Showing papers in "The Astrophysical Journal in 1989"

The relationship between infrared, optical, and ultraviolet extinction

Abstract: The parameterized extinction data of Fitzpatrick and Massa (1986, 1988) for the ultraviolet and various sources for the optical and near-infrared are used to derive a meaningful average extinction law over the 3.5 micron to 0.125 wavelength range which is applicable to both diffuse and dense regions of the interstellar medium. The law depends on only one parameter R(V) = A(V)/E(B-V). An analytic formula is given for the mean extinction law which can be used to calculate color excesses or to deredden observations. The validity of the law over a large wavelength interval suggests that the processes which modify the sizes and compositions of grains are stochastic in nature and very efficient.

The Star Formation Law in Galactic Disks

Abstract: Measurements of the distribution of H-alpha emission in galaxies are combined with published H I and CO data in order to reassess the dependence of the massive star formation rate (SFR) on the density and dynamics of the interstellar gas. The disk-averaged H-alpha surface brightness is correlated with the mean atomic and total gas surface densities, but is only weakly correlated with the mean molecular gas density inferred from CO emission. Radial profiles of gas and H-alpha emission in 15 galaxies are used to define the relationship between the SFR and gas surface density. In dense regions, the SFR and total gas density are well-represented by a Schmidt power-law relation. This Schmidt law breaks down, however, at densities below a critical threshold value. Massive star formation is completely suppressed at surface densities well below the threshold, while at densities near the threshold value the slope of the SFR-density relation is much steeper than a normal Schmidt law. A simple Toomre disk stability model predicts threshold densities and radii which are in excellent agreement with observations. 111 refs.

Formation and eruption of solar prominences

Abstract: A model for the magnetic field associated with solar prominences is considered. It is shown that flux cancellation at the neutral line of a sheared magnetic arcade leads to the formation of helical field lines which are capable, in principle, of supporting prominence plasma. A numerical method for the computation of force-free, canceling magnetic structures is presented. Starting from an initial potential field we prescribe the motions of magnetic footpoints at the photosphere, with reconnection occurring only at the neutral line. As more and more flux cancels, magnetic flux is transferred from the arcade field to the helical field. Results for a particular model of the photospheric motions are presented. The magnetic structure is found to be stable: the arcade field keeps the helical field tied down at the photosphere. The axis of the helical field moves to larger and larger height, suggestive of prominence eruption. These results suggest that prominence eruptions may be trigered by flux cancellation.

Molecule Formation and Infrared Emission in Fast Interstellar Shocks. III. Results for J Shocks in Molecular Clouds

Abstract: The structure and emission spectrum of J shocks in molecular gas are studied over a broad range of conditions. It is found that at high densities chemistry has a profound effect on the emission spectrum: the density behind the shock is sufficiently high that some of the internal energy of the newly formed H2 molecules is transformed to the gas as heat by collisional deexcitations, producing the H2 formation plateau. In this temperature plateau, endothermal reactions and neutral-neutral chemical reactions with activation energies can proceed efficiently, producing significant quantities of warm H2, CO, OH, and H2O and enhanced columns of warm atoms and ions. The heat generated by the H2 formation is radiated in collisionally excited atomic fine-structure lines.

Fast algorithm for spectral analysis of unevenly sampled data

Abstract: On montre que les transformations de Fourier rapides peuvent servir pour faire des calculs sur ordinateur de l'ordre de 10 2 N Log N

Is every quasar beamed

Abstract: A companion paper (Barthel et al.) reports the discovery of superluminal motion in the core of 4C 34.47, the largest known quasi-stellar radio source. Superluminal motion, the apparent continuity of small- and large-scale one-sided radio jets, and the recently established depolarization asymmetry in luminous quasars find a natural explanation in the effects of relativistic beaming. In the frameworkof of this relativistic beaming model, the available data imply that all radio-loud quasars are beamed toward the earth. Powerful radio galaxies may well form the unbeamed parent population. Radio-quiet QSOs and powerful infrared galaxies can be unified using similar orientation effects. 76 references.

Further experimental tests of relativistic gravity using the binary pulsar PSR 1913+16

Abstract: Fourteen-year observations of the binary pulsar PSR 1913 + 16 provided data consistent with a straightforward model allowing for the motion of the earth, special and general relativistic effects within the solar system, dispersive propagation in the interstellar medium, relativistic motion of the pulsar in its orbit, and deterministic spin-down behavior of the pulsar itself. The results indicate that at the present level of precision, the PSR 1913 + 16 can be modeled dynamically as a pair of orbiting point masses. Five Keplerian and five post-Keplerian orbital parameters are therefore mostly determined with remarkably high precision. The masses of the pulsar and its companion are determined to be m1 = 1.442 + or - 0.003 and m2 = 1.386 + or - 0.003 times the mass of the sun, respectively, and the orbit is found to be decaying at a rate equal to 1.01 + or - 0.01 times the general relativistic prediction for gravitational damping. The results represent the first experimental tests of gravitation theory not restricted to the weak-field, slow-motion limit in which nonlinearities and radiation effects are negligible. Excellent agreement between observation and theory indicates conclusively that gravitational radiation exists, at the level predicted by general relativity.more » 70 refs.« less

Continuum energy distribution of quasars: Shapes and origins

Abstract: Continuum observations from ~0.3 nm to 6 cm (10^(9.7)-10^(18) Hz) are presented for 109 bright quasars from the Palomar-Green (PG) survey. Two-thirds of the quasars have been detected in the infrared at wavelengths between 10 and 100 μm. All of the PG quasars appear to emit the bulk of their luminosity (typically more than 90%) between 3 nm and 300 μm (10^(12)-10^(17) Hz). The total luminosity at wavelengths longer than 1 μm is typically 20%-40% of that at wavelengths shortward of 1 μm. The gross shape of the energy distributions between 3 nm and 300 μm is remarkably similar for all the quasars except the flat-spectrum radio-loud quasars like 3C 273 and can plausibly be fitted by two broad components of thermal emission. In this interpretation the emission in the spectral range ~ 10 nm to 0.3 μm, the "big blue bump," is dominated by 10,00-100,000 K thermal emission from an accretion disk. The emission between 2 μm and 1 mm, the "infrared bump," is made up of reradiation from dust in a distorted disk extending from 0.1 pc to more than 1 kpc. The fairly small range in the relative sizes of the bumps suggests that the covering factor in most of the PG quasars is similar. There is no obvious connection between the strengths of the blue and infrared peaks and whether or not the quasar is radio quiet or radio loud. The mass of infrared emitting dust is estimated to be ~0.01 M_☉ at 2 μm and ~ 10^5 M_☉ at 60 μm. The radiation from 0.5 μm is thermal emission from the portion of the disk between 0.1 and 1 pc, illuminated primarily by the clouds of the broad-line region. The radiation from 5 μm to 1 mm is reradiation from a warped disk at distances greater than 1 pc from the central source, which is heated directly by radiation from the central source. Optically thin atomic emission (free-free and partially thermalized lines and bound-free) from gas within 1 pc of the central source, whose dust has sublimated, probably contributes to the flux from 0.5 to 2 μm. We believe that there is no convincing evidence for energetically significant nonthermal radiation in the wavelength range 3 nm to 300 μm in the continua of the radio-quiet and steep-spectrum radio-loud PG quasars.

Exact vacuum solution to conformal Weyl gravity and galactic rotation curves

Abstract: The complete, exact exterior solution for a static, spherically symmetric source in locally conformal invariant Weyl gravity is presented. The solution includes the familiar exterior Schwarzschild solution as a special case and contains an extra gravitational potential term which grows linearly with distance. The obtained solution provides a potential explanation for observed galactic rotation curves without the need for dark matter. The solution also has some interesting implications for cosmology.

Photoionization-regulated Star Formation and the Structure of Molecular Clouds

Abstract: A model for the rate of low-mass star formation in Galactic molecular clouds and for the influence of this star formation on the structure and evolution of the clouds is presented. The rate of energy injection by newly formed stars is estimated, and the effect of this energy injection on the size of the cloud is determined. It is shown that the observed rate of star formation appears adequate to support the observed clouds against gravitational collapse. The rate of photoionization-regulated star formation is estimated and it is shown to be in agreement with estimates of the observed rate of star formation if the observed molecular cloud parameters are used. The mean cloud extinction and the Galactic star formation rate per unit mass of molecular gas are predicted theoretically from the condition that photionization-regulated star formation be in equilibrium. A simple model for the evolution of isolated molecular clouds is developed.

Oxygen Abundances in Nearby Dwarf Irregular Galaxies

Abstract: Oxygen abundances are obtained by optical spectrophotometry of H II regions in seven nearby dwarf irregular galaxies. All of these yield oxygen abundances of less than 1/10 of the solar value, and most are in the range of 3-5 percent of the solar value. This suggests that observations of nearby dwarf galaxies may provide an effective means for studying the chemical evolution of low-mass galaxies and, possibly, the primordial helium abundance. A strong correlation is found between the oxygen abundances and absolute magnitudes for nearby irregular galaxies. This correlation will be useful for estimating abundances of irregular galaxies without observable H II regions, and possibly as a distance indicator for irregular galaxies with known abundances. It is inferred from this relationship that infall is no more important in irregular galaxies with extremely large H I halos than in typical irregular galaxies. 72 refs.

Evolutionary models of the rotating sun

Abstract: This paper reviews current work on the evolution of a differentially rotating solar model. Although we discuss global features of the evolution with rotation in general terms, the specific models described are those computed with the new Yale Rotating Evolution Code (YREC). YREC uses the Kippenhahn and Thomas (1970, KT) formalism as implemented by Endal and Sofia (1976), although the numerical formulation of our code is totally new. Particular calculations that we describe include the effects of different initial total angular momentum, the consequences of varying the properties and magnitude of angular momentum losses by wind torquing, and the consequences of specific composition and angular momentum redistribution mechanisms. This paper is a progress report which points out the complexity of the problem, and the need for a broad-based observational program to solve it. Because the final solution is not yet in hand, we outline the steps that, in our estimation, need to be undertaken in order to make progress.

The tidal disruption of a star by a massive black hole

Abstract: Results are reported from a three-dimensional numerical calculation of the tidal disruption of a low-mass main-sequence star on a parabolic orbit around a massive black hole (Mh = 10 to the 6th stellar mass). The postdisruption evolution is followed until hydrodynamic forces becomes negligible and the liberated gas becomes ballistic. Also given is the rate at which bound mass returns to pericenter after orbiting the hole once. The processes that determine the time scale to circularize the debris orbits and allow an accretion torus to form are discussed. This time scale and the time scales for radiative cooling and accretion inflow determine the onset and duration of the subsequent flare in the AGN luminosity.

Massive stars embedded in molecular clouds - Their population and distribution in the galaxy

Abstract: A new method for identifying massive stars inside molecular clouds is described which makes use of their characteristic FIR flux density distribution. A two-color selection criterion has been developed and carefully calibrated using stars which are known to be embedded in molecular clouds. When applied to the sensitive IRAS all-sky survey, a direct measure of the total number and distribution of embedded massive stars in the Galaxy is obtained. A lower limit is derived on the current O star formation rate in the Galaxy, the contribution O stars make to the FIR luminosity of the Galaxy is estimated, and it is shown that the number of IRAS sources found is consistent with a Galactic supernova rate of about one every 25 yrs.

The rotation curve of the Milky Way to 2 R(0)

Abstract: A unified analysis of the Galactic rotation curve is presented using H I CO, and spectrophotometric data obtained in the northern hemisphere are used to determine the rotation curve from R = 3-17 kpc A number of different functional forms are examined to fit the data, and the errors and uncertainties that go into its determination are discussed The results are compared with nine recent studies of the rotation curve, and specific recommendations are made on the choice of curves to use in kinematic studies and mass modeling 36 refs

Studies in astronomical time series analysis. III - Fourier transforms, autocorrelation functions, and cross-correlation functions of unevenly spaced data

Abstract: This paper develops techniques to evaluate the discrete Fourier transform (DFT), the autocorrelation function (ACF), and the cross-correlation function (CCF) of time series which are not evenly sampled. The series may consist of quantized point data (e.g., yes/no processes such as photon arrival). The DFT, which can be inverted to recover the original data and the sampling, is used to compute correlation functions by means of a procedure which is effectively, but not explicitly, an interpolation. The CCF can be computed for two time series not even sampled at the same set of times. Techniques for removing the distortion of the correlation functions caused by the sampling, determining the value of a constant component to the data, and treating unequally weighted data are also discussed. FORTRAN code for the Fourier transform algorithm and numerical examples of the techniques are given.

Overpressured cocoons in extragalactic radio sources

Abstract: It is shown that the cocoons of shocked gas which surround powerful double radio sources can have significantly higher pressures than the surrounding intergalactic medium. The pressures can be high enough to confine the jets in these sources, obviating the need for magnetic confinement. The cocoon pressure and the age of a radio source may be estimated from observable quantities, as demonstrated here for the radio galaxy Cygnus A. It is suggested that overpressured cocoons in high-redshift radio galaxies engulf and compress circumgalactic clouds, driving them over the Jeans limit and triggering star formation. It is proposed that this process leads to the observed alignments of optical continuum emission with radio source axes.

Neutron Star Accretion in a Supernova

Abstract: An estimate can be made of the effect of a central mass on an explosion through the study of self-similar blast waves in a medium having an inverse-square-of-r density profile, as is approximately the case in the inner core of the progenitor of SN 1987A; its parameters are such that the nucleon density is substantially constant within the shock front. In SN explosions with a massive envelopes, the core material is decelerated by its interaction with the envelope, and a reverse shock wave propagates back to the center. 42 refs.

The disk-halo interaction - Superbubbles and the structure of the interstellar medium

Abstract: A chimney model for the Galaxy is presented and its underlying physical principles are explained. The crucial point of the model is that the Galactic disk and halo are connected by chimneys which are a consequence of superbubbles bursting out of the disk forming these collimated structures through which the global mass and energy exchange flows from disk to halo. For canonical Galaxy parameters, the chimney phase is found to be associated with a mass flow rate of 0.3-3 solar masses/yr and a global power input of 10 to the 40th to 10 to the 42nd ergs/s. 72 refs.

The photoevaporation of interstellar clouds. I: Radiation-driven implosion

Abstract: An approximate analytical solution is developed for the evolution of an interstellar neutral cloud exposed to the ionizing radiation of a newly formed star. The structure of a steady photoevaporation gas flow off a spherical ionization front is derived even for cases where the ionization front cannot be considered thin, and the downstream flow is not in ionization and thermal equilibrium. Under a wide range of conditions, the ionization front is approximately D-critical, and the ionized gas expands supersonically into the intercloud medium. The ionization front typically drives a strong shock into the initial cloud.

Cosmic-ray transport and acceleration. I - Derivation of the kinetic equation and application to cosmic rays in static cold media. II - Cosmic rays in moving cold media with application to diffusive shock wave acceleration

Abstract: The propagation of energetic charged particles in a zero electric field and an ordered uniform magnetic field (B0) in a cold background medium is investigated theoretically, with a focus on the case where Alfven waves propagating parallel or antiparallel to B0 are superposed. The derivations of the governing equations of a quasi-linear theory for pitch-angle and momentum diffusion and for its applications to cosmic-ray protons in a cold medium at rest or in nonrelativistic motion parallel to B0 are given in great detail; numerical predictions are presented in tables and graphs and discussed with reference to published observational data. 83 references.

Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes

Abstract: Production of polycyclic aromatic hydrocarbons in carbon-rich circumstellar envelopes was investigated using a kinetic approach. A detailed chemical reaction mechanism of gas-phase PAH formation and growth, containing approximately 100 reactions of 40 species, was numerically solved under the physical conditions expected in cool stellar winds. The chemistry is based on studies of soot production in hydrocarbon pyrolysis and combustion. Several first-ring and second-ring cyclization processes were considered. A linear lumping algorithm was used to describe PAH growth beyond the second aromatic ring. PAH production using this mechanism was examined with respect to a grid of idealized constant velocity stellar winds as well as several published astrophysical models. The basic result is that the onset of PAH production in the interstellar envelopes is predicted to occur within the temperature interval of 1100 to 900 K. The absolute amounts of the PAHs formed, however, are very sensitive to a number of parameters, both chemical and astrophysical, whose values are not accurately known. Astrophysically meaningful quantities of PAHs require particularly dense and slow stellar winds and high initial acetylene abundance. It is suggested that most of the PAHs may be produced in a relatively small fraction of carbon-rich red giants.

Spectropolarimetry of 'narrow-line' Seyfert 1 galaxies

Abstract: Spectropolarimetry is used here to investigate the properties of 'narrow-line' Seyfert 1 galaxies. In general, continuum polarization arising to the blue and/or the lack of significant broadening of the polarized emission lines points to dust reflection as the primary polarigenic mechanism. There is some evidence that the dust is associated with the forbidden O III-emitting region. Where line polarizations can be compared, H-beta is found to be more highly polarized than the forbidden O III 4959, 5007 A lines A, demonstrating that these lines come from spatially distinct within the nucleus. Simultaneous spectrophotometry obtained by the spectropolarimetry is used to demonstrate that X-ray selected Seyfert 1s show significantly smaller H-beta equivalent widths than a sample of optically selected narrow-line 1s and Seyfert 2s. 51 refs.

Deuterium fractionation in dense interstellar clouds

Abstract: The time-dependent gas-phase chemistry of deuterium fractionation in dense interstellar clouds ranging in temperature between 10 and 70 K was investigated using a pseudo-time-dependent model similar to that of Brown and Rice (1986). The present approach, however, considers much more complex species, uses more deuterium fractionation reactions, and includes the use of new branching ratios for dissociative recombinations reactions. Results indicate that, in cold clouds, the major and most global source of deuterium fractionation is H2D(+) and ions derived from it, such as DCO(+) and H2DO(+). In warmer clouds, reactions of CH2D(+), C2HD(+), and associated species lead to significant fractionation even at 70 K, which is the assumed Orion temperature. The deuterium abundance ratios calculated at 10 K are consistent with those observed in TMC-1 for most species. However, a comparison between theory and observatiom for Orion, indicates that, for species in the ambient molecular cloud, the early-time results obtained with the old dissociative recombination branching ratios are superior if a temperature of 70 K is utilized. 60 refs.

The infrared response of molecular hydrogen gas to ultraviolet radiation: high-density regions

Abstract: A study is presented of the behavior of the H2 infrared emission spectra that are produced in dense static photodissociation regions exposed to UV radiation which both heats and excites the H2 gas. The theory of H2 excitation by UV radiation and the construction of one-dimensional steady state models which are used to compute the H2 infrared spectra for a wide range of cloud conditions are discussed. The results of a number of illustrative model calculations are presented. 110 refs.

Dynamical, physical, and chemical properties of emission-line nebulae in cooling flows

Abstract: The results are reported of a program of long-slit spectroscopy, narrow-band imaging, and VLA mapping of the optical emission-line gas and nonthermal radio continuum emission associated with the dominant galaxy in nine galaxy clusters with cooling flows. All the observed emission-line nebulae are very strongly centrally concentrated with typical characteristic radii of 5 kpc or less. If the mass inflow rate varies linearly with radius in cooling flows out to large radii, then the H-alpha surface brightness does not trace the cooling rate. The velocity field of the emission-line gas provides little evidence for organized rotation or shear. This result can be understood if the emission-line filaments condense out of an intracluster medium in which angular momentum has been transported radially outward. Electron densities are measured over spatially resolved regions and are found to by typically a few hundred per cu cm. Spatial variations in the emission-line ratios in the nebulae agree better with shock models than with photoionization models. The various possible mechanisms for heating and ionizing the emission-line nebulae are discussed. 87 refs.

A Pressure and Metallicity Dependence for Molecular Cloud Correlations and the Calibration of Mass

Abstract: The correlation of molecular cloud mass with the fourth power of the velocity dispersion, the square of the radius, and the inverse of the density squared is addressed using a modified version of the proposal by Maloney (1988) and Fleck (1988) that the correlation is due to virialized internal motions and a uniform external pressure. The average column density of the molecular part of a large cloud is found to depend almost exclusively on the cloud velocity dispersion and very little on the peak temperature. The mass of an unresolved aggregate of clouds in another galaxy is also derived in terms of the integrated CO luminosity in the telescope beam. The influence of metallicity on the calibration of mass is discussed and applied to recent observations of CO in dwarf galaxies, small Virgo galaxies, and the LMC. An expression is given for the fractional mass of a cloud that is in the molecular core. 55 refs.

Properties of H II region populations in galaxies. II. The H II region luminosity function

Abstract: H-alpha emission-line fluxes have been measured for all of the detected H II regions in 30 nearby spiral and irregular galaxies, and these data are used to study the behavior of the H II region luminosity functions (LFs). In most galaxies, the LF is well represented by a power law function. Both the normalization and the shape of the LF change systematically with Hubble type. Early-type spirals possess fewer H II regions of all luminosities, a steeper LF, and in many cases an upper cutoff or turnover in the LF. Taken together, these effects account for the virtual absence of 'supergiant' H II regions such as 30 Doradus in Sa-Sb galaxies. Most of the massive stars in late-type spiral and irregular galaxies form in large H II/OB complexes, while in early-type spirals the bulk of the massive star formation occurs in small regions ionized by no more than a few OB stars. 59 references.

Eccentric gravitational instabilities in nearly Keplerian disks

Abstract: The growth of global gravitational instabilities in young stellar objects (YSOs) with associated circumstellar disks is studied. The possibility that the accretion ultimately owes its origin to the growth of spiral gravitational instabilities is explored. The results indicate that YSO disks will be unstable to the growth of eccentric distortions which have growth rates comparable to the orbital frequency at the outer edge of the disk. Thus, the distortions grow on nearly a dynamical time scale. Perturbations with m = 1 force the star to move from the center of mass and thereby transfer angular momentum to the stellar orbit. Depending on whether or not an axisymmetric stability parameter Q barrier exists near the corotation radius of the disturbance, this coupling may lead to mass accretion or to the formation of a binary companion from the disk, or both.