of the effect of the impact of the supernova shell on different companion stars to predict the amount of mass stripped and its distribution in velocity and solid angle for the types of binary scenarios that have been proposed as Type Ia progenitor models. We present several high-resolution two-dimensional numerical simulations of the impact of a Type Ia supernova explosion with hydrogen-rich main-sequence, subgiant, and red giant companions. The binary parameters were chosen to represent several classes of single-degenerate Type Ia progenitor models that have been suggested in the literature. We use realistic stellar models and supernova debris profiles to represent each binary system. For each scenario, we explore the hydrodynamics of the supernova-secondary interaction, calculate the amount of stellar material stripped from the secondary and the kick delivered by the impact, and construct the velocity and solid angle distributions of the stripped material. We find that the main-sequence and subgiant companions lose 0.15-0.17 M? as a result of the impact of the supernova shell, 15% of their mass. The red giant companions lose 0.53-0.54 M?, 96%-98% of their envelopes. The characteristic velocity of the stripped hydrogen is less than 103 km s-1 for all the scenarios: 420-590 km s-1 for the red giant companions, 820 km s-1 for the main-sequence companion, and 890 km s-1 for the subgiant companion. The stripped hydrogen and helium contaminate a wide solid angle behind the companion: 115? from the downstream axis for the red giant, 66? for the main-sequence star, and 72? for the subgiant. With such low velocities, the bulk of the stripped hydrogen and helium is embedded within the low-velocity iron of the supernova ejecta. The hydrogen and helium may be visible in the late-time spectra as narrow emission lines.?????Although most of the stripped material is ejected at low velocities, all the numerical simulations yield a small high-velocity tail. The main-sequence, subgiant, and red giant companions are just under the upper limit determined by Della Valle et al. in 1996 from observations of SN 1990M taken near maximum light. The main-sequence companion receives a kick of 86 km s-1, and the subgiant receives a kick of 49 km s-1. In all cases, the kick to the remnant is smaller than the original orbital velocity. Because it is too small to intercept more than a negligible amount of momentum, the red giant core will not receive an appreciable kick.?????The impact of the supernova ejecta with the secondary star creates a hole in the supernova debris with an angular size of ~30? in the high-velocity ejecta and with an angular size of ~40? in the low-velocity ejecta. This corresponds to 7%-12% of the ejecta's surface. Because we explore binary scenarios that are close enough, or almost close enough, to be in Roche lobe overflow, the degree of asymmetry is similar for all the models. The asymmetry in the supernova debris could have observational consequences beyond the change in morphology of the supernova remnant. The asymmetry in the supernova atmosphere will result in distorted P Cygni profiles that might indicate the presence of the companion star, but it will be difficult to use the degree of asymmetry alone to discriminate between a main-sequence, subgiant, or red giant companion.?????The impact of the supernova shell will have consequences for the future evolution of the secondary star. After the impact, the main-sequence star is puffed up, much like a pre-main-sequence star. The luminosity will rise dramatically, to as high as ~5000 L?, as the extended envelope relaxes back into thermal equilibrium. The subgiant companion will follow a similar sequence of events. The star will not be contaminated by much supernova debris from the initial impact, but it may accrete low-velocity iron-group elements (or oxygen and silicon if the ejecta is radially mixed) at late times.?????A He pre-white dwarf will be left behind after a supernova explosion with a red giant companion. Almost all of the envelope will be ejected by the impact, but a residual amount of material (~0.02 M?) will form an extended, hydrogen-rich envelope around the degenerate core. The star will evolve away from the red giant branch on a timescale of 105-106 yr. On its post-red giant track, it may appear as an underluminous O or B star before passing through an sdO or sdB phase on its way to a standard He white dwarf cooling track. This binary scenario could be a possible pathway for the formation of a subset of single, low-mass He white dwarfs.

The first bright quasar survey. ii. 60 nights and 1200 spectra later

Abstract: We have used the Very Large Array (VLA) FIRST survey and the Automated Plate Measuring Facility (APM) catalog of the Palomar Observatory Sky Survey I (POSS-I) plates as the basis for constructing a new radio-selected sample of optically bright quasars. This is the first radio-selected sample that is competitive in size with current optically selected quasar surveys. Using only two basic criteria, radio-optical positional coincidence and optical morphology, quasars and BL Lac objects can be identified with 60% selection efficiency; the efficiency increases to 70% for objects fainter than 17 mag. We show that a more sophisticated selection scheme can predict with better than 85% reliability which candidates will turn out to be quasars. This paper presents the second installment of the FIRST Bright Quasar Survey (FBQS), with a catalog of 636 quasars distributed over 2682 deg2. The quasar sample is characterized and all spectra are displayed. The FBQS detects both radio-loud and radio-quiet quasars out to redshift z > 3. We find a large population of objects of intermediate radio loudness; there is no evidence in our sample for a bimodal distribution of radio characteristics. The sample includes ~29 broad absorption line quasars, both high and low ionization, and a number of new objects with remarkable optical spectra.

How exactly did the universe become neutral

Abstract: We present a re—ned treatment of H, He I, and He II recombination in the early universe. The diUer- ence from previous calculations is that we use multilevel atoms and evolve the population of each level with redshift by including all bound-bound and bound-free transitions. In this framework we follow several hundred atomic energy levels for H, He I, and He II combined. The main improvements of this method over previous recombination calculations are (1) allowing excited atomic level populations to depart from an equilibrium distribution, (2) replacing the total recombination coefficient with recombi- nation to and photoionization from each level calculated directly at each redshift step, and (3) correct treatment of the He I atom, including the triplet and singlet states. We —nd that is approximately 10% smaller at redshifts than in previous calcu- x e (4 n e /n H ) (800 lations, as a result of the nonequilibrium of the excited states of H that is caused by the strong but cool radiation —eld at those redshifts. In addition, we —nd that He I recombination is delayed compared with previous calculations and occurs only just before H recombination. These changes in turn can aUect the predicted power spectrum of microwave anisotropies at the few percent level. Other improvements, such as including molecular and ionic species of H, including complete heating and cooling terms for the evolution of the matter temperature, including collisional rates, and including feedback of the secondary spectral distortions on the radiation —eld, produce negligible change to the ionization fraction. The lower at low z found in this work aUects the abundances of H molecular and ionic species by 10%¨25%. x e However, this diUerence is probably not larger than other uncertainties in the reaction rates. Subject headings: atomic processescosmic microwave backgroundcosmology: theory ¨ early universe

The Formation of Stellar Clusters: Gaussian Cloud Conditions. I.

Abstract: The isothermal dynamical evolution of a clumpy molecular cloud region and its fragmentation into a protostellar cluster is investigated numerically. The initial density distributions are generated from different realizations of a Gaussian random field with power spectrum P(k) ∝ k-2. During the evolution of the system, the one-point probability distribution functions (PDFs) of the gas density and of the line-of-sight velocity centroids develop considerable distortions away from the initial Gaussian behavior. The density PDF can be best described by power-law distributions, whereas the velocity PDF exhibits extended tails. As a result of the interplay between gas pressure and gravitational forces, a quasi-equilibrium clump mass spectrum emerges with a power-law distribution dN/dM ∝ M-1.5. Being part of a complex network of filaments, individual clumps are elongated, centrally condensed objects with 2 : 1 to 4 : 1 axis ratios with outer r-2 density distributions. Dense, Jeans-unstable gas clumps collapse and form protostellar cores that evolve through competitive accretion and N-body interactions with other cores. In contrast to the clumps, the core mass spectrum is best described by a lognormal distribution that peaks approximately at the average Jeans mass of the system. Scaled to physical conditions typical for star-forming molecular clouds, the mass function is in good agreement with the initial mass function (IMF) of multiple stellar systems. The final dynamical state of the newly formed stellar cluster closely resembles observed young stellar clusters. It has a core/halo structure, which is typical for collision dominated N-body systems. The two-point correlation function of the spatial stellar distribution can be described by two power laws with a break in the slope at the transition point from the binary to the large-scale clustering regime. The protostellar cluster is marginally bound and would be easily disrupted if the conversion of cores into stars were inefficient.

Discovery of DampedLyα Systems at Redshifts Less than 1.65 and Results on Their Incidence and Cosmological Mass Density

Abstract: We present results from an efficient, nontraditional survey to discover damped Lyα (DLA) absorption systems with neutral hydrogen column densities N ≥ 2 × 1020 atoms cm-2 and redshifts z 0.5 A are DLA systems. (2) The incidence of DLA systems per unit redshift, nDLA, decreases as a function of decreasing redshift. The low-redshift data are consistent with the larger incidence of DLA systems seen at high redshift and the inferred low incidence for DLA at z = 0 derived from 21 cm observations of gas-rich spirals. However, the errors in our determination are large enough that it is not clear if the decrease per comoving volume begins to be significant at z ≈ 2 or possibly does not set in until z ≈ 0.5. (3) On the other hand, the cosmological mass density of neutral gas in low-redshift DLA absorbers, ΩDLA, is observed to be comparable to that observed at high redshift. In particular, there is no observed trend that would indicate that ΩDLA at low redshift is approaching the value at z = 0, which is a factor of ≈4-6.5 lower than ΩDLA. (4) The low-redshift DLA absorbers exhibit a larger fraction of very high column density systems in comparison to determinations both at high redshift and at z = 0. In addition, at no redshift is the column density distribution of DLA absorbers observed to fall off in proportion to ~N with increasing column density, a trend that is theoretically predicted for disklike systems. We discuss this and other mounting evidence that DLA absorption arises not solely in luminous disks but in a mixture of galaxy types. Although we have doubled the sample of confirmed low-redshift DLA systems, we are still confronted with the statistics of small numbers. As a result, the errors in the low-redshift determinations of nDLA and ΩDLA are substantial. Therefore, aside from the above evolutionary trends, we also discuss associated limitations caused by small-number statistics and the robustness of our results. In addition, we note concerns due to gravitational lensing bias, reliance on the Mg II statistics, dust obscuration, and the sensitivity of local H I 21 cm emission surveys.

A Three-Position Spectral Line Survey of Sagittarius B2 between 218 and 263 GHZ. II. Data Analysis

Abstract: We present an analysis of the data from our Swedish-ESO Submillimetre Telescope molecular line survey in the 1.3 mm band of the N, M, and NW positions in the Sgr B2 molecular cloud. The line emissions from 42 molecular species, and some of their isotopomers, were analyzed assuming a single temperature and a homogeneous source. In cases where a source size much smaller than the antenna beam (23'') could be estimated, optical depth effects were also accounted for. In this way rotation temperatures, molecular column densities, and in several cases also source sizes, were determined. Observed and modeled intensities are presented in rotation diagrams. A few complex molecular species, NH2CHO, CH3CHO, C2H3CN, C2H5CN, and CH3OCHO, mainly in the N source, exhibit anomalously strong intensities in their intrinsically weak b- and c-type lines. We argue that this effect can hardly be explained by high optical depths alone, and therefore propose that the possibility of radiative pumping via the low-lying vibrational states of these molecules should be investigated as an alternative explanation. The highest rotation temperatures, up to about 500 K, were found for large molecules toward Sgr B2(N), closely followed by Sgr B2(M). In Sgr B2(NW), which samples the cloud envelope, the rotation temperatures are 15-50 K. For molecules with too few observed transitions to allow determination of rotation temperatures, the column density was calculated assuming optically thin emission, and we adopted rotation temperatures of 50, 50, and 20 K in M, N, and NW, respectively. Column density ratios of isotopomers were determined. After a critical discussion of the resolution-dependent H2 column density toward the observed positions, abundances relative to H2 were calculated. We discuss the chemical differences between the three observed cloud positions and compare with the hot core, compact ridge, and outflow in Orion A. Hot core-type molecules like CH2NH, NH2CN, CH3CN, C2H3CN, and C2H5CN, as well as H2CS, are more abundant in Sgr B2(N) by factors of 3-8 as compared to the M position. Large oxygen-containing species like CH3OH, CH3CHO, CH3OCHO, CH3OCH3, and NH2CHO, of compact ridge-type, show similar or slightly enhanced abundances in N as compared to M. The C2H5OH abundances are similar in N, M, and also in NW. The SO2 and SO abundances in the M core (4 × 10-7 and 1 × 10-7, respectively) are 13 and 5 times higher than in N and are very enhanced (103 and 102 times) as compared to NW. Such high SO2 and SO abundances are also found in the prominent Orion A outflow source. In M the SO18O and S18O data suggest a 16O/18O ratio of 120. The HOCO+ ion is detected in all three positions and appears to be 3 times more abundant in the NW position. HCNH+ is seen only in NW.

Tracing the Mass during Low-Mass Star Formation. I. Submillimeter Continuum Observations

Abstract: We have obtained 850 and 450 μm continuum maps of 21 low-mass cores with SEDs ranging from pre-protostellar to Class I (18 K < Tbol < 370 K), using SCUBA at the JCMT. In this paper we present the maps, radial intensity profiles, and photometry. Pre-protostellar cores do not have power-law intensity profiles, whereas the intensity profiles of Class 0 and Class I sources can be fitted with power laws over a large range of radii. A substantial number of sources have companion sources within a few arcminutes (two out of five pre-protostellar cores, nine out of 16 Class 0/I sources). The mean separation between sources is 10,800 AU. The median separation is 18,000 AU including sources without companions as a lower limit. The mean value of the spectral index between 450 and 850 μm is 2.8 ± 0.4, with pre-protostellar cores having slightly lower spectral indices (2.5 ± 0.4). The mean mass of the sample, based on the dust emission in a 120'' aperture, is 1.1 ± 0.9 M☉. For the sources fitted by power-law intensity distributions (Iν(b)/Iν(0) = (b/b0)m), the mean value of m is 1.52 ± 0.45 for Class 0 and I sources at 850 μm and 1.44 ± 0.25 at 450 μm. Based on a simple analysis, assuming the emission is in the Rayleigh-Jeans limit and that Td(r) ∝ r-0.4, these values of m translate into power-law density distributions (n ∝ r-p) with p ~ 2.1. However, we show that this result may be changed by more careful consideration of effects such as beam size and shape, finite outer radii, more realistic Td(r), and failure of the Rayleigh-Jeans approximation.

Atomic Data for Resonance Absorption Lines. II. Wavelengths Longward of the Lyman Limit for Heavy Elements

Abstract: This compilation extends the 1991 listing of atomic data by Morton to the heavier stable elements from germanium to bismuth Technetium, thorium, and uranium are added because they can live long enough to be astrophysically detectable The tabulation emphasizes resonance lines, ie, lines the lower level of which is the ground state, or an excited fine-structure state of the ground term, and is restricted to wavelengths longward of the H I Lyman limit at 91175 A This paper has attempted to review all data published by mid-1999 and includes some later material The tables contain the best data available to the author on level designations, ionization potentials, vacuum and air wavelengths, lower and upper energy levels, statistical weights, transition probabilities, natural damping constants (reciprocal lifetimes), oscillator strengths, and the often-used combinations of log gf and log λf All ion stages with classified lines are included Individual components resulting from isotope shift and hyperfine structure are listed explicitly for Rb I, Cs I, Hg I and Hg II, Ti II, and Pb II The accompanying text provides references, explanations for the critical selection of data, and notes indicating where new measurements or calculations are needed This compilation should be particularly useful in the analysis of interstellar and quasar absorption lines and other astrophysical sites where the density of particles and radiation is low enough to excite only the lowest atomic levels The data also are relevant to the study of stellar atmospheres, particularly those with enhanced abundances of heavy elements

Spectrophotometry of Nearby Field Galaxies: The Data

Abstract: We have obtained integrated and nuclear spectra as well as U, B, R surface photometry for a representative sample of 196 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (MB = -14 to -22). Here we present the spectrophotometry for these galaxies. The selection of the sample and the U, B, R surface photometry is described in a companion paper. Our goals for the project include measuring the current star formation rates and metallicities of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high redshift. We describe the observing, data reduction, and calibration techniques and demonstrate that our spectrophotometry agrees well with that of Kennicutt. The spectra span the range 3550-7250 A at a resolution (FWHM) of ~6 A and have an overall relative spectrophotometric accuracy of ~±6%. We present a spectrophotometric atlas of integrated and nuclear rest-frame spectra as well as tables of equivalent widths and synthetic colors. The atlas and tables of measurements will be made available electronically. We study the correlations of galaxy properties determined from the spectra and images. Our findings include: (1) galaxies of a given morphological class display a wide range of continuum shapes and emission-line strengths if a broad range of luminosities are considered, (2) emission-line strengths tend to increase and continua tend to get bluer as the luminosity decreases, and (3) the scatter on the general correlation between nuclear and integrated Hα emission-line strengths is large.

Interstellar Hydroxyl Masers in the Galaxy. I. The VLA Survey

Abstract: Interstellar OH masers are bright signposts for recently formed massive stars, and the maser emission can be used to study the kinematic and physical conditions of dense molecular material surrounding these stars. We present interferometric maps of 91 interstellar OH maser sources in one or both of the ground-state, main-line, 2Π3/2J = 3/2 OH transitions near 18 cm wavelength. The maps comprising this large, uniformly processed, survey have a spectral resolution of 0.14 km s-1 and an angular resolution of ≈15. We measured the absolute positions of the masers to an accuracy of ≈03 in the E-W direction and ≈05 in the N-S direction, except for those sources with declinations below about -30°, and relative positions of isolated OH maser spots within each source and OH transition to an accuracy of ≈001. This survey forms a nearly complete sample of interstellar OH masers that are stronger than 1 Jy in both right- and left-circular polarization in at least one of the ground-state OH transitions.

Prompt and Delayed Emission Properties of Gamma-Ray Bursts Observed with BeppoSAX

Abstract: We investigated the spectral evolution in the 2-700 keV energy band of gamma-ray bursts (GRBs) detected by the Gamma-Ray Burst Monitor (GRBM) and localized with the Wide Field Cameras (WFCs) aboard the BeppoSAX satellite before 1998 May. Most of them have been followed up with the Narrow Field Instruments aboard the same satellite. In the light of these results we discuss open issues on the GRB phenomenon. We find that the optically thin synchrotron shock model (SSM) provides an acceptable representation of most of the time-resolved GRB spectra extending down to 2 keV, except in the initial phases of several bursts and during the whole duration of the quite strong GRB 970111, where a low-energy photon depletion with respect to the thin SSM spectrum is observed. A strong and time-variable low-energy cutoff, consistent with absorption effect, is observed during the prompt emission of GRB 980329. We find that the X-ray afterglow starts at about 50% of the GRB duration and that its fluence, as computed from the WFC light curve, is consistent with the decay law found from the afterglow NFI observations. We also investigate the hydrodynamical evolution of the GRB in our sample and their associated afterglow, when it was detected. We find that the photon index of the latest spectrum of the GRB prompt emission is correlated with the index of the afterglow fading law, when available, as expected on the basis of an external shock of a relativistic fireball. We also find that for most of the GRBs in our sample the late emission is consistent with a slow cooling of the shock. Adiabatic shocks appear more likely than radiative shocks. Parameters of the shocks at earliest times have been derived.

Two-dimensional Models of Hydrodynamical Accretion Flows into Black Holes

Abstract: We present a systematic numerical study of two-dimensional axisymmetric accretion flows around black holes. The flows have no radiative cooling and are treated in the framework of the viscous hydrodynamic approximation. The models calculated in this study cover the large range of the relevant parameter space. There are four types of flows, determined by the values of the viscosity parameter α and the adiabatic index γ: convective flows, large-scale circulations, pure inflows, and bipolar outflows. Thermal conduction introduces significant changes to the solutions but does not create a new flow type. Convective accretion flows and flows with large-scale circulations have significant outward-directed energy fluxes, which have important implications for the spectra and luminosities of accreting black holes.

Criteria for Scaled Laboratory Simulations of Astrophysical MHD Phenomena

Abstract: We demonstrate that two systems described by the equations of the ideal magnetohydrodynamics (MHD) evolve similarly, if the initial conditions are geometrically similar and certain scaling relations hold. The thermodynamic properties of the gas must be such that the internal energy density is proportional to the pressure. The presence of the shocks is allowed. We discuss the applicability conditions of the ideal MHD and demonstrate that they are satisfied with a large margin both in a number of astrophysical objects, and in properly designed simulation experiments with high-power lasers. This allows one to perform laboratory experiments whose results can be used for quantitative interpretation of various effects of astrophysical MHD.

A Database of Cepheid Distance Moduli and Tip of the Red Giant Branch, Globular Cluster Luminosity Function, Planetary Nebula Luminosity Function, and Surface Brightness Fluctuation Data Useful for Distance Determinations

Abstract: We present a compilation of Cepheid distance moduli and data for four secondary distance indicators that employ stars in the old stellar populations: the planetary nebula luminosity function (PNLF), the globular cluster luminosity function (GCLF), the tip of the red giant branch (TRGB), and the surface brightness fluctuation (SBF) method. The database includes all data published as of 1999 July 15. The main strength of this compilation resides in the fact that all data are on a consistent and homogeneous system: all Cepheid distances are derived using the same calibration of the period-luminosity relation, the treatment of errors is consistent for all indicators, and measurements that are not considered reliable are excluded. As such, the database is ideal for comparing any of the distance indicators considered, or for deriving a Cepheid calibration to any secondary distance indicator, such as the Tully-Fisher relation, the Type Ia supernovae, or the fundamental plane for elliptical galaxies. This task has already been undertaken by Ferrarese et al., Sakai et al., Kelson et al., and Gibson et al. Specifically, the database includes (1) Cepheid distances, extinctions, and metallicities; (2) reddened apparent λ5007 A magnitudes of the PNLF cutoff; (3) reddened apparent magnitudes and colors of the turnover of the GCLF (in both the V and B bands); (4) reddened apparent magnitudes of the TRGB (in the I band) and V-I colors at 0.5 mag fainter than the TRGB; and (5) reddened apparent surface brightness fluctuation magnitudes measured in Kron-Cousin I, K', and Kshort, and using the F814W filter with the Hubble Space Telescope (HST) WFPC2. In addition, for every galaxy in the database we give reddening estimates from IRAS/DIRBE as well as H I maps, J2000 coordinates, Hubble and T-type morphological classification, apparent total magnitude in B, and systemic velocity.

Models for Old, Metal-Poor Stars with Enhanced α-Element Abundances. II. Their Implications for the Ages of the Galaxy's Globular Clusters and Field Halo Stars

Abstract: The ages of globular clusters (GCs) and post-turnoff field stars in the Galactic halo are derived using new stellar evolutionary models that explicitly take into account the observed abundances of the α-elements. Whereas the distances of the field subgiants considered in this study are based on Hipparcos parallax measurements, theoretical zero-age horizontal-branch (ZAHB) loci have been used to set the GC distance scale. (As reported in Paper I, the latter imply RR Lyrae luminosities that are within 0.1-0.15 mag, on the high side, of those found for field variables from Baade-Wesselink, trignometic parallax, and statistical parallax studies.) Both the field and cluster observations indicate that the most metal-deficient stars are 14 Gyr old while those with [Fe/H] -1.3 are 2-3 Gyr younger. Unless the O/Fe (or, more generally, the α/Fe) number abundance ratio rises quite steeply with decreasing [Fe/H], it seems unlikely that a significant age-metallicity relation can be avoided. Using what is effectively the ΔV method of determining relative cluster ages, we find that the dispersion in age at any [Fe/H] less than ≈-1.0 is small (10%-15%). Even Arp 2 and IC 4499, which had been previously categorized as young GCs, probably have near-normal ages for their metallicities—though a more definitive conclusion must await improved photometry for these systems. Ruprecht 106 appears no more than 1-1.5 Gyr younger than M3, as opposed to the ~4 Gyr age difference that others have found. However, Palomar 12 and Terzan 7 are undoubtedly young, and they provide strong evidence that, at [Fe/H] -1.0, the ages of GCs differ by as much as 4 Gyr (or ~25%). (A much larger sample of metal-rich clusters must be studied to ascertain whether the distribution of ages at higher metallicities is broad or narrow: Pal 12 and Ter 7 could well be atypical and represent the tail of a distribution that is strongly peaked near 12 Gyr.) Importantly, for the best-observed systems, there is no obvious conflict between the relative age estimates based on the ΔV method, on the one hand, and the Δ(B-V)TO,RGB technique, on the other. However, there are some exceptions, like M68, M53, M13, and NGC 288 for which the two approaches give slightly different results, indicating that something in addition to, or besides, age must be playing a role. From isochrone/ZAHB fits to the C-M diagrams (CMDs) of the outer-halo GCs Pal 3, Pal 4, and Eridanus, we conclude that there is no more than a small dependence of age on Galactocentric distance. Pal 4 seems to have very close to the same age as NGC 362, NGC 1261, and NGC 1851, while Eridanus and Pal 3 appear to be 1 Gyr younger than most inner-halo systems of the same metallicity. Unless our understanding of the HB phase of evolution is seriously in error, cluster-to-cluster age differences (at a given metallicity) are much too small for age to be the most important of the possible second parameters in determining the morphology of the horizontal branch (the first parameter being [Fe/H]). Finally, we suggest that the GC distance scale as inferred from studies of nearby RR Lyraes is not necessarily in conflict with that based on local subdwarfs. A reconciliation of the two approaches is possible simply by adopting a particular metallicity scale for the globular clusters—one that is much closer to the Zinn-West scale than to that recently proposed by Carretta & Gratton. Indeed, such a simple way of achieving consistency between subdwarf-based distances and those inferred from RR Lyraes seems compelling. If this suggestion is correct, then the long distance scale and low estimates of GC ages are not tenable. In fact, the adoption of the short distance scale, implying ages 15 Gyr for the most metal-poor GCs (and 18.30 for the true distance modulus of the Large Magellanic Cloud), leads to much improved agreement between synthetic and observed CMDs in the vicinity of the turnoff. It does cause a mismatch between predicted and observed HB luminosities, in the sense that the models are too bright by ~0.1-0.15 mag, but this may simply be an indication that current conductive opacities or the assumed chemical abundance parameters (Y, [O/Fe], and/or [α/Fe]) are not quite right.

A Uniform Analysis of the Lyα Forest at z = 0-5. II. Measuring the Mean Intensity of the Extragalactic Ionizing Background Using the Proximity Effect

Abstract: Moderate-resolution data for 40 quasi-stellar objects (QSOs) at z ≈ 2 were combined with spectra of comparable resolution of 59 QSOs with redshifts greater than 1.7 found in the literature to form a large, homogeneous sample of moderate-resolution (~1 A) QSO spectra. These spectra were presented and the statistics of the Lyα forest were discussed in Paper I. In this analysis, we demonstrate that a proximity effect is present in the data; i.e., there exists a significant (5.5 σ) deficit of lines at zabs ≈ zem. Within 1.5 h-1 Mpc of the QSO emission redshift, the significance does depend on QSO luminosity, in accordance with the theory that this effect is caused by enhanced ionization of hydrogen in the vicinity of the QSO from UV photons from the QSO itself. The photoionization model of Bajtlik, Duncan, & Ostriker permits an estimate of the mean intensity of the extragalactic background radiation at the Lyman limit. We compare the results of this standard analysis with those obtained using a maximum likelihood technique. If the spectrum of the background is assumed to be identical to that of each individual QSO, and if this background is assumed to be constant over the redshift range 1.7 < z < 3.8, then the best-fit value for J(ν0) is found to be 1.4 × 10-21 ergs s-1 cm-2 Hz-1 sr-1, using QSO redshifts based on the Lyα emission line. Systemic QSO redshifts based on the [O III] λ5007 emission line for 19 objects in our sample show an average redshift of ~400 km s-1 with respect to Lyα emission. Using redshifts based on [O III] or Mg II for the 35 objects for which they are measured and adding 400 km s-1 to the remaining QSO Lyα redshifts gives a lower value of J(ν0), 7.0 × 10-22 ergs s-1 cm-2 Hz-1 sr-1. This value is in reasonable agreement with the predictions of various models of the ionizing background based on the integrated quasar luminosity function. Allowing for the fact that individual QSOs have different spectral indices that may also be different from that of the background, we use the standard methods to solve for the H I photoionization rate, Γ, and the parameters describing its evolution with redshift. The best-fit value for the H I ionization rate we derive is 1.9 × 10-12 s-1, in good agreement with models of the background that incorporate QSOs only. Finally, we use simulated Lyα forest spectra including the proximity effect to investigate curve-of-growth effects in the photoionization model used in the analysis. We find that the presence of lines on the saturated part of the curve of growth could cause our estimates of the background intensity to be overestimated by a factor of 2-3. This large absorption-line sample and these techniques for measuring the background and understanding the systematics involved allow us to place what we believe are the firmest limits on the background at these redshifts.

The Bright SHARC Survey: The Cluster Catalog

Abstract: We present the Bright SHARC (Serendipitous High-Redshift Archival ROSAT Cluster) Survey, which is an objective search for serendipitously detected extended X-ray sources in 460 deep ROSAT PSPC pointings. The Bright SHARC Survey covers an area of 178.6 deg2 and has yielded 374 extended sources. We discuss the X-ray data reduction, the candidate selection and present results from our on-going optical follow-up campaign. The optical follow-up concentrates on the brightest 94 of the 374 extended sources and is now 97% complete. We have identified 37 clusters of galaxies, for which we present redshifts and luminosities. The clusters span a redshift range of 0.0696< z < 0.83 and a luminosity range of 0.065< LX< 8.31044 ergs s-1 [0.5-2.0 keV] (assuming H0=50 km s-1 Mpc-1 and q0=0.5). Twelve of the clusters have redshifts greater than z=0.3, eight of which are at luminosities brighter than LX=31044 ergs s-1. Seventeen of the 37 optically confirmed Bright SHARC clusters have not been listed in any previously published catalog. We also report the discovery of three candidate ``fossil groups'' of the kind proposed by Ponman et al. Based on data taken at the European Southern Observatory, Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, Canada-France-Hawaii, and Apache Point Observatory.

VLBA Observations of Radio Reference Frame Sources. III. Astrometric Suitability of an Additional 225 Sources

Abstract: We present simultaneous dual-frequency Very Long Baseline Array 2 and 8 GHz observations of 225 of the 560 extragalactic sources for which positions were reported by Johnston and coworkers and which are now part of the International Celestial Reference Frame. These observations represent the third and final in a series of observations intended to obtain single-epoch images of the entire set of sources presented by Johnston and coworkers and, together with previously reported observations, bring the total number of sources observed to 389. As with previous papers in this series, we use these data to quantify the magnitude of the expected effect of intrinsic source structure on astrometric bandwidth synthesis Very Long Baseline Interferometry observations and to calculate a source "structure index" for the observed sources. The structure index can be used as an estimate of the astrometric quality of the sources. Based on this indicator, correlations between the observed radio structure and the astrometric position accuracy and stability of the sources have been found. These correlations indicate that the more extended sources have larger position uncertainties and are less positionally stable than the more compact sources.

Massive Stars in the Range 13-25 M⊙: Evolution and Nucleosynthesis. II. The Solar Metallicity Models

Abstract: We present the evolutionary properties of a set of massive stellar models (namely 13, 15, 20, and 25 M⊙) from the main-sequence phase up to the onset of the iron core collapse. All these models have initial solar chemical composition, i.e., Y = 0.285 and Z = 0.02. A 179 isotope network, extending from neutron up to 68Zn and fully coupled to the evolutionary code has been adopted from the carbon burning onward. Our results are compared, whenever possible, to similar computations available in literature.

Low- and high-ionization absorption properties of Mg II absorption-selected galaxies at intermediate redshifts. I. General properties

Abstract: We present extensive metal-line absorption properties for 45 absorption systems that were selected by their Mg II absorption at redshifts between 0.4 and 1.4. For each system the properties of several chemical species are determined, including a wide range of ionization conditions. In the optical, the absorption systems have been observed at ~6 km s-1 resolution with HIRES/Keck, which covered Mg II, several Fe II transitions, Mg I, and in some cases (depending upon redshift) Ca II, Ti II, Mn II, and Al III. Ultraviolet, lower resolution (~230 km s-1) Faint Object Spectrograph data (1600-3275 ?) were obtained from the Hubble Space Telescope archive. These spectra covered Al II, Al III, Si II, Si III, Si IV, C II, C III, C IV, N V, O VI, and several Lyman series transitions, with coverage dependent upon the absorption system redshift. From these data, we infer that Mg II-absorbing galaxies at intermediate redshifts have multiphase gaseous structures.

Microwave Spectra of 11 Polyyne Carbon Chains

Abstract: A summary is given of the laboratory study of the rotational spectra of 11 recently detected carbon chain molecules. Classified according to their various end groups, these are the cyanopolyynes HC15N and HC17N; the isocyanopolyynes HC4NC and HC6NC; the methylcyanopolyynes CH3(C≡C)3CN, CH3(C≡C)4CN, and CH3(C≡C)5CN; and the methylpolyynes CH3(C≡C)4H, CH3(C≡C)5H, CH3(C≡C)6H, and CH3(C≡C)7H. Measured line frequencies and derived spectroscopic constants are given for each. The microwave laboratory astrophysics of the entire set is now complete in the sense that all the astronomically most interesting rotational transitions, including those with nitrogen quadrupole hyperfine structure, have now been directly measured or can be calculated from the derived constants to a small fraction of 1 km s-1 in equivalent radial velocity. All 11 carbon chains are candidates for astronomical discovery since they are closely related in structure and composition to ones that have already been discovered in space.

An Inexpensive Nuclear Energy Generation Network for Stellar Hydrodynamics

Abstract: We compare the nuclear energy generation rate and abundance levels given by an α-chain nuclear reaction network that contains only seven isotopes with a standard 13 isotope α-chain reaction network. The energy generation rate of these two small networks are also compared to the energy generation rate given by a 489 isotope reaction network with weak reactions turned on and off. The comparison between the seven isotope and α-chain reaction networks indicate the extent to which one can be replaced by the other, and the comparison with the 489 isotope reaction network roughly indicates under what physical conditions it is safe to use the seven isotope and α-chain reaction networks. The seven isotope reaction network reproduces the nuclear energy generation rate of the standard α-chain reaction network to within 30%, but often much better, during hydrostatic and explosive helium, carbon, and oxygen burning. It will also provide energy generation rates within 30% of an α-chain reaction network for silicon burning at densities less than 107 g cm-3. Provided there remains an equal number of protons and neutrons (Ye = 0.5) over the course of the evolution, and that flows through α-particle channels dominate, then both of the small reaction networks return energy generation rates that are compatible with the energy generation rate returned by the 489 reaction network. If Ye is significantly different from 0.5, or if there are substantial flows through neutron and protons channels, then it is not generally safe to employ any α-chain based reaction network. The relative accuracy of the seven isotope reaction network, combined with its reduction in the computational cost, suggest that it is a suitable replacement for α-chain reaction networks for parameter space surveys of a wide class of multidimensional stellar models.

The Dust-Radio Connection in 3CR Radio Galaxies

Abstract: We investigate dust in the nuclear regions of radio galaxies on the basis of 120 images of 3CR radio galaxies in the redshift range 0 < z < 0.5, taken with the WFPC-2 on the Hubble Space Telescope. At least 40 of these radio galaxies show evidence for absorption due to clumpy dust features; a large range of morphologies is observed for these dust features, including disklike and filamentary structures. No difference in host galaxy properties was found between the radio galaxies with clumpy dust and those without. Where dust absorption is detected, the morphological properties of the dust depend strongly upon the Fanaroff & Riley (FR) classification of the source. The dust in FR I host galaxies is generally situated in sharply defined disks on small (<2.5 kpc) scales, and for eight out of the nine FR I galaxies with dust disks, the radio source lies nearly perpendicular to the dust disk. In contrast, FR II host galaxies have dust that varies both in morphology and in linear size, and the correlation between the dust and radio orientation only exists (and less strongly) for the sources that have a significant mass of dust in disks within a distance of 2.5 kpc of the center of the galaxy. The derived dust masses also correlate with the FR type of the host galaxy: high masses of clumpy dust are not seen in FR I radio galaxies, while they are present in FR II radio galaxies. Further, FR I galaxies have derived dust masses that are typically larger than the dust masses found in a matched sample of radio-quiet ellipticals. We suggest that the observed differences between FR I and FR II radio galaxies are produced due to differences in their formation.

An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera

Abstract: We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE) stars observed in the IUE short-wavelength bandpass (1150-1980 A). Each low-resolution (R ~ 6 A) spectrum was visually inspected for source contamination and data quality, and then all good spectra were combined to form a single time-averaged spectrum for each star. Use of IUE Final Archive spectra processed with NEWSIPS reduces fixed pattern noise in individual spectra, allowing significant signal-to-noise ratio gains in our co-added spectra. For the TTS observed by IUE, we measured fluxes and uncertainties for 17 spectral features, including two continuum windows and four fluoresced H2 complexes. Thirteen of the 32 accreting TTS observed by IUE have detectable H2 emission, which until now had been reported only for T Tau. Using an empirical correlation between H2 and C IV line flux, we show that lack of sensitivity can account for practically all nondetections, suggesting that H2 fluorescence may be intrinsically strong in all accreting TTS systems. Comparison of IUE and GHRS spectra of T Tau show extended emission primarily, but not exclusively, in lines of H2. We also fit reddened main-sequence templates to 72 HAEBE stars, determining extinction and checking spectral types. Several of the HAEBE stars could not be fitted well or yielded implausibly low extinctions, suggesting the presence of a minority emission component hotter than the stellar photosphere, perhaps caused by white dwarf companions or heating in accretion shocks. We identified broad wavelength intervals in the far-UV that contain circumstellar absorption features ubiquitous in B5-A4 HAEBE stars, declining in prominence for earlier spectral types, perhaps caused by increasing ionization of metal resonance lines. For 61 HAEBE stars, we measured or set upper limits on a depth index that characterizes the strength of circumstellar absorption and compared this depth index with published IR properties.

Surface photometry of nearby field galaxies: the data

Abstract: We have obtained integrated spectra and multifilter photometry for a representative sample of similar to 200 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (M(B) = -14 to -22) and colors (B-R = 0.4-1.8). Here we describe the sample selection criteria and the U, B, R surface photometry for these galaxies. The spectrophotometric results will be presented in a companion paper. Our goals for the project include measuring the current star formation rates and metallicity of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high red-shift. We discuss the observing, data reduction, and calibration techniques and show that our photometry agrees well with previous work in those cases in which earlier data are available. We present an atlas of images, radial surface brightness profiles, and color profiles as well as tables of derived parameters. The atlas and tables of measurements will be made available electronically. We study the correlations of galaxy properties determined from the galaxy images. Our findings include the following: (1) colors determined within the effective radius correlate better with morphological type than with M(B) and (2) 50% of the low-luminosity galaxies are bluest in their centers.