Table of contents

We consider next-to-leading-order (one-loop) nonlinear corrections to the bispectrum and skewness of cosmological density fluctuations induced by gravitational evolution, focusing on the case of Gaussian initial conditions and scale-free initial power spectra, P(k) ∝ kⁿ. As has been established by comparison with numerical simulations, leading order (tree-level) perturbation theory describes these quantities at the largest scales. The one-loop perturbation theory provides a tool to probe the transition to the nonlinear regime on smaller scales. In this work, we find that, as a function of spectral index n, the one-loop bispectrum follows a pattern analogous to that of the one-loop power spectrum, which shows a change in behavior at a "critical index" n_c ≈ -1.4, where nonlinear corrections vanish. The tree-level perturbation theory predicts a characteristic dependence of the bispectrum on the shape of the triangle defined by its arguments. For n n_c, one-loop corrections increase this configuration dependence of the leading order contribution; for n n_c, one-loop corrections tend to cancel the configuration dependence of the tree-level bispectrum, in agreement with known results from n = -1 numerical simulations. A similar situation is shown to hold for the Zeldovich approximation, where n_c ≈ -1.75. We obtain explicit analytic expressions for the one-loop bispectrum for n = -2 initial power spectra, for both the exact dynamics of gravitational instability and the Zeldovich approximation. We also compute the skewness factor, including local averaging of the density field, for n = -2: S₃(R)=4.02+3.83σ(R) for Gaussian smoothing and S₃(R)=3.86+3.18σ(R) for top-hat smoothing, where σ²(R) is the variance of the density field fluctuations smoothed over a window of radius R. A comparison with fully nonlinear numerical simulations implies that, for n < -1, the one-loop perturbation theory can extend our understanding of nonlinear clustering down to scales where the transition to the stable clustering regime begins.

I study the efficiency of genus statistics in differentiating between different models of structure formation. Simple models that reproduce the salient features of the structure that are seeded by topological defects are examined. I consider accretion onto static point masses, modeling slow-moving cosmic string loops or other primordial pointlike sources. Filamentary structures and wakes are considered as models of the structures seeded by slow- and fast-moving strings, respectively. The predictions of genus statistics for Gaussian fluctuations are compared to genus curves obtained by the CfA Redshift Survey. A generic class of density models with wakes and filaments is found to provide results comparable to or better than Gaussian models for this suite of tests.

We study the uncertainty in galaxy cluster mass estimates derived from X-ray data, assuming hydrostatic equilibrium for the intracluster gas. Using a Monte Carlo procedure, we generate a general class of mass models allowing very massive clusters. We then compute the corresponding temperature profiles through the equation of hydrostatic equilibrium and compare them to observational data for some clusters. We find several massive clusters that pass the observational constraints, with integrated masses varying over quite a wide range. The resulting accuracy of the mass estimates is rather poor, the uncertainties larger than what is generally claimed. Despite the fact that the mass profile can be exactly determined mathematically from the temperature and surface brightness profiles, we find that very accurate measurements of both quantities are required to determine the actual mass with moderate accuracy. We argue that the tight constraints on cluster masses previously obtained arise from the fact that a too restricted class of mass density profiles has been investigated so far, without serious physical justification. Applying our procedure to the Perseus and then to the Coma clusters, we find that an improvement of the observational constraints results in a quite modest improvement in the accuracy of the mass estimate. For Coma, using the best current available data, we end up with an uncertainty of a factor of 2 for the mass within the Abell radius. This uncertainty rapidly increases at further radius.

In a previous paper, we pointed out the existence of some particular scalar-tensor gravity theories that are able to relax the nucleosynthesis constraint on the cosmic baryonic density. In this paper, we present an exhaustive study of primordial nucleosynthesis in the framework of such theories, taking into account the currently adopted observational constraints. We show that a wide class of theories allows for a baryonic density very close to that needed for closure of the universe. This class of theories converges soon enough toward general relativity and, hence, is compatible with all solar system and binary pulsar gravitational tests. In other words, we show that primordial nucleosynthesis does not always impose a very stringent bound on the baryon contribution to the density parameter.

For the quadruple gravitational lens PG 1115+080, we combine recent measurements of the time delays with new lens models to determine the Hubble constant H₀. We explore the effects of systematic uncertainties in the lens models on the estimates of H₀, and we discuss how the uncertainties can be reduced by future observations. We find that the lens cannot be fit by an isolated lens galaxy, but that it can be well fit by including a perturbation from the nearby group of galaxies. To understand the full range of systematic uncertainties, it is crucial to use an ellipsoidal galaxy and to let the group position vary. In this case, the existing constraints cannot break degeneracies in the models with respect to the profiles of the galaxy and group and to the position of the group. Combining the known time delays with a range of lens models incorporating some of the plausible systematic effects yields H₀=51^{+ 14}₋₁₃ km s^-1 Mpc^-1. The constraints on the lens models, and hence on H₀, can be improved by reducing the standard errors in the lens galaxy position from 50 mas to ~10 mas, reducing the the uncertainties in the time delays to ~0.5 days, and constraining the lens mass distribution using Hubble Space Telescope photometry and the fundamental plane. In particular, the time delay ratio r_ABC ≡ Δτ_AC/Δτ_BA may provide the best constraint on the mass profile of the galaxy.

A popular model of single microlensing light curves neglects the possibility that only a fraction of the light is due to the lensed star, the remaining part being due to a close, unresolved blend, which may be related or unrelated to the lens. Unfortunately, the effects of blending are significant, as all microlensing experiments choose very crowded fields as their targets. The propensity of blends among double lenses is the most direct evidence for the seriousness of the problem. In this paper, we point out a strong degeneracy of the fitting procedure for single lensing events, which makes it very difficult to detect the presence of a blend, and practically impossible to correct for it by purely photometric means, in a large part of the parameter space. Some blends may be detected by astrometric means, but the majority have to be corrected for statistically. It will be helpful to measure the luminosity function well below the ground-based detection limit, e.g., using the Hubble Space Telescope (HST). Binary stars in the target population affect blending and may be undetectable with HST; their presence will be revealed by repeating microlensing events, so it will be possible to correct for them statistically. If no correction is made, then the event timescales and the lens masses are systematically underestimated.

One of the largest uncertainties in understanding the effect of a background UV field on galaxy formation is the intensity and evolution of the radiation field with redshift. This work attempts to shed light on this issue by computing the quasi-hydrostatic equilibrium states of gas in spherically symmetric dark matter halos (roughly corresponding to dwarf galaxies) as a function of the amplitude of the background UV field. We integrate the full equations of radiative transfer, heating, cooling, and nonequilibrium chemistry for nine species: H, H⁺, H^-, H₂, H⁺₂, He, He⁺, He⁺⁺, and e^-. As the amplitude of the UV background is decreased, the gas in the core of the dwarf goes through three stages characterized by the predominance of ionized (H⁺), neutral (H), and molecular (H₂) hydrogen. Characterizing the gas state of a dwarf galaxy with the radiation field allows us to estimate its behavior for a variety of models of the background UV flux. Our results indicate that a typical radiation field can easily delay the collapse of gas in halos corresponding to 1 σ cold dark matter perturbations with circular velocities of less than 30 km s^-1.

This paper examines the gravitational stability against small perturbations of the quasi-steady state cosmological model. This model was first introduced by Hoyle et al., who in subsequent papers looked at its various theoretical and observational implications. Here we carry out a perturbation analysis of the exact solution of the field equations obtained by Sachs et al. in the noncreative mode which describes the oscillatory feature of this model. We show that the perturbations grow only to a limited amount and then fall off, thus confirming the stability of the solution. We discuss the implications of this result for structure formation in this cosmology.

We present the first results of an ongoing program to investigate the kinematic properties of high-redshift damped Lyα systems. Because damped Lyα systems are widely believed to be the progenitors of current massive galaxies, an analysis of their kinematics allows a direct test of galaxy formation scenarios. Specifically, the kinematic history of protogalactic gas is a sensitive discriminator among competing theories of galaxy formation.

We use the HIRES echelle spectrograph on the Keck 10 m telescope to obtain accurate, high-resolution spectra of 17 damped Lyα systems. We focus on unsaturated, low-ion transitions such as Si II 1808, since these accurately trace the velocity fields of the neutral gas dominating the baryonic content of the damped systems. The velocity profiles: (1) comprise multiple narrow components; (2) are asymmetric in that the component with strongest absorption tends to lie at one edge of the profile; and (3) exhibit a nearly uniform distribution of velocity widths between 20 and 200 km s^-1.

In order to explain these characteristics, we consider several physical models proposed to explain the damped Lyα phenomenon, including rapidly rotating "cold" disks, slowly rotating "hot" disks, massive isothermal halos, and a hydrodynamic spherical accretion model. Using standard Monte Carlo techniques, we run sight lines through these model systems to derive simulated low-ion profiles. We develop four test statistics that focus on the symmetry and velocity widths of the profiles to distinguish among the models. Comparing the distributions of test statistics from the simulated profiles with those calculated from the observed profiles, we determine that the models in which the damped Lyα gas is distributed in galactic halos and in spherically infalling gas, are ruled out at more than 99.9% confidence. A model in which dwarf galaxies are simulated by slowly rotating "hot" disks is ruled out at 97% confidence. More important, we demonstrate that the cold dark matter (CDM) model, as developed by Kauffmann (1996) is inconsistent with the damped Lyα data at more than the 99.9% confidence level. This is because the CDM model predicts the interception cross section of damped Lyα systems to be dominated by systems with rotation speeds too slow to be compatible with the data. This is an important result, because slow rotation speeds are generic traits of protogalaxies in most hierarchical cosmologies.

We find that models with disks that rotate rapidly and are thick are the only tested models consistent with the data at high confidence levels. A relative likelihood ratio test indicates disks with rotation speeds, v_rot < 180 km s^-1, and scale heights, h < 0.1 times the radial scale length R_d, are ruled out at the 99% confidence level. The most likely values of these parameters are v_rot = 225 km s^-1 and h = 0.3R_d. We also find that these disks must be "cold," since models in which σ_cc/v_rot > 0.1 are ruled out with 99% confidence, where σ_cc is the velocity dispersion of the gas. We describe an independent test of the "cold" disk hypothesis. The test makes use of the redshift of emission lines sometimes detected in damped Lyα systems, as well as the absorption profiles. The test potentially distinguishes between damped systems that are (1) large rotating disks detected in absorption and emission, in which case a systematic relation exists between emission redshift and absorption velocity profile, and (2) emitting galaxies surrounded by satellite galaxies detected in damped Lyα absorption, in which case the relation between emission and absorption redshifts is random.

Finally, we emphasize a dilemma stimulated by our findings. Specifically, while the kinematics of the damped Lyα systems strongly favor a "cold" disklike configuration, the low metallicities and Type II supernova abundance patterns of damped Lyα systems argue for a "hot" halo-like configuration. We speculate on how this dilemma might be resolved.

Cosmological expansion cannot produce the reported correlations of the gamma-ray burst timescale and spectral energy with peak flux if the burst model reproduces the Burst and Transient Source Experiment (BATSE) 3B peak flux distribution for a nonevolving burst source density. The required ratios of time dilation and redshift factors are only produced by monoluminous models at peak fluxes below the BATSE threshold, and they are never produced by power-law luminosity models. Monoluminous models produce acceptable fits to the peak flux distribution only for very specific combinations of the spectral and cosmological parameters. The redshift of gamma-ray bursts at the BATSE threshold is z ≈ 1.5. Power-law luminosity distribution models ∝L^-β produce acceptable fits to the data for most values of the spectral parameters when β < 1.6. In this model, gamma-ray bursts of a given peak flux have a distribution of redshifts, with a maximum redshift of 3 for peak fluxes near the BATSE threshold and with an average redshift of <1 for all values of peak flux. This qualitative behavior occurs whenever the luminosity distribution determines the shape of the peak flux distribution, regardless of whether source density evolution occurs. The reported correlations of the burst timescale and the spectral energy with peak flux are systematically 1 standard deviation above the monoluminous model and 1.5-2 standard deviations above the power-law luminosity model. These results suggest that an intrinsic correlation of burst timescale and spectral energy with luminosity is present. Studies of the peak flux distribution for bursts selected by E_peak or the hardness ratio provide a test for this intrinsic correlation.

We have used the two-year Differential Microwave Radiometer data from the Cosmic Background Explorer (COBE) satellite to systematically search for millimetric (31-90 GHz) emission from the gamma-ray bursts (GRBs) in the Burst and Transient Source Experiment (BATSE) GRB 3B catalog. The large beam size of the COBE instrument (7° FWHM) allows for an efficient search of the large GRB positional error boxes, although it also means that fluxes from (point-source) GRB objects will be somewhat diluted. A likelihood analysis has been used to look for a change in the level of millimetric emission from the locations of 81 GRB events during the first two years (1990 and 1991) of the COBE mission. The likelihood analysis determined that we did not find any significant millimetric signal before or after the occurrence of the GRB. We found 95% confidence level upper limits of 175, 192, and 645 Jy or, in terms of fluxes, of 9.6, 16.3, and 54.8 × 10^-13 erg cm^-2 s, respectively at 31, 53, and 90 GHz. We also looked separately at three different classes of GRBs, including the top 10 (in peak flux), "short-burst," and "long-burst" subsets and found a similar upper limits for each subset. While these limits may be somewhat higher than one would like, we estimate that using this technique with future missions could push these limits down to ~1 mJy.

The narrow emission line spectra of active galactic nuclei are not accurately described by simple photoionization models of single clouds. Recent Hubble Space Telescope images of Seyfert 2 galaxies show that these objects are rich with ionization cones, knots, filaments, and strands of ionized gas. Here we extend to the narrow-line region the "locally optimally emitting cloud" (LOC) model, in which the observed spectra are predominantly determined by powerful selection effects. We present a large grid of photoionization models covering a wide range of physical conditions and show the optimal conditions for producing many of the strongest emission lines. We show that the integrated narrow-line spectrum can be predicted by an integration of an ensemble of clouds, and we present these results in the form of diagnostic line ratio diagrams making comparisons with observations. We also predict key diagnostic line ratios as a function of distance from the ionizing source and compare these with observations. The predicted radial dependence of the [O III]/[O II] ratio may be matched to the observed one in NGC 4151, if the narrow-line clouds see a more intense continuum than we see. The LOC scenario when coupled with a simple Keplerian gravitational velocity field will quite naturally predict the observed line width versus critical density relationship. The influence of dust within the ionized portion of the clouds is discussed, and we show that the more neutral gas is likely to be dusty, although a high-ionization dust-free region is most likely present too. This argues for a variety of narrow-line region cloud origins.

In the innermost part of an accretion disk around a black hole, electron scattering could provide the dominant opacity, and so the X-ray radiation originating from this hot region is expected to be partially linearly polarized. If strong magnetic fields are present on or above the disk, then synchrotron radiation from electrons may also contribute to the polarization, although different orientations of the magnetic field and Faraday rotation might reduce the effect. Both observational and theoretical studies suggest that the inner disk region is unstable and could appear "clumpy." In this paper we investigate polarization features due to polarized orbiting clumps around a black hole. It is found that, in contrast to the Newtonian case, rapid polarization variability can be produced by those regions emitting extra radiation, and that the variability amplitudes of both the degree of polarization and the angle of the polarization plane are energy dependent, i.e., the polarization variability amplitudes are larger at higher energy. This feature will not appear if the central object is not gravitationally strong, even when polarized clumps rotate around the object, and this trend depends only weakly on the local physics, such as the specific polarization mechanism or optical depth of the sources.

Energy-dependent polarization variability is a direct result of near-field bending of light rays by the central black hole, and it is unique to black hole systems involving accretion disks. Since accretion disks around black holes are expected in both active galactic nuclei and X-ray binaries, we look for future X-ray polarimetry missions to confirm our prediction of this phenomenon.

We study the stability of stellar dynamical equilibrium models for M32. Kinematic observations show that M32 has a central dark mass of ~3 × 10⁶ M_☉, most likely a black hole, and a phase-space distribution function that is close to the "two-integral" form f = f(E, L_z). M32 is also rapidly rotating; 85%-90% of the stars have the same sense of rotation around the symmetry axis. Previous work has shown that flattened, rapidly rotating two-integral models can be bar-unstable. We have performed N-body simulations to test whether this is the case for M32. This is the first stability analysis of two-integral models that have both a central density cusp and a nuclear black hole.

Particle realizations with N = 512,000 were generated from distribution functions that fit the photometric and kinematic data of M32. We constructed equal-mass particle realizations and also realizations with a mass spectrum to improve the central resolution. Models were studied for two representative inclinations, i = 90° (edge-on) and i = 55°, corresponding to intrinsic axial ratios of q = 0.73 and q = 0.55, respectively. The time evolution of the models was calculated with a "self-consistent field" code on a Cray T3D parallel supercomputer. We find both models to be dynamically stable. This implies that they provide a physically meaningful description of M32 and that the inclination of M32 (and hence its intrinsic flattening) cannot be strongly constrained through stability arguments.

Previous work on the stability of f(E, L_z) models has shown that the bar mode is the most common unstable mode for systems rounder than q ≈ 0.3 (i.e., E7) and that the likelihood for this mode to be unstable increases with flattening and rotation rate. The f(E, L_z) models studied for M32 are not bar-unstable, and M32 has a higher rotation rate than nearly all other elliptical galaxies. This suggests that f(E, L_z) models constructed to fit data for real elliptical galaxies will generally be stable, at least for systems rounder than q 0.55, and possibly for flatter systems as well.

The Hα peak intensity, velocity shift, and velocity dispersion maps of the giant H II region NGC 604 in M33, obtained by two-dimensional high spatial resolution Fabry-Perot observations at the 4.2 m William Herschel Telescope in Spain, are analyzed via two-point correlation functions. The whole system seems to rotate as a rigid body on scales from 50 to 80 pc (the largest studied scale), with a period of ≈ 85 Myr. We demonstrate that the cloud seems to be comprised of eddies with varying characteristic scale lengths which range from 10 pc to the largest observed scales. The calculated kinetic energy spectrum may be interpreted either as a manifestation of a double cascading spectrum of forced two-dimensional turbulence or as a Kolmogorov three-dimensional turbulence (although this last possibility seems unlikely). According to the first interpretation, turbulence is being forced at scales of ≈ 10 pc, while an enstrophy (mean square vorticity) cascade has developed down to the smallest scales resolved and an inverse kinetic energy cascade extends up to scales of ≈ 70 pc, where a low wavenumber turnover is observed; if this interpretation is correct, this would be the first time that such a phenomenon has been observed outside the solar system. As for the second interpretation, energy should be injected at the largest scales, ≈ 70 pc. In both cases the average intrinsic optical depth consistent with the results is ≈ 20 pc.

We compare the detailed distributions of H I, Hα, and 150 nm far-UV (FUV) continuum emission in the spiral arms of M81 at a resolution of 9'' (linear resolution 150 pc at 3.7 Mpc distance). The bright Hα emission peaks are always associated with peaks in the FUV emission. The converse is not always true; there are many regions of FUV emission with little corresponding Hα. The H I and the FUV are always closely associated, in the sense that the H I is often brightest around the edges of the FUV emission.

The effects of extinction on the morphology are small, even in the FUV. Extensive FUV emission, often with little corresponding Hα, indicates the presence of many "B stars," which produce mostly nonionizing UV photons. These FUV photons dissociate a small fraction of an extensive layer of H₂ into H I.

The observed morphology can be understood if "chimneys" are common in the spiral arms of M81, where holes are blown out of the galactic disk, exposing the bright H II regions and the corresponding FUV associated with vigorous star formation. These "naked" star-forming regions show little obscuration. H₂ is turned into H I by UV photons impinging on the interior surfaces of these chimneys.

The intensity of the FUV radiation measured by the Ultraviolet Imaging Telescope can dissociate the underlying H₂ with a typical density of ~10 H nuclei cm^-3 to produce the observed amount of H I in the spiral arms of M81. Except for thin surface layers locally heated in these photodissociation regions that are close to the FUV sources, the bulk of the molecular gas in the inner disk of M81 is apparently too cold to produce much ¹²CO(1-0) emission.

We show that the Galactic cosmic-ray source (GCRS) composition is best described in terms of (1) a general enhancement of the refractory elements relative to the volatile ones, and (2) among the volatile elements, an enhancement of the heavier elements relative to the lighter ones. This mass dependence most likely reflects a mass-to-charge (A/Q) dependence of the acceleration efficiency; among the refractory elements, there is no such enhancement of heavier species, or only a much weaker one. We regard as coincidental the similarity between the GCRS composition and that of the solar corona, which is biased according to first ionization potential. In a companion paper, this GCRS composition is interpreted in terms of an acceleration by supernova shock waves of interstellar and/or circumstellar (e.g.,²²Ne-rich Wolf-Rayet wind) gas-phase and, especially, dust material.

We present a quantitative model of Galactic cosmic-ray (GCR) origin and acceleration, wherein a mixture of interstellar and/or circumstellar gas and dust is accelerated by a supernova remnant blast wave. The gas and dust are accelerated simultaneously, but differences in how each component is treated by the shock leave a distinctive signature, which we believe exists in the cosmic-ray composition data. A reexamination of the detailed GCR elemental composition, presented in a companion paper, has led us to abandon the long-held assumption that GCR abundances are somehow determined by first ionization potential. Instead, volatility and mass (presumably mass-to-charge ratio) seem to better organize the data: among the volatile elements, the abundance enhancements relative to solar increase with mass (except for the slightly high H/He ratio); the more refractory elements seem systematically overabundant relative to the more volatile ones in a quasi-mass-independent fashion. If this is the case, material locked in grains in the interstellar medium must be accelerated to cosmic-ray energies more efficiently than interstellar gas-phase ions. Here we present results from a nonlinear shock model that includes (1) the direct acceleration of interstellar gas-phase ions, (2) a simplified model for the direct acceleration of weakly charged grains to ~100 keV amu^-1 energies, simultaneously with the acceleration of the gas ions, (3) the energy losses of grains colliding with the ambient gas, (4) the sputtering of grains, and (5) the simultaneous acceleration of the sputtered ions to GeV and TeV energies. We show that the model produces GCR source abundance enhancements of the volatile, gas-phase elements that are an increasing function of mass, as well as a net, mass-independent enhancement of the refractory, grain elements over protons, consistent with cosmic-ray observations. We also investigate the implications of the slightly high H/He ratio. The GCR²²Ne excess may also be accounted for in terms of the acceleration of ²²Ne-enriched presupernova Wolf-Rayet star wind material surrounding the most massive supernovae. We also show that cosmic-ray source spectra, at least below ~10¹⁴ eV, are well matched by the model.

No cosmic antinucleus has yet been detected in the primary cosmic radiation. Only upper limits to the antinucleus fluxes have been measured in various rigidity ranges in different experiments. Calculations of the flux reduction experienced by cosmic antihelium traversing the Milky Way are reported. A significant depression of the antihelium flux has been found in the low-momentum band 2-6 GeV/c compared to that postulated to exist in the intergalactic space. The relevant results presented here are based on simple observational data regarding the total matter column swept by cosmic rays in the Milky Way and the expected properties of antihelium interactions with the interstellar hydrogen. The calculations have been made by the simulation code LEASA (Low Energy Antinucleus Simulation Algorithms) developed to describe antinucleus interactions with matter. Important implications of these results on past and future experiments giving upper limits on antihelium-to-helium flux ratios are discussed.

The Galactic nova rate is poorly known, with estimates in the literature ranging from as few as 11 to as many as 260 yr^-1. At the lower end of the spectrum (50 yr^-1), the predictions are based on scalings from extragalactic nova surveys, while estimates based on extrapolations of Galactic nova observations suggest rates that are significantly higher, in the range 50-100 yr^-1 or more. In an attempt to reconcile this difference, the nova rate, based on Galactic nova observations, is recomputed. If the stellar mass distribution is axisymmetric about the Galactic center, a new estimate of the Galactic nova rate of ~35 ± 11 yr^-1 is deduced. Although this value is marginally consistent with the highest estimates based on extragalactic surveys, the agreement is not entirely satisfactory. It is pointed out that a departure from axial symmetry, such as that caused by the presence of a Galactic bar, can potentially lower the estimated nova rate (perhaps approaching 50%) if the bar is sufficiently large, and has its long axis pointed toward the Sun. Alternatively, or perhaps in addition, it is possible that previous extragalactic surveys may have missed a significant fraction of novae (up to a factor of ~2) because of extinction internal to the galaxies. If this latter possibility can be ruled out, for example, through infrared surveys of nearby galaxies, the Galactic nova observations may provide additional support for the existence of a Galactic bar.

This paper presents a 2' × 2' map of 60 μm polarization from the core of the massive molecular cloud Sagittarius B2, located near the center of the Galaxy. The measurements were obtained from the Kuiper Airborne Observatory with the array polarimeter Stokes. The polarization (up to 8%) is large compared to the polarization at 115 μm and, in contrast to other molecular clouds, is greatest toward the peaks in the dust column density. We argue that the far-infrared polarization is due to absorption by aligned dust grains. The magnetic field in the absorbing cloud, as projected onto the plane of the sky, is roughly north-south, making an agle of 54° with the Galactic plane.

We discuss the dynamics of dust grains subjected to torques arising from H₂ formation. In particular, we discuss grain dynamics when a grain spins down and goes through a "crossover" event. As first pointed out by Spitzer & McGlynn, the grain angular momentum before and after a crossover event are correlated, and the degree of this correlation critically affects the alignment of dust grains by paramagnetic dissipation. We calculate the correlation including the important effects of thermal fluctuations within the grain material. These fluctuations limit the degree to which the grain angular momentum J is coupled with the grain principal axis a₁ of maximal inertia. We show that this imperfect coupling of a₁ with J plays a critical role during crossovers and can substantially increase the efficiency of paramagnetic alignment for grains larger than 0.1 μm. As a result, we show that for reasonable choices of parameters, the observed alignment of a 0.1 μm grains could be achieved by paramagnetic dissipation in suprathermally rotating grains, if radiative torques caused by starlight were not present. We also show that the efficiency of mechanical alignment in the limit of long alignment times is not altered by the thermal fluctuations in the grain material.

Redshifted Lyα absorption toward α Cen has been interpreted by Linsky & Wood and Frisch et al. as evidence for decelerated interstellar hydrogen piled up on the upstream side of the heliosphere. We utilize newly developed two-dimensional multifluid models of the solar wind interaction with the interstellar material to corroborate this interpretation by synthesizing the Lyα absorption profile predicted for this "hydrogen wall." Both subsonic and supersonic inflow into the heliosphere are considered, corresponding to one-shock and two-shock global morphologies, respectively. We find that these two extremes give observably different redward absorption characteristics in the Lyα profiles, and our preliminary conclusion is that the Lyα profiles seen toward α Cen favor a barely subsonic model (Mach number 0.9). For such a model to hold, additional interstellar pressure terms, such as cosmic-ray or magnetic pressures, must contribute. To make this conclusion more certain, an extended-model parameter survey is required, coupled with Lyα data along other lines of sight.

The nonlinear evolution of unstable C-type shocks in weakly ionized plasmas is studied by means of time-dependent, magnetohydrodynamic simulations. This study is limited to shocks in magnetically dominated plasmas (in which the Alfvén speed in the neutrals greatly exceeds the sound speed), and microphysical processes such as ionization and recombination are not followed. Both the two-dimensional simulations of initially planar perpendicular and oblique C-type shocks and the fully three-dimensional simulation of a perpendicular shock are presented.

For the cases studied here, the instability results in the formation of dense sheets of gas elongated in the direction of shock propagation and oriented perpendicular to the magnetic field. The formation of a weak J-type front is associated with the growth of the instability from an equilibrium shock structure. After saturation, the magnetic field structure consists of arches that bow outward in the direction of shock propagation and are anchored by the enhanced ion-neutral drag in the dense sheets. Analogous to the magnetic buoyancy (Parker) instability, saturation occurs when the magnetic tension in the distorted field lines is balanced by drag in the sheets. For the magnetically dominated shocks studied here, the distortions in the magnetic field that produce saturation are very small. Nonetheless, the enhancements of the ion and neutral densities in the sheets are very large, between 2 and 3 orders of magnitude compared with the preshock values. At these high densities, recombination processes may be important. The sheets evolve slowly in time, so that shocks propagating in a homogeneous medium may leave behind a network of intersecting filaments and sheets of dense gas elongated in the direction of shock propagation and perpendicular to the mean field. The temperature structure and emission properties of unstable C-type shocks in the nonlinear regime are presented in a companion paper.

We have modeled the gas temperature structure in unstable C-type shocks and obtained predictions for the resultant CO and H₂ rotational line emissions, using numerical simulations of the Wardle instability presented in Paper I. Our model for the thermal balance of the gas includes ion-neutral frictional heating; compressional heating; radiative cooling due to rotational and ro-vibrational transitions of the molecules CO, H₂O, and H₂; and gas-grain collisional cooling. We obtained results for the gas temperature distribution in—and H₂ and CO line emission from—shocks of neutral Alfvénic Mach number 10 and velocity 20 or 40 km s^-1 in which the Wardle instability has saturated. Both two- and three-dimensional simulations were carried out for shocks in which the preshock magnetic field is perpendicular to the shock propagation direction, and a two-dimensional simulation was carried out for the case in which the magnetic field is obliquely oriented with respect to the shock propagation direction. Although the Wardle instability profoundly affects the density structure behind C-type shocks, most of the shock-excited molecular line emission is generated upstream of the region where the strongest effects of the instability are felt. Thus the Wardle instability has a relatively small effect on the overall gas temperature distribution in—and the emission-line spectrum from—C-type shocks, at least for the cases that we have considered. In none of the cases that we have considered thus far did any of the predicted emission-line luminosities change by more than a factor of 2.5, and in most cases the effects of instability were significantly smaller than that. Slightly larger changes in the line luminosities seem likely for three-dimensional simulations of oblique shocks, although such simulations have yet to be carried out and lie beyond the scope of this study. Given the typical uncertainties that are always present when model predictions are compared with real astronomical data, we conclude that Wardle instability does not imprint any clear observational signature on the shock-excited CO and H₂ line strengths. This result justifies the use of one-dimensional steady shock models in the interpretation of observations of shock-excited line emission in regions of star formation. Our three-dimensional simulations of perpendicular shocks revealed the presence of warm filamentary structures that are aligned along the magnetic field, a result that is of possible relevance to models of water maser emission from C-type shocks.

We explore the evolution of collisionally merged stars in the blue straggler region of the H-R diagram. The starting models for our stellar evolution calculations are the results of the smoothed particle hydrodynamics (SPH) simulations of parabolic collisions between main-sequence stars performed by Lombardi, Rasio, & Shapiro. Since SPH and stellar evolution codes employ different and often contradictory approximations, it is necessary to treat the evolution of these products carefully. The mixture and disparity of the relevant timescales (hydrodynamic, thermal relaxation, and nuclear burning) and of the important physical assumptions between the codes makes the combined analysis of the problem challenging, especially during the initial thermal relaxation of the star. In particular, the treatment of convection is important and semiconvection must be modeled in some detail.

The products of seven head-on collisions are evolved through their initial thermal relaxation and then through the main-sequence phase to the base of the giant branch. Their evolutionary tracks are presented. In contrast to what was assumed in previous work, these collision products do not develop substantial convective regions during their thermal relaxation and therefore are not mixed significantly after the collision.

Long-slit spectroscopic observations along the major axes of four planetary nebulae with interesting jets and FLIERs (Hb 4, IC 4634, NGC 6369, and NGC 7354) have been conducted with the Palomar 5 m telescope. Chemical abundances and physical conditions (n, T) in microstructures were derived along their structural axes. No evidence of conspicuous shock activity or N/O abundance anomalies is seen in most cases, unlike some earlier studies of similar features in other planetary nebulae. Microstructures seem to be a heterogeneous class of structures aside from their low ionization and generally supersonic motions.

Systematic variations with wavelength in the position angle of interstellar linear polarization of starlight may be indicative of multiple cloud structure along the line of sight. We use polarimetric observations of two stars (HD 29647 and HD 283809) in the general direction of TMC-1 in the Taurus dark cloud to investigate grain properties and cloud structure in this region. We show the data to be consistent with a simple two-component model in which general interstellar polarization in the Taurus cloud is produced by a widely distributed cloud component with relatively uniform magnetic field orientation; light from stars close to TMC-1 suffers additional polarization arising in one (or more) subcloud(s) with larger average grain size and magnetic field directions different from the general trend. Toward HD 29647 in particular, we show that the unusually low degree of visual polarization relative to extinction is due to depolarization associated with the presence of distinct cloud components in the line of sight with markedly different magnetic field orientations. Stokes parameter calculations allow us to separate the polarization characteristics of the individual components. Results are fitted with the Serkowski empirical formula to determine the degree and wavelength of maximum polarization. Whereas λ_max values in the widely distributed material are similar to the average (0.55 μm) for the diffuse interstellar medium, the subcloud in the line of sight to HD 283809, the most heavily reddened star in our study, has λ_max ≈ 0.73 μm, indicating the presence of grains ~30% larger than this average. Our model also predicts detectable levels of circular polarization toward both HD 29647 and HD 283809.

We have detected linear polarization in the 115 μm continuum radiation from the giant molecular cloud Sagittarius B2. We found polarization at nine positions in the dense cloud core and at 15 positions in the less-dense envelope. The polarization in the core is due to absorption by magnetically aligned grains and that in the envelope is due to emission from magnetically aligned grains. The inferred magnetic field direction is roughly north-south everywhere, but with spatially smooth variations of up to 30°. By considering our data together with Zeeman splitting observations we are able to set a conservative lower limit of 150 μG on the strength of the large-scale field in the envelope. If large-scale fields this strong are common in Galactic center clouds, they could be detectable via large-beam Zeeman measurements. For positions in the envelope that are furthest from the core, the field is nearly parallel to the plane of the Galaxy. This is consistent with the idea of a globally azimuthal magnetic field in the Galactic center neutral gas layer, which is expected if gravitational forces dominate magnetic forces.

We have obtained new narrowband CCD images, spatially resolved intermediate- and high-dispersion spectroscopic observations of the Ring Nebula (NGC 6720). These data reveal that the bright main nebula is not a real ring, but a closed shell. A prolate ellipsoidal geometry for this shell is inferred from the observed tilts in the [N II] velocity ellipses at different position angles. The shell has enhanced densities near the equator; the shell surface is fragmented with protruding bubbles and outflows. The high-resolution spatially resolved echellograms allow us to identify the kinematic components associated with small-scale morphological features. The morphologically identified inner and outer halos show neither distinct kinematic discontinuity at the transition nor different chemical abundances, indicating that they have a common origin, the red giant wind.

The kinematic and chemical properties of the Ring Nebula do not support the hypothesis that the Ring is a nearly pole-on bipolar nebula. We propose that the Ring Nebula contains a bubbling prolate ellipsoidal shell and a halo of remnant red giant wind, and that the combination of a nonisotropic excitation and the interaction of the main shell's bubbles and outflows with the surrounding red giant wind produces the petal-like morphology in the inner halo.

The central region of the Monoceros R2 molecular cloud has been studied using molecular line maps, maps in continuum emission, and an M-band (4.7 μm) absorption spectrum toward Mon R2 IRS 3. Maps were made in the emission lines CO (3-2) H₂CO (5_1,5-4_1,4), and HCN (4-3), all with a 14'' beam size. CO (2-1) and ¹³CO (3-2) spectra were obtained at a dozen positions. Maps of continuum emission were made at 1300 μm (25'' resolution), 1100 μm (20'' resolution), 800 μm (14'' resolution), and 450 μm (14'' resolution). The M-band spectrum of IRS 3 has a velocity resolution of 5.2 km s^-1 and shows fundamental vibrational band absorption lines of CO and ¹³CO over a range of rotational states. The CO map has numerous intensity peaks which, if interpreted as clumps, have masses from 0.1 to 3 M_☉. The large velocity dispersion of these structures implies that they cannot be gravitationally bound. The brightest CO-emitting gas shows no bipolar distribution with velocity. Diffuse CO-emitting gas with low velocities does have a generally bipolar distribution, but there are no collimated lobes pointing to a particular source. We conclude that the source (or sources) of the very extended Mon R2 outflow is (are) now inactive. The highest velocity gas is found toward the embedded young stellar object IRS 3, suggesting that IRS 3 is the source of a compact outflow, unresolved at our 14'' resolution. The presence of blueshifted CO in the absorption spectrum supports the interpretation of IRS 3 as an outflow source. The H₂CO and HCN maps demonstrate that much of the dense gas is distributed within three structures having different velocities. The fundamental band absorption lines of ¹³CO show two gas temperatures in the line of sight to IRS 3. The colder (45 K) is identified as gas in the clump surrounding IRS 3, which is seen in emission lines of CO, H₂CO, and HCN. The warmer (310 K) we interpret as gas very close to IRS 3. From the submillimeter continuum maps we identify 11 clumps whose masses lie in the range 3-10 M_☉. A clump that is prominent in the continuum maps but not in the molecular line maps is attributed to heated dust inside the compact H II region, where molecules have been destroyed.

At any one time, approximately one-quarter of the most rapidly rotating normal A-type dwarfs (V sin i ≥ 200 km s^-1) show shell lines of Ti II in the near-ultraviolet. Our observations during 22 years show that the lines appear and disappear on timescales of decades but do not display significant changes within 1 year. This implies that they are not remnants of the star formation but rather are probably caused by sporadic mass-loss events. A working hypothesis is that all A-type stars that are rotating near their limits have these shells, but for only one-quarter of the time. Because these lines do not appear in stars with smaller sin i, the shells must be disks. These are hot inner disks that may or may not be related to the cool outer disks seen by Smith and Terrile around β Pic or through infrared excesses around Vega and other A-type dwarfs. The similar, limited line widths indicate that the disks are ~7 R_* above the stellar surfaces.

We propose a self-consistent mechanism to estimate the size of the acceleration region in the outer magnetosphere of pulsars (outer gap) and calculate the high-energy radiation produced by the synchrocurvature mechanism from the outer gap. We find that a power-law energy distribution of the accelerated particles can be obtained if the outer gap is thick enough that E inside the gap can be approximately proportional to (Ωr/c)^1/2B(r). We apply our model to explain X-rays and γ-rays from Geminga and PSR B1055-52, whose outer gaps may occupy ~70% of the outer magnetosphere region. If the radius of curvature near the light cylinder of the medium outer gap is larger than the dipolar structure, then perhaps this model may also apply to PSR B1951+32, PSR B1706-44, and others.

I report the results of the first all-sky survey of Hα emission in the spectra of O-type binaries. The survey includes 26 systems, of which 10 have emission that extends clearly above the continuum. This is the first report of emission for four of these. An additional three systems show small distortions in the Hα profile that may result from weak emission. I compare the distribution of emission systems in H-R diagrams for both binary and single stars, using a survey of single O-type stars done by Conti (1974). Emission in main-sequence systems is extremely rare and is completely absent in my sample of binary stars. Among binary stars, 78% of the systems containing giants show some emission, while no single giants in Conti's sample do. In the case of supergiants, 78% of single stars show emission, while all supergiant binaries show strong emission. Hα emission may come from a variety sources, but the fact that binaries have a higher incidence and strength of emission in post-main-sequence stages may indicate that wind interactions are a common source of emission in massive binaries. To ascertain whether or not colliding winds have been observed, it will be necessary to study the Hα line profile throughout several orbits of each candidate colliding wind system and look for recurring orbital-phase-related variations. Such a study is underway.

Recent observations of the Crab pulsar by the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory show that the high-energy gamma-ray light curve has changed little over the lifetime of the instrument. Previous data collected by SAS 2 and COS B in the years 1972-1982, along with earlier EGRET data, suggested a 14 yr sinusoidal variation in the flux ratio between the first and second peaks. The new data from EGRET indicate that the flux ratio is constant.

The broadband (2-500 keV) data for Cygnus X-1 from observations by EXOSAT, OSSE, and the XMPC balloon, have been fitted to the transition disk model. In this model the emission is from the inner region of an accretion disk where the temperature is a rapidly varying function of radius and the radiative mechanism is saturated Comptonization. We fit the data to an empirical model and obtain the temperature profile that would give rise to the observed spectrum. Then we solve for the disk structure using this profile and show that the analysis is self-consistent. An advantage of this method is that the viscosity mechanism need not be specified.

We find that the transition model spectrum seems to be a better fit compared to a power law with exponential cutoff. In particular, a second component (with peak around 100 keV) that has been used in the past to explain the spectrum is not required here. We emphasize the need to conduct simultaneous broadband observations of this source in order to test ideas such as those presented here.

We report the results of high-speed optical white-light photometry of the black hole candidate GX 339-4, obtained at the Cerro Tololo Inter-American Observatory during 1996 April. We searched the data for strictly periodic features, quasi-periodic oscillations (QPOs), and aperiodic variability (noise) using Fourier and wavelet techniques. (1) We found a QPO with amplitude 4.5%-6%, frequency f ~ 0.064 Hz, and width Δf/f ~ 0.3-0.5. The QPO was long-lived in that it was present in all segments of the data, even in sets acquired one day apart. (2) In addition to the 0.064 Hz QPO there are indications of low-amplitude QPOs at f = 0.02, 0.03, 0.08, 0.3, and 3 Hz. None of the features appeared in more than one day of data (except for the 0.08 Hz feature) and none of the features were as strong as the 0.064 Hz QPO, and so they should be taken with some caution. (3) We found power in excess of counting statistics out to frequencies of several hertz on both nights. The origin of this excess power is not clear.

We present new, detailed polarimetric measurements of the Galilean satellites of Jupiter with U, B, V, and R filters at phase angles ranging from 12° to nearly 0°. The polarization phase curves of Io, Europa, and Ganymede in the B, V, and R filters clearly show the presence of the polarization opposition effect in the form of a sharp peak of negative polarization centered at a very small phase angle of 06-07 and superimposed on the regular negative polarization branch. This phase angle is comparable to the width of the spikelike photometric opposition effect observed for Europa, thus indicating that both opposition phenomena are likely to be produced by the coherent backscattering mechanism. The U filter values of |P_min| for Io and Europa are close to 0.60% and 0.47%, respectively, and exceed the respective BVR values by a factor of almost 2. The BVR polarization for the trailing hemispheres of Io, Europa, and, especially, Ganymede is systematically stronger than for the respective leading hemispheres. For Callisto, the leading hemisphere polarization is significantly stronger than for the trailing hemisphere. The inversion angles for Io, Europa, and Ganymede are nearly wavelength independent and close to 100, 86, and 88, respectively. The inversion angle for the trailing hemisphere of Callisto is also wavelength independent and is in the range of 12°-13°.

We report on the absolute antiproton flux and the antiproton to proton ratio in the energy range 0.62-3.19 GeV at the top of the atmosphere, measured by the balloon-borne experiment CAPRICE flown from Lynn Lake, Manitoba, Canada, on 1994 August 8-9. The experiment used the New Mexico State University WiZard/CAPRICE balloon-borne magnet spectrometer equipped with a solid radiator Ring Imaging Cherenkov (RICH) detector and a silicon-tungsten calorimeter for particle identification. This is the first time a RICH is used together with an imaging calorimeter in a balloon experiment, and it allows antiprotons to be clearly identified over the rigidity range 1.2-4 GV. Nine antiprotons were identified in the energy range 0.62-3.19 GeV at the top of the atmosphere. The data were collected over 18 hr at a mean residual atmosphere of 3.9 g cm^-2. The absolute antiproton flux is consistent with a pure secondary production of antiprotons during the propagation of cosmic rays in the Galaxy.

The magnetic field in the solar photosphere evolves as flux concentrations fragment in response to sheared flows, merge when they collide with others of equal polarity, or (partially) cancel against concentrations of opposite polarity. Newly emerging flux replaces the canceled flux. We present a quantitative statistical model that is consistent with the histogram of fluxes contained in concentrations of magnetic flux in the quiet network for fluxes exceeding ≈ 2 × 10¹⁸ Mx, as well as with estimated collision frequencies and fragmentation rates. This model holds for any region with weak gradients in the magnetic flux density at scales of more than a few supergranules. We discuss the role of this dynamic flux balance (i) in the dispersal of flux in the photosphere, (ii) in sustaining the network-like pattern and mixed-polarity character of the network, (iii) in the formation of unipolar areas covering the polar caps, and (iv) on the potential formation of large numbers of very small concentrations by incomplete cancellation. Based on the model, we estimate that as much flux is cancelled as is present in quiet-network elements with fluxes exceeding ≈ 2 × 10¹⁸ Mx in 1.5 to 3 days, which is compatible with earlier observational estimates. This timescale is close to the timescale for flux replacement by emergence in ephemeral regions, so that this appears to be the most important source of flux for the quiet-Sun network; based on the model, we cannot put significant constraints on the amount of flux that is injected on scales that are substantially smaller than that of the ephemeral regions. We establish that ephemeral regions originate in the convection zone and are not merely the result of the reemergence of previously cancelled network flux. We also point out that the quiet, mixed-polarity network is generated locally and that only any relatively small polarity excess is the result of flux dispersal from active regions.

We investigate the resistive processes of plasmoid dynamics in eruptive flares by performing 2.5-dimensional resistive MHD numerical simulations. We start with a linear force-free field arcade and impose the localized resistive perturbation on the symmetry axis of the arcade. Then the magnetic fields begin to dissipate, producing inflows toward this region. These inflows make the magnetic fields convex to the symmetry axis and hence a neutral point is formed on this axis, leading to a formation of a magnetic island around the symmetry axis. At the first stage, the magnetic island slowly rises by the upflow produced by the initial resistive perturbation. Then, once the anomalous resistivity sets in, the magnetic island begins to be accelerated. This acceleration stops after the fast MHD shock is formed at the bottom of the magnetic island, which implies that the upflow around the central part of the magnetic island is no longer strong. These three stages in the evolution of the plasmoid are confirmed to exist in the observational results. Moreover, a time lag between the start time when the magnetic island begins to be accelerated and the peak time of the neutral-point electric field can be explained by the inhibition of magnetic reconnection by the perpendicular magnetic field. We also study the difference of the initial rise motion of the plasmoid between the simulation results and the observational ones, and we conclude that, in actual situations, the initial resistive perturbation proceeds very weakly and at many positions inside the arcade.

We present time-distance analyses of several active regions and a region of quiet Sun observed with the Global Oscillation Network Group (GONG). Analyzing temporal correlations between the p-mode oscillation signal observed within the sunspots with the signals integrated within surrounding annuli, we confirm the recent finding of Duvall and his colleagues that travel times (τ⁺) for outward propagating p-modes are smaller by approximately 1 minute than corresponding inward travel times (τ^-). We also analyze correlations of the oscillation signal integrated within annuli of different radii. By varying the radius of the inner annulus (that which is closer to the target) we show that the radial extent of the region giving rise to the travel time perturbations is coincident with the outer boundary of the sunspot penumbrae.

A comparison of independent methods designed to determine the mean travel time perturbations of p-modes passing through the sunspots is made. We find the surprising result that time-distance correlations that do not utilize the signal within the sunspot itself (employing "two-skip" trajectories) yield mean travel times that differ substantially from the average of τ⁺ and τ^- and that are significantly closer in agreement with times predicted from scattering phase shifts measured by Hankel decomposition techniques. These observations suggest that it unlikely that Doppler shifts caused by subsurface flows are responsible for the travel time differences determined from center-annuli correlations targeted on sunspots.

Recent R-matrix calculations of electron impact excitation rates in Ar IV are used to calculate the emission-line ratio: ratio diagrams (R₁, R₂), (R₁, R₃), and (R₁, R₄), where R₁ = I(4711 Å)/I(4740 Å), R₂ = I(7238 Å)/I(4711 + 4740 Å), R₃ = I(7263 Å)/I(4711 + 4740 Å), and R₄ = I(7171 Å)/I(4711 + 4740 Å), for a range of electron temperatures (T_e = 5000-20,000 K) and electron densities (N_e = 10-10⁶ cm^-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of T_e and N_e from measurements of the [Ar IV] lines in a spectrum. Plasma parameters deduced for a sample of planetary nebulae from (R₁, R₃) and (R₁, R₄), using observational date obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope at the Lick Observatory, are found to show excellent internal consistency and to be in generally good agreement with the values of T_e and N_e estimated from other line ratios in the echelle spectra. These results provide observational support for the accuracy of the theoretical ratios and, hence, the atomic data adopted in their derivation. In addition, they imply that the 7171 Å line is not as seriously affected by telluric absorption as previously thought. However, the observed values of R₂ are mostly larger than the theoretical high-temperature and density limit, which is due to blending of the Ar IV 7237.54 Å line with the strong C II transition at 7236 Å.

The J = 1-0 transitions of ⁶³CuH and ⁶⁵CuH were measured in the 468 GHz region with a source modulation submillimeter-wave spectrometer. The CuH molecule was generated by sputtering from copper powder in a DC glow discharge through hydrogen gas. The observed rotational transitions show the line broadening caused by the hyperfine structure due to the copper nucleus (I = 3/2). The central transition frequency, the nuclear electric quadrupole coupling constant, and the nuclear magnetic spin-rotation coupling constant were determined for each isotopomer by line-shape simulation.