Showing papers in "The Astrophysical Journal in 1988"

Ultraluminous infrared galaxies and the origin of quasars

Abstract: The Infrared Astronomical Satellite (IRAS) has discovered a class of ultraluminous infrared galaxies (L ir > 1012 L ⊙) that are bolometrically as luminous, and slightly more numerous than optically selected quasars in the local universe (z ≲ 0.2). Ground-based observations of a complete sample of 10 of these infrared galaxies suggest that they represent the initial dust-enshrouded stages of quasars. A second group of ultraluminous objects selected on the basis of ‘warm’ infrared colors are found to be preferentially Seyfert galaxies. We suggest that these ultraluminous ‘warm’ objects represent the transition phase between the infrared selected and optically selected quasars.

Nanoflares and the solar X-ray corona

Abstract: Observations of the sun with high time and spatial resolution in UV and X-rays show that the emission from small isolated magnetic bipoles is intermittent and impulsive, while the steadier emission from larger bipoles appears as the sum of many individual impulses. We refer to the basic unit of impulsive energy release as a nanoflare. The observations suggest, then, that the active X-ray corona of the sun is to be understood as a swarm of nanoflares. This interpretation suggests that the X-ray corona is created by the dissipation at the many tangential discontinuities arising spontaneously in the bipolar fields of the active regions of the sun as a consequence of random continuous motion of the footpoints of the field in the photospheric convection. The quantitative characteristics of the process are inferred from the observed coronal heat input.

Cosmology with a Time Variable Cosmological Constant

Abstract: Description d'une cosmologie basee sur une constante cosmologique Λ decroissante avec le temps quand l'energie potentielle du champ scalaire φ possede une queue en loi de puissance V∼φ − α pour φ eleve

Slim Accretion Disks

Abstract: In a recent paper, Abramowicz, Czerny, Lasota and Szuszkiewicz (1988) have found a new branch of equilibrium solutions for stationary accretion discs around black holes. These solutions correspond to moderately super-Eddington accretion rates. The existence of the new branch is a consequence of cooling due to general relativistic Roche lobe overflow and horizontal advection of heat. On an accretion rate versus surface density plane the new branch forms, together with the two “standard” branches (corresponding to to the Shakura-Sunyaev accretion discs models) a characteristically S-shaped curve. The relation between this “S-curve” and the variability of accretion discs is under study and seems to allow, at least in principle, for a limit cycle behaviour.

The discrete-dipole approximation and its application to interstellar graphite grains

Abstract: The discrete dipole approximation (DDA), a flexible method for computing scattering of radiation by particles of arbitrary shape, is extended to incorporate the effects of radiative reaction and to allow for possible anisotropy of the dielectric tensor of the material. Formulas are given for the evaluation of extinction, absorption, scattering, and polarization cross sections. A simple numerical algorithm based on the method of conjugate gradients is found to provide an efficient and robust method for obtaining accurate solutions to the scattering problem. The method works well for absorptive, as well as dielectric, grain materials. Two validity criteria for the DDA are presented. The DDA is then used to compute extinction cross sections for spherical graphite grains and to calculate extinction cross sections for nonspherical graphite grains with three different geometries. It is concluded that the interstellar 2175 A extinction feature could be produced by small graphite grains which should have aspect ratios not far from unity. 35 references.

The discrete correlation function: a new method for analyzing unevenly sampled variability data

Abstract: A method for measuring correlation functions without interpolating in the temporal domain is proposed which provides an assumption-free representation of the correlation measured in the data and allows meaningful error estimates. Physical interpretation of the cross-correlation function of two series believed to be related by a convolution is shown to require knowledge of the input function's fluctuation power spectrum. Application of the method to two systems reveals no correlation for the optical data of Akn 120, but a strong correlation for the UV data of NGC 4151, placing bounds of between 1.2 and 20 light days on the size of the line-emitting region.

Simulation of magnetohydrodynamic flows: A Constrained transport method

Abstract: Description d'une nouvelle technique numerique, appelee methode du transport contraint, pour etudier l'evolution de l'equation d'induction en fonction du flux magnetique. Cette methode fonctionne aussi bien a 3 dimensions qu'a 2 et l'expression des equations dans le cadre de la relativite generale ne pose pas de problemes non plus. Des resultats de simulations obtenus par cette methode sont compares a ceux d'autres modeles numeriques

The photodissociation and chemistry of interstellar CO

Abstract: Recent work on the vacuum UV absorption spectrum of CO to the description of the photodissociation of interstellar CO and its principal isotopic varieties is discussed. The effects of line broadening, self-shielding, shielding by H and H2, and isotope-selective shielding are examined as functions of depth into interstellar clouds. The photodissociation rates of the isotopic species are larger than that of (C-12)O inside the clouds by up to one to two orders of magnitude. A simple approximation to the attenuation by line absorption is given in tabular form. Computed abundances of CO and related species C and C+ are presented for a variety of interstellar clouds ranging from diffuse clouds to dense photodissociation regions. Several series of models of translucent clouds are presented which illustrate how the CO abundance increases rapidly with total cloud thickness. The variations of the isotopic abundances with depth and their sensitivity to temperature and total cloud thickness are explored in detail.

Time-dependent models of radiatively driven stellar winds. I - Nonlinear evolution of instabilities for a pure absorption model

Abstract: Numerical radiation-hydrodynamics simulations of the nonlinear evolution of instabilities in radiatively driven stellar winds have been performed. The results show a strong tendency for the unstable flow to form rather sharp rarefactions in which the highest speed material has very low density. The qualitative features of the model agree well with the reqirements of displaced narrow absorption components in UV lines.

Molecular tori in Seyfert galaxies - Feeding the monster and hiding it

Abstract: The principal properties of the tori of gas which surround Seyfert nuclei are discussed. The internal state of the clouds and their size distribution function are examined, and it is shown that the Jeans mass scale results in clouds which are individually sufficiently opaque to block out the nucleus, and that the balance of processes which controls their size distribution function also forces the covering factor to be of the order of or greater than unity. Where the gravitational potential is dominated by stars, cloud-cloud collisions keep the molecular clouds close to the equatorial plane. Stirring by stellar processes is never strong enough to compete with collisional losses. The position of the inner edge of the torus is determined by a balance between the inward flow of clouds and the rate at which the nuclear continuum can evaporate them.

The equation of state for stellar envelopes. I - An occupation probability formalism for the truncation of internal partition functions

Abstract: An equation of state for material in stellar envelopes, subject to the limits of temperature less than about 10 to the 7th K and density less than about .01 g/cu cm is presented. The equation makes it possible to express free energy as the sum of several terms representing effects such as partial degeneracy of the electron, Coulomb interactions among charged particles, finite-volume, hard sphere repulsion, and van der Waals attraction. An occupation probability formalism is used to represent the effects of the plasma in establishing a finite partition function. It is shown that the use of the static screened Coulomb potential to calculate level shifts and to estimate the cutoff of the internal partition function is invalid. For most of the parameter space relevant to stellar envelopes, perturbations arising from the plasma ions are shown to be dominant in establishing the internal partition function.

Warm ultraluminous galaxies in the IRAS survey - The transition from galaxy to quasar?

Abstract: Of 90 extragalactic objects chosen from the IRAS catalog on the basis of brightness at 60 mum and "warm" infrared color [f_ ν(25 µm)/f_ ν(60 µm) >0.2], 12 have luminosities characteristic of quasars. These 12 ultraluminous objects have Seyfert spectra, and nine of them show morphological evidence for recent collisions or mergers - they plausibly represent a transition stage between cooler ultraluminous infrared galaxies and optical quasars.

OJ 287 - Binary pair of supermassive black holes

Abstract: A historical light curve of the BL Lacertae object OJ 287 is constructed in the optical V band using observations between 1890 and the present The curve exhibits periodic outbursts at intervals of 116 yr or 9 yr The individual outbursts show an initial maximum with decaying submaxima at intervals of about 1 month It is proposed that these characteristics are caused by a binary in the nucleus of OJ 287, with the binary members having an accretion disk around them The outbursts at 9-yr intervals would be caused by the tidal action of the companion on the disk of the larger black hole A computer simulation is used to demonstrate that the inflow into the center of such a disk during the repeated periastron passages of a companion will produce an outburst similar to the ones observed Based on indirect evidence, probable masses of 5 billion solar and 20 million solar are determined for the two objects 38 references

Angular momentum loss in low-mass stars

Abstract: The wind models discussed by Mestel (1984) are used here to formulate a general expression for the rate of angular momentum loss by magnetic stellar winds as a function of magnetic field configuration, rotation rate, and stellar model properties. The sensitivity of the rotation velocity to the various wind model parameters, the initial angular momenta, and the time dependence of the angular velocity for each mass is shown. The theoretical results are compared with observational ones, and it is found that the existence of very rapidly rotating stars in young clusters implies that low-mass stars are formed with a large spread of angular momentum. The high efficiency of angular momentum loss through magnetic stellar winds causes the rotation velocity to become less dependent on initial angular momentum J0 with time; by 300 million yur, the rotation velocity becomes independent of J0. This results in a decrease with time in the spread of rotation velocities as a function of stellar mass in young clusters.

Dynamics of radiative supernova remnants

Abstract: A high-resolution numerical simulation is used to study the evolution of a SNR evolving in a homogeneous uniform medium. Emphasis is placed on the transition from the adiabatic stage to the radiative pressure-driven snowplow stage, along with the possible further establishment of a momentum-conserving snowplow state. In most cases the momentum-conserving snowplow is found to be delayed beyond the merger of the remnant with the interstellar medium. 39 references.

Superbubbles in disk galaxies

Abstract: Correlated supernovae from an OB association create a superbubble: a large, thin, shell of cold gas surrounding a hot pressurized interior. Because supernova blast waves usually become subsonic before reaching the walls of the shell or cooling radiatively, the energy input from supernovae may be reasonably approximated as a continuous luminosity. Using the Kompaneets (thin-shell) approximation, the growth of superbubbles in various stratified atmospheres is numerically modeled. A dimensionless quantity predicts whether a superbubble will blow out of the H I disk of a spiral galaxy (and begin to accelerate upward) or collapse. Superbubbles blow out of the H I layer when they have a radius in the plane between one and two scale heights. They blow out only one side of a disk galaxy if their centers are more than 50-60 p above the plane and the gas layer has density and scale height typical of the Milky Way. Fingers of warm interstellar gas intrude into the hot interior when the superbubble overtakes dense clouds.

The formation of dark halos in a universe dominated by cold dark matter

Abstract: Etude de la formation des halos galactiques dans un univers plat domine par de la matiere sombre froide en utilisant des simulations n-corps pour des regions cubiques de taille actuelle ∼14 Mpc et de densite variee, en suivant ces regions depuis Z=6 jusqu'au present avec une resolution lineaire qui decroit de ∼2 kpc a ∼14 kpc. L'etude est centree sur la structure interne, l'abondance et la distribution en masse des halos sombres, ces objets devant representer les halos observes dans les galaxies et les groupes de galaxies; on etudie egalement les trajets d'evolution qui ont conduit aux halos presents, ces trajets pouvant suggerer quand et comment les galaxies se sont formees et la dependance de leur structure sur l'accretion recente et l'amalgame

Precise Proper-Motion Measurement of Solar Granulation

Abstract: : We describe a powerful cross-correlation technique for the precise measurement of the proper motion of tracers seen on successive images of a time series of solar granulation. The cross correlation is defined as a function of position in the image, within a spatially localized apodization window. The time average of the spatially localized cross correlation gives a measure of the displacement that is not biased by atmospheric seeing. The window size and the seeing define the effective resolution of the vector displacement determination. We use this cross-correlation technique to analyze an 80 minute run of white- light observations made at the Sacramento Peak Vacuum Tower Telescope. Even though geometric distortion due to atmospheric seeing is instantaneously at least 10-20 times larger than the observed scale of the large-scale solar displacements, 100-1000 m/s , its net contribution to the 80 minute average of proper motions is RMS < 20 m/s. The measured vector displacement clearly show solar mesogranulation and super-granulation flows having spatial scales from 10 min to 40 min. The measured amplitude of these flows is significantly larger than the RMS 100 m/s noise which we attribute principally to solar granulation evolution. Keywords: Solar atmospheric motion. Reprints.

Encounters of disk/halo galaxies

Abstract: The first numerical models of interacting galaxies are presented in which all dynamical components of the participants are modeled as fully three-dimensional N-body systems with thousands of particles each and the equations of motion are integrated by a self-consistent algorithm without arbitrary restrictions on the geometry or spatial extent of the mass distribution. Several encounters between multicomponent galaxy models are described, starting with a lookalike for the 'Antennae', NGC 4038/9. The influence of massive halos is shown by contrasting models run with and without dark matter. The results show that encounters involving pairs of disk/halo galaxies can produce impressive tidal tails while permitting the visible galaxies to merge on a short time scale, mimicking 'double-tailed' objects. Extensive dark halos are very effective at soaking up binding energy and angular momentum from other components, significantly altering the distribution and dynamics of the luminous material. 43 references.

Dynamical modeling of long-period variable star atmospheres

Abstract: Using a new flexible computer program, numerical calculations were performed to investigate the dynamical structure and behavior of a spherical model atmosphere for cool pulsating Mira-like stars with masses from 0.8 to 2.0 solar masses and fundamental-mode pulsation periods from 175 to 1000 days. In particular, the response of the model to periodic driving at its inner boundary was examined for a considerable range of stellar parameters for both fundamental and overtone modes, various driving amplitudes, and various assumptions about the physical processes involved; radiation pressure on dust and time-dependent temperature relaxation were included. Stable steady state models with periodic shocks were obtained in all cases. Both shocks and dust were found to be essential for rapid mass loss. The shocks increase the density and thus greatly increase the amount of dust formed in the cool outer atmosphere. Radiation pressure accelerates the grains, which drive a slow cool wind. Calculated mass-loss rates appear to be of the right order of magnitude to agree with observations and show how mass loss can be expected to depend on the stellar parameters. 42 references.

Effects of Cold Matter in Active Galactic Nuclei: A Broad Hump in the X-Ray Spectra

Abstract: Recent observations and interpretations of the strong UV emission from active galactic nuclei (AGNs) suggest that relatively cold, thermal matter coexists with the hot, X-ray-emitting matter near the centers of these objects. A fraction of the X-rays will be reprocessed by the cold material, and the composite X-ray spectrum should help diagnose the conditions of this material and its energy source. In a variety of situations, reprocessing of the X-rays should lead to a composite X-ray spectrum with a broad hump between about 10 keV and about 300 keV. The lower limit of this energy range is determined by atomic absorption and the upper limit by electron scattering in the cold material. Where available, observed spectra are consistent with such a broad hump; however, the predicted amplitude of the hump is about 0.1-0.5, and observations with smaller error bars are clearly needed. 30 references.

The temperature structure, mass, and energy flow in the corona and inner solar wind

Abstract: Remote-sensing and in situ data are used to constrain a radiative energy balance model in order to study the radial variations of coronal temperatures, densities, and outflow speeds in several types of coronal holes and in an unstructured quiet region of the corona. A one-fluid solar wind model is used which takes into account the effects of radiative and inward conductive losses in the low corona and the chromospheric-coronal transition region. The results show that the total nonradiative energy input in magnetically open coronal regions is 5 + or - 10 to the 5th ergs/sq cm, and that most of the energy heating the coronal plasma is dissipated within 2 solar radii of the solar surface.

Accretion Disks around T Tauri Stars

Abstract: The extent to which the continuum spectral energy distributions of T Tauri stars from 0.2 to 10 microns can be explained by a simple model consisting of an active PMS star and active accretion disk is considered. The disk contributes both an IR excess due to accretion energy dissipation and stellar light reprocessing and an ultraviolet excess from the boundary layer between disk and star where half of the total accretion luminosity is generated. This model is shown to be good at predicting the range of continuum excesses observed in T Tauri stars, with accretion rates implied up to a few times 10 to the -7th solar mass/yr. 83 references.

The X-ray spectra of the extragalactic sources in the Einstein extended medium sensitivity survey

Abstract: The X-ray energy distribution of a large sample of faint extragalactic X-ray sources is studied. Two independent methods are used to determine that the average spectral index for the sources is about 0.95. The intrinsic dispersion of the spectral index distribution is about 0.36. The sources are characterized by a range of power law slopes in the soft X-ray band with a mean of 1.03 + 0.05 or - 0.06 and an intrinsic dispersion of about 0.36. The present analysis extends knowledge of the AGN spectral properties to sources which are about three times more distant and more than 10 times fainter than those previously studied. There is no evidence in the data for any change in the mean spectral index of either extragalactic sources as a whole or AGN in particular, as a function of the X-ray flux in the flux range 10 to the -11th to 10 to the -13th ergs/sq cm/s.

The equation of state for stellar envelopes. II - Algorithm and selected results

Abstract: A free-energy-minimization method for computing the dissociation and ionization equilibrium of a multicomponent gas is discussed. The adopted free energy includes terms representing the translational free energy of atoms, ions, and molecules; the internal free energy of particles with excited states; the free energy of a partially degenerate electron gas; and the configurational free energy from shielded Coulomb interactions among charged particles. Internal partition functions are truncated using an occupation probability formalism that accounts for perturbations of bound states by both neutral and charged perturbers. The entire theory is analytical and differentiable to all orders, so it is possible to write explicit analytical formulas for all derivatives required in a Newton-Raphson iteration; these are presented to facilitate future work. Some representative results for both Saha and free-energy-minimization equilibria are presented for a hydrogen-helium plasma with N(He)/N(H) = 0.10. These illustrate nicely the phenomena of pressure dissociation and ionization, and also demonstrate vividly the importance of choosing a reliable cutoff procedure for internal partition functions.

Corrections to virial estimates of molecular cloud masses

Abstract: The application of the virial theorem to the determination of giant molecular cloud masses is reconsidered. It is argued that it is necessary to transform the results to equivalent velocity widths for the optically thin line of (C-13)O rather than that of (C-12)O. The results lead to a substantial lowering of H2 mass and values of X closer to that of the gamma-ray analysis of Bhat et al. (1986). The total mass of H2 in the Inner Galaxy is significantly less than that of atomic hydrogen. 27 references.

I(CO)/N(H2) conversions and molecular gas abundances in spiral and irregular galaxies

Abstract: Observations of emission in the J = 1-0 rotational transition of interstellar CO are used to obtain column densities and masses of hydrogen. By taking into account the effects of variations in molecular cloud parameters on conversion factors between integrated CO intensity and molecular hydrogen column density, it is shown that conversion factors are very sensitive to the kinetic temperature of the emitting gas. Results indicate that the gas temperatures in systems with high star formation rates can be quite high, and it is suggested that use of a standard conversion factor will lead to systematic overestimation of the amount of molecular gas.

A search for substellar companions to solar-type stars

Abstract: Les donnees d'observations des vitesses radiales relatives de 16 etoiles (12 naines rouges et 4 sous-geantes) sont analysees en detail pour tenter de detecter la presence de composantes planetaires (de type naine brune par exemple).Deux d'entre elles, χ 1 ori A et γ Cep presentent de grandes variations de vitesses (quelques km/s) probablement causees par des compagnons stellaires. Sept autres etoiles presentent de petites variations a long terme dont l'attribution a des composantes secondaires ayant plusieurs fois la masse de Jupiter est discutee