Showing papers in "The Astrophysical Journal in 1996"

The Structure of cold dark matter halos

Abstract: High resolution N-body simulations show that the density profiles of dark matter halos formed in the standard CDM cosmogony can be fit accurately by scaling a simple “universal” profile. Regardless of their mass, halos are nearly isothermal over a large range in radius, but significantly shallower than r -2 near the center and steeper than r -2 in the outer regions. The characteristic overdensity of a halo correlates strongly with halo mass in a manner consistent with the mass dependence of the epoch of halo formation. Matching the shape of the rotation curves of disk galaxies with this halo structure requires (i) disk mass-to-light ratios to increase systematically with luminosity, (ii) halo circular velocities to be systematically lower than the disk rotation speed, and (iii) that the masses of halos surrounding bright galaxies depend only weakly on galaxy luminosity. This offers an attractive explanation for the puzzling lack of correlation between luminosity and dynamics in observed samples of binary galaxies and of satellite companions of bright spiral galaxies, suggesting that the structure of dark matter halos surrounding bright spirals is similar to that of cold dark matter halos.

Updated Opal Opacities

Abstract: The reexamination of astrophysical opacities has eliminated gross discrepancies between a variety of observations and theoretical calculations; thus allowing for more detailed tests of stellar models. A number of such studies indicate that model results are sensitive to modest changes in the opacity. Consequently, it is desirable to update available opacity databases with recent improvements in physics, refinements of element abundance, and other such factors affecting the results. Updated OPAL Rosseland mean opacities are presented. The new results have incorporated improvements in the physics and numerical procedures as well as corrections. The main opacity changes are increases of as much as 20{percent} for Population I stars due to the explicit inclusion of 19 metals (compared to 12 metals in the earlier calculations) with the other modifications introducing opacity changes smaller than 10{percent}. In addition, the temperature and density range covered by the updated opacity tables has been extended. As before, the tables allow accurate interpolation in density and temperature as well as hydrogen, helium, carbon, oxygen, and metal mass fractions. Although a specific metal composition is emphasized, opacity tables for different metal distributions can be made readily available. The updated opacities are compared to other work. {copyright} {ital 1996 Themore » American Astronomical Society.}« less

A Line of sight integration approach to cosmic microwave background anisotropies

Abstract: We present a new method for calculating linear cosmic microwave background (CMB) anisotropy spectra based on integration over sources along the photon past light cone. In this approach the temperature anisotropy is written as a time integral over the product of a geometrical term and a source term. The geometrical term is given by radial eigenfunctions which do not depend on the particular cosmological model. The source term can be expressed in terms of photon, baryon and metric perturbations, all of which can be calculated using a small number of differential equations. This split clearly separates between the dynamical and geometrical effects on the CMB anisotropies. More importantly, it allows to significantly reduce the computational time compared to standard methods. This is achieved because the source term, which depends on the model and is generally the most time consuming part of calculation, is a slowly varying function of wavelength and needs to be evaluated only in a small number of points. The geometrical term, which oscillates much more rapidly than the source term, does not depend on the particular model and can be precomputed in advance. Standard methods that do not separate the two terms and require a much higher number of evaluations. The new method leads to about two orders of magnitude reduction in CPU time when compared to standard methods and typically requires a few minutes on a workstation for a single model. The method should be especially useful for accurate determinations of cosmological parameters from CMB anisotropy and polarization measurements that will become possible with the next generation of experiments. A programm implementing this method can be obtained from the authors.

Atomic data for astrophysics. II. New analytic fits for photoionization cross sections of atoms and ions

Abstract: We present a complete set of analytic fits to the nonrelativistic photoionization cross sections for the ground states of atoms and ions of elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization thresholds, the fits are based on the Opacity Project theoretical cross sections interpolated and smoothed over resonances. At higher energies, the fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections. {copyright} {ital 1996 The American Astronomical Society.}

Radiative Transfer in a Clumpy Universe. II. The Ultraviolet Extragalactic Background

Abstract: We present a detailed calculation of the propagation of AGN-like ionizing radiation through the intergalactic space. We model the ionization state of absorbing clouds, and show that the universe will be more opaque above $4\,$Ryd than previously estimated. Singly ionized helium in \Lya forest clouds and Lyman-limit systems is found to be very efficient in reprocessing soft X-ray, helium-ionizing photons into ultraviolet, hydrogen-ionizing ones. We demonstrate that a significant fraction of the absorbed primary photons (emitted, e.g., by quasar sources) will be reradiated by the photoionized gas through \Lya line emission, two-photon continuum, and recombination continuum radiation. In the light of new data and recent studies, we also reassess the contribution of the QSOs observed in optical surveys to the UV extragalactic background, and find that the stochastic reprocessing of quasar Lyman continuum radiation by hydrogen and helium along the line of sight will significantly affect the amplitude, spectral shape, and fluctuations properties of the metagalactic flux. In a scenario in which QSOs are the primary source of ionizing photons in the universe, the integrated \HI \Lya emission at $z=0$ from photoionized \Lya clouds and Lyman-limit systems is found to be at a level of less than 5\% of current observational limits on the far-UV extragalactic radiation flux. We show that $J_{912}$ increases from $\approx 10^{-23} \uvunits$ at the present epoch to $\approx 5\times 10^{-22}\uvunits$ at $z=2.5$. The attenuated direct quasar emission plus recombination radiation from intergalactic gas appears to provide enough hydrogen-ionizing photons to satisfy the proximity effect at large redshift. The \HeII/\HI ratio in the diffuse intergalactic medium and the \Lya clouds increases from $\approx25$ at

The Cosmic Microwave Background spectrum from the full COBE FIRAS data set

Abstract: We have refined the analysis of the data from the FIRAS (Far-InfraRed Absolute Spectrophotometer) on board the COBE (COsmic Background Explorer). The FIRAS measures the difference between the cosmic microwave background and a precise blackbody spectrum. We find new, tighter upper limits on general deviations from a blackbody spectrum. The rms deviations are less than 50 parts per million of the peak of the cosmic microwave background radiation. For the Comptonization and chemical potential, we find |y| < 15 × 10–6 and |μ| < 9 × 10–5 (95% confidence level [CL]). There are also refinements in the absolute temperature, 2.728 ± 0.004 K (95% CL), the dipole direction, (1, b)/(26414 ± 0.30, 4826 ± 0.30) (95% CL), and the amplitude, 3.372 ± 0.014 mK (95% CL). All of these results agree with our previous publications.

Gasdynamics and starbursts in major mergers

Abstract: Using numerical simulation, we study the development of gaseous inflows and triggering of starburst activity in mergers of comparable-mass disk galaxies. In all encounters studied, the galaxies experience strong gaseous inflows and moderate to intense starburst activity. We find that galaxy structure plays a dominant role in regulating activity. The gaseous inflows are strongest when galaxies with dense central bulges are in the final stages of merging, while inflows in bulgeless galaxies are weaker and occur earlier in the interaction. Orbital geometry plays only a relatively modest role in the onset of collisionally-induced activity. Through an analysis of the torques acting on the gas, we show that these inflows are generally driven by gravitational torques from the host galaxy (rather than the companion), and that dense bulges act to stabilize galaxies against bar modes and inflow until the galaxies merge, at which point rapidly varying gravitational torques drive strong dissipation and inflow of gas in the merging pair. The strongest inflows (and associated starburst activity) develop in co-planar encounters, while the activity in inclined mergers is somewhat less intense and occurs slightly later during the merger. The starbursts which develop in mergers of galaxies with central bulges represent an increase in the star formation rate of two orders of magnitude over that in isolated galaxies. We find that the gaseous and stellar morphology and star-forming properties of these systems provide a good match to those of observed ultraluminous infrared galaxies. Our results imply that the internal structure of the merging galaxies, rather than orbital geometry, may be the key factor in producing ultraluminous infrared galaxies.

Transformations of Galaxies. II. Gasdynamics in Merging Disk Galaxies

Abstract: In mergers of disk galaxies, gas plays a role quite out of proportion to its relatively modest contribution to the total mass. To study this behavior, we have included gasdynamics in self-consistent simulations of collisions between equal-mass disk galaxies. The large-scale dynamics of bridge- and tail-making, orbit decay, and merging are not much altered by the inclusion of a gaseous component. However, tidal forces during encounters cause otherwise stable disks to develop bars, and the gas in such barred disks, subjected to strong gravitational torques, flows toward the central regions where it may fuel the kiloparsec-scale starbursts seen in some interacting disk systems. Similar torques on the gas during the final stages of a collision yield massive gas concentrations in the cores of merger remnants, which may be plausibly identified with the molecular complexes seen in objects such as NGC 520 and Arp 220. This result appears insensitive to the detailed microphysics of the gas, provided that radiative cooling is permitted. The inflowing gas can dramatically alter the stellar morphology of a merger remnant, apparently by deepening the potential well and thereby changing the boundaries between the major orbital families.

The Canada-France Redshift Survey: The Luminosity Density and Star Formation History of the Universe to z ~ 1

Abstract: The comoving luminosity density of the universe, (λ), is estimated from the Canada-France Redshift Survey (CFRS) faint galaxy sample in three wave bands (2800 A, 4400 A, and 1 μm) over the redshift range 0 < z < 1. In all three wave bands, increases markedly with redshift. For a (q0 = 0.5, Ω = 1.0) cosmological model, the comoving luminosity density increases as (1 + z)2.1 ± 0.5 at 1 μm, as (1 + z)2.7 ± 0.5 at 4400 A, and as (1 + z)3.9 ± 0.75 at 2800 A, these exponents being reduced by 0.43 and 1.12 for (0.05, 0.1) and (-0.85, 0.1) cosmological models, respectively. The (λ)-τ relation can be reasonably well modeled by an actively evolving stellar population with a Salpeter initial mass function (IMF) extending to 125 M☉, and a star formation rate declining as τ-2.5 with a turn-on of star formation at early epochs. A Scalo IMF extending to the same mass limit produces too many long-lived low-mass stars. This rapid evolution of the star formation rate and comoving luminosity density of the universe is in good agreement with the conclusions of Pei & Fall from their analysis of the evolving metallicity of the universe. One consequence of this evolution is that the physical luminosity density at short wavelengths has probably declined by 2 orders of magnitude since z ~ 1.

The Soft Gamma Repeaters as Very Strongly Magnetized Neutron Stars. II. Quiescent Neutrino, X-Ray, and Alfvén Wave Emission

Abstract: We calculate the quiescent X-ray, neutrino, and Alfven wave emission from a neutron star with a very strong magnetic field, Bdipole ~ 1014 − 1015 G and Binterior ~ (5–10) × 1015 G. These results are compared with observations of quiescent emission from the soft gamma repeaters and from a small class of anomalous X-ray pulsars that we have previously identified with such objects. The magnetic field, rather than rotation, provides the main source of free energy, and the decaying field is capable of powering the quiescent X-ray emission and particle emission observed from these sources. New features that are not present in the decay of the weaker fields associated with ordinary radio pulsars include fracturing of the neutron star crust, strong heating of its core, and effective suppression of thermal conduction perpendicular to the magnetic field. As the magnetic field is forced through the crust by diffusive motions in the core, multiple small-scale fractures are excited, as well as a few large fractures that can power soft gamma repeater bursts. The decay rate of the core field is a very strong function of temperature and therefore of the magnetic flux density. The strongest prediction of the model is that these sources will show no optical emissions associated with X-ray heating of an accretion disk.

Template ultraviolet to near-infrared spectra of star-forming galaxies and their application to K-corrections

Abstract: Template UV-optical spectra of quiescent and starburst galaxies are presented and used to derive Kcorrections as a function of morphological type and redshift. IUE observations and archival data are used for the UV template spectra. The optical spectra are from ground-based observations obtained in apertures that match closely the 200 arcsec 2 IUE aperture. The templates of quiescent galaxies are built according to morphological type, elliptical, bulge, SO, Sa, Sb, and So, and the templates of starburst galaxies according to color excess. The unprecedented characteristics of these templates is that UV and optical spectra have been obtained in matched apertures to produce consistent spectral information from 1200 to 10,000 A. Despite the relatively small IUE aperture, the galaxy stellar populations are well represented in the elliptical, SO, Sa, and Sc, and in the starburst templates. The spectra are available digitally. The UV-optical templates can be applied to the classification of high-redshift galaxies and to the identification of the host galaxies of quasars. The templates predict that observed magnitudes from traditional ground-based photometric surveys can be uniquely interpreted. For example, U, B, and I magnitudes uniquely determine both the redshift and the morphological type of a galaxy. The template spectra are also used to calculate K-corrections for galaxies as a function of morphological type and redshift, up to z = 2. These improved K-corrections are not sufficient to explain the excess counts in faint blue galaxies. A subset of our galaxy templates are linked with published data from the radio to the X-ray for galaxies and quasars. A comparison between the quiescent galaxies and the quasars suggests that, in the optical band, the host galaxy is a factor of 10-100 fainter in flux than the quasar. Subject headings: galaxies: distances and redshifts --galaxies: photometry -galaxies: starburst -galaxies: stellar content -quasars: general

Four-Year COBE* DMR Cosmic Microwave Background Observations: Maps and Basic Results

Abstract: In this Letter we present a summary of the spatial properties of the cosmic microwave background radiation based on the full 4 yr of COBE Differential Microwave Radiometer (DMR) observations, with additional details in a set of companion Letters. The anisotropy is consistent with a scale-invariant power-law model and Gaussian statistics. With full use of the multifrequency 4 yr DMR data, including our estimate of the effects of Galactic emission, we find a power-law spectral index of n = 1.2 ± 0.3 and a quadrupole normalization Qrms-PS = 15.3−2.8+3.8 μK. For n = 1 the best-fit normalization is Qrms-PS|n=1 = 18 ± 1.6 μK. These values are consistent with both our previous 1 yr and 2 yr results. The results include use of the l = 2 quadrupole term; exclusion of this term gives consistent results, but with larger uncertainties. The final DMR 4 yr sky maps, presented in this Letter, portray an accurate overall visual impression of the anisotropy since the signal-to-noise ratio is ~2 per 10° sky map patch. The improved signal-to-noise ratio of the 4 yr maps also allows for improvements in Galactic modeling and limits on non-Gaussian statistics.

Small scale cosmological perturbations: An Analytic approach

Abstract: Through analytic techniques verified by numerical calculations, we establish general relations between the matter and cosmic microwave background (CMB) power spectra and their dependence on parameters on small scales. Fluctuations in the CMB, baryons, cold dark matter (CDM), and neutrinos receive a boost at horizon crossing. Baryon drag on the photons causes alternating acoustic peak heights in the CMB and is uncovered in its bare form under the photon diffusion scale. Decoupling of the photons at last scattering and of the baryons at the end of the Compton drag epoch freezes the diffusion-damped acoustic oscillations into the CMB and matter power spectra at different scales. We determine the dependence of the respective acoustic amplitudes and damping lengths on fundamental cosmological parameters. The baryonic oscillations, enhanced by the velocity overshoot effect, compete with CDM fluctuations in the present matter power spectrum. We present new exact analytic solutions for the cold dark matter fluctuations in the presence of a growth-inhibiting radiation and baryon background. Combined with the acoustic contributions and baryonic infall into CDM potential wells, this provides a highly accurate analytic form of the small-scale transfer function in the general case.

The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars

Abstract: Using the results of the most recent stellar atmosphere models applied to a sample of hot stars, we construct calibrations of effective temperature (T(sub eff)), and gravity (log(sub g)) with a spectral type and luminosity class for Galactic 0-type and early B-type stars. From the model results we also derive an empirical relation between the bolometric correction and T(sub eff) and log g. Using a sample of stars with known distances located in OB associations in the Galaxy and the Large Magellanic Cloud, we derive a new calibration of M(sub v) with spectral class. With these new calibrations and the stellar atmosphere models of Kurucz, we calculate the physical parameters and ionizing photon luminosities in the H(0) and He(0) continua for O and early B-type stars. We find substantial differences between our values of the Lyman- continuum luminosity and those reported in the literature. We also discuss the systematic discrepancy between O-type stellar masses derived from spectroscopic models and those derived from evolutionary tracks. Most likely, the cause of this 'mass discrepancy' lies primarily in the atmospheric models, which are plane parallel and hydrostatic and therefore do not account for an extended atmosphere and the velocity fields in a stellar wind. Finally, we present a new computation of the Lyman-continuum luminosity from 429 known O stars located within 2.5 kpc of the Sun. We find the total ionizing luminosity from this population ((Q(sub 0)(sup T(sub ot))) = 7.0 x 10(exp 51) photons/s) to be 47% larger than that determined using the Lyman continuum values tabulated by Panagia.

Spectroscopic Confirmation of a Population of Normal Star-forming Galaxies at Redshifts z > 3*

Abstract: We report the discovery of a substantial population of star-forming galaxies at 3.0 z 3.5. These galaxies have been selected using color criteria sensitive to the presence of a Lyman continuum break superposed on an otherwise very blue far-UV continuum, and then confirmed with deep spectroscopy on the W. M. Keck telescope. The surface density of galaxies brighter than = 25 with 3.0 z 3.5 is 0.4 ± 0.07 galaxies arcmin-2, approximately 1.3% of the deep counts at these magnitudes; this value applies both to "random" fields and to fields centered on known QSOs. The corresponding comoving space density is approximately half that of luminous (L L*) present-day galaxies. Our sample of z > 3 galaxies is large enough that we can begin to detail the spectroscopic characteristics of the population as a whole. The spectra of the z > 3 galaxies are remarkably similar to those of nearby star-forming galaxies, the dominant features being strong low-ionization interstellar absorption lines and high-ionization stellar lines, often with P Cygni profiles characteristic of Wolf-Rayet and O star winds. Lyα emission is generally weak (less than 20 A rest equivalent width) and is absent for more than 50% of the galaxies. We assign approximate mass scales to the galaxies using the strengths of the heavily saturated interstellar features and find that, if the line widths are dominated by gravitational motions within the galaxies, the implied velocity dispersions are 180 km s-1 ≤ σ ≤ 320 km s-1, in the range expected for massive galaxies. The star formation rates, which can be measured directly from the far-UV continua, lie in the range 4-25 h−250 M☉ yr-1 (for q0 = 0.5), with 8.5 h−250 M☉ yr-1 being typical. Together with the morphological properties of the z > 3 galaxy population, which we discuss in a companion paper, all of these findings strongly suggest that we have identified the high-redshift counterparts of the spheroid component of present-day luminous galaxies. In any case, it is clear that massive galaxy formation was already well underway by z ~ 3.5.

Linear Regression for Astronomical Data with Measurement Errors and Intrinsic Scatter

Abstract: Two new methods are proposed for linear regression analysis for data with measurement errors. Both methods are designed to accommodate intrinsic scatter in addition to measurement errors. The first (BCES) is a direct extension of the ordinary least squares (OLS) estimator to allow for measurement errors. It is quite general, allowing a) for measurement errors on both variables, b) the measurement errors for the two variables to be dependent, c) the magnitudes of the measurement errors to depend on the measurements, and d) other `symmetric' lines such as the bisector and the orthogonal regression can be constructed. The second method is a weighted least squares (WLS) estimator, which applies only in the case where the `independent' variable is measured without error and the magnitudes of the measurement errors on the 'dependent' variable are independent from the measurements. Several applications are made to extragalactic astronomy: The BCES method, when applied to data describing the color-luminosity relations for field galaxies, yields significantly different slopes than OLS and other estimators used in the literature. Simulations with artificial data sets are used to evaluate the small sample performance of the estimators. Unsurprisingly, the least-biased results are obtained when color is treated as the dependent variable. The Tully-Fisher relation is another example where the BCES method should be used because errors in luminosity and velocity are correlated due to inclination corrections. We also find, via simulations, that the WLS method is by far the best method for the Tolman surface-brightness test, producing the smallest variance in slope by an order of magnitude. Moreover, with WLS it is not necessary to ``reduce'' galaxies to a fiducial surface-brightness, since this model incorporates intrinsic scatter.

Opal equation-of-state tables for astrophysical applications

Abstract: OPAL opacities have recently helped to resolve a number of long-standing discrepancies between theory and observation. This success has made it important to provide the associated equation-of-state (EOS) data. The OPAL EOS is based on an activity expansion of the grand canonical partition function of the plasma in terms of its fundamental constituents (electrons and nuclei). The formation of composite particles and many-body effects on the internal bound states occur naturally in this approach. Hence, pressure ionization is a consequence of the theory. In contrast, commonly used approaches, all of which are based on minimization of free energy, are forced to assert the effect of the plasma on composite particles and must rely on an ad hoc treatment of pressure ionization. Another advantage of the OPAL approach is that it provides a systematic expansion in the Coulomb coupling parameter that includes subtle quantum effects generally not considered in other EOS calculations. Tables have been generated that provide pressure, internal energy, entropy, and a variety of derivative quantities. These tables cover a fairly broad range of conditions and compositions applicable to general stellar-evolution calculations for stars more massive than {approximately}0.8 {ital M}{sub {circle_dot}}. An interpolation code is provided along with themore » tables to facilitate their use. {copyright} {ital 1996 The American Astronomical Society.}« less

A Precise distance indicator: Type Ia supernova multicolor light curve shapes

Abstract: We present an empirical method that uses multicolor light-curve shapes (MLCSs) to estimate the luminosity, distance, and total line-of-sight extinction of Type Ia supernovae (SNe Ia). The empirical correlation between the MLCSs and the luminosity is derived from a " training set" of nine SN Ia light curves with independent distance and reddening estimates. We find that intrinsically dim SN Ia's are redder and have faster light curves than the bright ones, which are slow and blue. By 35 days after maximum, the intrinsic color variations become negligible. A formal treatment of extinction employing Bayes's theorem is used to estimate the best value and its uncertainty. Applying the MLCS method to both light curves and to color curves provides enough information to determine which supernovae are dim because they are distant, which are intrinsically dim, and which are dim because of extinction by dust. The precision of the MLCS distances is examined by constructing a Hubble diagram with an independent set of 20 SN Ia's. The dispersion of 0.12 mag indicates a typical distance accuracy of 5% for a single object, and the intercept yields a Hubble constant on the Sandage et al. Cepheid distance scale of H0 = 64 ± 3 (statistical) km s–1 Mpc–1 (±6 total error). The slope of 0.2010 ± 0.0035 mag over the distance interval 32.2 < μ < 38.3 yields the most precise confirmation of the linearity of the Hubble law.

Structure of Stationary Photodissociation Fronts

Abstract: The structure of stationary photodissociation fronts is revisited. H_2 self- shielding is discussed, including the effects of line overlap. We find that line overlap is important for N(H_2) > 10^{20} cm^{-2}. We compute multiline UV pumping models, and compare these with simple analytic approximations for the effects of self-shielding. The overall fluorescent efficiency of the photodissociation front is obtained for different ratios of chi/n_H (where chi characterizes the intensity of the incident UV) and different dust extinction laws. The dust optical depth tau_{pdr} to the point where 50% of the H is molecular is found to be a simple function of a dimensionless quantity phi_0 depending on chi/n_H, the rate coefficient for H_2 formation on grains, and the UV dust opacity. The fluorescent efficiency of the PDR also depends primarily on phi_0 for chi 10^4K, but shows some sensitivity to the v-J distribution of newly-formed H_2. The 1-0S(1)/2-1S(1) and 2-1S(1)/6-4Q(1) intensity ratios, the ortho/para ratio, and the rotational temperature in the $v$=1 and $v$=2 levels are computed as functions of the temperature and density, for different values of chi and n_H. We apply our models to the reflection nebula NGC 2023. We are best able to reproduce the observations with models having chi=5000, n_H=10^5 cm^{-3}.

Mass estimates of X-ray clusters

Abstract: We use cosmological gasdynamic simulations to investigate the accuracy of galaxy cluster mass estimates based on X-ray observations. The experiments follow the formation of clusters in different cosmological models and include the effects of gravity, pressure gradients, and hydrodynamical shocks. A subset of our ensemble also allows for feedback of mass and energy from galactic winds into the intracluster medium. We find that mass estimates based on the hydrostatic, isothermal {beta}-model are remarkably accurate when evaluated at radii where the cluster mean density is between 500 and 2500 times the critical density. At lower densities, radial temperature information becomes important. In the quoted radial regime, the distribution of the estimated-to-true mass ratio, derived from 174 artificial images constructed from the simulations, is nearly unbiased and has a standard deviation of 14{percent}{endash}29{percent}. The scatter can be considerably reduced (to 8{percent}{endash}15{percent}) by using an alternative mass estimator that exploits the tightness of the mass-temperature relation found in the simulations. The improvement over {beta}-model estimates is due to the elimination of the variance contributed by the gas outer slope parameter. We discuss these findings and their implications for recent measurements of cluster baryon fractions. {copyright} {ital 1996 The American Astronomical Society.}

First Results from the All-Sky Monitor on the Rossi X-Ray Timing Explorer

Abstract: The all-sky monitor on the Rossi X-Ray Timing Explorer has been monitoring the sky in the 1.5-12 keV band since late February. The instrument consists of three coded-aperture cameras that can be rotated to view different regions by a motorized drive assembly. Intensities of ~100 known sources are obtained via least-squares fits of shadow patterns to the data and compiled to form X-ray light curves. Six orbital periodicities and four long-term periodicities, all previously known, have been detected in these light curves. Searches for additional sources have also been conducted. X-ray light curves for the Crab Nebula, Cyg X-1, 4U 1705-44, GRO J1655-40, and SMC X-1 are reported. They illustrate the quality of the results and the range of observed phenomena.

Mass Flow through Gaps in Circumbinary Disks

Abstract: We demonstrate through smoothed particle hydrodynamics simulations that a circumbinary disk can supply mass to the central binary through gas streams that penetrate the disk gap without closing it. The conditions for an efficient flow typically require the disk thickness-to-radius ratio z/r 0.05, if the turbulent viscosity parameter α is greater than 0.01. This mass flow may be important for both the individual systems and their statistics. It occurs preferentially onto the lower mass object and acts toward equalization of component masses. The less massive component may be more luminous and easier to detect, owing to its larger accretion luminosity. For eccentric binaries, the mass flow is strongly modulated in time, providing diagnostics for both the disk and the binary. In the protostellar disks, the flow could be detected as shock emission phased with the binary orbit, resulting from stream impact with the circumstellar disks and/or the young stars. In the (super)massive black hole binaries in nuclei of galaxies, the flow may result from the surrounding interstellar medium and produce nearly periodic emission, as observed in quasar OJ 287. For star-planet-disk systems, our results show that the opening of a gap around a planet is not always sufficient for the termination of its growth. This suggests that planets supplied by gas streams from protoplanetary disks may outgrow Jupiter to become "superplanets" with properties heretofore reserved for stars.

Nucleosynthesis in Neutrino-Driven Winds. I. The Physical Conditions

Abstract: During the first 20 s of its life, the enormous neutrino luminosity of a neutron star drives appreciable mass loss from its surface. Previous investigations have shown that this neutrino-driven wind could be the site where the r-process occurs. The nucleosynthesis is sensitive to four physical parameters that characterize the wind: its mass-loss rate, the entropy per baryon, the electron fraction, and the dynamic timescale. Different authors, using numerical models for supernovae, have arrived at qualitatively different values for these key parameters. Here we derive their values analytically and test our analytic results by numerical calculations using an implicit hydrodynamic code. Employing our analytic and numerical methods, we also investigate how various factors can affect our results. The derived entropy typically falls short, by a factor of 2?3, of the value required to produce a strong r-process. Various factors that might give a higher entropy or a more rapid expansion in the wind are discussed.