Table of contents

We have obtained new mathematical results for the theory of clustering of peaks in a Gaussian random field and for the theory of percolation. This new approach can be used for the development of methods for the analysis of the cosmic microwave background. We have simulated 10° × 10° sky maps of the cosmic microwave background anisotropy expected from different cosmological models with 05-1° resolution and have demonstrated the usefulness of the topological method to distinguish between different models of the noise.

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.

We derive the metric for a simple class of isothermal inhomogeneous universes in which the nonzero pressure balances gravity. These models are spherical and static. There exists a family of such models in which the isothermal universe is a special case. These models may represent the ultimate state of an Einstein-de Sitter universe that undergoes a phase transition caused by gravitational clustering.

The associations of the angular positions of background quasars with foreground galaxies, clusters of galaxies, and quasars are often attributed to statistical lensing by gravitational potentials of the matter along the lines of sight, although it is known that the presence of none of the individual objects (galaxies, clusters, or quasars) is sufficient to explain fully the reported amplitudes of the quasar number enhancements. This probably arises from the fact that the gravitational lensing effect of the environmental matter surrounding these objects has been ignored. In this paper we conduct an extensive study of the influence of the environmental matter on the prediction of the quasar enhancement factor by employing a spatial two-point correlation function. Assuming a singular isothermal sphere for the mass density profile in galaxies and cluster of galaxies, we estimate the average surface mass density bar-Σ around galaxies, clusters, and quasars from the galaxy-galaxy, cluster-cluster, cluster-galaxy, and quasar-galaxy correlations. Our results show that the bar-Σ induced quasar number enhancement in the scenario of gravitational magnification depends critically on the mass density parameters of galaxies (Ω_g) and clusters of galaxies (Ω_c) in the universe. For a flat cosmological model with Ω₀ = 1 the environmental matter can indeed play an important role in the lensing origin of the quasar-quasar and quasar-galaxy associations if Ω_g ~ Ω_c ~ Ω₀, while it is unlikely that bar-Σ is sufficient to account for the reported quasar overdensity behind quasars/galaxies if galaxies and clusters of galaxies contribute no more than 25% to the matter of the universe. Nonetheless, the recently observed quasar-cluster associations on a scale of ~ 10' cannot be the result of gravitational lensing by the cluster environmental matter even if Ω_g = Ω_c = Ω₀.

We use an Eulerian hydrodynamic cosmological simulation to model the Lyα forest in a spatially flat, COBE-normalized, cold dark matter model with) Ω = 0.4. We find that the intergalactic, photoionized gas is predicted to collapse into sheetlike and filamentary structures which give rise to absorption lines having characteristics similar to the observed Lyα forest. A typical filament is ~500 h^–1 kpc long with thickness ~50 h^–1 kpc (in proper units), and baryonic mass ~ 10¹⁰h⁻¹M_☉. In comparison our cell size is (2.5, 9) h⁻¹ kpc in the two simulations we perform, with true resolution perhaps a factor of 2.5 worse than this. The gas temperature is in the range 10⁴-10⁵ K, and it increases with time as structures with larger velocities collapse gravitationally.

We show that the predicted distributions of column densities, b-parameters, and equivalent widths of the Lyα forest clouds agree reasonably with observations, and that their evolution is consistent with the observed evolution, if the ionizing background has an approximately constant intensity between z = 2 and z = 4. A new method of identifying lines as contiguous regions in the spectrum below a fixed flux threshold is suggested to analyze the absorption lines, given that the Lyα spectra arise from a continuous density field of neutral hydrogen rather than discrete clouds. We also predict the distribution of transmitted flux and its correlation along a spectrum and on parallel spectra, and the He ii flux decrement as a function of redshift. We predict a correlation length of ~80 h⁻¹ kpc perpendicular to the line of sight for features in the Lyα forest.

In order to reproduce the observed number of lines and average flux transmission, the baryon content of the clouds may need to be significantly higher than in previous models because of the low densities and large volume-filling factors we predict. If the background intensity J_{H I} is at least that predicted from the observed quasars, Ω_b needs to be as high as ~0.25 h⁻². The model also predicts that most of the baryons at z > 2 are in Lyα clouds, and that the rate at which the baryons move to more overdense regions is slow. A large fraction of the baryons which are not observed at present in galaxies might be intergalactic gas in the currently collapsing structures, with T ~ 10⁵–10⁶ K.

All our results on the statistical properties of the simulated spectra are predictions that can be directly tested by applying the same methods to observed spectra. We are making the simulated spectra electronically available.

The Las Campanas Redshift Survey (LCRS) contains 23, 697 galaxies, with an average redshift z = 0.1, distributed over six 15 × 80° slices in the north and south galactic caps. We have computed the power spectrum P(k) for magnitude-limited samples of LCRS galaxies over wavelengths λ = 2π/k = 5–400 h⁻¹ Mpc. The LCRS P(k) may be approximated as ∝ k^−1.8±0.1 for small scales λ = 5–30 h⁻¹ Mpc, changing to ∝k^{1 ± 1} for large scales λ ≈ 200-400 h⁻¹ Mpc. The overall amplitude corresponds to σ₈ = 1.0 ± 0.1 in redshift space.

Comparisons to the power spectra of other redshift surveys will be presented; the LCRS results agree best with those from the combined Center for Astrophysics (CfA2) and Southern Sky redshift surveys (SSRS2). For λ ≳ 100 h⁻¹ Mpc, the LCRS results are consistent with those of other surveys, given the large errors among all the surveys on these scales. For λ ≲ 100 h⁻¹ Mpc, the LCRS P(k) is well determined and similar in shape to the P(k) of other surveys, but with an amplitude differing from some of the other samples, possibly because of inherent clustering differences among different types of galaxies. In particular, power spectrum measurements for volume-limited LCRS samples show that galaxies brighter than about M* − 1 appear about 50% more strongly clustered than those fainter. Also, a sample of LCRS emission galaxies shows 30% weaker clustering than the full LCRS sample.

Comparisons to N-body models show that the LCRS power spectrum lies intermediate between that of a standard flat Ω₀h = 0.5 cold dark matter (CDM) model and an open Ω₀h = 0.2 model, both normalized to σ₈ = 1 for galaxies. On large scales λ ≳ 40 h⁻¹ Mpc, we have fit the LCRS results to various linear CDM models, and find that a number of them could meet the constraints set by the LCRS power spectrum, the Hubble constant range 0.5 ≲ h ≲ 0.8, the abundance of galaxy clusters, and the reasonable assumption that LCRS galaxies are roughly unbiased tracers of the mass, relative to the normalization provided by the 4 year COBE DMR data. The possibilities include open CDM or flat nonzero cosmological-constant CDM models with Ω₀ ≈ 0.4–0.6 and shape parameter Γ ≈ Ω₀h ≈ 0.2–0.3, as well as flat Ω₀ = 1 models with massive neutrino density Ω_v ≈ 0.2–0.3 or a spectral tilt n ≈ 0.7–0.8.

We have used the photon energy spectra of 100 bright gamma-ray bursts (GRBs) obtained from data from the Large Area Detectors of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory to examine the effects of spectral shape on cosmological models of the burst number-intensity distribution (log N – log P), where N is the number of bursts with peak photon flux greater than P. Since it has become common to use an assumed burst photon spectrum in computations of this type, we have examined the consequences of this assumption by using a range of observed burst spectra to create theoretical intensity distributions. We used a conventional Friedmann cosmology to create the models, and assumed that there is no burst source rate density evolution and that the sources are monoluminous (standard candles). This enabled us to focus on the effects of spectral shape on the log N – log P model parameters (peak luminosity and redshift). The shape of the burst spectrum has an influence on the maximum redshift consistent with the BATSE data. The variation in the burst photon spectra causes the model maximum redshifts to vary by an average of ~20%. However, the maximum redshift can differ among log N – log P models incorporating different spectra by more than a factor of 2. Thus, the use of an assumed burst spectrum in modeling the number-intensity distribution yields results that are moderately dependent upon the assumed spectral shape.

We present a striking new Hubble Space Telescope (HST) observation of the rich cluster Abell 2218 taken with the Wide Field Planetary Camera 2. HST's restored image quality reveals a sizable number of gravitationally lensed features in this cluster, significantly more than had been identified by using ground-based telescopes. The brightest arcs are resolved by HST and show internal features that enable us to identify multiply imaged examples, confirming and improving the mass models of the cluster determined from ground-based observations. Although weak lensing has been detected statistically in this and other clusters from ground-based data, the superlative resolution of HST enables us to individually identify weakly distorted images more reliably than hitherto, with important consequences for their redshift determination. Using an improved mass model for the cluster calibrated with available spectroscopy for the brightest arcs, we demonstrate how inversion of the lensing model can be used to yield the redshift distribution of ~ 80 faint arclets to R ≃ 25. We present a new formalism for estimating the uncertainties in this inversion method and review prospects for interpreting our results and verifying the predicted redshifts.

A generally accepted approach to the interpretation of the damped Lyα absorption systems seen in QSO spectra is to attribute them to cases in which the observer's sight line passes through high column density gaseous disks of galaxies, i.e., spiral galaxies or the progenitors of spirals. Here we consider an alternative natural possibility, consistent with available observational data, namely that the absorption is simply associated with neutral gas in giant hydrogen clouds that could be associated with any type of gaseous galaxy or protogalaxy. At high redshift such galaxies could be the progenitors of different types of galaxies (e.g., ellipticals, spirals, etc.) that are observed at the present epoch. We show that the observational data, which include low to moderate redshift data recently obtained with HST and moderate to high-redshift data compiled using large ground-based telescopes over the last decade, coupled with some reasonable assumptions about the properties of giant hydrogen clouds in galaxies, can be used to form the two-dimensional distribution for the number of damped Lyα systems in redshift and column density, d²/dz dN_{H I}, over the redshift interval 0.1 < z < 3.5. This can be further extended to redshift z = 0 using information provided by 21 cm observations of nearby galaxies. By combining this result with the assumption that damped Lyα absorbers behave like the giant hydrogen clouds in our Galaxy and neighboring galaxies, we show that the mass spectrum of clouds responsible for the damped Lyα absorption systems can be derived. This mass spectrum can be used to study the evolution of the giant hydrogen cloud population associated with galaxies over cosmic time. An interesting feature of the mass spectrum is that its steepness increases systematically as the redshift decreases over the interval 3 < z < 0.75. This effect is likely to be related to intensive star formation processes, which lead to the destruction of high-mass clouds and the formation of numerous low-mass clouds.

A simple method for mapping the temperature distribution in extended sources is developed for application to ASCA observations of galaxy clusters. Unlike the conventional approach to spatially resolved spectral analysis, this method does not require nonlinear minimization and is computationally fast and stable. Therefore, it can be implemented for a large number of regions or on a fine spatial grid. Although based on a Taylor expansion over the nonlinear parameter, the method is found to be accurate in many practical situations, the relative error for the temperature estimate being less than 2%-4% when the plasma temperature exceeds ~2 keV. This method is not intended to replace conventional spectral analysis but to supplement it, providing relatively fast and easy construction of temperature maps, which may be used as a guide to further detailed analysis of particularly interesting regions using conventional spectral fitting.

Conventional spectral analysis in the case of moderate and low numbers of counts is discussed. A practical recipe for unbiased parameter estimation is suggested and verified in Monte Carlo simulations for commonly used spectral models. A simple modification of the χ² statistic (calculation of weights based on the smoothed observed spectrum) yields nearly unbiased parameter estimates and correct confidence interval determination with no need for regrouping (binning) the energy channels even in the case of low statistics (~50-100 counts in the observed spectrum with several hundred channels).

We analyze X-ray spatial and spectral data on the giant elliptical galaxy NGC 4472, the brightest galaxy in the Virgo cluster. The X-ray contours of NGC 4472 are elongated in the northeast-southwest direction, perhaps as a result of motion through the Virgo intracluster gas. A bow shock-like structure is evident on the galaxy's north side. The temperature at a given radius in this bow shock region is slightly higher than the temperature at the same radius on the galaxy's southwest side. Away from this bow shock region, the surface brightness profile of NGC 4472 can be traced out to a radius of 260 kpc in the southwest direction. Beyond 260 kpc, we find evidence for emission from both the Virgo cluster and the Galactic North Polar Spur (believed to be the rim of a hot Galactic superbubble).

NGC 4472 is interacting with the dwarf irregular galaxy UGC 7636. We do not detect any excess or deficit in the X-ray emission toward this galaxy. An H i cloud, detected previously in the 21 cm line and located midway between the two galaxies, appears to have been removed from the irregular galaxy through either tidal interaction or ram pressure stripping. We find a marginally significant hole in the ROSAT HRI and PSPC X-ray images at the position of this cloud, suggesting that the cloud lies at the front side of NGC 4472. If the hole in the X-ray images is due to soft X-ray absorption, the total gaseous mass of the cloud must be at least 1.7 × 10⁹M_☉, far greater than its 21 cm H I mass. This suggests that the majority of the material in the cloud is molecular.

The galaxy population in the intermediate-redshift (z = 0.228) rich cluster Abell 2390 is investigated. We present velocities, colors, and morphological information for an exceptionally large sample of 323 galaxies (216 cluster members) in a 46' × 7' (6 h⁻¹ Mpc × 1 h⁻¹ Mpc) strip centered on the cD galaxy. This sample of confirmed cluster members is second only to that for the Coma cluster in terms of sample size and spatial coverage in the cluster rest frame and it is the first to trace the transition between a rich cluster and the field at intermediate redshift. The galaxy population in the cluster changes gradually from a very evolved, early-type population in the inner 0.4 h^–1 Mpc of the cluster to a progressively later type population in the extensive outer envelope of the cluster from 1 to 3 h^–1 Mpc in radius. Radial gradients in galaxy g - r color, 4000 Å break, Hδ and [O II] line strengths, and morphology are seen in the cluster and are investigated by comparing the data to models computed with the GISSEL spectral synthesis package. The results suggest that the cluster has been built up gradually by the infall of field galaxies over ~8 Gyr and that star formation has been truncated in infalling galaxies during the accretion process. The morphological composition of the cluster is shown to be consistent with such a scenario. If true for other clusters, infall-truncated star formation as seen in Abell 2390 may explain both the Butcher-Oemler effect and the large fraction of S0 galaxies in clusters. Only ≲5% of the galaxies observed in Abell 2390 exhibit evidence for star formation at levels stronger than those seen in typical late-type systems. This suggests that starbursts do not play a major role in driving cluster galaxy evolution at the redshift of Abell 2390, although infall-induced starbursts leading to truncated star formation may have played a role in the earlier history of the cluster. Evidence is found for at least one subcomponent on the west side of the cluster, which is likely to be infalling at the epoch of observation.

Infrared surveys have discovered a significant population of bright (K ≲ 19) extremely red (R—K ≳ 6) objects. Little is known about the properties of these objects on account of their optical faintness (R ≳ 24). Here we report deep infrared imaging and spectroscopy of one of the extremely red objects (EROs) discovered by Hu & Ridgway in the field of the z = 3.79 quasar PC 1643 + 4631A. The infrared images were obtained in 0''.5 seeing and show that the object (denoted HR 10) is not a dynamically relaxed elliptical galaxy dominated by an old stellar population as was previously suspected, but instead has an asymmetric morphology suggestive of either a disk or an interacting system. The infrared spectrum of HR 10 shows a single, possibly broad emission feature at 1.60 μm, which we identify as Hα + [N II] at z = 1.44. The luminosity and width of this emission line indicates either intense star formation (~20 h⁻²M_☉ yr⁻¹) or the presence of an active nucleus. Based on the rest frame UV-optical spectral energy distribution, the luminosity of HR 10 is estimated to be 3–8 L*. The colors of HR 10 are unusually red for a galaxy (at z = 1.44 the age of HR 10 is at most 2–8 Gyr depending on cosmology) and indicate that HR 10 is dusty. HR 10 is detected weakly at radio wavelengths; this is consistent with either the starburst or active galactic nucleus hypothesis. If HR 10 is a typical representative of its class, EROs are numerous and represent a significant component of the luminous objects in the universe at z ≈ 1.5.

This paper is mainly concerned with the prediction of absorption-line spectral features in cool stars, to be used as input in stellar population synthesis projects. From a detailed comparison with the solar intensity spectrum, we have refined the main parameters of atomic and molecular absorption lines that are prominent in the 4850-5400 Å wavelength interval. This line list was used to compute an extensive library of synthetic stellar spectra at high resolution in the temperature range T_eff = 4000–8000 K, surface gravity interval log g = 1.0–5.0 dex, metallicities ([M/H]) from −1.0 to + 0.5 dex, and microturbulent velocity ξ = 2 km s⁻¹. The computations were performed by using the latest release of Kurucz's model atmospheres and numerical codes. The library contains a total of 693 synthetic spectra, from which iron and magnesium indices were obtained, together with the corresponding "pseudocontinuum" fluxes.

We illustrate the behavior of five "Lick-like" spectral indices, namely, Mg₁ Mg₂, Mg b, Fe5270, and Fe5335, in terms of the main atmospheric parameters, namely, effective temperature, surface gravity, and metallicity. The trend of the indices with microturbulent velocity is also illustrated by means of an additional set of spectra computed at different microturbulent velocities. Transformation equations of the theoretical grid into the Lick/IDS observational database are presented, showing the full consistency of our grid with the empirical database.

We report on the results of a 5 year coordinated program of spectroscopic monitoring of the luminous Seyfert 1 galaxy Markarian 509. The Hβ and He II λ4686 emission lines are found to respond to continuum variations with time lags of ~80 and ~60 days, respectively, considerably longer than the emission-line lags measured for other Seyfert galaxies.

We have used a set of equations developed by Pringle to follow the evolution of a viscous twisted disk in a galaxy-like potential that is stationary or tumbling relative to inertial space. In an axisymmetric potential, the disk settles to the equatorial plane at a rate largely determined by the coefficient ν₂, associated with shear perpendicular to the local disk plane. If the disk is initially close to the galaxy equator, then the rate at which the inclination decays is well described by the analytic formula of Steiman-Cameron & Durisen; in a highly inclined disk, "breaking waves" of curvature steepen as they propagate through the disk, rendering the numerical treatment untrustworthy. In a triaxial potential that is stationary in inertial space, settling is faster than in an oblate or prolate galaxy, since the disk twists simultaneously about two perpendicular axes. If the figure of the potential tumbles about one of its principal axes, the viscous disk can settle into a warped state in which gas at each radius follows a stable tilted orbit, which precesses in such a way as to remain stationary relative to the underlying galaxy.

We derive all relevant equations needed for constructing a global general relativistic model of advectively cooled, very hot, optically thin accretion disks around black holes and present solutions that describe advection-dominated flows in the gravitational field of a Kerr black hole.

The MACHO project has been monitoring about 10 million stars in the Large Magellanic Cloud (LMC) in the search for gravitational microlensing events caused by massive compact halo objects (MACHOs) in the halo of the Milky Way. In our standard analysis, we have searched this data set for well-sampled, long-duration microlensing light curves, detected several microlensing events consistent with MACHOs in the 0.1 M_☉ ≲ m ≲ 1.0 M_☉ mass range, and set limits on the abundance of objects with masses 10⁻⁵M_☉ ≲ m ≲ 10⁻¹M_☉. In this paper, we present a different type of analysis involving the search for very short timescale brightenings of stars, which is used to set strong limits on the abundance of lower mass MACHOs. Our analysis of the first 2 years of data toward the LMC indicates that MACHOs with masses in the range 2.5 × 10^–7M_☉ < m < 5.2 × 10⁻⁴M_☉ cannot make up the entire mass of a standard spherical dark halo. Combining these results with those from the standard analysis, we find that the halo dark matter cannot be comprised of objects with masses 2.5 × 10⁻⁷M_☉ < m < 8.1 × 10⁻²M_☉.

We present a search for bright, transient emission lines near 0.5 MeV in nearly two years of data from the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory. Such features have been reported from black hole candidates Nova Muscae and 1E 1740.7-2942 and from the Crab, lasting for ≈ 1 day. Our survey covers the whole sky and is sensitive to events with durations from 0.5-3.0 day. No transients are observed, and the systematic errors are low enough that the upper limits are significantly below the fluxes of the two most significant events previously reported.

A number of globular clusters appear to have undergone core collapse, in the sense that their predicted collapse times are much shorter than their current ages. Simulations with gas models and the Fokker-Planck approximation have shown that the central density of a globular cluster after the collapse undergoes nonlinear oscillation with a large amplitude (gravothermal oscillation). However, the question whether such an oscillation actually takes place in real N-body systems has remained unsolved because an N-body simulation with a sufficiently high resolution would have required computing resources of the order of several GFLOPS-yr. In the present paper, we report the results of such a simulation performed on a dedicated special-purpose computer, GRAPE-4. We have simulated the evolution of isolated point-mass systems with up to 32,768 particles. The largest number of particles reported previously is 10,000. We confirm that gravothermal oscillation takes place in an N-body system. The expansion phase shows all the signatures that are considered to be evidence of the gravothermal nature of the oscillation. At the maximum expansion, the core radius is ~1% of the half-mass radius for the run with 32,768 particles. The maximum core size, r_c, depends on N as ⟨r_c⟩ ∝ N^−1/3.

The standard model of tidal capture predicts that ~45 moderately bright cataclysmic binaries should exist today in 47 Tuc. To test this prediction, we have conducted a search for erupting dwarf novae in the center of 47 Tuc with 12 separate epochs of Hubble Space Telescope (HST) images in visual through near-UV bands. We have found a second eruption of the dwarf nova V2 discovered by Paresce and De Marchi. In addition, we have discovered a faint blue variable with period ~4.7 hr and amplitude 1.5 mag, which may be a cataclysmic variable. Detailed simulations demonstrate that our areal and temporal coverage and detection sensitivity is sufficient to have detected one-third of all dwarf novae (via their eruptions) in the center of 47 Tuc; even the very faint U Gem-type objects. We therefore claim that there are probably no more than three dwarf novae in the core of 47 Tuc, in significant disagreement with a key prediction of the standard model of tidal capture, unless the properties of dwarf novae in globulars differ (e.g., in outburst frequency) from those in the field. In addition, the cluster color-magnitude diagram reveals zero novalike variable candidates.

Interstellar molecular clouds have power-law size L and mass M distributions of the form n(L) dL = L^−α_LdL and n(M) dM = M^−α_MdM, where M ∝ L^k is also a power law. These relations are shown to result from the fractal and scale-free nature of interstellar gas with power indices that are independent of distance. The results are α_L = 1 + D and α_M = 1 + D/κ for interstellar fractal dimension D = 2.3 ± 0.3 and a value of κ in the range 2.4-3.7, as determined from cloud surveys in the literature. The same fractal dimension also results from the expected relation D = κ when the M(L) correlation includes many different surveys, spanning a range of 10¹⁰ in mass. These results imply that interstellar CO clouds are the unresolved parts of a pervasive fractal structure in the interstellar gas. The similarity between n(M) for interstellar clouds and n(M) for globular clusters suggests that the clusters formed inside fractal progenitor clouds at a nearly constant efficiency.

The shapes of isolated Bok globules and embedded dense cores of molecular clouds are analyzed using a nonparametric kernel method, assuming the alternate hypotheses that they are randomly oriented prolate objects or that they are randomly oriented oblate objects. In all cases, the prolate hypothesis gives a better fit to the data. If Bok globules are oblate spheroids, they must be very flattened; the average axis ratio is ⟨γ⟩ ≈ 0.3, and no globules can have γ ≳ 0.7. If Bok globules are prolate, their intrinsic flattening is not as great, with a mean axis ratio ⟨γ⟩ ≈ 0.5. For most data samples of dense cores embedded within molecular clouds, the randomly oriented oblate hypothesis can be rejected at the 99% one-sided confidence level. If the dense cores are prolate, their mean axis ratio is in the range ⟨γ⟩ = 0.4–0.5. Analysis of the data of Nozawa et al. reveals that dense cores are significantly different in shape from the clouds in which they are embedded. The shapes of dense cores are consistent with their being moderately flattened prolate spheroids; clouds have flatter apparent shapes and are statistically inconsistent with a population of axisymmetric objects viewed at random angles.

We present fully three-dimensional hydrodynamic simulations of radiative cooling jets propagating into stratified isothermal ambient media with power-law density and pressure distributions. The parameters used are mainly suitable for protostellar jets, but results applicable to extragalactic jets are also presented. Comparisons are made with previous simulations of jets through homogeneous media. We find that, for radiative cooling jets propagating into regions where the ambient medium has an increasing density (and pressure) gradient, the ambient gas tends to compress the cold, low-pressure cocoon of shocked material that surrounds the beam and destroy the bow shock-like structure at the head. The compressing medium collimates the jet and promotes the development of Kelvin-Helmholtz instabilities, which cause beam focusing, wiggling, and the formation of internal traveling shocks, close to the head, via pinching along the beam. This remarkably resembles the structure of some observed systems (e.g., the Haro 6-5B northern and HH 24G jets). These effects are larger for jets with a smaller density ratio η between jet and environment (tested for η = 1, 3, and 10) and larger Mach numbers M_a = v_j/c_a (tested for M_a = 12 and 24, where v_j is the jet velocity and c_a is the ambient sound speed). In an ambient medium of decreasing density (and pressure), the beam is poorly collimated and relaxes, becoming faint. This could explain "invisible" jet sections, like the gap between the parent source and collimated beam (e.g., in the HH 30 jet). Although, on average, jets propagating into an increasing (decreasing) density environment are decelerated (accelerated) by the increasing (decreasing) ram pressure of the ambient medium, we find that their propagation velocities have an oscillatory pattern. The internal traveling shocks that develop in jets propagating into positive density gradient environments display a similar velocity variation, in qualitative agreement with recent measurements of fluctuations in the tangential velocity of the knots of the Haro 6-5B jet. Finally, runs of adiabatic jets into similar stratified environments indicate that they are less affected by the effects of stratification than the cooling jets because their higher pressure cocoons are better able to preserve the beam structure.

High-resolution echelle spectra have been obtained for low-mass pre-main-sequence stars in the Taurus-Auriga, Orion, Chamaeleon, Ophiuchus, and other star formation regions. Using temperature-sensitive individual metallic line ratios, new effective temperatures are determined for a sample of 30 G and K pre-main-sequence stars in nearby star-forming clouds to an accuracy of ± 200 K or better in most cases. Comparison of these values with previous spectral classifications using low-resolution spectra reveals occasional large discrepancies in spectral type. Microturbulences derived for 23 "weak line" T Tauri stars range as high as 3.1 ± 0.4 km s^–1. The mean microturbulent velocity of the sample is 1.6 ± 0.6 km s^–1. Using these new effective temperatures and microturbulences, iron abundances have been determined for 30 pre-main-sequence stars in several northern and southern sky star-formation regions. By determining the mean [Fe/H] for at least five stars in each cloud, bulk metallicities have been derived for the Taurus-Auriga, Orion, Chamaeleon, and Ophiuchus molecular clouds. [Fe/H] is approximately solar in all the regions surveyed.

We present the C¹⁸O(J = 1–0) observation of the Cep OB3 molecular cloud. The observation was made for a week in 1993 December with the new 4 m millimeter telescope at Nagoya University. The extent of the C¹⁸O(J = 1–0) emission indicates that total area coverage of the map is substantially less than 28 × 14 structure. A portion of the Cep OB3 molecular cloud has been mapped in the C¹⁸O(J = 1–0) line on a completely sampled grid with a 20 spacing. The physical parameters of the C¹⁸O(J = 1–0) core are derived. The C¹⁸O(J = 1–0) emission is also tracing the dense gas in the core. The large dense core appears to have been disturbed significantly by the star formation activity within it.

We present high-sensitivity images of the H92α, He92α, and C92α recombination line emission from H II regions in the Galactic radio source W3. Our 8.3 GHz VLA observations have 4'' resolution and sufficient sensitivity to image the carbon recombination line in four of the eight H II regions in the complex. Portions of W3A, the largest and probably most evolved of the H II regions, have a helium abundance as high as 20% by number (nearly twice the cosmic abundance) which confirms previous observations made with lower signal-to-noise ratios. W3A also shows large velocity gradients in the H92α line; we conclude that the gradient is probably caused by the evaporation of the dense molecular core associated with IRS 5. Near the edges of W3B and W3C, strong C⁺ emission is found where no hydrogen or helium line emission is detected. This C⁺ emission comes from the interface zone between the H II region and the surrouding molecular cloud. We show that the C⁺ velocity is more typical of the surrounding molecular cloud while the H⁺ and He⁺ velocities trace the ionized gas. A H92α velocity gradient is used as a probe of the gasdynamics in W3B; a model is proposed which explains the gradient. Despite the fact that W3C and W3D are separate H II regions, they apparently share a common C⁺ region; the C92α line intensity is up to 70% that of the H92α line. Three independent methods provide an electron temperature in the range of T_e = 8000-9000 K for W3A; a wider range of values is derived for W3B and W3D. W3B shows signs (10 σ) of a decreasing radial electron temperature gradient.

The supernova remnants G327.1 –1.1 and G327.4 + 0.4 (Kes 27) are located 15 apart in the constellation Norma. In 1980, Einstein IPC observations discovered that both were irregular filled-center X-ray sources with possible point sources superposed. This paper describes new ROSAT PSPC observations which both map the diffuse structure and clearly show several unresolved sources in each field. Both remnants have bright emitting regions inside the limb which might indicate the presence of high-energy electrons accelerated by a pulsar. The interior region is more prominent in G327.1-1.1 than in Kes 27.

The spectra are relatively strongly absorbed, as expected from distant remnants close to the Galactic plane. Comparison of the X-ray and radio maps of each remnant allows us to attribute some emission to a shell and some to the interior. With this information, a blast-wave model is used to derive approximate ages and energy release. Indications are that the Kes 27 supernova deposited ~10⁵¹ ergs in the surrounding medium. The G327.1-1.1 event probably deposited a factor of 3-10 less.

In order to determine the location of the complex molecules CH₃OH, CH₂CHCN, HCOOCH₃, CH₃CH₂CN, CH₃CH₂OH, and CH₃OCH₃ and in the G34.3 + 0.2 star-forming complex, BIMA Array observations of transitions near 104 and 107 GHz were carried out with 13'' × 8'' resolution. One site of complex molecule emission was detected. This core is located near the cometary H II region G34.3 + 0.2 C. The proximity of this core containing complex species to an ultracompact H II region is quite similar to the Sgr B2(N) core. This similarity suggests that, as appears to be the case in Sgr B2(N), a nearby, newly-formed massive star has evaporated grain mantles, releasing large amounts of complex species into the gas phase.

A series of two-dimensional numerical simulations of explosive nuclear burning is presented for white dwarfs near the Chandrasekhar mass. We assume that the burning begins as a slow deflagration front at or near the center of the star and continues until the density in the burning regions has declined to about 10⁷ g cm^–3, where the flame is essentially extinguished. We employ a novel numerical representation of the turbulent flame brush based upon ideas previously developed for modeling laboratory combustion and explore in some detail the sensitivity of the outcome to the manner in which burning is initiated. In particular, we simulate (1) a centrally ignited deflagration, (2) off-center ignition at a single "point," and (3) simultaneous off-center ignition at five "points." We find that the amount of ⁵⁶Ni that is produced and other observable properties depend sensitively upon how the fuel is ignited. Because of the immediate onset of buoyant acceleration, the burning region in models ignited off center rises toward the surface more quickly than in the (commonly assumed) case of central ignition. With the exception of the model that ignited off-center at a single point, all models are unbound at the end of the computations, and between 0.59 M_☉ (central ignition) and 0.65 M_☉ (ignition at multiple "points") of matter are processed into nuclear-burning products. These results would guarantee an observable, though weak, Type Ia supernova. Our results are expected to change for simulations in three dimensions, especially for the off-center ignitions discussed in this paper, and late detonations driven by pulsations are not unambiguously excluded. We can, however, state that the chances for a direct transition to a detonation appear small because, in all our models, the turbulent velocity of the burning front remains very subsonic.

We report evidence from the Third BATSE Gamma-Ray Burst Catalog that long (T₉₀ > 10 s) and short (T₉₀ < 10 s) gamma-ray bursts (GRBs) represent distinct source populations. Their spatial distributions are significantly different, with long bursts having ⟨ V/V_max ⟩ = 0.282 ± 0.014 but short bursts having ⟨ V/V_max ⟩ = 0.385 ± 0.019, differing by 0.103 ± 0.024, significant at the 4.3 σ level. This implies different spatial origin and physical processes for long and short bursts. Long bursts may be explained by accretion-induced collapse. Short bursts require another mechanism, for which we suggest neutron star collisions. These are capable of producing neutrino bursts as short as a few milliseconds, consistent with the shortest observed timescales in GRBs. We briefly investigate the parameters of clusters in which neutron star collisions may occur, and discuss the nuclear evolution of expelled and accelerated matter.

We examine the speed of inward-traveling cooling fronts in accretion disks. We show that their speed is determined by the rarefaction wave that precedes them and is approximately α_Fc_F(H/r)^q, where α_F is the dimensionless viscosity, c_F is the sound speed, r is the radial coordinate, H is the disk thickness, and all quantities are evaluated at the cooling front. The scaling exponent q lies in the interval [0, 1], depending on the slope of the (T, Σ) relation in the hot state. For a Kramer's law opacity and α ∝ (H/r)ⁿ, where n is of order unity, we find that q ~ 1/2. This supports the numerical work of Cannizzo, Chen, & Livio and their conclusion that n ≈ 3/2 is necessary to reproduce the exponential decay of luminosity in black hole X-ray binary systems. Our results are insensitive to the structure of the disk outside of the radius where rapid cooling sets in. In particular, the width of the rapid cooling zone is a consequence of the cooling front speed rather than its cause. We conclude that the exponential luminosity decay of cooling disks is probably compatible with the wave-driven dynamo model. It is not compatible with models with separate, constant values of α for the hot and cold states.

We describe a non-LTE photoionization code that calculates the atmospheric structure and emergent spectrum of a red giant illuminated by the hot component of a symbiotic binary system. The model assumes hydrostatic, radiative, and statistical equilibrium for the red giant atmosphere and solves the radiative transfer equation with a local escape probability method. We compute non-LTE level populations for a variety of ions and predict the variation of emission-line fluxes as function of the temperature and luminosity of the hot component.

Our models produce strong emission lines only when the hot component has a high effective temperature, T_h ≳ 100,000 K, for hot component luminosities, L_h ≳ 630 L_☉. Predicted electron densities and temperatures for the photoionized atmosphere agree with observations. The models also produce reasonably large continuum variations that are consistent with the light curves of some symbiotic stars. However, predictions for most optical and ultraviolet emission-line fluxes fall well below those observed in typical symbiotic stars. We conclude that the hot component must illuminate a red giant wind to reproduce observed line fluxes. Hydrostatic red giant atmospheres simply do not have enough material beyond the photosphere to account for the emission features observed in most symbiotics.

Illumination can modify the structure of a red giant atmosphere even when the emitted spectrum changes very little. Energetic photons from the hot component cause the atmosphere to expand by several percent for large hot component luminosities. This expansion is insufficient to increase the red giant mass-loss rate, except in systems where the giant already fills or nearly fills its Roche lobe.

Optical spectroscopy of the nova-like variable V1315 Aql shows strong single-peaked emission lines, high-velocity s-waves in the wings of the lines, and two distinct absorption components. In order to explain all the features, I suggest that disk overflow accretion has to be combined with an accretion disk wind. The absorption is probably caused by the accretion stream as it flows over the disk and by P Cygni troughs produced by the wind. If correct, this model can account for the peculiarities of the emission lines in SW Sex stars. Both disk overflow and the strong wind might result from a high mass transfer rate.

We have completed a new program that calculates both synthetic spectra and synthetic light curves for binary systems that contain a standard model optically thick accretion disk. Our initial model is physically and geometrically simple. It is capable of substantial elaboration. By basing both synthetic light curves and synthetic spectra on a single self-consistent model, we anticipate useful tests of systems for which both types of data are available. Alternatively, explicit simulations now become possible for systems for which only light curves or only spectra are available.

The strong optical emission of the Crab pulsar allows us to study the polarization properties in detail. Optical pulsed emission can best be understood as synchrotron radiation of relativistic electron-positron pairs in the outer magnetosphere. We calculate the polarization properties of the optical pulses based on the outer gap model, which has explained much of the observed properties of this pulsar. We find that the calculated polarization profiles exhibit the same qualitative behaviors as the observations for a large range in parameter space. It seems that the optical polarizations do not constrain strongly on the Crab pulsar's emission geometry.

We present results of observations at 47 and 95 μm from the Kuiper Airborne Observatory of several "Vega-like" stars. Spatial cuts and aperture photometry are presented for β Pictoris and Fomalhaut, and for HD 135344, HD 139614, HD 142527, and HD 169142, four stars that had been suggested to possibly represent more distant examples of the Vega phenomenon by Walker & Wolstencroft. We have modeled the dust around β Pic and Fomalhaut with a spatially and optically thin disk to determine the constraints our new observations place on the properties of the dust disks that are required to explain the infrared and optical properties of these two stars. For β Pic we find that models similar to those proposed by Backman, Gillett, & Witteborn can fit our data quite well. For Fomalhaut we find that very different models are required which have much "blacker" dust with a much shallower density distribution, surface density ∝r^−0.5, than for β Pic. Our observations of the four HD stars are consistent with their being spatially unresolved. Because of their distance, this does not allow us to put any new constraints on their circumstellar shells.

We report high spatial resolution HST imagery and photometry of three well-studied, intense Galactic X-ray binaries, X2129 + 470, CAL 87, and GX 17 + 2. All three sources exhibit important anomalies that are not readily interpreted by conventional models. Each source also lies in a severely crowded field, and in all cases the anomalies would be removed if much of the light observed from the ground in fact came from a nearby, thus far unresolved superposed companion. For V1727 Cyg (X2129] 470), we find no such companion. We also present an HST FOS spectrum and broadband photometry which is consistent with a single, normal star. The supersoft LMC X-ray source CAL 87 was already known from ground-based work to have a companion separated by 09 from the optical counterpart; our HST images clearly resolve these objects and yield the discovery of an even closer, somewhat fainter additional companion. Our photometry indicates that contamination is not severe outside eclipse, where the companions only contribute 20% of the light in V, but during eclipse more than half of the V light comes from the companions. The previously determined spectral type of the CAL 87 secondary may need to be reevaluated due to this significant contamination, with consequences on inferences of the mass of the components. We find no companions to NP Ser (= X1813–14, = GX 17 + 2). However, for this object we point out a small but possibly significant astrometric discrepancy between the position of the optical object and that of the radio source which is the basis for the identification. This discrepancy needs to be clarified.

ASCA observed the bright Herbig star HD 104237 (A4e) for 30 ks in 1995 April, providing the best X-ray spectrum yet obtained of an intermediate mass pre-main-sequence star. The objective was to identify the physical mechanism responsible for the X-ray emission, with emphasis on discriminating between the softer emission that is characteristic of shocks and the harder emission (≥ 1 keV) that is normally associated with magnetic activity. Spectral fits using optically thin plasma models show that most of the emission comes from a cool component at 0.2-0.4 keV (~2-4 MK) and a hotter component whose temperature is not tightly constrained but is above 1.6 keV (~18 MK). We consider several possible emission mechanisms, including wind shocks, accretion shocks, a wind-fed magnetosphere, and a corona. Our main conclusion is that the X-ray emission most likely arises in a corona. However, coronal X-ray emission is unanticipated since Herbig stars are thought to lack the convection zones needed to sustain magnetic activity via a solar-like dynamo. We examine two possible solutions to this apparent paradox, namely (1) a corona around the Herbig star itself, sustained by a nonsolar shear-induced dynamo (Tout & Pringle 1995), and (2) a corona around a faint late-type companion whose presence is suspected on the basis of recent infrared observations.

Evidence for coronal heating by chromospheric evaporation in flares of active dMe stars is presented through observations of the Neupert effect in high-frequency microwaves and soft X-rays. The Neupert effect, as originally found in solar flares, manifests itself in a close similarity between the soft X-ray light curve and the time integral of the simultaneous microwave light curve. It is interpreted as the signature of the accumulation of hot plasma due to heating by accelerated electrons in the chromosphere.

We used the ROSAT and ASCA soft X-ray observatories and the Very Large Array (VLA) radio telescope (at 6 cm and 3.6 cm wavelengths) to monitor simultaneously the nearby dMe flare star binary Gliese 65 A+ B = UV Ceti during 9 hours on each of two consecutive days. We find several weakly polarized radio events that start contemporaneously (within a few minutes) with X-ray flares and then peak and decay as the X-ray flares develop gradually. A striking similarity to the temporal evolution of solar gradual events is found. We argue that the Neupert effect is best observed in relatively hard bands of the soft X-ray emission, but that its presence can be inferred from the much softer bands commonly used for stellar observations by use of the solar analogy. Together with spectral hardness observations of soft X-rays, the data suggest the operation of chromospheric evaporation on UV Cet. The observations thus indicate a causal relation between the nonthermal and thermal energies of the underlying electron populations.

We find that stellar flares are, relative to solar flares, X-ray-weak. The ratio between the total energy radiated into the radio and the soft X-ray bands closely matches the corresponding ratio between the quiescent luminosities of active stars, perhaps implying similar mechanisms and similar efficiencies for the quiescent emission and for larger, single flares. Estimating the total kinetic energy in the electrons from the radio flux, we find that only a part is observed in soft X-rays, a discrepancy well known from solar flares.

Slow resonant MHD waves are studied in a compressible plasma with strongly anisotropic viscosity and thermal conductivity. It is shown that anisotropic viscosity and/or thermal conductivity removes the slow singularity which is present in the linear ideal MHD equations. Simple analytical solutions to the linear dissipative MHD equations are obtained which are valid in the dissipative layer and in two overlap regions to the left and the right of the dissipative layer. Asymptotic analysis of the dissipative solutions enables us to obtain connection formulae specifying the variations or jumps of the different wave quantities across the dissipative layer. These connection formulae coincide with those obtained previously for plasmas with isotropic viscosity and finite electrical conductivity. The thickness of the dissipative layer is inversely proportional to the Reynolds number, in contrast to the case of isotropic dissipative coefficients, where it is inversely proportional to the cube root of the Reynolds number. The behavior of the perturbations in the dissipative layer is described in terms of elementary functions of complex argument.

We present the results of a study of the scaling properties of solar photospheric motions. We use time series of Doppler images obtained in good seeing conditions with the San Fernando Observatory 28 cm vacuum telescope and vacuum spectroheliograph in video spectra-spectroheliograph mode. Sixty line-of-sight Doppler images of an area of the quiet Sun near disk center are investigated. They were taken at 60 s intervals over a 1 hr time span at ~2'' resolution.

After filtering to remove 5 minute acoustic oscillations, the time-spatial spectrum of the velocity is calculated. To study the turbulence of photospheric flows in the mesogranulation scale range, we estimate two scaling parameters in the spectrum: the exponent of the spatial part of the power spectrum and the exponent governing the scaling of time correlations of each spatial mode. These parameters characterize the type of diffusion involved and the fractal dimension of the diffusion front. Our results indicate that the turbulent diffusion produced by motions in this scale range is not normal diffusion but superdiffusion.

"Helioseismic tomography" is a method using observations to construct slices of the Sun's internal structure. It is based on a reduction of observations to time-distance surfaces and hypersurfaces. We present a procedure for measuring time-distance surfaces and hypersurfaces, and thereby a method of studying localized inhomogeneities in the interior of the Sun, such as abnormalities in the sound speed (e.g., a thermal shadow, Parker 1987a), or local subsurface flows, or magnetic fields. We also present a simulation of measuring time-distance surfaces and illustrate how to measure the size of an inhomogeneity, its location in depth, and the deviation of its sound speed compared to its local surroundings.

Avalanche models of solar flares successfully reproduce the power-law distribution of flare frequency as a function of energy. However, the model distributions have been produced for a single numerical grid, representing a single active region on the Sun. Here we convolve the distribution owing to an avalanche grid with each of two observationally determined active region size distributions. The resulting energy distributions are power laws (with index α ≈ 1.5) below about 10³¹ ergs, but they gradually steepen with energy. The resulting distributions are compared with a flare energy distribution derived from International Cometary Explorer satellite observations. Qualitative agreement is found between the model and observed distributions, although the observations favor a simple power-law model distribution with a somewhat steeper index (≈1.71).

In this paper, we analyze first the configuration, especially the reversal features in the Hβ chromospheric longitudinal magnetograms in a very flare-active δ group (NOAA 6659). We discuss the possibility of large-scale reversal chromospheric magnetic structures overlapped on the photospheric umbrae after the comparison between the photospheric vector magnetograms and chromospheric longitudinal magnetograms and their forming height difference. We present the possible spatial configuration of the magnetic shear and the relationship with the electric current, and we and that some highly sheared magnetic features of opposite polarity in the active region deviated from the constant-α force-free field. The interaction of the newly emerging magnetic flux of opposite polarity with large-scale magnetic structures is the triggering mechanism of a series of powerful fiares formed in the lower solar atmosphere.

The Large Angle Spectrometric Coronagraph (LASCO) C1 coronagraph on board the Solar and Heliospheric Observatory (SOHO) is designed to image the corona from 1.1 to 3.0 R_☉. The resolution of C1 is defined by the size of its CCD pixels, which correspond to 56, and not by the diffraction limit of the optical system, which may be as small as 3''. The resolution of C1 can be improved using the technique of "dynamic imaging"—the process of acquiring successive images of the same scene using subpixel displacements of the steerable primary mirror. We developed a technique we call the fractional pixel restoration (FPR) algorithm that utilizes these observations to construct an image with improved resolution. Simulations were used to test this algorithm and to explore its limitations. We also applied the direct co-addition and FPR algorithms to laboratory preflight images of a wire mesh grid. These results show that the resolution of the C1 coronagraph can be significantly enhanced, even in the presence of noise and modest differences between successive images. In some cases, the results can even reach the diffraction limit of the telescope.

The laboratory submillimeter wave rotational spectrum of the ¹³CH₃CN, CH₃¹³CN, and CH₃C¹⁵N isotopomers of methyl cyanide has been observed in natural abundance in the 294 to 607 GHz region. The maximum J and K values are 34 and 14, respectively. Fifteen additional CH₃CN transitions up to K = 21 were also measured. The transitions of all four species are fitted to a symmetric top Hamiltonian, and the rotation and distortion constants are determined. The ¹⁴N quadrupole and spin rotation coupling constants are also calculated and presented. Suggested values for many other parameters, which could not be directly determined from the isotope spectra, are calculated from the normal species values and isotope relationships. The determined and calculated constants should predict the spectrum of the three isotopomers to well over 1 THz accurately enough for astronomical assignments.

Erratum to ApJ, 462, 614 (1996).

We present 4 years of spectrophotometric monitoring data for two radio-quiet quasars, PG 0804+762 and PG 0953+414, with typical sampling intervals of several months. Both sources show continuum and emission-line variations. The variations of the Hβ line follow those of the continuum with a time lag, as derived from a cross-correlation analysis, of 93 ± 30 days for PG 0804+762 and 111 ± 55 days for PG 0953+414. This is the first reliable measurement of such a lag in active galactic nuclei with luminosity L > 10⁴⁵ ergs s^-1. The broad-line region (BLR) size that is implied is almost an order of magnitude larger than that measured in several Seyfert 1 galaxies and is consistent with the hypothesis that the BLR size grows as L^0.5.

Hubble Space Telescope WFPC2 images show that the low-luminosity elliptical galaxy companion to M87, NGC 4486B, has a double nucleus that resembles the one discovered in M31. The NGC 4486B nucleus comprises two peaks separated by ~015 or 12 pc. Neither peak is coincident with the galaxy photocenter, which falls between them. The nuclear morphology is independent of color; thus, the double structure is not likely to arise from dust absorption. It is also unlikely that the peaks are a binary stellar system (such as an ongoing merger of the nucleus of a less luminous system with the nucleus of NGC 4486B), since the decay timescale is short ( < 10⁸ yr) and the present environment of NGC 4486B should inhibit mergers. We suggest that the nuclear morphology of NGC 4486B may be explained by the eccentric-disk model of Tremaine, which was originally advanced to account for the central structure of M31. This model requires that NGC 4486B contains a central massive dark object, which is suggested by the spectroscopic observations of Kormendy et al. The eccentric disk might be related to the symmetric disk seen at larger radii.

Using X-ray temperature and surface brightness profiles of the hot intracluster medium (ICM) derived from ASCA (Astro-D) and ROSAT observations, we place constraints on the dark matter (DM) and baryon fraction distributions in the poor clusters Abell 1060 (A1060) and AWM 7. Although their total mass distributions are similar, AWM 7 has twice the baryon fraction of A1060 in the best-fit models. The functional form of the DM distribution is ill determined; however, mass models where the baryon fractions in A1060 and AWM 7 significantly overlap are excluded. Such variations in baryon fraction are not predicted by standard models and imply that some mechanism in addition to gravity plays a major role in organizing matter on cluster scales.

Using the William Herschel Telescope ISIS spectrograph, deep long-slit spectra were obtained of the ionized gas across the blue compact dwarf galaxy I Zw 18 and in an irregular galaxy located 15'' to the northwest. Heliocentric radial velocities of Hα and Hβ emission lines were measured over a 50'' length spanning the main body of I Zw 18 all the way to a central bright emission knot of the companion galaxy. This emission shows a smoothly varying double-sinusoidal variation in radial velocity over the length of the slit, which corresponds to about 2.5 kpc in the plane of the sky for an adopted distance of 10 Mpc for I Zw 18. Our primary result is that the radial velocity of an Hα knot near the center of the companion galaxy is the same as the systemic radial velocity of the ionized gas in the center of the main body of I Zw 18. We also observe a smoothly varying velocity field in the ionized gas between, which we interpret to be strong evidence for the two stellar systems' physical closeness to each other.

We calculate the spectra expected from internal shocks in unsteady relativistic wind models of gamma-ray bursts, suitable for events of arbitrary duration and light curve. The spectral energy distribution of the burst is calculated over photon energies spanning from eV to TeV, for a range of event durations and variability timescales. The relative strength of the emission at different wavelengths can provide valuable information on the particle acceleration, radiation mechanisms, and the possible types of models.

I consider a model of γ-ray bursts in which they arise right before the merging of binary pulsars. A binary pulsar moving through its companion's magnetic field experiences a large, motional electric field E = v × B/c, which leads to the release in the pulsar's magnetosphere of a pair cascade and the acceleration of a wind of pure pairs. The energy and energy deposition rate of the wind are those of γ-ray bursts, provided the pulsars have a field of ≈ 10¹⁵ G. Baryon contamination is small and dominated by tidal heating, leading to M_baryon ≈ 10^-6M_☉, as required by the dirty-fireball model of Mészáros, Laguna, & Rees.

Many current models of Type Ia supernovae involve detonations at some phase of the explosion. Experimental and computational results for detonations in terrestrial fuel-air mixtures show that multidimensional effects are critical to understanding the initiation, propagation, and quenching of real detonations. Multidimensional calculations produce structures that can lead to unsteady detonation propagation in simulations of fuel-air mixtures, in agreement with experiments. We present here the first results to demonstrate the presence of multidimensional effects in detonations in degenerate carbon-oxygen matter. Our simulations show that perturbations induce transverse waves in the carbon-burning layer that interact and create pockets of incompletely burned material. This increases the effective size of the burning region, reduces the detonation velocity, and produces a different composition distribution in the detonated material relative to those predicted by one-dimensional calculations.

We present key sample results of a systematic survey of the pulsation properties of models of hot B subdwarfs. We use equilibrium structures taken from detailed evolutionary sequences of solar metallicity (Z = 0.02) supplemented by grids of static envelope models of various metallicities (Z = 0.02, 0.04, 0.06, 0.08, and 0.10). We consider all pulsation modes with l = 0, 1, 2, and 3 in the 80-1500 s period window, the interval currently most suitable for fast photometric detection techniques. We establish that significant driving is often present in hot B subdwarfs and is due to an opacity bump associated with heavy-element ionization. We find that models with Z ≥ 0.04 show low radial order unstable modes; both radial and nonradial (p, f, and g) pulsations are excited. The unstable models have T_eff ≲ 30,000 K and log g ≲ 5.7, depending somewhat on the metallicity. We emphasize that metal enrichment need only occur locally in the driving region. On this basis, combined with the accepted view that local enrichments and depletions of metals are commonplace in the envelopes of hot B subdwarfs, we predict that some of these stars should show luminosity variations resulting from pulsational instabilities.

Observations of the Merrill-Sanford bands of SiC₂ in the 4750-5050 Å spectral region, seen in absorption in some carbon stars and in emission in IRAS 12311-3509, are reported. Vibrational band assignments based on recent laser-based laboratory electronic spectroscopy of this molecule constitute substantial revisions compared with earlier work. Vibrational hot bands are very strong when present in carbon stars, but were greatly weakened in T Mus for a time in 1994, indicating that the bands were then formed in a cooler region than that typical of carbon star photospheres. Emission bands of SiC₂, with intensities that are consistent with optical excitation of cool gas, appear in IRAS 12311-3509 which is probably a J-type carbon star occulted by a dusty disk so that the star is seen by reflection from material out of the plane of the disk.

We report the first detection of the western bow shock component from IRAS 2 in NGC 1333 along with observations of previously detected shocks and outflow winds from this source and those from IRAS 4. We compare the shock and outflow distributions from these two young stellar objects (YSOs), and the locations of other YSOs, with the overall distribution of the dense molecular gas in the star-forming core using high spatial resolution observations of CS (J = 2 → 1, 3 → 2, and 5 → 4) emission made with the Institut de Radio Astronomie Millimétrique 30 m antenna. These comparisons provide a new picture of the morphology and dynamics of the star-forming core of NGC 1333. The CS maps show (1) a large cavity with many YSOs just at the inner edge of the cavity, (2) a dense, compressed shell at 8 km s^-1; and (3) a gas layer at 7 km s^-1 probably located inside the cavity. We find that the IRAS 2 and IRAS 4 outflows impact different gas layers as indicated by the spatial association of the red- and blueshifted lobes, and that IRAS 2 is located near the front edge of the CS shell. The burst of star formation that has shaped NGC 1333 is occurring in the compressed shell traced by CS and now appears to be in a late evolutionary stage.