Showing papers in "The Astrophysical Journal in 1976"

An analytic expression for the luminosity function for galaxies

Abstract: A new analytic approximation for the luminosity function for galaxies is proposed, which shows good agreement with both a luminosity distribution for bright nearby galaxies and a composite luminosity distribution for cluster galaxies. The analytic expression is proportional to L/sup -5///sup 4/e/sup -//sup L*/, where L* is a characteristic luminosity corresponding to a characteristic absolute magnitude M*/sub B0//sub -//sub 2//sub 0//sub .6/For an individual cluster, the characteristic magnitude may be determined with an accuracy of approx.0.25 mag, suggesting its use as a standard candle. The analytic expression is used to compute an expected richness-absolute magnitude correlation for first ranked cluster galaxies and an expected dispersion, which are compared with the data of Sandage and Hardy. (AIP)

Velocity dispersions and mass-to-light ratios for elliptical galaxies.

Abstract: The close correlation between luminosity and dynamical properties for normal ellipticals is further evidence that the ellipticals are very nearly a one-parameter family with total mass as the most important independent variable. (AIP)

A two-temperature accretion disk model for Cygnus X-1: structure and spectrum.

Abstract: We present a model for Cygnus X-1, involving an accretion disk around a black hole, which can explain the observed X-ray spectrum from 8 to 500 keV. In particular we construct a detailed model of the structure of an accretion disk whose inner region is considerably hotter and geometrically thicker than previous disk models. The inner region of the disk is optically thin to absorption, is gas-pressure dominated, and yields, from first principles, electron temperatures of 10/sup 9/ K and ion temperatures 3--300 times hotter. The spectrum above 8 keV is produced by inverse Compton scattering of soft X-ray photons in the two-temperature inner region of the disk. This spectrum is computed by numerical integration of the Kompane'ets equation, modified to account for escape of photons from a region of finite (order unity) electron scattering optical depth. (AIP)

Star distribution around a massive black hole in a globular cluster.

Abstract: To predict the likely distribution of stars around a massive black hole in the core of a cluster of stars, an equation of the Fokker-Planck type is derived that describes the diffusion of stars in the 1/r gravitational well of the black hole, by star-star gravitational collisions. The main assumptions are: (1) the distribution of stars is described by a single-particle distribution function that is shperically symmetric in coordinate space and approximately isotropic in velocity space; (2) the stars have equal masses; (3) star mass very-much-less-thanblack-hole massvery-much-less-thancluster-core mass; (4) a star is destroyed by star-star collisions or by tidal forces when its binding energy in the well exceeds a specified large value; (5) binaries are unimportant. Numerical solutions for the time-dependent equations indicate that the equilibrium star density, closely approached within a collision time, approximates an r/sup -7///sup 4/ power law throughout most of the well. The same equilibrium power law obtains for nonisotropic distribution functions whose anisotropy is independent of r. Stars in bound orbits about a black hole diffuse slowly into its gravitational well under equilibrium conditions. A black hole of approximately-less-than10/sup 3/ M/sub sun/ may accrete stars primarily by capture from unbound orbits.Theshape of the star distribution more » near the cluster center observed with small diaphragms is predicted. As a function of diaphragm size, the velocity dispersion and line profile that might be measured spectroscopically is calculated. Some calculations are also presented for an open slit configuration. For globular clusters in our Galaxy, one might be able to detect black holes with masses approximately-greater-than5 x 10/sup 3/ M/sub sun/ and, with a large space telescope, masses approximately-greater-than10/sup 3/ M/sub sun/. An approximate formula for the mean distance of a massive black hole from the center of mass of the unbound stars is also presented. (AIP) « less

Parameter estimation in X-ray astronomy

Abstract: The problems of model classification and parameter estimation are examined, with the objective of establishing the statistical reliability of inferences drawn from X-ray observations. For testing the validities of classes of models, the procedure based on minimizing the chi/sup 2/ statistic is recommended; it provides a rejection criterion at any desired significance level. Once a class of models has been accepted, a related procedure based on the increase of chi/sup 2/ gives a confidence region for the values of the model's adjustable parameters. The procedure allows the confidence level to be chosen exactly, even for highly nonlinear models. Numerical experiments confirm the validity of the prescribed technique.The chi/sup 2//sub min/+1 error estimation method is evaluated and found unsuitable when several parameter ranges are to be derived, because it substantially underestimates their joint errors. The ratio of variances method, while formally correct, gives parameter confidence regions which are more variable than necessary. (AIP)

Consequences of a New Hot Component of the Interstellar Medium

Abstract: The suggestion that the observed 0.25 keV X-ray background and 0 vi absorption lines are produced by a new, hot (Tapprox.10/sup 6/ K), diffuse component of the interstellar medium is examined in the context of both a steady-state and a time-dependent model. It is concluded that such a component can explain the two observations only if the pressure of the cooler interstellar medium is approx.10 times higher than the previously estimated p/kapprox. =2000 cm/sup -3/ K; a mechanism such as the proposed convective-radiative ''galactic fountain'' exists to cool the hot gas; and another component is responsible for the presence of 0 ii at temperatures below 10/sup 6/ K. (AIP)

Energy spectra of X-ray clusters of galaxies

Abstract: A procedure for estimating the ranges of parameters that describe the spectra of X-rays from clusters of galaxies is presented. The applicability of the method is proved by statistical simulations of cluster spectra; such a proof is necessary because of the nonlinearity of the spectral functions. Implications for the spectra of the Perseus, Coma, and Virgo clusters are discussed. The procedure can be applied in more general problems of parameter estimation.

Surface Brightness and Evolution of Galaxies

Abstract: It is well known that before the redshift-magnitude diagram of galaxies could be used for determination of the cosmological parameters one must know the evolution of the galaxies. We propose use of apparent surface brightness: which depends only on the redshift and is independent of the cosmological model and the inhomogeneities in the universe: for observational determination of the evolution of galaxies. The needed observations are isophotal angular diameters and apparent magnitudes within this or any other reasonable angular diameter. The application of the results for determination of q/sub 0/ is discussed briefly. (AIP)

An infall model for the T Tauri phenomenon

Abstract: An infall model which produces P Cygni type II emission lines is presented. The blueshifted apparent absorption feature in these emission lines is interpreted in this model as the overlap region between a strong emission line centered near the rest velocity of the star and a weaker emmission line at a velocity near -150 km s/sup -1/. According to this model the energy source for the continuum and line-emission features observed in T Tauri stars is the kinetic energy of the infalling matter. A weak flux of soft X-rays is produced in the high-temperature postshock gas. These X-rays may be responsible for producing the nebulae observed near some T Tauri stars. (AIP)

The tidal disruption of neutron stars by black holes in close binaries.

Abstract: The formation of close, doubly compact binary systems from close massive binaries is considered. Massive X-ray binaries are shown to be possible progenitors of these systems. The recent observation of the binary pulsar indicates that doubly compact binary systems may exist. Reasonable scenarios can be developed in which at least one of the compact stars, in many cases, is a black hole. In general, these objects have evolved to separations less than 10 R/sub sun/, and thus their orbits decay (within 10/sup 10/ yr) via the emission of gravitational radiation. Black-hole-neutron-star collisions may therefore be possible in these systems. A model for the tidal disruption of a neutron star by the black hole in these systems is constructed, and shows that mass ejection to infinity from 1.3 M/sub sun/ neutron stars (a reasonable value suggested by evolutionary calculations) spiraling into black holes with masses less than 8--17 M/sub sun/ is possible, this range depending upon the neutron star equation of state. For much larger black hole masses, the neutron star breakup occurs inside the Schwarzschild radius of the neutron star. A statistical estimate for the frequency of black-hole-neutron-star collisions is given, and possible implications for nucleosynthesis and antineutrino emission aremore » discussed. (AIP)« less

Gamma rays from primordial black holes

Abstract: This paper examines the possibilities of detecting hard ..gamma..-rays produced by the quantum-mechanical decay of small black holes created by inhomogeneities in the early universe. Observations of the isotropic ..gamma..-ray background around 100 MeV place an upper limit of 10/sup 4/ pc/sup -3/ on the average number density of primordial black holes with initial masses around 10/sup 15/ g. The local number density could be greater than this by a factor of up to 10/sup 6/ if the black holes were clustered in the halos of galaxies. The best prospect for detecting a primordial black hole seems to be to look for the burst of hard ..gamma..-rays that would expected in the final stages of the evaporation of the black hole. Such observations would be a great confirmation of general relativity and quantum theory and would provide information about the early universe and about strong-interaction physics. (AIP)

W Ursae Majoris systems with marginal contact.

Abstract: The A-type systems are suggested to be W UMa systems that have been able to achieve thermal equilibrium as a result of nuclear evolution. The relationship of W UMa systems to other short-period binaries is also discussed. (AIP)

Nonhomologous contraction and equilibria of self-gravitating, magnetic interstellar clouds embedded in an intercloud medium: star formation. II. Results.

Abstract: The paper studies the equilibrium of self-gravitating isothermal models of interstellar clouds with a frozen-in magnetic field linked smoothly to the field of a hot tenuous intercloud medium. Equilibrium states are determined which can be reached by clouds contracting nonhomologously from a spherical uniform initial state. Three free parameters characterize the problem: a dimensionless initial radius related to the Jeans length of the cloud, the initial ratio of the magnetic and gas pressures in the cloud, and the initial ratio of the intercloud and cloud pressures. The dependence of the solutions on each of these parameters is investigated. It is found that: (1) the frozen-in field causes the cloud to become oblate with its major axis normal to the field lines; (2) the flattening increases as the magnetic-field strength, gravitational forces, or intercloud pressure increases; (3) increasing intercloud pressure eventually leads to gravitational collapse; and (4) the cloud can reach equilibrium only if its radius does not exceed some critical value. The observed inefficiency of the star-formation process within massive clouds is examined and explained in terms of magnetic phenomena in a collapsing cloud.

Photonuclear interactions of ultrahigh energy cosmic rays and their astrophysical consequences.

Abstract: Results of detailed Monte Carlo calculations of the interaction histories of ultrahigh energy cosmic-ray nuclei with intergalactic radiation fields are presented. Estimates of these fields and empirical determinations of photonuclear cross sections are used, including multinuclear disintegrations for nuclei up to 56Fe. Intergalactic and galactic energy loss rates and nucleon loss rates for nuclei up to 56Fe are also given. Astrophysical implications are discussed in terms of expected features in the cosmic-ray spectrum between quintillion and sextillion eV for the universal and supercluster origin hypotheses. The results of these calculations indicate that ultrahigh energy cosmic rays cannot be universal in origin regardless of whether they are protons or nuclei. Both the supercluster and galactic origin hypotheses, however, are possible regardless of nuclear composition.

Empirical effective temperatures and bolometric corrections for early-type stars

Abstract: An empirical effective temperature for a star can be found by measuring its apparent angular diameter and absolute flux distribution The angular diameters of 32 bright stars in the spectral range O5f to F8 have recently been measured with the stellar interferometer at Narrabri Observatory, and their absolute flux distributions have been found by combining observations of ultraviolet flux from the Orbiting Astronomical Observatory (OAO-2) with ground-based photometry In this paper, these data have been combined to derive empirical effective temperatures and bolometric corrections for these 32 stars

Arrival-time analysis for a pulsar in a binary system.

Abstract: A method is described for analyzing the arrival times of pulses from the binary pulsar PSR 1913+16, in terms of the orbital elements and their possible secular variations. Estimates are given for the times necessary to measure such secular changes and to detect various relativistic effects. If measurements errors approx.1 ms are the dominant source of error and approx.1000 independent observations are made per year, then approx.5 years of observations are necessary for a dynamical determination of the component masses accurate to 10 percent and approx.15 years for the possible detection of gravitational radiation. Other sources of error are briefly discussed. (AIP)

Discovery of intense x-ray bursts from the globular cluster NGC 6624

Abstract: A new type of time variation of cosmic X-ray sources has been found from the Astronomical Netherlands Satellite (ANS) observations of the source 3U 1820-30, associated with the globular cluster NGC 6624. Two bursts in the 1-30 keV X-ray intensity of this source are reported. Each displayed a rapid rise in flux (no more than 1 s) by a factor of 20-30 followed by an approximately 8-s exponential decay. These bursts appear to be qualitatively different from short time variations previously reported for X-ray sources. Analysis for further source variability, energy spectra, and position is presented. The characteristics of these events may imply the existence of a collapsed core in the globular cluster.

Accretion onto magnetized neutron stars: structure and interchange instability of a model magnetosphere.

Abstract: A self-consistent model is analyzed for the spherical infall of weakly magnetized plasma into the magnetosphere of a slowly rotating, strongly magnetized neutron star. It is shown that spherical infall is probably a good approximation for X-ray sources which accrete from a stellar wind. The location of the standoff shock which halts the hypersonic infall is estimated along with the emission from the shocked layer. The location of the equilibrium magnetopause and the structure of the magnetic field within it are calculated; it is found that the magnetic poles are true cusps and that the entry of gas due to equilibrium flow across a cusp is almost certainly dominated by the interchange instability near the magnetic equator. The energy principle is applied to derive necessary conditions for the occurrence of this instability. The results indicate that the strong magnetic-pressure gradient stabilizes the gas unless moderately strong radiative cooling takes place and that the cooled plasma enters the magnetosphere as long filaments capable of moving between field lines. The rate at which the equilibrium magnetopause can 'absorb' mass and momentum is derived, the validity of the approximations employed is discussed, and the likely evolution of the sinking filaments is outlined to show that the spatial distribution of the plasma is determined mainly by the dynamics and thermodynamics of the filaments rather than the magnetic-field structure.

Systematic variations in the wavelength dependence of interstellar linear polarization

Abstract: New observations of the wavelength dependence of interstellar linear polarization [p(λ)] made to investigate the influence of the environment on the effective size distribution of the aligned polarizing particles are presented. Optical and infrared measurements were obtained simultaneously in eight photometric passbands between U and K, giving a coherent data set for a total of 105 reddened stars. The p(λ) data were modeled using the usual Serkowski relation with three parameters: p max , λ max , and K

Evidence for interplanetary acceleration of nucleons in corotating interaction regions

Abstract: Simultaneous measurements of protons and helium in the energy range 0.5-2 MeV per nucleon on IMP-8 (at 1 AU) and Pioneer-10 and Pioneer-11 (between 1 and 8 AU) have revealed a correlation which develops beyond about 2.5 AU between the positions of double peaks in recurring proton intensity increases and the leading and trailing edges of recurring regions of fast solar wind streams and enhanced magnetic field. From these correlations and the study of energy spectra and the proton-to-helium abundance ratios, it is concluded that there is local acceleration of nucleons, probably near the boundary regions where shocks are observed, even if the nucleons are injected initially at the sun.

A Cyclic Thermal Instability in Contact Binary Stars

Abstract: It is shown analytically that contact binary stars coupled by a common convective envelope, the Lucy model, are almost invariably unstable when subjected to mass transfer: if either component begins to exchange mass, it will continue to do so. A detailed sequence of models is calculated which follows the thermal evolution of a 2 M/sub sun/ contact binary of normal Population I abundances (X=0.70, Z=0.02), starting at nearly equal mass. The initial instability develops into a cyclic exchange of mass with the mass fraction oscillating between 0.56< or =M/sub 1//(M/sub 1/+M/sub 2/) < or =0.62 with a period of approx.10/sup 7/ years. The system achieves thermal equilibrium on the average, but individually both stars are perpetually out of equilibrium. The instability is of a general nature and such oscillating systems can satisfactorily populate the short-period, red region of the period-color relation for W UMa stars. (AIP)

The hydrodynamics of Type II supernove

Abstract: Observations of Type II supernovae indicate the presence of a moderately cool expanding photosphere. This situation can result from an explosion in a star with an extended envelope. The evolutionary phases of an explosion are described. Information on the propagation of the shock wave through the star can be obtained from ..gamma..=4/3 blast wave solutions. If the photon mean free path becomes large compared to the length scales of the flow, a thermal wave moves out from the shock wave and a dense shell is formed behind the shock. The arrival of the shock wave at the photosphere is accompanied by ultraviolet and X-ray burst. As the star expands, a rarefaction wave converts internal energy into kinetic energy. Detailed hydrodynamic models have been calculated, assuming an initial radius compatible with stellar evolution and an energy compatible with the observed velocities. The observed values of photospheric radius and temperature near maximum light are reproduced. Features of the models which are consistent with observation are: the ejection of a detached shell; the cooling of the photosphere from 10,000 K to 6000 K in tens of days after maximum visual light; the shape of the light curve around maximum; the decrease in themore » velocity of the gas at the photosphere in tens of days after maximum; and a photospheric radius of about 10/sup 14/ cm after several hundred days. (AIP)« less

Interstellar H$sub 2$: The population of excited rotational states and the infrared response to ultraviolet radiation

Abstract: Molecular hydrogen in interstellar clouds absorbs ultraviolet radiation in lines of the Lyman and Werner systems. The subsequent fluorescence leads to dissociation or to the population of excited rotational-vibrational levels of the ground electronic state. The rotational-vibrational levels decay by means of quadrupole transitions which result in the emission of infrared photons and populate excited rotational levels of the lowest vibrational state. An additional input into the cascade is provided by the formation of molecules on grain surfaces. A concise tabular description of the rotation-vibration cascade is presented. Calculations of the infrared emission spectrum of H$sub 2$ are given for an illustrative cloud model which includes the processes of fluorescence and hot molecule formation. In favorable circumstances, some of the infrared lines may have detectable intensities. Due to distribution of lines, it may prove possible to detect interstellar H$sub 2$ using narrow-band filter photometry at a wavelength of 2.4 $mu$. (AIP)

The evolution of rotating stars. I - Method and exploratory calculations for a 7-solar-mass star

Abstract: A method is developed which allows the evolution of rotating stars to be studied well beyond the main-sequence stage. Four different cases of redistribution of angular momentum in an evolving star are considered. Evolutionary sequences for a 7-solar-mass star, rotating according to these different cases, were computed from the zero-age main-sequence to the double-shell-source stage. Each sequence was begun with a (typical) equatorial rotational velocity of 210 km/s. On the main sequence, the effects of rotation are of minor importance. However, as the core contracts during later stages, important effects arise in all physically plausible cases. The outer regions of the cores approach critical velocities and develop unstable angular-velocity distributions. The effects of these instabilities should significantly alter the subsequent evolution.

The binary pulsar: physical processes, possible companions, and evolutionary histories.

Abstract: A study of some aspects of the binary pulsar, PSR 1913+16, is presented in the light of recently reported observational results. If the companion to the pulsar gives a Newtonian contribution to the observed apsidal motion (a helium star or a rotating white dwarf), then it will probably be observable: the former either optically or through the dispersive effects of a stellar wind, and the latter through a secular change in the observed inclination of the orbit. If alternatively the companion behaves dynamically as a point mass (apsidal motion caused solely by general relativity), observations of the O (..nu../c)/sup 2/ frequency shift will furnish a measurement of the masses of the components. In addition, with a tenfold improvement in timing accuracy, the post-Newtonian corrections to the Keplerian ellipse and the effects of aberration could in principle be measured. This would then provide a determination of the orientation of the pulsar spin axis and allow the observation of geodetic spin precession to become a test of general relativity. We also examine the test based on the detection of orbital period changes due to gravitational radiation.Possible evolutionary histories are discussed. The most probable present system consists of two neutron stars, the endmore » result of an X-ray binary phase followed by a double core star phase. In this scenario the long timing age and short period of PSR 1913+16 are shown to be interrelated if the pulsar was the first neutron star formed. To show that other companions are not ruled out on evolutionary grounds, we construct alternate histories leading to either He star, white dwarf, or black hole companions. (AIP)« less