Showing papers in "Astrophysical Journal Supplement Series in 1985"

Self-similar secondary infall and accretion in an Einstein-de Sitter universe

Abstract: Similarity solutions have been found for secondary infall and accretion onto an initially overdense perturbation in an Einstein--de Sitter (..cap omega.. = 1) universe. After the initial collapse of a positive density perturbation, bound shells continue to turn around and fall in, with the radius of the shell currently turning around increasing as t/sup 8/9/ and the mass within this radius increasing as t/sup 2/3/. The secondary infall approaches a self-similar form, with the exact behavior depending on the kind of gas and on central boundary conditions. If there is a central black hole, it grows by accretion, with the density having the power-law form rhoproportionalr/sup 01.5/ near the center. If there is no central black hole, a rhoproportionalr/sup 02.25/ density profile results, with infalling matter added to successively larger radii.

Stellar core collapse - Numerical model and infall epoch

Abstract: A numerical model based on hydrodynamics coupled to radiation transport of all neutrino types is developed for calculating stellar core collapse. General relativistic hydrodynamic equations for spherically symmetric systems including neutrino flow are obtained and presented in a form paralleling the adiabatic equations of May and White (1967). A multigroup, flux-limited diffusion scheme, used to evolve ..nu../sub e/'s, nu-bar/sub e/'s, and ..nu../sub ..mu../ and ..nu../sub tau/ pairs independently, is derived from the neutrino Boltzmann equation. Expressions for the zeroth and first Legendre moments of all important neutrino interactions are derived from the standard model of electroweak interactions. Numerical values are obtained from these expressions (by numerical integration when necessary) during core-collapse calculations. An implicit numerical scheme for directly solving the neutrino equations, including the neutrino-electron scattering and neutrino thermal production terms, is presented. This scheme includes a consistent treatment of neutrino-matter decoupling in the neutrino transparent regimes. At the present time, the equation of state of Lamb et al. (1978, 1981) up to nuclear density, and that of Friedman and Pandharipande (1981) above nucleon density, are incorporated in the numerical model.

The chemistry of galaxies. I. The nature of giant extragalactic H II regions.

Abstract: Spectrophotometric observations of 99 H II regions in 20 spiral and irregular galaxies are presented and discussed. The emission properties are shown to form a one-parameter sequence. The vast majority of the nebulae must be ionization-bounded. The most effective spectral sequencing index composed of line intensities from one metal is log )((O II)+(O III))/H..beta..). Theoretical dust-free models of H II regions are constructed to explain the variations of (O II)/H..beta.., (O III)/H..beta.., (N II)/(O II), and (S II)/(O II). The oxygen abundance is found to be underlying physical determinant of the spectrum of a giant extragalactic H II region. The ionizing radiation field hardens with decreasing abundance. Above an oxygen abundance of about 0.2 times solar, the abundances of oxygen and nitrogen appear to be closely coupled. Nitrogen grows like a secondary product of nucleosynthesis (with respect to oxygen) on top of a base abundance (relative to hydrogen) of order 1 x 10/sup -6/. Sulfur appears to grow as a primary product of nucleosynthesis (with respect to oxygen). The models are used to revise the calibration of (O II)+(O III) with oygen abundance. The visual continuum of the H II regions seems to be contributed mainly by nonionizing supergiants inmore » the embedded OB associations.« less

Numerical techniques for large cosmological N-body simulations

Abstract: Techniques for carrying out large N-body simulations of the gravitational evolution of clustering in the fundamental cube of an infinite periodic universe are described and compared. The accuracy of the forces derived from several commonly used particle mesh schemes is examined, showing how submesh resolution can be achieved by including short-range forces between particles by direct summation techniques. The time integration of the equations of motion is discussed, and the accuracy of the codes for various choices of 'time' variable and time step is tested by considering energy conservation as well as by direct analysis of particle trajectories. Methods for generating initial particle positions and velocities corresponding to a growing mode representation of a specified power spectrum of linear density fluctuations are described. The effects of force resolution are studied and different simulation schemes are compared. An algorithm is implemented for generating initial conditions by varying the number of particles, the initial amplitude of density fluctuations, and the initial peculiar velocity field.

Solar-stellar outer atmospheres and energetic particles, and galactic cosmic rays

Abstract: The heavy element compositions of the solar corona, solar wind (SW), solar energetic particles (SEP, ''mass-unbiasedd baseline''; Paper I) and galactic cosmic-ray sources (GCRS) are remarkably similar. They all show the same pattern as compared to standard local galactic (or solar photospheric) composition: an underabundance of heavy elements with first ionization potential (FIP) > or approx. =9 eV relative to elements with lower FIP, by factors of approx.4-6. Only C is clearly more abundant in GCR sources than in SEP (by a factor of approx.2-3), as well as possibly O.

CCD surface photometry of field galaxies. II: Bulge/disk decompositions

Abstract: Major- and minor-axis profiles given previously for 105 galaxies of all morphological types are decomposed into bulge and disk components. In addition, three model-independent parameters which measure the mean surface brightness, scale radius, and degree of light concentration are derived. The best correlations are found between Hubble type, concentration, bulge/disk ratio, and mean surface brightness. Correlations between the individual bulge and disk parameters generally show large scatter. The properties of S0 galaxies are inconsistent with their having been formed from spiral galaxies via gas depletion; their properties are intermediate between those of ellipticals and spirals. Most elliptical galaxies probably do not form by the merging of disk galaxies. The difficulty of distinguishing between elliptical and S0 galaxies in some cases is emphasized.

Thermodynamics of clathrate hydrate at low and high pressures with application to the outer solar system

Abstract: The thermodynamic stability of clathrate hydrate is calculated under a wide range of temperature and pressure conditions applicable to solar system problems, using a statistical mechanical theory developed by van der Waals and Platteeuw (1959) and existing experimental data on properties of clathrate hydrates and their components. At low pressure, dissociation pressures and partition functions (Langmuir constants) for CO clathrate (hydrate) have been predicted, using the properties of clathrate containing, as guests, molecules similar to CO. The comparable or higher propensity of CO to incorporate in clathrate relative to N_2 is used to argue for high CO-to-N_2 ratios in primordial Titan if N_2 was accreted as clathrate. The relative incorporation of noble gases in clathrate from a solar composition gas at low temperatures is calculated and applied to the case of giant-planet atmospheres and icy satellites. It is argued that nonsolar but well-constrained noble gas abundances will be measured by Galileo in the Jovian atmosphere if the observed carbon enhancement is due to bombardment of the atmosphere by clathrate-bearing planetesimals sometime after planetary formation. The noble gas abundances in Titan's atmosphere are also predicted under the hypothesis that much of the satellite's methane accreted as clathrate. Double occupancy of clathrate cages by H_2 and CH_4 in contact with a solar composition gas is examined, and it is concluded that potentially important amounts of H_2 may have incorporated in satellites as clathrate. The kinetics of clathrate formation is also examined, and it is suggested that, under thermodynamically appropriate conditions, essentially complete clathration of water ice could have occurred in high-pressure nebulae around giant planets but probably not in the outer solar nebula; comets probably did not aggregate as clathrate. At moderate pressures, the phase diagram for methane clathrate hydrate in the presence of 15% ammonia (relative to water) is constructed, and application to the early Titan atmospheric composition is described. The high-pressure stability of CH_4, N_2, and mixed CH_4-N_2 clathrate hydrate is calculated; conversion back to water and CH_4 and/ or N_2 fluids or solids is predicted for pressures ≳ 12 kilobars (independent of temperature) and temperatures ≳ 320 K (independent of pressure). The effect of ammonia is to shrink the T-P stability field of clathrate with increasing ammonia concentration. These results imply that (1) clathrate is stable throughout the interior of Oberon- and Rhea-sized icy satellites, and (2) clathrate incorporated in the innermost icy regions of Titan would have decomposed, perhaps allowing buoyant methane to rise. Brief speculation on the implications of this conclusion for the origin of surficial methane on Titan is given. A list of suggested experiments and observations to test the theory and its predictions is presented.

Tabulated optical properties of graphite and silicate grains

Abstract: On donne les fonctions dielectriques complexes pour le graphite et le «silicate astronomique» pour les longueurs d'onde λ comprises entre 2000 μm et 200 A, ainsi que Q abs , l'albedo et pour des spheres a=0,01 μm et a=0,1 μm

On the evolution of close binaries with components of initial mass between 3 solar masses and 12 solar masses

Abstract: The evolution of components of close binary systems of intermediate mass (component masses in the range 3--12 M/sub sun/) and of Population I composition (X = 0.7, Z = 0.02) is studied numerically. We assume that a model star first fills its Roche lobe after it has developed a helium core and becomes a giant and that mass loss from the model continues until model radius decreases below a predetermined limit. How much of the matter that is lost from the Roche-lobe--filling model is transferred to its companion and how much is lost from the system are left undetermined. In the mass loss/exchange process, a model of initial mass in the 3--10 M/sub sun/ range is transformed into a compact ''helium'' star (which, nevertheless, retains some hydrogen-rich matter in its envelope). In our experiments, models of initial mass 3, 4, 5 6.95, and 9.85 M/sub sun/ produce remnants of mass 0.378, 0.523, 0.765, 1.11, and 1.95 M/sub sun/, respectively. The lifetime of the core helium-burning phase of each remnant ranges from 200% (.0378 M/sub sun/ remnant) to 20% (1.95 M/sub sun/ remnant) of the main-sequence lifetime of its progenitor, and the possibility arises that the primordial secondary, if it ismore » initially massive enough or if it has accreted sufficient matter during the first mass-transfer episode, will expand and transfer matter back to the primary remnant while this remnant is still relatively compact. We show that, then, most of the matter outside the helium core of the secondary will be ejected from the system.« less

Old stellar populations. II. an analysis of K-giant spectra.

Abstract: Etude de onze caracteres d'absorption forte sur un echantillon de 110 etoiles geantes K, a partir de spectres pris a 9 A de resolution

Spectra and linear polarizations of extragalactic variable sources at centimeter wavelengths

Abstract: On presente les resultats d'observations multifrequences des densites de flux et des polarisations lineaires de 97 sources extragalactiques. Les degres de polarisation excedent rarement 15% et sont generalement voisins de 5% ou moins. Pour de nombreuses sources, mais pas toutes, les variations sont plus rapides et de plus grande amplitude aux plus hautes frequences

The baseline composition of solar energetic particles

Abstract: We analyze all existing spacecraft observations of the highly variable heavy element composition of solar energetic particles (SEP) during non-/sup 3/He-rich events. All data show the imprint of an ever-present basic composition pattern (dubbed ''mass-unbiased baseline'' SEP composition) that differs from the photospheric composition by a simple bias related to first ionization potential (FIP). In each particular observation, this mass-unbiased baseline composition is being distorted by an additional bias, which is always a monotonic function of mass (or Z). This latter bias varies in amplitude and even sign from observation to observation. To first order, it seems related to differences in the A/Z* ratio between elements (Z* = mean effective charge).

A search for «dwarf» Seyfert 1 nuclei. I: The initial data and results

Abstract: Recherche de noyaux de Seyfert 1 de faible luminosite a partir de spectres ayant une resolution ∼2,5 A et des rapports signal-bruit tres eleves (∼100/1) dans le domaine λλ6200-6880. Les observations concernent 75 galaxies proches, brillantes

JPL pulsar timing observations. III. Pulsar rotation fluctuations.

Abstract: Pulsar arrival-time data from a timing program carried out at the Jet Propulsion Laboratory from 1968 to 1982 at 2390 MHz were analyzed to study the precision of neutron star rotation rates. Most of the 24 objects in the sample show 'timing activity' as deviations from the smooth spin-down expected from a simple electromagnetic torque mechanism. About 20 events in both frequency and frequency derivative across the sample of objects are identified, whose properties differ from those of the large-amplitude 'glitches' observed from a few objects. Prospects are good for the millisecond pulsar PSR 1937 + 214 being better than terrestrial maser clocks on time scales of a few years.

M supergiants in the Milky Way and the Magellanic Clouds Colors, spectral types, and luminosities

Abstract: On presente une nouvelle photometrie BVRI pour 116 candidates supergeantes rouges dans le Petit Nuage de Magellan et 11 supergeantes rouges lumineuses dans le Grand Nuage de Magellan. On presente egalement une nouvelle photometrie IR de 65 candidates supergeantes rouges dans le PNM et 96 dans le GNM. On donne la confirmation spectroscopique pour 65 des candidates du PNM. Les donnees obtenues sont combinees avec des donnees photometriques et spectroscopiques obtenues precedemment par Humphreys

Long-slit spectroscopy of gas in the cores of X-ray luminous clusters

Abstract: The results of long-slit spectroscopy obtained for the core regions of 14 clusters of galaxies are reported. The data are presented in detail. It is shown that the presence of optical emission is tied to the properties of the hot gas in the cluster and not to the morphology of the central galaxy or cluster, demonstrating that the optical systems are indeed formed by the cooling of hot gas. Cooling flows occur when the gas density exceeds a critical central value which corresponds to a cooling time scale which, it is argued, weakly favors low values of H(0). The kinematics of the gas flows are discussed. The excitation mechanisms, correlation of optical emission with radio properties, and upper limits on coronal line strengths from the hot gas are discussed.

Molecular line survey of Orion A from 215 to 247 GHz

Abstract: Molecular line emission from the core of the Orion molecular cloud has been surveyed from 215 to 247 GHz to an average sensitivity of about 0.2 K. A total of 544 resolvable lines were detected, of which 517 are identified and attributed to 25 distinct chemical species. A large fraction of the lines are partially blended with other identified transitions. Because of the large line width in the Orion core, the spectrum is near the confusion limit for the weakest lines identified (≈ 0.2 K). The most abundant complex molecules present are HCOOCH_3, CH_30CH_3, and C_2H_5CN, with beam-averaged column densities of about 3X10^(15) cm^(-2). Together with the simpler species S0_2, CH_30H, and CH_3CN, they account for approximately 70% of the lines in the spectrum. Relatively few unidentified lines are present. There are 27 lines clearly present in the spectrum which are currently unidentified. However, many of these are thought to be high-lying transitions of complex asymmetric rotors such as CH_30H. Present spectroscopic data are inadequate to predict the frequencies of such transitions with sufficient accuracy.

Population studies. I. The Bidelman-MacConnell "weak-metal" stars.

Abstract: We present BVRI and DDO photometry for 309 of the Bidelman-MacConnell ''weak-metal'' stars, together with radial velocities for most of the stars that have (Fe/H)< or =-0.8. Photometric taxonomy is used to classify the sample as giants, dwarfs, red horizontal-branch stars, UV-bright stars, etc., and various calibrations have been adopted to determine (Fe/H), M/sub V/, space motions, and orbital eccentricity e.

Evolution of 0.7-3.0 M stars having -1.0 <= Fe/H 0.0.

Abstract: Five grids of stellar models have been calculated for masses ranging from 0.7 to 3.0 M/sub sun/ assuming, in turn, a metal abundance (Fe/H) = -1.0, -0.76, -0.46, -0.23, and 0.0. All of the calculations are based on a value of Y = 0.25 for the helium content and ..cap alpha.. = 1.6 for the ratio of the mixing length to the pressure scale height. The latest improvements in opacity data and nuclear reaction rates have been incorporated into the computations. Moreover, model atmospheres have been used to provide the boundary conditions for the stellar interior calculations as well as to transpose the isochrones, computed for ages from 0.3 x 10/sup 9/ to 15 x 10/sup 9/ yr, from the theoretical to the (M/sub v/, B-V)-plane. Cousins V-I and V-R colors are also predicted for each of the model sequences, which are extensively tabulated.

The nuclear activity of interacting galaxies

Abstract: On presente les resultats de la recherche de noyaux galactiques actifs parmi les galaxies en interaction. On a observe spectrophotometriquement dans le domaine 4700-7100 A, un echantillon de galaxies asymetriques et en interaction. Les resultats sont compares aux donnees d'observations d'un echantillon de galaxies isolees

Theoretical isochrones for globular clusters with predicted BVRI and Stromgren photometry.

Abstract: On presente de nouvelles sequences d'evolution stellaire et des isochrones, calcules pour des âges et des metallicites correspondant aux amas globulaires de la Galaxie, dans les systemes photometriques UBV de Johnson, RI de Cousin et uvby de Stromgren. On compare a des donnees d'observations recentes de M4, M15 et 47 Tuc

Estimated number of field stars toward Galactic globular clusters and Local Group Galaxies

Abstract: Field star densities are estimated for 89 fields with /b/ greater than 10 degrees based on the Galaxy model of Bahcall and Soneira (1980, 1984; Bahcall et al. 1985). Calculated tables are presented for 76 of the fields toward Galactic globular clusters, and 16 Local Group Galaxies in 13 fields. The estimates can be used as an initial guide for planning both ground-based and Space Telescope observations of globular clusters at intermediate-to-high Galactic latitudes.

A catalog of ultraviolet interstellar extinction excesses for 1415 stars

Abstract: Ultraviolet interstellar extinction excesses are presented for 1415 stars with spectral types B7 and earlier. The excesses with respect to V are derived from Astronomical Netherlands Satellite (ANS) 5-channel UV photometry at central wavelengths of approximately 1550, 1800, 2500, and 3300 A. A measure of the excess extinction in the 2200-A extinction bump is also given. The data are valuable for investigating the systematics of peculiar interstellar extinction and for studying the character of UV interstellar extinction in the general direction of stars for which the extinction-curve shape is unknown.

Structure and stability of constant-alpha force-free fields

Abstract: The Woltjer-Taylor hypothesis suggests that constant-..cap alpha.. force-free fields may arise naturally as a result of magnetic helicity conservation during field relaxation. Thus, an analysis of magnetic helicity is important in the understanding of these fields. We present a general expression for the helicity in spherical and planar geometries, and apply it to constant-..cap alpha.. fields. Inhomogeneous boundary conditions (B x nnot =0) are explicitly allowed for by use of the gauge-invariant relative helicity integral. The relation between relative helicity and energy is given particular attention.

Theoretical models for H II regions. I. Diagnostic diagrams

Abstract: We have computed an extensive, homogeneous grid of theoretical photoionization models with conditions appropriate to observed H II regions. This grid has been used to develop a comprehensive set of theoretical diagnostic diagrams for H II regions which employ ratios of prominent emission lines to determine the ionization parameter within the nebula, and the ionization temperature of its exciting star(s). It is also possible to estimate the element-averaged metallicity in the nebula and stars from these diagnostics.

Star-forming properties and histories of dwarf irregular galaxies: down but not out

Abstract: Star formation processes and their relationships to other galactic properties are investigated in a sample of low optical surface brightness dwarf irregular galaxies. New observations made for this study include B and R images taken with a CCD detector; flux-calibrated, H..cap alpha.., narrow-band digital imagery; large-aperture spectrophotometry; and spectrophotometry of individual H II regions. These data provide the basis for several measurements of star-forming characteristics of dwarf irregular galaxies: (1) Total current star formation rates are deduced from H..cap alpha.. luminosities. (2) Properties of typical star-forming centers are derived from the sizes, luminosities, and surface brightness of H II regions. (3) Star formation histories are estimated from the ratio of H..cap alpha.. to blue luminosities and from the stellar content models. (40 The metallicity of the gas comes from oxygen emission lines. (5) Images provide information on the optical light distributions.

Spectrophotometric and model-atmosphere analyses of the hot DO and DAO white dwarfs from the Palomar-Green survey

Abstract: Recent optical and spectrophotometric observations of all currently known DAO and DO hot white dwarf candidates from the Palomar-Green Survey are presented. The optical observations were carried out using the Kitt Peak National Observatory Intensified Image Dissector Scanner (IIDS) system, the Multiple Mirror Telescope (MMT), and two photon-counting Reticon systems. Ultraviolet spectrophotometry was obtained using the International Ultraviolet Explorer (IUE) short-wavelength and long-wavelength cameras. The following spectroscopic subgroups were identified in the observations: the cool DO stars with spectra showing H I features together with He II + H blends; the hot DO stars, with spectra exhibiting weak He II (+H) lines; and hydrogen-rich DAO stars with broad H lines and sharp H II lines at 4686 A. A fourth subgroup of stars resembling PG 1159 with He at He II lines at 4686, and a C, N ion blend in the optical spectrum were also found. The spectroscopic data were analyzed in terms of a grid of hot, high-gravity LTE model atmospheres composed of a mixture of hydrogen and helium, and atmospheric parameters are deduced for each class of objects. On the basis of the model atmosphere analysis, the local space density of the DO stars is estimated to be 1.9-3.0 x 10 to the -7th per cubic pc, depending on the scale height.

Supernova remnants in the Magellanic Clouds. III

Abstract: As part of a continuing study of supernova remnants in the Magellanic Clouds, narrow-band optical images were obtained of seven SNR candidates in the LMC selected on the basis of radio and/or X-ray observations. Four of the candidates are confirmed as new SNRs. The object 0536-692 appears to be a superbubble resulting from one or more supernovae and the stellar winds from the large OB stellar association, NGC 2044, within its interior. The latest results bring the total number of SNRs with optical identifications in the Large Magellanic Cloud to 32. 16 references.