Showing papers in "The Astrophysical Journal in 1993"

Gamma-ray bursts from stellar mass accretion disks around black holes

Abstract: A cosmological model for gamma-ray bursts is explored in which the radiation is produced as a broadly beamed pair fireball along the rotation axis of an accreting black hole. The black hole may be a consequence of neutron star merger or neutron star-black hole merger, but for long complex bursts, it is more likely to come from the collapse of a single Wolf-Rayet star endowed with rotation ('failed' Type Ib supernova). The disk is geometrically thick and typically has a mass inside 100 km of several tenths of a solar mass. In the failed supernova case, the disk is fed for a longer period of time by the collapsing star. At its inner edge the disk is thick to its own neutrino emission and evolves on a viscous time scale of several seconds. In a region roughly 30 km across, interior to the accretion disk and along its axis of rotation, a pair fireball is generated by neutrino annihilation and electron-neutrino scattering which deposit approximately 10 exp 50 ergs/s.

BATSE observations of gamma-ray burst spectra. I: Spectral diversity

Abstract: We studied the time-averaged gamma-ray burst spectra accumulated by the spectroscopy detectors of the Burst and Transient Source Experiment. The spectra are described well at low energy by a power-law continuum with an exponential cutoff and by a steeper power law at high energy. However, the spectral parameters vary from burst to burst with no universal values. The break in the spectrum ranges from below 100 keV to more than 1 MeV, but peaks below 200 keV with only a small fraction of the spectra breaking above 400 keV; it is therefore unlikely that a majority of the burst spectra are shaped directly by pair processes, unless bursts originate from a broad redshift range. The correlations among burst parameters do not fulfill the predictions of the cosmological models of burst origin. No correlations with burst morphology or the spatial distribution were found. We demonstrate the importance of using a complete spectral description even if a partial description (e.g., a model without a high-energy tail) is statistically satisfactory.

The Absolute Magnitudes of Type IA Supernovae

Abstract: Absolute magnitudes in the B, V, and I bands are derived for nine well-observed Type Ia supernovae, using host galaxy distances estimated via the surface brightness fluctuations or Tully-Fisher methods. These data indicate that there is a significant intrinsic dispersion in the absolute magnitudes at maximum light of Type Ia supernovae, amounting to +/- 0.8 mag in B, +/- 0.6 mag in V, and +/- 0.5 mag in I. Moreover, the absolute magnitudes appear to be tightly correlated with the initial rate of decline of the B light curve, with the slope of the correlation being steepest in B and becoming progressively flatter in the V and I bands. This implies that the intrinsic B - V colors of Type Ia supernovae at maximum light are not identical, with the fastest declining light curves corresponding to the intrinsically reddest events. Certain spectroscopic properties may also be correlated with the initial decline rate. These results are most simply interpreted as evidence for a range of progenitor masses, although variations in the explosion mechanism are also possible. Considerable care must be exercised in employing Type Ia supernovae as cosmological standard candles, particularly at large redshifts where Malmquist bias could be an important effect.

Identification of two classes of gamma-ray bursts

Abstract: We have studied the duration distribution of the gamma-ray bursts of the first BATSE catalog. We find a bimodality in the distribution, which separates GRBs into two classes: short events (less than 2 s) and longer ones (more than 2 s). Both sets are distributed isotropically and inhomogeneously in the sky. We find that their durations are anticorrelated with their spectral hardness ratios: short GRBs are predominantly harder, and longer ones tend to be softer. Our results provide a first GRB classification scheme based on a combination of the GRB temporal and spectral properties.

Bias and variance of angular correlation functions

Abstract: We present a general method for calculating the bias and variance of estimators for w(θ) based on galaxy-galaxy (DD), random-random (RR), and galaxy-random (DR) pair counts and describe a procedure for quickly estimating these quantities given an arbitrary two-point correlation function and sampling geometry. These results, based conditionally upon the number counts, are accurate for both high and low number counts. We show explicit analytical results for the variances in the estimators DD/RR, DD/DR, which turn out to be considerably larger than the common wisdom Poisson estimate and report a small bias in DD/DR in addition to that due to the integral constraint. Further, we introduce and recommend an improved estimator (DD−2DR+RR)/RR, whose variance is nearly Poisson

Spectral evolution of stellar populations using isochrone synthesis

Abstract: We combine the photometric model of isochrone synthesis recently published by Charlot & Bruzual with an updated library of stellar spectra to predict the spectral evolution of stellar populations with solar metallicity. The library of spectra assembled here supersedes other existing libraries (Bruzual; Guiderdoni & Rocca-Volmerange; Buzzoni) by its spectral range (extreme ultraviolet to far-infrared), its complete coverage of the color-magnitude diagram, and its inclusion of observed near-infrared spectra out to 2.56 μm. Also, the spectra are distributed on the stellar evolutionary tracks using optical/near-infrared color calibrations, as an improvement over models that used a single color of the effective temperature of the stars alone

Pulsar distances and the galactic distribution of free electrons

Abstract: The present quantitative model for Galactic free electron distribution abandons the assumption of axisymmetry and explicitly incorporates spiral arms; their shapes and locations are derived from existing radio and optical observations of H II regions. The Gum Nebula's dispersion-measure contributions are also explicitly modeled. Adjustable quantities are calibrated by reference to three different types of data. The new model is estimated to furnish distance estimates to known pulsars that are accurate to about 25 percent.

Submillimeter Continuum Observations of rho Ophiuchi A: The Candidate Protostar VLA 1623 and Prestellar Clumps

Abstract: We have mapped the submillimeter continuum emission from the rich star-forming core, ρ Oph A, at 350, 450, 800, and 1300 μm using the JCMT and IRAM 30 m telescopes. In addition to the diffuse emission from the cloud core itself, these observations reveal four well-defined clumps, which, suprisingly, are barely visible in the single-dish molecular maps obtained so far in this region. The gas may have partially frozen out onto the dust grains, due to low temperature and/or high density in the clumps. We discuss the possibility that these clumps are prestellar or extremely young, low-mass protostars

Evolution of Low- and Intermediate-Mass Stars to the End of the Asymptotic Giant Branch with Mass Loss

Abstract: Stars with initial masses in the range 0.89≤M/M ○. ≤5.0 have been evolved from the main-sequence phase through to the end of the asymptotic giant branch (AGB). The calculations were done with metallicities Z of 0.016, 0.008, 0.004, and 0.001 to allow comparison with Galactic and Magellanic Cloud stars. The novel feature of these calculations is the inclusion of mass loss on the AGB using an empirical formula relating the mass-loss rate to the pulsation period. The calculations show that a superwind phase develops naturally on the AGB, but only over the last 2-3 thermal pulse cycles

Spectroscopic constraints on the properties of dust in active galactic nuclei

Abstract: The optical properties of graphite, silicate, and SiC grains over the wavelength range 1000 microns - 1 A are calculated for grains in the 0.005-10-micron size range. Both graphite + silicate (MRN) and graphite + SiC grain mixtures are considered, with various grain size distributions. A detailed radiative transfer calculation is performed to obtain constraints on the emission properties of dust in AGN in either 'optically thin' or optically thick configurations. Warm graphite + silicate or graphite + SiC dust (T is greater than 200 K) with the MRN size distribution and a column density less than about 10 exp 23/sq cm produces a strong silicate or SiC emission feature. Such dust cannot be responsible for the observed IR emission from AGN. MRN dust with a high optical depth at 10 microns produces an emission feature with an amplitude of about 57 percent, in excess of the typical observational limit for most objects. Reddening of the broad emission lines and continuum is unlikely to be common.

Neutron star dynamos and the origins of pulsar magnetism

Abstract: Neutron star convection is a transient phenomenon and has an extremely high magnetic Reynolds number In this sense, a neutron star dynamo is the quintessential fast dynamo The convective motions are only mildly turbulent on scales larger than the approximately 100 cm neutrino mean free path, but the turbulence is well developed on smaller scales Several fundamental issues in the theory of fast dynamos are raised in the study of a neutron star dynamo, in particular the possibility of dynamo action in mirror-symmetric turbulence It is argued that in any high magnetic Reynolds number dynamo, most of the magnetic energy becomes concentrated in thin flux ropes when the field pressure exceeds the turbulent pressure at the smallest scale of turbulence In addition, the possibilities for dynamo action during the various (pre-collapse) stages of convective motion that occur in the evolution of a massive star are examined, and the properties of white dwarf and neutron star progenitors are contrasted

Model for the high-energy emission from blazars

Abstract: Approximate formulae for the instantaneous scattered radiation spectra are derived in the Thomson and Klein-Nishina regimes, assuming that the relativistic electrons are described by an isotropic angular and power-law energy distribution in the outflowing fluid frame. The soft photon source is assumed to be axially symmetric. We find that the Thomson-limit luminosity enhancement in the direction that the scattered radiation is most intense varies as Γ 6 for photons entering from the side and as Γ 2 for photons entering directly from behind, where Γ is the bulk Lorentz factor of the outflowing plasma cloud. Energy loss and flow are treated in a model where energetic electrons are instantaneously injected with a power-law energy distribution at some height above a point or disk photon source, and instantaneous and time-average spectra are calculated

X-Ray Spectra from Two-Phase Accretion Disks

Abstract: A model for the X-ray emission from radio-quiet active galactic nuclei (AGNs) is considered. The X-rays are produced via inverse Compton (IC) emission in a hot corona embedding a colder accretion disk. Soft thermal photons emitted by the cold layer provide the main source of cooling for the hot electrons in the corona. At the same time, about half of the high-energy photons produced in the corona are effective in heating the underlying dense layers. The local viscous dissipation within the disk is considered negligible. The energy coupling between the disk and the corona forces the electron temperature to adjust so as to maintain comparable luminosities in the soft and hard components of the emitted radiation, independently of the optical depth of the scattering medium

Energy balance in the solar transition region. III - Helium emission in hydrostatic, constant-abundance models with diffusion

Abstract: In our previous papers we described the mathematical formalism and the computed results for energy-balance hydrostatic models of the solar transition region. In this paper we discuss in some detail the limitations of the hydrostatic and one-dimensional assumptions used. Then we analyze the determination of helium emission when diffusion is included. We use transport coefficients estimated from kinetic theory to determine the helium departures from local ionization balance. We calculate the helium spectra for each of our models and evaluate the role of helium in the energy transport. Also, we investigate the effects of coronal illumination on the structure of the transition region and upper chromosphere, and show how coronal illumination affects various EUV lines and the He I 10830 A line. Comparing with both absolute intensities and detailed line profiles, we show that our models are consistent not only with the observed hydrogen spectra but also with the available helium spectra.

A new method for the identification of non-Gaussian line profiles in elliptical galaxies

Abstract: A new parameterization for the line profiles of elliptical galaxies, the Gauss-Hermite series, is proposed. This approach expands the line profile as a sum of orthogonal functions which minimizes the correlations between the errors in the parameters of the fit. This method also make use of the fact that Gaussians provide good low-order fits to observed line profiles. The method yields measurements of the line strength, mean radial velocity, and the velocity dispersion as well as two extra parameters, h3 and h4, that measure asymmetric and symmetric deviations of the line profiles from a Gaussian, respectively. The new method was used to derive profiles for three elliptical galaxies which all have asymmetric line profiles on the major axis with symmetric deviations from a Gaussian. Results confirm that elliptical galaxies have complex structures due to their complex formation history.

Mapping the dark matter with weak gravitational lensing

Abstract: We consider the problem of reconstructing the projected mass distribution in clusters from coherent distortions of background galaxies. The ellipticity of a background galaxy provides an estimate of the trace-free components of the tidal field ∂ 2 Φ/∂θ y ∂θ j where Φ(θ) is the two-dimensional potential generated by the surface density Σ(θ). We present a technique for inverting this problem. The resulting surface density contains a strong, but incoherent, random component arising from the random intrinsic galaxy ellipticities. This can be removed by filtering

The Tip of the Red Giant Branch as a Distance Indicator for Resolved Galaxies

Abstract: We show that the I magnitude of the tip of the first-ascent red giant branch (TRGB) of low-mass stars is a distance indicator for resolved galaxies with metal-poor ([Fe/H]<−0.7 dex) old populations, having a precision comparable to primary distance indicators such as Cepheids and RR Lyraes. A comparison of the distances to resolved galaxies based on the TRGB with those derived from Cepheids and RR Lyraes shows that they agree within ±0.1 mag. Advantages of the TRGB method are discussed

Dense cores in dark clouds. VIII - Velocity gradients

Abstract: An analysis of motions consistent with uniform rotation in dense cores is presented. Twenty-nine of the 43 cores studied have a statistically significant gradient. Some gradients are spatially continuous and are consistent with uniform rotation, but other apparent gradients are caused by clump-clump motion, or sharp localized gradients, within a map. The motions in L1495, B217, L1251, L43, B361, and L1551 are discussed in detail. In L1551, the residuals of the fit to the NH3 velocity field indicate an outflow from IRS5 in the same direction as the CO outflow. Gradient orientation appears to be preserved over a range of density, as evidenced by comparing results of NH3 to fits of (C-18)O and CS maps. The specific angular momentum is found to scale roughly as F exp 3/2, where R represents the diameter of the FWHM intensity contour in a map.

Relativistic fireballs and their impact on external matter: Models for cosmological gamma-ray bursts

Abstract: We discuss the production of cosmological gamma-ray bursts intense enough to be detected at cosmological distances. Events such as the coalescence of compact binaries can create sufficient energy on time scales much less than 1 s. A short 'primary' burst is expected when the resultant fireball becomes optically thin, but this may be weak because the bulk of the radiative energy has been converted into kinetic energy while still trapped within the fireball. But when this expanding material impacts on an external medium, its bulk kinetic energy can be rerandomized. The requirements on the composition of the fireball itself are less stringent than for other interpretations of cosmological gamma-ray bursts. Moreover, our model suggests that the spectra and time structure of the bursts may depend in interesting ways on the environment in which the energy-generating event occurs.

Rapidly Rotating Neutron Stars in General Relativity: Realistic Equations of State

Abstract: We construct equilibrium sequences of rotating neutron stars in general relativity. We compare results for fourteen nuclear matter equations of state. We determine a number of important physical parameters for such stars, including the maximum mass and maximum spin rate. The stability of the configurations to quasi-radial perturbations is assessed. We employ a numerical scheme particularly well-suited to handle rapid rotation and large departures from spherical symmetry. We provide an extensive tabulation of models for future reference. Two classes of evolutionary sequences of fixed baryon rest mass and entropy are explored: normal sequences, which behave very much like Newtonian evolutionary sequences, and supramassive sequences, which exist solely because of relativistic effects. Adiabatic dissipation of energy and angular momentum causes a star to evolve in a quasi-stationary fashion along an evolutionary sequence. Supramassive sequences have masses exceeding the maximum mass of a nonrotating neutron star. A supramassive star evolves towards eventual catastrophic collapse to a black hole. Prior to collapse, the star actually spins up as it loses angular momentum, an effect that may provide an observable precursor to gravitational collapse to a black hole. Keywords: pulsar, stars, neutron, rotation

Heating of galactic disks by mergers

Abstract: The paper considers dynamical aspects of mergers between disks and small satellite companions, with special attention given to: the self-consistent response of disks to satellite mergers, which generalizes an earlier investigation by Quinn & Goodman (1986); the evolution of satellite orbits and debris; and the long-term structure and kinematics of thickened disks as compared with observed properties of disk galaxies. For satellites with masses in the range 0.04-0.2 M sub disk on initially circular inclined prograde orbits merging with disks similar to the thin disk of the Galaxy, it is found that the dynamical coupling between disks and satellites leads to orbital decay. Disks respond to mergers by spreading both radially and vertically. The radial heating is driven largely by the spiral response of the dynamically cool disks to satellite perturbations.

A solar dynamo surface wave at the interface between convection and nonuniform rotation

Abstract: A simple dynamo surface wave is presented to illustrate the basic principles of a dynamo operating in the thin layer of shear and suppressed eddy diffusion beneath the cyclonic convection in the convection zone of the sun. It is shown that the restriction of the shear delta(Omega)/delta(r) to a region below the convective zone provides the basic mode with a greatly reduced turbulent diffusion coefficient in the region of strong azimuthal field. The dynamo takes on the character of a surface wave tied to the lower surface z = 0 of the convective zone. There is a substantial body of evidence suggesting a fibril state for the principal flux bundles beneath the surface of the sun, with fundamental implications for the solar dynamo.

Relativistic bulk motion in active galactic nuclei

Abstract: We discuss the evidence for relativistic bulk motion of the emitting plasma in the nuclei of ∼100 radio sources, which include BL Lacertae objects, radio quasars, and radio galaxies, with published VBLI measurements of the core angular dimension and radio flux. Comparing the predicted and observed high-frequency (X-ray) flux, in the framework of the synchrotron self-Compton model, we derive the beaming or Doppler factor for all sources. This is compared with other beaming indicators, such as the value of the expansion velocity (mostly superluminal and available for ∼40% of the objects) and the ratio of the core to the extended radio flux (available for all but two sources)

The Pre-Main-Sequence Evolution of Intermediate-Mass Stars

Abstract: We calculate numerically the structure and evolution of pre-main-sequence stars with masses from 1 to 6 M ○ These stars are assumed to originate from protostars accreting within molecular clouds For masses from 2 to 4 M ○ , the stellar luminosity first increases markedly during a protracted epoch of nonhomologous contraction and thermal relaxation Deuterium burns in a subsurface shell throughout this period More massive objects are thermally relaxed from the start; they contract homologously, with fully radiative interiors Stars with masses greater then 8 M ○ have no pre-main-sequence phase, since they are already burning hydrogen by the time protostellar accretion has ended

The alpha -enhanced Isochrones and Their Impact on the FITS to the Galactic Globular Cluster System

Abstract: We analyze in detail the effect produced by the enhancement of the α-elements O, Ne, Mg, Si, S, and Ca on the evolutionary properties of low-mass, low-metallicity stars. In particular we address the evolutionary phases which extend from the main sequence up to the H reignition on the asymptotic giant branch. We find that the α-elements other than O cannot be neglected

Biasing and Hierarchical Statistics in Large-Scale Structure

Abstract: In this paper we consider the consequences for galaxy formation of an arbitrary, effectively local biasing transformation of a hierarchical underlying matter distribution. We show that a general form of such a transformation preserves the hierarchical properties and the shape of the dispersion in the limit of small fluctuations, i.e., on large scales, although the values of the hierarchical amplitudes may change arbitrarily. We present expressions for the induced hierarchical amplitudes S(g,j) of the galaxy distribution in terms of the matter amplitudes S(j) and biasing parameters for j = 3-7. For higher order correlations, j greater than 2, restricting to a linear bias is not a consistent approximation even at very large scales. To draw any conclusions from the galaxy distribution about matter correlations of order j, properties of biasing must be specified completely to order j - 1.

Ultraviolet Radiation from Evolved Stellar Populations. I. Models

Abstract: This series of papers comprises a systematic exploration of the hypothesis that the far-ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the zero-age horizontal branch (ZAHB) through to a point late in post-HB evolution or a point on the white dwarf cooling track. The grid will be used to produce synthesized UV fluxes for the interpretation of existing and future short-wavelength (900-3000 A) observations

A catalog of intracluster gas temperatures

Abstract: We have searched the Einstein Monitor Proportional Counter (MPC) data base for observations of clusters of galaxies. By coadding the MPC spectra obtained during all pointed observations of clusters with IPC count rates greater than 0.1 counts/s, we have obtained sufficient photon statistics to estimate the X-ray temperature of 84 clusters. Combining the MPC results with EXOSAT and Ginga results reported in the literature yields a combined sample of 104 clusters with known X-ray temperatures. One of the best studied X-ray correlations between clusters is that between their X-ray luminosity and gas temperature. We show that the best-fit power-law relation for our combined cluster sample can be explained by the observed increase in the gas-to-stellar mass ratio between low- and high-temperature clusters. The statistical significance of any evolution in our combined X-ray sample has been examined and compared with the statistical properties of clusters culled from optical catalogs. We find that there is strong evidence for a decrease in the X-ray luminosity of optically rich clusters beyond z approximately about 0.06. This result is used to estimate the normalization of the primordial power spectrum of density fluctuations.

Beyond Clausius-Mossotti: Wave Propagation on a Polarizable Point Lattice and the Discrete Dipole Approximation

Abstract: We derive the dispersion relation for electromagnetic waves propagating on a lattice of polarizable points. From this dispersion relation we obtain a prescription for choosing dipole polarizabilities so that an infinite lattice with finite lattice spacing will mimic a continuum with dielectric constant. The discrete dipole approximation is used to calculate scattering and absorption by a finite target by replacing the target with an array of point dipoles. We compare different prescriptions for determining the dipole polarizabilities. We show that the most accurate results are obtained when the lattice dispersion relation is used to set the polarizabilities.

Massive star formation near the Galactic center and the fate of the stellar remnants

Abstract: Several points are made regarding massive stars, star formation, and stellar remnants in the Galactic center region, particularly the inner 1-10 parsecs. First, in light of the processes which act to inhibit or suppress star formation there, it is argued that those stars which do form tend to be formed by an externally caused compression of their parent clouds rather than by spontaneous cloud collapse. As a result of this, and because of the particular characteristics of the interstellar medium near the Galactic center, it is likely that the initial mass function (IMF) favors more massive stars than that in the Galactic disk, or at least that the lower mass cutoff of the Galactic center IMF is relatively large