Showing papers in "The Astrophysical Journal in 1987"

Mass, luminosity, and line width relations of Galactic molecular clouds

Abstract: The paper presents an analysis of the cloud sizes, velocity line widths, viral masses, and CO luminosities of 273 Galactic molecular clouds which utilizes the higher resolution Massachusetts-Stony Brook Galactic plane CO survey. It is shown that the molecular clouds are in or near viral equilibrium and are not confined by pressure equilibrium with a warm or hot phase of interstellar matter. Allowance is made for the mass-luminosity law in a cloud model consisting of a large number of optically thick clumps in viral equilibrium each with a thermal internal velocity dispersion; the clouds are optically thin at a fixed velocity along the line of sight. 32 references.

Fundamental Properties of Elliptical Galaxies

Abstract: The global properties of elliptical galaxies, such as luminosity, radius, projected velocity dispersion, projected luminosity, etc., form a two-dimensional family. This 'fundamental plane' of elliptical galaxies can be defined in observable terms by the velocity dispersion and mean surface brightness. Its thickness is given by the present measurement error bars, and there are no significant indications of nonlinearity or higher dimensionality. This is indicative of a strong regularity in the process of galaxy formation. The equations of the plane can be used as new, substantially improved distance indicators for elliptical galaxies. However, all morphological parameters which describe the shape of the light distribution (ellipticity, elippticity gradient, isophotal twist rate, slope of the surface brightness profile) and reflect dynamical anisotropies of stars are completely independent of this fundamental plane. Thus, the elliptical galaxies are actually a '2 + N' parameter family. The M/L ratios correlate only with the velocity dispersions and show a small intrinsic scatter, perhaps only about 30 percent, in a luminosity range spanning some four orders of magnitude; this suggests a constant fraction of the dark matter contribution in elliptical galaxies. 33 references.

Spectroscopy and photometry of elliptical galaxies. I: a new distance estimator

Abstract: Kinematic and photometric data have been obtained for 97 elliptical galaxies in six rich clusters. These data show that ellipticals describe a plane in three dimensions which, when viewed edge-on, projects a smaller scatter than the Faber-Jackson relationship between luminosity and velocity dispersion sigma. This plane is approximately given by L ∝ sigma8/3Sigmae-3/5, where Sigmae is the surface brightness within the effective radius Ae. The authors present a new photometric parameter Dn, the diameter which encloses an integrated surface brightness Sigma, that correlates as well with sigma as any linear combination of L and Sigma. It is shown that the Dn-sigma relation can be used to find relative distances of ellipticals with rms errors of l25% for a single galaxy and l10% for rich clusters. A poorer correlation between the line strength indicator Mg2 and Dn provides an independent, though less accurate, distance indicator. The new sigma and Mg2 distance estimators are used to determine an infall of the Local Group toward the Virgo Cluster. Associated Articles Part 1 Part 4 Part 2 Part 3 Part 5 Part 6 Part 7

Molecular abundances in OMC-1 - the chemical composition of interstellar molecular clouds and the influence of massive star formation

Abstract: We present here an investigation of the chemical composition of the various regions in the core of the Orion molecular cloud (OMC-1) based on results from the Caltech Owens Valley Radio Observatory (OVRO) millimeter-wave spectral line survey (Sutton et al.; Blake et al.). This survey covered a 55 GHz interval in the 1.3 mm (230 GHz) atmospheric window and contained emission from over 800 resolved spectral features. Of the 29 identified species 14 have a sufficient number of detected transitions to be investigated with an LTE "rotation diagram" technique, in which large numbers of lines are used to estimate both the rotational excitation and the overall abundance. The rotational temperatures and column densities resulting from these fits have then been used to model the emission from those remaining species which either have too few lines or which are too weak to be so analyzed. When different kinematic sources of emission are blended to produce a single feature, Gaussian fits have been used to derive the individual contributions to the total line profile. The uniformly calibrated data in the unique and extensive Caltech spectral line survey lead to accurate estimates of the chemical and physical parameters of the Orion molecular cloud, and place significant constraints on models of interstellar chemistry. A global analysis of the observed abundances shows that the markedly different chemical compositions of the kinematically and spatially distinct Orion subsources may be interpreted in the framework of an evolving, initially quiescent, gas-phase chemistry influenced by the process of massive star formation. The chemical composition of the extended Orion cloud complex is similar to that found in a number of other objects, but the central regions of OMC-1 have had their chemistry selectively altered by the radiation and high-velocity outflow from the young stars embedded deep within the interior of the molecular cloud. Specifically, the extended ridge clouds are inferred to have a low (subsolar) gas-phase oxygen content from the prevalence of reactive carbon-rich species like CN, CCH, and C_3H_2 also found in more truly quiescent objects such as TMC-1. The similar abundances of these and other simple species in clouds like OMC-1, Sgr B2, and TMC-1 lend support to gas-phase ion-molecule models of interstellar chemistry, but grain processes may also play a significant role in maintaining the overall chemical balance in such regions through selective depletion mechanisms and grain mantle processing. In contrast, the chemical compositions of the more turbulent plateau and hot core components of OMC-1 are dominated by high-temperature, shock-induced gas and grain surface neutral-neutral reaction processes. The high silicon/sulfur oxide and water content of the plateau gas is best modeled by fast shock disruption of smaller grain cores to release the more refractory elements followed by a predominantly neutral chemistry in the cooling postshock regions, while a more passive release of grain mantle products driven toward kinetic equilibrium most naturally explains the prominence of fully hydrogenated N-containing species like HCN, NH_3 , CH_3CN, and C_2H_5CN in the hot core. The clumpy nature of the outflow is illustrated by the high-velocity emission observed from easily decomposed molecules such as H_2CO. Areas immediately adjacent to the shocked core in which the cooler, ion-rich gas of the surrounding molecular cloud is mixed with water/oxygen rich gas from the plateau source are proposed to give rise to the enhanced abundances of complex internal rotors such as CH_30H, HCOOCH_3 , and CH_30CH_3 whose line widths are similar to carbon-rich species such as CN and CCH found in the extended ridge, but whose rotational temperatures are somewhat higher and whose spatial extents are much more compact.

Spectral evolution of young stellar objects

Abstract: An evolutionary sequence, from protostars to pre-main sequence stars, for the classification of young stellar objects is derived by comparing the predictions of the theoretical protostar models of Adams and Shu (AS, 1986) with the morphological classification scheme of Lada and Wilking (1984). It is shown that the AS models adequately explain the emergent spectral energy distributions of unidentified objects with negative spectral indices in the mid-IR and near-IR in both Taurus and Ophiuchus. If the infalling dust envelope is then completely removed, the spectra of the underlying stars and nebular disks used by AS provide a natural explanation for the near-IR and mid-IR excesses and the positive spectral indices of embedded T Tauri stars. It is found that the addition of a simple physical model for residual dust envelopes can reproduce the far-IR excesses found in some of these T Tauri stars.

Spectral energy distributions of T Tauri stars - Disk flaring and limits on accretion

Abstract: The Adams et al. (1987) conclusion that much of the IR excess emission in the spectral energy distribution of T Tauri stars arises from reprocessing of stellar radiation by a dusty circumstellar disk is presently supported by analyses conducted in light of various models of these stars' spectra. A low mass reprocessing disk can, however, produce these spectra as well as a massive accretion disk. The detection of possible boundary layer radiation in the optical and near-UV regions poses the strongest limits on accretion rates. Disk accretion in the T Tauri phase does not significantly modify stellar evolution.

A Composite CO survey of the entire Milky Way

Abstract: Large-scale CO surveys of the entire Galactic plane and specific nearby clouds have been combined to produce a panorama of the entire Milky Way in molecular clouds at an angular resolution of 1/2 deg. This survey is the only molecular line survey to date with sky coverage and resolution comparable to that of the early 21 cm surveys. The telescopes and the observing techniques used for the individual surveys and the methods of reconciling the survey results are described. The composite survey is presented as a spatial map integrated over velocity and as a longitude-velocity map integrated over latitude; individual clouds and large-scale features of the maps are briefly discussed. The distribution and properties of molecular clouds within 1 kpc of the sun are investigated, and a comparison with other Population I tracers is used to demonstrate that the composite survey probably provides a nearly complete inventory of nearby molecular clouds. 92 references.

Hot Dust and the Near-Infrared Bump in the Continuum Spectra of Quasars and Active Galactic Nuclei

Abstract: It is shown here that thermal radiation by dust can reproduce the overall shape of the bump seen in the near-infrared continua of many QSOs and AGN. A simple model in which dust grains are heated by the primary nuclear optical/ultraviolet continuum produces the required emission at short wavelengths. The model naturally explains the onset of the bump at about 2 microns. This wavelength corresponds to the optically thin emission peak for the hottest possible grains, i.e., graphite grains at their evaporation temperature near 1500 K. Emission longward of 2 microns is due to cooler grains farther from the central source. 33 references.

Fluorescent excitation of interstellar H2

Abstract: The infrared emission spectrum of H2 excited by ultraviolet absorption, followed by fluorescence, was investigated using comprehensive models of interstellar clouds for computing the spectrum and to assess the effects on the intensity to various cloud properties, such as density, size, temperature, and the intensity of the UV radiation field. It is shown that the absolute H2 IR line intensities depend primarily on the density of the cloud and the strength of the incident UV radiation, and to a much lesser exent on the temperature of the gas, the total thickness of the cloud, and the optical properties of the grains. A variety of recent observational results are discussed with reference to theoretical models. It is shown that the rich H2 emission spectrum of the reflection nebula NGC 2023 can be reproduced by a model with density of about 10,000/cu cm, temperature of about 80 K, and UV flux approximately 300 times that of the Galactic background starlight.

Angular momentum from tidal torques

Abstract: The origin of the angular momentum of bound objects in large N-body simulations is studied using three sets of models. One model with white-noise initial conditions is analyzed as well as two in which the initial conditions have more power on large scales, as predicted in models with cold dark matter. The growth and distribution of angular momentum in individual objects is studied and it is found that the specific angular momentum distribution of bound clumps increases in a near linear fashion with radius while the orientation of the angular momentum in the inner high-density regions is often poorly correlated with that of the outer parts. It is also found that the dimensionless spin parameter is insensitive to the initial perturbation spectrum and has a median value of about 0.05. 61 references.

Resonant Enhancements in Weakly Interacting Massive Particle Capture by the Earth

Abstract: The exact formulas for the capture of weakly interacting massive particles (WIMPs) by a massive body are derived. Capture by the earth is found to be significantly enhanced whenever the WIMP mass is roughly equal to the nuclear mass of an element present in the earth in large quantities. For Dirac neutrino WIMPs of mass 10-90 GeV, the capture rate is 10-300 times that previously believed. Capture rates for the sun are also recalculated and found to be from 1.5 times higher to 3 times lower than previously believed, depending on the mass and type of WIMP. The earth alone or the earth in combination with the sun is found to give a much stronger annihilation signal from Dirac neutrino WIMPs than the sun alone over a very large mass range. This is particularly important in the neighborhood of the mass of iron where previous analyses could not set any significant limits.

Rate of energy gain and maximum energy in diffusive shock acceleration

Abstract: The problem of diffusive shock acceleration of fast charged particles is reexamined with emphasis on the rate of energy gain, and the maximum energy which can be attained in a given circumstance. The direction of the average magnetic field at the shock is shown to have a large effect. If the perpendicular diffusion coefficient is much smaller than the parallel coefficient, particles can gain much more energy if the shock is quasi-perpendicular than if it is quasi-parallel. The maximum energy attainable can be substantially higher (by a factor of 100 or more) than previous discussions would predict, in cases where the shock is quasi-perpendicular. The energy gain increases as kappa-perpendicular decreases. The principal limitation comes from the requirement that diffusion be a valid approximation to the particle motion, and that the particle be able to diffuse fast enough to encounter the shock many times.

Radiative Bow Shock Models of Herbig-Haro Objects

Abstract: The bow-shock models of HH objects presently constructed from a collection of 43 radiative-planar-shock models are used to predict the line ratios and line profiles expected from HH objects. It is found that a radiating-bow-shock model formed around a 'bullet' of dense gas plowing into the ambient medium accounts for existing observations sufficiently well to constitute a unifying model. For particular bow-shock orientations, double-peaked profiles are predicted in spatially unresolved spectra. A simple analytic formula is presented that can be used to estimate the shock velocity and bow-shock orientation from a single high-resolution observation of a low-excitation line. 63 references.

Filamentary structure in the Orion molecular cloud

Abstract: A large-scale (C-13)O map (containing 33,000 spectra on a 1-arcmin grid) is presented for the giant molecular cloud located in the southern part of Ori which contains the Ori Nebula, NGC 1977, and the L1641 dark cloud complex. The overall structure of the cloud is filamentary, with individual features having a length up to 40 times their width. The northern portion of the cloud is compressed, dynamically relaxed, and supports massive star formation. In contrast, the southern part of the Ori A cloud is diffuse, exhibits chaotic spatial and velocity structure, and supports only intermediate- to low-mass star formation. This morphology may be the consequence of the formation and evolution of the Ori OB I association centered north of the molecular cloud. The entire cloud, in addition to the 5000-solar-mass filament containing both OMC-1 and OMC-2, exhibits a north-south velocity gradient. Implications of the observed cloud morphology for theories of molecular cloud evolution are discussed. 14 references.

The IRAS bright galaxy sample. II - The sample and luminosity function

Abstract: A complete sample of 324 extragalactic objects with 60 μm flux densities greater than 5.4 Jy has been selected from the IRAS catalogs. Only one of these objects can be classified morphologically as a Seyfert nucleus; the others are all galaxies. The median distance of the galaxies in the sample is ~ 30 Mpc, and the median luminosity vL,(60 μm) is ~ 2 x 10^(10) L_☉ . This infrared selected sample is much more "infrared active" than optically selected galaxy samples. The range in far-infrared luminosities of the galaxies in the sample is 10^8 L_☉ -2 x 10^(12) L_☉ The far-infrared luminosities of the sample galaxies appear to be independent of the optical luminosities, suggesting a separate luminosity component. As previously found, a correlation exists between 60 μm/100 μm flux density ratio and far-infrared luminosity. The mass of interstellar dust required to produce the far-infrared radiation corresponds to a mass of gas of 108-10^(10) M_☉ for normal gas to dust ratios. This is comparable to the mass of the interstellar medium in most galaxies. The infrared luminous galaxies are found to be an important component of extraglactic objects, being the most numerous objects in the local universe at luminosities L > 10^(11) L_☉, and producing a luminosity density of ~ that of the observed starlight in normal galaxies. Approximately 60%-80% of the far-infrared luminosity of the local universe is likely attributed to recent or ongoing star formation. If the infrared active phase (L_(FIR) > 10^(11) L_☉ ) is a nonrecurring event of duration less than 108 yr in galaxy evolution, then more than 10%, and perhaps all of the galaxies with blue luminosities greater than 10^(10) L_☉ must undergo such an event.

Collisional charging of interstellar grains

Abstract: Collisional charging of small interstellar particles is reexamined, including effects due to electrostatic polarization of grains by the electric fields of approaching charged particles. The interaction potentials and resulting energy-dependent collisional cross sections are discussed and used to obtain the collisional charging rate for grains of specified charges in a thermal plasma. The probability distribution of the grain charges is obtained, and the application of the results to the interstellar medium is illustrated by considering the power-law grain size distribution of Mathis, Rumpl, and Nordsieck (1977). It is found that dust grains with such a size distribution can contribute appreciably to the recombination of metal ions in dark clouds.

Supershells and propagating star formation

Abstract: Stellar winds and repeated supernovae from an OB association will create a cavity of coronal gas in the interstellar medium, with radius greater than 100 pc, surrounded by a dense, expanding shell of cool interstellar gas. If the association has a typical initial mass function, its supernovae explosions will inject energy into the supershell at a nearly constant rate for about 50 Myr. The supershell loses its interior pressure and enters the snowplow phase when radiative cooling becomes important or when the shell bursts through the gas disk of a galaxy, typically after a few times 10 Myr and with a radius of 100-300 pc. At approximately the same time, the supershell becomes gravitationally unstable, forming giant molecular clouds which are sites for new star formation. There is widespread evidence for supershells in the Galaxy and other spiral and irregular galaxies from 21-cm emission-line surveys, optical emission-line surveys, and studies of supernova remnants. The gravitational instability of the supershells provides a physical mechanism for induced star formation and may account for bursts of star formation, especially in irregular galaxies.

The Structure of Young Star Clusters in the Large Magellanic Cloud

Abstract: The surface brightness profiles presented for 10 rich star clusters in the LMC extend over 8-10 mag in surface brightness, and to radii of 4 arcmin. The crossing times are shorter than the ages of the clusters, and the two-body relaxation times, except in a few of the cores, are noted to be significantly longer than the ages; the clusters are therefore well mixed, but not relaxed through stellar encounters. The possibility that the expansion of a newly formed cluster through either mass loss or violent relaxation could form a halo of unbound stars is examined in light of a calculation of the LMC's tidal field. 90 references.

Conditions for the formation of massive stars

Abstract: Upper limits on the masses of stars that can form are reexamined and models for the inflow of matter through cocoons around stars of 60, 100, and 200 solar masses are calculated. Consideration is given to the general conditions that must hold at the inner and outer boundaries of a protostellar cocoon; limits on the dust-to-gas ratios and mass inflow rates that will permit inflow onto very massive stars are determined. It is found that inflow can occur if intermediate-sized grains (0.05-0.25 micron) are missing from the initial gas/dust mixture. The existence of massive stars in certain locations in galaxies indicates that preconditioning of the interstellar medium by shocks or turbulence is necessary for massive star formation.

An independent method for determining the age of the universe

Abstract: An age of 9.3 + or - 2.0 Gyr is derived for the Galactic disk on the basis of comparisons between the sudden drop in the observed luminosity distribution and theoretical evolutionary white dwarf models and allowance for a mean prewhite-dwarf lifetime of 0.3 Gyr. To obtain the age of the universe, the time between the big bang and the first appearance of stars in the Galactic disk is added. The age of the universe is estimated to be 10.3 + or - 2.2 Gyr.

Synchrotron Emission from Shock Waves in Active Galactic Nuclei

Abstract: The origin of the sharp near infrared cutoff in the continuous energy distribution of many compact non-thermal sources (BL Lacs, OVVs, red quasars and certain jets) is considered under the assumption that particle acceleration takes place in shocks. This model predicts a highest frequency v* of electron synchrotron emission which depends principally on the shock velocity and the ratio a of photon to magnetic energy density in the acceleration region. For near relativistic flows and reasonable values of a a spectral cutoff is predicted in the range 3 1014 < v < 2 1015 Hz. The model thus leads to 1) near relativistic flows, 2) a gradual steepening of optical continuum spectra as one follows a jet outwards, 3) a correlation between spectral hardening and luminosity, 4) a correlation between optical spectrum and X-ray emission, 5) a possible synchrotron contribution to the X-ray emission in Quasars from secondary particles, and 6) the production of very high energy particles such as observed in cosmic rays, of up to ~1012 GeV.

Hydrogen molecules and the radiative cooling of pregalactic shocks

Abstract: Detailed results for the hydrodynamical, thermal, ionization, and molecular formation history of postshock cooling flows behind steady state shocks in a primordial gas at redshifts z = 5, 10, and 20 are presented and analyzed for a wide range of shock velocities from 50 to 400 km/s. The nonequilibrium results indicate that, for a significant range of shock velocities, if the shock-heated gas can cool to 10,000 K within the age of the universe, then it quite commonly forms an H2 fraction in excess of 0.001 and cools at nearly constant pressure to less than 100 K. The presence of an external flux of ionizing and dissociating radiation can, for a range of fluxes similar to that expected from a background of quasars, actually increase the peak H2 concentration to values of order 10 to the -1.5 or higher; it also increases the cooling time to 100 K. 78 references.

Stokes profile analysis and vector magnetic fields. I. Inversion of photospheric lines

Abstract: Improvements are proposed for the Auer et al. (1977) method for the analytic inversion of Stokes profiles via nonlinear least squares. The introduction of additional physics into the Mueller absorption matrix (by including damping wings and magnetooptical birefringence, and by decoupling the intensity profile from the three-vector polarization profile in the analysis) is found to result in a more robust inversion method, providing more reliable and accurate estimates of sunspot vector magnetic fields without significant loss of economy. The method is applied to sunspot observations obtained with the High Altitude Observatory polarimeter. 29 references.

Thresholds for rapid mass transfer in binary systems. I. Polytropic models

Abstract: The adiabatic responses to mass loss of simple and compound polytropic star models are explored. The results show that stars in thermal equilibrium with isentropic surface layers are linearly unstable if the mass ratio exceeds a finite critical threshold q(0). Unless the mass fraction in these surface layers is quite small, q(0) is of the order of unity. On the other hand, stars in thermal equilibrium with subadiabatic surface layers are linearly stable for any value of the mass ratio. A finite-amplitude instability exists to which radiative stars are subject. If the initial phases of mass transfer are sufficiently rapid, such a star may undergo a delayed transition to dynamical time scale mass loss. Red giants with small core mass fractions behave very much like fully convective stars, expanding adiabatically as they lose mass. 32 references.

The influence of the Coriolis force on flux tubes rising through the solar convection zone

Abstract: In order to study the effect of the Coriolis force due to solar rotation on rising magnetic flux, the authors consider a flux ring, azimuthally symmetric around the rotation axis, starting from rest at the bottom of the convection zone, and then follow the trajectory of the flux ring as it rises. If it is assumed that the flux ring remains azimuthally symmetric during its ascent, then the problem can be described essentially in terms of two parameters: the value of the initial magnetic field in the ring when it starts, and the effective drag experienced by it. For field strengths at the bottom of the convection zone of order 10,000 G or less, it is found that the Coriolis force plays a dominant role and flux rings starting from low latitudes at the bottom are deflected and emerge at latitudes significantly poleward of sunspot zones.

Galaxy Formation by Gravitational Collapse

Abstract: Galaxies may form by the collapse of density perturbations in a universe dominated by dark matter. In this scenario, the growth of density and velocity distributions around collapsed mass peaks is investigated, taking into account the random velocities imparted by substructure. Structures resembling galactic halos arise naturally from density perturbations in the cold dark matter scenario. When the halo structures are adiabatically compressed by the dissipating baryonic component, the resulting rotation curves are flat, over the full range of amplitudes studied here. In all cases the baryons are dynamically important in the centers of the structures, but in no case do they dominate. A peak of amplitude somewhat less than three sigma on the scale 10 to the ninth solar masses produces a structure with a rotation velocity comparable to that of the Galaxy; it is argued that peaks of about this amplitude and smaller should account for most of the mass in bound structures. 28 references.