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We measure the linear power spectrum of mass-density fluctuations at redshift z = 2.5 from the Lyα forest absorption in a sample of 19 QSO spectra, using the method introduced by Croft et al. The P(k) measurement covers the range 2π/k ~ 450-2350 km s^-1 (2-12 comoving h^-1 Mpc for Ω = 1), limited on the upper end by uncertainty in fitting the unabsorbed QSO continuum and on the lower end by finite spectral resolution (0.8-2.3 Å FWHM) and by nonlinear dynamical effects. We examine a number of possible sources of systematic error and find none that are significant on these scales. In particular, we show that spatial variations in the UV background caused by the discreteness of the source population should have negligible effect on our P(k) measurement. We estimate statistical errors by dividing the data set into ten subsamples. The statistical uncertainty in the rms mass-fluctuation amplitude, σ ∝ [P(k)]^1/2, is ~20%, and is dominated by the finite number of spectra in the sample. We obtain consistent P(k) measurements (with larger statistical uncertainties) from the high- and low-redshift halves of the data set, and from an entirely independent sample of nine QSO spectra with mean redshift z = 2.1. A power-law fit to our results yields a logarithmic slope n = -2.25 ± 0.18 and an amplitude Δ²_ρ(k_p) = 0.57^+0.26_-0.18, where Δ²_ρ is the contribution to the density variance from a unit interval of ln k and k_p = 0.008(km s^-1)^-1. Direct comparison of our mass P(k) to the measured clustering of Lyman break galaxies shows that they are a highly biased population, with a bias factor b ~ 2-5. The slope of the linear P(k), never previously measured on these scales, is close to that predicted by models based on inflation and cold dark matter (CDM). The P(k) amplitude is consistent with some scale-invariant, COBE-normalized CDM models (e.g., an open model with Ω₀ = 0.4) and inconsistent with others (e.g., Ω = 1). Even with limited dynamic range and substantial statistical uncertainty, a measurement of P(k) that has no unknown "bias factors" offers many opportunities for testing theories of structure formation and constraining cosmological parameters.

We propose a general formalism for galaxy biasing and apply it to methods for measuring cosmological parameters, such as regression of light versus mass, the analysis of redshift distortions, measures involving skewness, and the cosmic virial theorem. The common linear and deterministic relation g = bδ between the density fluctuation fields of galaxies g and mass δ is replaced by the conditional distribution P(g|δ) of these random fields, smoothed at a given scale at a given time. The nonlinearity is characterized by the conditional mean ⟨g|δ⟩ ≡ b(δ)δ, while the local scatter is represented by the conditional variance σ²_b(δ) and higher moments. The scatter arises from hidden factors affecting galaxy formation and from shot noise unless it has been properly removed. For applications involving second-order local moments, the biasing is defined by three natural parameters: the slope of the regression of g on δ, a nonlinearity , and a scatter σ_b. The ratio of variances b²_var and the correlation coefficient r mix these parameters. The nonlinearity and the scatter lead to underestimates of order ²/² and σ²_b/² in the different estimators of β (~ Ω^0.6/). The nonlinear effects are typically smaller. Local stochasticity affects the redshift-distortion analysis only by limiting the useful range of scales, especially for power spectra. In this range, for linear stochastic biasing, the analysis reduces to Kaiser's formula for (not b_var), independent of the scatter. The distortion analysis is affected by nonlinear properties of biasing but in a weak way. Estimates of the nontrivial features of the biasing scheme are made based on simulations and toy models, and strategies for measuring them are discussed. They may partly explain the range of estimates for β.

We consider the long-standing problem of predicting the hierarchical clustering amplitudes S_p in the strongly nonlinear regime of gravitational evolution. N-body results for the nonlinear evolution of the bispectrum (the Fourier transform of the three-point density correlation function) suggest a physically motivated Ansatz that yields the strongly nonlinear behavior of the skewness, S₃, starting from leading-order perturbation theory. When generalized to higher order (p > 3) polyspectra or correlation functions, this Ansatz leads to a good description of nonlinear amplitudes in the strongly nonlinear regime for both scale-free and cold dark matter models. Furthermore, these results allow us to provide a general fitting formula for the nonlinear evolution of the bispectrum that interpolates between the weakly and strongly nonlinear regimes, analogous to previous expressions for the power spectrum.

It is shown that in cosmological models based on a vacuum energy decaying as a^-2, where a is the scale factor of the universe, the fate of the universe in regard to whether it will collapse in the future or expand forever is determined not by the curvature constant k but by an effective curvature constant k_eff. It is argued that a closed universe with k = 1 may expand forever, in other words, simulate the expansion dynamics of a flat or an open universe because of the possibility that k_eff = 0 or -1, respectively. Two such models, in one of which the vacuum does not interact with matter and in another of which it does, are studied. It is shown that the vacuum equation of state p_vac = -ρ_vac may be realized in a decaying vacuum cosmology provided the vacuum interacts with matter. The optical depths for gravitational lensing as a function of the matter density and other parameters in the models are calculated at a source redshift of 2. The age of the universe is discussed and shown to be compatible with the new Hipparcos lower limit of 11 Gyr. The possibility that a time-varying vacuum energy may serve as dark matter is suggested.

The observed redshifts and magnitudes of the host galaxies of gamma-ray bursts (GRBs) are compared with the predictions of three basic GRB models, in which the comoving rate density of GRBs is (1) proportional to the cosmic star formation rate density, (2) proportional to the total integrated stellar density, and (3) constant. All three models make the assumption that at every epoch the probability of a GRB occurring in a galaxy is proportional to that galaxy's broadband luminosity. No assumption is made that GRBs are standard candles or even that their luminosity function is narrow. All three rate-density models are consistent with the observed GRB host galaxies to date, although model (2) is slightly disfavored relative to the others. Models (1) and (3) make very similar predictions for host galaxy magnitude and redshift distributions; these models probably will not be distinguished without measurements of host galaxy star formation rates. The fraction of host galaxies fainter than 28 mag may constrain the faint end of the galaxy luminosity function at high redshift, or, if the fraction is observed to be low, may suggest that the bursters are expelled from low-luminosity hosts. In all models, the probability of finding a z < 0.008 GRB among a sample of 11 GRBs is less than 10^-4, strongly suggesting that GRB 980425, if associated with SN 1998bw, represents a distinct class of GRBs.

Current theories of galaxy formation have tended to focus on hierarchical structure formation, which is the most likely scenario for cosmological models with a good deal of power at small scales (e.g., standard cold dark matter). Models with little small-scale power lead to scenarios closer to spherical collapse. Recently favored power spectra (e.g., CDM+Λ) lie somewhere in between, which suggests that both types of processes are important and may vary over time owing to gaseous reheating. From this viewpoint this paper explores a very simple inside-out scenario for galaxy formation. This scenario is a natural result of synthesizing earlier work on dark matter halos, spherical collapse, and gas redistribution via angular momentum. Although this model is highly simplified and is not designed to describe the detailed formation of any individual galaxy accurately, it does (by design) predict the overall features of galaxies. In addition, old bulges and young disks are almost unavoidable results of these very simple models. This scenario may provide a useful framework for both observers and theoreticians to think about galaxy formation.

Observations with the Wide-Field/Planetary Camera-2 of the Hubble Space Telescope are presented for three radio-loud quasars: 3C 48 (z = 0.367), B2 1425+267 (z = 0.366), and 3C 345 (z = 0.594). All three quasars have luminous (~4 × L*) galaxies as hosts, which are either elliptical (B2 1425+267 and 3C345) or interacting (3C 48), and all hosts are 0.5-1.0 mag bluer in V-I than other galaxies with the same overall morphology at redshifts similar to those of the quasars. The host of 3C 48 has many H II regions and a very extended tidal tail. All nine of the radio-loud quasars studied here and in a previous paper by Bahcall et al. either have bright elliptical hosts or occur in interacting systems. There is a robust correlation between the radio emission of the quasar and the luminosity of host galaxy; the radio-loud quasars reside in galaxies that are on average ~1 mag brighter than hosts of the radio-quiet quasars.

We examine the relationship between the mass and X-ray gas temperature of galaxy clusters using data drawn from the literature. Simple theoretical arguments suggest that the mass of a cluster is related to the X-ray temperature as M ∝ T_X^3/2. Virial theorem mass estimates based on cluster galaxy velocity dispersions seem to be accurately described by this scaling with a normalization consistent with that predicted by the simulations of Evrard, Metzler, and Navarro. X-ray mass estimates that employ spatially resolved temperature profiles also follow a T_X^3/2 scaling although with a normalization about 40% lower than that of the fit to the virial masses. However, the isothermal β-model and X-ray surface brightness deprojection masses follow a steeper ∝ T_X^1.8-2.0 scaling. The steepness of the isothermal estimates is due to their implicitly assumed dark matter density profile of ρ(r) ∝ r^-2 at large radii, whereas observations and simulations suggest that clusters follow steeper profiles [e.g., ρ(r) ∝ r^-2.4].

We present ROSAT HRI data of the distant and X-ray-luminous [L_X(bol) = 2.6^+0.4_-0.2 × 10⁴⁵ergs s^-1] cluster of galaxies 3C 295. We fit both a one-dimensional and a two-dimensional isothermal β-model to the data, the latter taking into account the effects of the point spread function (PSF). For the error analysis of the parameters of the two-dimensional model, we introduce a Monte Carlo technique. Applying a substructure analysis by subtracting a cluster model from the data, we find no evidence for a merger, but we see a decrement in emission southeast of the center of the cluster, which might be due to absorption. We confirm previous results by Henry & Henriksen that 3C 295 hosts a cooling flow. The equations for the simple and idealized cooling flow analysis presented here are based solely on the isothermal β-model, which fits the data very well, including the center of the cluster. We determine a cooling flow radius of 60-120 kpc and mass accretion rate of = 400-900 M_☉ yr^-1, depending on the applied model and temperature profile. We also investigate the effects of the ROSAT PSF on our estimate of , which tends to lead to a small overestimate of this quantity if not taken into account. This increase of (10%-25%) can be explained by a shallower gravitational potential inferred by the broader overall profile caused by the PSF, which diminishes the efficiency of mass accretion. We also determine the total mass of the cluster using the hydrostatic approach. At a radius of 2.1 Mpc, we estimate the total mass of the cluster (M_tot) to be 9.2 ± 2.7 × 10¹⁴M_☉. For the gas-to-total mass ratio, we get M_gas/M_tot = 0.17-0.31, in very good agreement with the results for other clusters of galaxies, giving strong evidence for a low-density universe.

The origin of jets and hot spots in FR II double radio sources are explained by a model in which a supermassive black hole possessing twin beams moves out of the host galaxy with a speed greater than the escape speed. The lobe structure arises as a combination of two trails: the trail of the forward pointing beam is the lobe itself while the backward-pointing beam creates the so-called "jet" (narrow linear high-brightness feature). We show that the formation of the "jet" depends on the beam angle relative to the line of motion of the black hole. The same angle also controls the amount of entrainment of thermal gas in the lobe, and correlations between the component depolarization and the existence of a jet may arise through this common factor. The high-resolution samples of FR II radio galaxies at z < 0.3 by Hardcastle et al. and a representative sample of radio quasars by Bridle et al. are studied for evidence of the beam angle correlation with the jet formation.

We present a two-dimensional map of the gas temperature distribution in the Centaurus cluster, based on Advanced Satellite for Cosmology and Astrophysics (ASCA) observations derived using a novel approach to account for the energy-dependent point-spread function. Along with a cool region centered on NGC 4696, asymmetric temperature variations of moderate amplitude are detected. The hottest region roughly coincides with the position of the second-brightest galaxy, NGC 4709, known to be the dominant galaxy of one of the subgroups in the Centaurus cluster. ROSAT images show faint surface brightness emission also centered on this galaxy. The imaging and spectral results suggest that a subcluster centered on NGC 4709 is merging with the main cluster centered on NGC 4696, in agreement with earlier suggestion that these two systems are located at the same distance, despite their different line-of-sight velocities.

The Hawaii Imaging Fabry-Perot Interferometer (HIFI) on the University of Hawaii 2.2 m telescope was used to map the Hα and [O III] λ5007 emission-line profiles across the entire disk of the edge-on Sb galaxy NGC 4388. We confirm a rich complex of highly ionized gas that extends ~4 kpc above the disk of this galaxy. Low-ionization gas associated with star formation is also present in the disk. Evidence for bar streaming is detected in the disk component and is discussed in a companion paper. Nonrotational blueshifted velocities of 50-250 km s^-1 are measured in the extraplanar gas northeast of the nucleus. The brighter features in this complex tend to have more blueshifted velocities. A redshifted cloud is also detected 2 kpc southwest of the nucleus. The velocity field of the extraplanar gas of NGC 4388 appears to be unaffected by the inferred supersonic (Mach number M ≈ 3) motion of this galaxy through the ICM of the Virgo cluster. We argue that this is because the galaxy and the high-|z| gas lie behind a Mach cone with opening angle ~80°. The shocked ICM that flows near the galaxy has a velocity of ~500 km s^-1 and exerts insufficient ram pressure on the extraplanar gas to perturb its kinematics. We consider several explanations of the velocity field of the extraplanar gas. Velocities, especially blueshifted velocities on the north of the galaxy, are best explained as a bipolar outflow that is tilted by >12° from the normal to the disk. The observed offset between the extraplanar gas and the radio structure may be due to buoyancy or refractive bending by density gradients in the halo gas. Velocity substructure in the outflowing gas also suggests an interaction with ambient halo gas.

The spectrum of the diffuse isotropic component of cosmic X-rays over the 13-180 keV range was determined by the UCSD/MIT Hard X-Ray and Gamma-Ray instrument (HEAO 1 A-4) on board the High Energy Astronomical Observatory 1 (HEAO 1). The instrument consists of a complex of actively shielded and collimated scintillation counters, including the low-energy detector set from which the data reported here were obtained. These data join smoothly with the spectrum at lower energies reported by the GSFC HEAO 1 A-2 instrument and with that measured to 400 keV by the HEAO 1 A-4 medium-energy detectors. The HEAO 1 data set also joins the recent results from COMPTEL on the Compton Gamma Ray Observatory (CGRO) in the 1-10 MeV range, which failed to confirm the existence of an "MeV bump" in this range. Although the spectrum over the entire range 3 keV ≤ E ≤ 100 GeV can be fitted by a simple empirical analytic expression, the origin is likely due to a number of distinct source components. The prevailing idea for the origin is that the hard X-ray spectrum is due to X-rays from various active galactic nucleus components, particularly Seyfert galaxies extending to cosmological distances, and that the low-energy gamma rays may be due to emission from Type Ia supernovae, which is also integrated to cosmological distances. The higher energy gamma-ray spectrum defined by EGRET, also on the CGRO, may be due to unresolved gamma-ray-emitting blazars. Models of production by these source components, extrapolated to the present epoch, must reproduce the observationally derived spectrum.

We examine the 0.1-10.0 keV X-ray spectrum of the bright nuclear LINER galaxy NGC 1052, one of two elliptical galaxies known to contain a luminous H₂O maser. The observed 2.0-10.0 keV spectrum is unusually flat (photon index Γ ~ 0.2) and is best described as intrinsically power-law shaped nuclear flux that is either (1) attenuated by a complex absorber with ~70% of the nuclear flux absorbed by a column density of N_H ~ 3 × 10²³ cm^-2 and ~30% absorbed by a column density of N_H ~ 3-5 × 10²² cm^-2 or (2) reprocessed, with the nuclear source blocked and the X-rays Compton reflected in our direction by high column density (≥10²⁴ cm^-2) gas. The moderate equivalent width of the Fe Kα line favors the dual absorption model as the most likely scenario. The 0.1-2.0 keV spectrum does not resemble the few times 10⁶-10⁷ K thermal emission typically found in other elliptical galaxies, but instead is best described as nuclear X-rays leaking through a patchy absorber or scattered in our direction by low-density, ionized gas plus a 15%-20% contribution from a thermal component, which is most likely due to the galaxy. The absorption-corrected 2-10 keV luminosity of the nuclear source is L_X ~ 8 × 10⁴¹ ergs s^-1 or L_X ~ 2 × 10⁴³ ergs s^-1 for the dual-absorption and Compton-reflection models, respectively. The absorbing and H₂O masing gases appear to be spatially separate in this galaxy.

We investigate microlensing amplification of faint stars in the dense stellar cluster in the Galactic center by the supermassive black hole, which is thought to coincide with the radio source Sgr A*. Such amplification events would appear very close to the position of Sgr A* and could be observed, in principle, during the monitoring of stellar proper motions in the Galactic center.

We use observations of the near-infrared K-band (2.2 μm) luminosity function in the Galactic center and in Baade's window, as well as stellar population synthesis computations, to construct empirical and theoretical K luminosity function models for the inner 300 pc of the Galaxy. These, together with the observed dynamical properties of the central cluster and inner bulge, are used to compute the rates of microlensing events that amplify stars with different intrinsic luminosities above specified detection thresholds.

We present computations of the lensing rates as functions of the event durations, which range from several weeks to a few years, for detection thresholds ranging from K₀ = 16 to 19 mag. We find that events with durations shorter than a few months dominate the lensing rate because of the very high stellar densities and velocities very near the black hole, where the effective lens size is small. For the current detection limit of K₀ = 17 mag, the total microlensing rate is 3 × 10^-3 yr^-1. The rate of events with durations ≥1 yr is 1 × 10^-3 yr^-1. The median value of the peak amplification for short events is ΔK ~ 0.75 mag above the detection threshold and is only weakly dependent on K₀. Long events are rarer and are associated with more distant stars, stars at the low-velocity tail of the velocity distribution or stars that cross closer to the line of sight to Sgr A*. Therefore, the median peak amplifications of long events are larger and attain values ΔK ~ 1.5 mag above the threshold.

Recent proper-motion studies of stars in the Galactic center have revealed the possible presence of one or two variable K-band sources very close to, or coincident with, the position of Sgr A*. These sources may have attained peak brightnesses of K ≈ 15 mag, about 1.5-2 mag above the observational detection limits, and appear to have varied on a timescale of ~1 yr. This behavior is consistent with long-duration microlensing amplification of faint stars by the central black hole. However, we estimate that the probability that a single such event could have been detected during the course of the recent proper-motion monitoring campaigns is ~0.5%. A 10-fold improvement in the detection limit and 10 yr of monthly monitoring could increase the total detection probability to ~20%.

The H II region W40 harbors a small group of young, hot stars behind roughly 9 mag of visual extinction. We have detected gaseous carbon monoxide (CO) and diatomic carbon (C₂) in absorption toward the star W40 IRS 1a. The J = 2-0 R0, R1, and R2 lines of ¹²CO at 2.3 μm were measured using the CSHELL on the NASA Infrared Telescope Facility (with upper limits placed on R3, R4, and R5) yielding an N_CO of (1.1 ± 0.2) × 10¹⁸ cm^-2. Excitation analysis indicates T_kin > 7 K. The Phillips system of C₂ transitions near 8775 Å was measured using the Kitt Peak 4 m telescope and echelle spectrometer. Radiative pumping models indicate a total C₂ column density of (7.0 ± 0.4) × 10¹⁴ cm^-2, two excitation temperatures (39 and 126 K), and a total gas density of n ~ 250 cm^-3. The CO ice band at 4.7 μm was not detected, placing an upper limit on the CO depletion of δ < 1%. We postulate that the sight line has multiple translucent components and is associated with the W40 molecular cloud. Our data for W40 IRS 1a, coupled with other sight lines, shows that the ratio of CO/C₂ increases from diffuse through translucent environs. Finally, we show that the hydrogen-to-dust ratio seems to remain constant from diffuse to dense environments, whereas the CO-to-dust ratio apparently does not.

Hyperfine structure in the rotational transitions of the C₅H, C₆H, and C₈H carbon chain radicals has been measured with a Fourier transform microwave spectrometer. Enough transitions (four to six) between 7 and 22 GHz were measured for each radical to determine the hydrogen hyperfine coupling constants to high precision. The radio spectrum of each radical can now be calculated to better than 0.30 km s^-1 in equivalent radial velocity at frequencies less than 50 GHz, adequate for radio astronomical studies in very narrow line sources like TMC-1.

We have observed seven ultracompact H II regions in hydrogen recombination lines in the millimeter band. Toward four of these regions there is a high-velocity (FWHM 60-80 km s^-1) component in the line profiles. The high-velocity gas accounts for 35%-70% of the emission measure within the beam. We compare these objects with an additional seven similar sources we have found in the literature. The broad recombination line objects (BRLOs) make up about 30% of all sources in complexes containing ultracompact H II regions. Comparison of spectral line and continuum data implies that the BRLOs coincide with sources with rising spectral indices, ≥0.4 up to 100 GHz. Both the number of BRLOs and their frequency of occurrence within H II region complexes, when coupled with their small size and large internal motions, mean that the apparent contradiction between the dynamical and the population lifetimes for BRLOs is even more severe than for ultracompact H II regions as a whole. We evaluate a number of possible models for the origin of the broad recombination line emission. The lifetime, morphology, and rising spectral index of the sources argue for photoevaporated disks as the cause for BRLOs. Existing models for such regions, however, do not account for the large amounts of ionized gas observed at supersonic velocities.

Refraction of pulsar radiation by electron density irregularities in the interstellar medium sometimes produces multiple imaging of pulsars, which can lead to periodic oscillations of intensity in pulsar dynamic spectra records. Such events can be used as tools to resolve the emission regions in pulsar magnetospheres. Here we describe results from the recent observation of a double imaging event for PSR B1133+16, which place fairly tight constraints on the location of the emission regions. Our analysis constrains the location of the scattering object to the shell of the Local Bubble. The phase of the oscillations shows significant variations across the pulse. The minimum value for the transverse separation of the emitting regions at the two edges of the pulse is inferred to be 3 × 10⁵ m. This translates to a minimum emission altitude of 2.6 × 10⁶ m. The nonmonotonic variations of the fringe phase with pulse longitude are interpreted as variations of the altitude of the emission regions for the orthogonal polarization modes of this pulsar. This is in agreement with theories where propagation effects, such as refraction, are responsible for the orthogonal modes.

Studies of the abundances of deuterium in different astrophysical sites are of fundamental importance to answering the question about how much deuterium was produced during big bang nucleosynthesis and what fraction of it was destroyed later. With this in mind, we used the Interstellar Medium Absorption Profile Spectrograph (IMAPS) on the ORFEUS-SPAS II mission to observe at a wavelength resolution of 4 km s^-1 (FWHM) the Lyδ and Ly absorption features produced by interstellar atomic deuterium in the spectrum of δ Ori A. A χ² analysis indicated that 0.96 < N(D I) < 1.45 × 10¹⁵ cm^-2 at a 90% level of confidence, and the gas is at a temperature of about 6000 K. In deriving these results, we created a template for the velocity profile defined by seven different N I transitions recorded at a high signal-to-noise ratio. Extra free parameters in the analysis allowed for the additional uncertainties that could arise from various sources of systematic error. To derive a value for D/H, we measured the Lyα absorption features in 57 spectra of δ Ori in the IUE archive, with the objective of arriving at an H I column density more accurate than those reported by other investigators. From our measurement of N(H I) = 1.56 × 10²⁰ cm^-2, we found that N(D I)/N(H I) = 7.4^+1.9_-1.3 × 10^-6 (90% confidence). Systematic errors in the derivation of N(H I) probably dominate over the very small formal error, but their relative value should be smaller than that for N(D I). Our result for D/H contrasts with the more general finding along other lines of sight that D/H ≈ 1.5 × 10^-5. The underabundance of D toward δ Ori A is not accompanied by an overabundance of N or O relative to H, as one might expect if the gas were subjected to more stellar processing than usual.

We present a study of the high-energy diffuse emission observed toward Orion by EGRET on the Compton Gamma Ray Observatory. The total exposure by EGRET in this region has increased by more than a factor of 2 since a previous study. A simple model for the diffuse emission adequately fits the data; no significant point sources are detected in the region studied (l = 195° to 220° and b = -25° to -10°) in either the composite data set or in two separate groups of EGRET viewing periods considered. The gamma-ray emissivity in Orion is found to be (1.65 ± 0.11) × 10^-26 s^-1 sr^-1 for E > 100 MeV, and the differential emissivity is well described as a combination of contributions from cosmic-ray electrons and protons with approximately the local density. The molecular mass calibrating ratio is N(H₂)/W_CO = (1.35 ± 0.15) × 10²⁰ cm^-2 (K km s^-1)^-1.

We present a new analysis of the transport of cosmic rays in a turbulent magnetic field that varies in all three spatial dimensions. The analysis utilizes a numerical simulation that integrates the trajectories of an ensemble of test particles from which we obtain diffusion coefficients based on the particle motions. We find that the diffusion coefficient parallel to the mean magnetic field is consistent with values deduced from quasi-linear theory, in agreement with earlier work. The more interesting and less understood transport perpendicular to the average magnetic field is found to be enhanced (above the classical scattering result) by the random walk, or braiding, of the magnetic field. The value of κ_⊥ obtained is generally larger than the classical scattering value but smaller than the quasi-linear value. The computed values of κ_⊥/κ_∥, for a representation of the interplanetary magnetic field, are 0.02-0.04; these values are of the same general magnitude as those assumed in recent numerical simulations of cosmic-ray modulation and transport in the heliosphere, and give reasonable agreement with spacecraft observations of cosmic rays. Some consequences of these results for the interpretation of heliospheric observations are discussed.

We obtained 3.6-20 μm photometry of 38 bright [IRASF_ν(12 μm) > 0.7 Jy] main-sequence stars with the Infrared Space Observatory (ISO). Observations were conducted with the ISOPHOT instrument, in the single-pointing photometry mode, through filters at 3.6, 11.5, and 20.0 μm. We searched for excess (Vega-type) emission from dust at temperatures ≳100 K, located at ~1-60 AU from the stars. We thus sampled dust at warm, terrestrial material temperatures and at cool (~100 K) temperatures of possible Kuiper Belt-type regions in these systems. We detected 20 μm excesses from ~14% of our sources, but we did not detect 11.5 μm excesses from any of them. We present single-temperature blackbody models of the location and density of dust emission around 10 stars, two of them (29 Cyg and Gl 816) with excesses newly reported here. We make a thorough comparison of ISO and IRAS data on our target stars and propose a new calibration procedure for ISOPHOT staring measurements at 3.6, 11.5, and 20 μm.

The HCN 1-0 hyperfine lines have been observed toward 24 young stellar objects (YSOs) of class 0 and I. The hyperfine lines are well separated in most cases and show such rich structures as asymmetric double peaks and strong wings. We examined how their line shapes and velocity shifts vary along with their relative optical depths, and compared them with those of the CS 2-1, H₂CO 2₁₂-1₁₁, and HCO⁺ 4-3 and 3-2 transitions previously observed by Mardones et al. and Gregersen et al. We find that all these molecular species do not always exhibit the same sense of line asymmetry, and the correlation of velocity shift is better between HCN and CS than between HCN and H₂CO. The most opaque transition of HCN F = 2-1 has about the same velocity shift as that of CS despite the larger beam size of this study, which suggests that the HCN F = 2-1 line could be more sensitive to the internal motion of YSOs than CS line. Systematic changes of the velocity shift are noted for many sources as one goes from F = 0-1 to 2-1. The monotonic decrease of velocity (blueshift) is apparently more frequent. A detailed model of radiative transfer allowing line overlap of HCN is employed to L483, which shows convincing signatures of infall on a scale of ~0.1 pc. It appears that the observed line is not compatible with the standard Shu model, but is fitted with augmentations of density and infall velocity, by factors of 6 and 0.5, respectively, and with the inclusion of a diffuse, static, turbulent, and geometrically thick envelope. The distribution of hyperfine line intensity ratios for these YSOs does not accord with the LTE condition and is essentially the same as ones previously noted in cold dark clouds or small translucent cores. Although this anomaly may be explained in terms of radiative transfer effects in the cores, which are either static or under systematic motion, some of them seem to invoke the existence of a scattering envelope. It is confirmed that HCN is detected more selectively in class 0 and I sources than in starless cores or class II objects, which implies that the YSO(s) form a dense (~10⁶ cm^-3) envelope with a significant HCN abundance in a narrow time span of their evolution.

Space-based gravitational wave detectors will have the ability to observe continuous low-frequency gravitational radiation from binary star systems. They can determine the direction to continuous sources with an angular resolution approaching tens of arcminutes. This resolution should be sufficient to identify binary sources as members of some nearby globular clusters. Thus, gravitational radiation can be used to determine the population of hard binaries in globular clusters. For particularly hard binaries, the orbital period may change as a result of gravitational wave emission. If one of these binaries can be identified with a globular cluster, then the distance to that cluster can be determined. Thus, gravitational radiation may provide reddening-independent distance measurements to globular clusters and the RR Lyrae stars that inhabit them.

Doppler measurements of two G-type main-sequence stars, HD 210277 and HD 168443, reveal Keplerian variations that imply the presence of companions with masses (M sin i) of 1.28 and 5.04 M_J (where M_J is the mass of Jupiter) and orbital periods of 437 and 58 days, respectively. The orbits have large eccentricities of e = 0.45 and e = 0.54, respectively. All nine known extrasolar planet candidates with a = 0.2-2.5 AU have orbital eccentricities greater than 0.1, higher than that of Jupiter (e = 0.05). Eccentric orbits may result from gravitational perturbations imposed by other orbiting planets or stars, by passing stars in the dense star-forming cluster, or by the protoplanetary disk. Based on published studies and our near-IR adaptive optics images, HD 210277 appears to be a single star. However, HD 168443 exhibits a long-term velocity trend consistent with a close stellar companion, as yet undetected directly.

We extend and reconcile recent work on turbulence and particle heating in advection-dominated accretion flows. For approximately equipartition magnetic fields, the turbulence primarily heats the electrons. For weaker magnetic fields, the protons are primarily heated. The division between electron and proton heating occurs between β ~ 5 and β ~ 100 (β is the ratio of gas to magnetic pressure), depending on unknown details of how Alfvén waves are converted into whistlers on scales of the proton Larmor radius. We also discuss the possibility that magnetic reconnection could be a significant source of electron heating.

Gravitomagnetic precession near neutron stars and black holes has received much recent attention, particularly as a possible explanation of 15-60 Hz quasi-periodic brightness oscillations (QPOs) from accreting neutron stars in low-mass X-ray binaries, and of somewhat higher frequency QPOs from accreting stellar-mass black holes. Previous analyses of this phenomenon have either ignored radiation forces or assumed for simplicity that the radiation field is isotropic, and in particular that there is no variation of the radiation field with angular distance from the rotational equatorial plane of the compact object. However, in most realistic accretion geometries (e.g., those in which the accretion proceeds via a geometrically thin disk) the radiation field depends on latitude. Here we show that in this case radiation forces typically have an important, even dominant, effect on the precession frequency of test particles in orbits that are tilted with respect to the star's rotational equator. Indeed, we find that even for accretion luminosities only a few percent of the Eddington critical luminosity, the precession frequency near a neutron star can be changed by factors of up to ~10. Radiation forces must therefore be included in analyses of precession frequencies near compact objects in such varied contexts as low-frequency QPOs, warp modes of disks, and trapped oscillation modes. We discuss specifically the impact of radiation forces on models of low-frequency QPOs involving gravitomagnetic precession, and we show that such models are rendered much less plausible by the effects of radiation forces.

We study systematically the ≃0.1-1200 Hz quasi-periodic oscillations (QPOs) and broad noise components observed in the power spectra of nonpulsing neutron star and black hole low-mass X-ray binaries. We show that among these components we can identify two, occurring over a wide range of source types and luminosities, whose frequencies follow a tight correlation. The variability components involved in this correlation include neutron star kilohertz QPOs and horizontal-branch oscillations, as well as black hole QPOs and noise components. Our results suggest that the same types of variability may occur in both neutron star and black hole systems over 3 orders of magnitude in frequency and with coherences that vary widely but systematically. Confirmation of this hypothesis will strongly constrain theoretical models of these phenomena and provide additional clues to understanding their nature.

The nature of the 5-12 s "anomalous" X-ray pulsars (AXPs) remains a mystery. Among the models that have been proposed to explain the properties of AXPs, the most likely are (1) isolated accreting neutron stars evolved from the Thorne-Żytkow objects (TŻOs) due to complete spiral-in during the common envelope (CE) evolution of high-mass X-ray binaries (HMXBs), and (2) magnetars, which are neutron stars with ultrahigh (~10¹⁴-10¹⁵ G) surface magnetic fields. We have critically examined the predicted change of a neutron star's spin in the accretion model, and found that it is unable to account for the steady spin-down observed in AXPs. A simple analysis also shows that any accretion disk around an isolated neutron star has an extremely limited lifetime. A more promising explanation for such objects is the magnetar model.

We consider the X-ray luminosity difference between neutron star and black hole soft X-ray transients (NS and BH SXTs) in quiescence. The current observational data suggest that BH SXTs are significantly fainter than NS SXTs. The luminosities of quiescent BH SXTs are consistent with the predictions of binary-evolution models for the mass transfer rate if (1) accretion occurs via an advection-dominated accretion flow (ADAF) in these systems and (2) the accreting compact objects have event horizons. The luminosities of quiescent NS SXTs are not consistent with the predictions of ADAF models when combined with binary-evolution models, unless most of the mass accreted in the ADAF is prevented from reaching the neutron star surface. We consider the possibility that mass accretion is reduced in quiescent NS SXTs because of an efficient propeller and develop a model of the propeller effect that accounts for the observed luminosities. We argue that modest winds from ADAFs are consistent with the observations, while strong winds are probably not.

We present the results of timing and spectral studies of the dipping X-ray source XB 1916-053, observed by ASCA during its performance verification phase. The detected dipping activity is consistent with previous observations, with a period of 3008 s and an intermittent secondary dip observed roughly 0.4 out of phase with the primary dip. The energy spectra of different intensity states are fitted with a power law with partial covering fraction absorption and interstellar absorption. The increase in the hardness ratio during the primary and secondary dips, and the increase in the covering fraction and column density with decreasing X-ray intensity, all imply that the dipping is caused by the photoabsorbing materials that have been suggested to be where the accreted flow hits the outer edge of the disk materials. The spectra at all intensity levels show no apparent evidence for Fe or Ne emission lines. This may be due to the low metal abundance in the accretion flow. Alternatively, the X-ray luminosity of the central source may be too weak to excite emission lines, which are assumed to be produced by X-ray photoionization of the disk materials.

We calculate spectral models of advection-dominated accretion flows, taking into account the possibility that significant mass may be lost to an outflow/wind. We apply the models to the soft X-ray transient V404 Cyg in quiescence, the Galactic center source Sgr A*, and the nucleus of NGC 4649. We show that there are qualitative degeneracies between the mass-loss rate in the wind and parameters characterizing the microphysics of the accretion flow; of particular importance is δ, the fraction of the turbulent energy which heats the electrons. For small δ, current observations suggest that at most 90% of the mass originating at large radii is lost to a wind, so that at least ~10% reaches the central object. For large δ ~ 0.3, however, models with significantly more mass loss are in agreement with the observations. We conclude by highlighting future observations which may clarify the importance of mass loss in sub-Eddington accretion flows.

We report the ASCA results of the Great Annihilator 1E 1740.7-2942 obtained with five pointing observations in a time span of 3.5 yr. The X-ray spectrum for each period is well fitted with a single power law absorbed by a high column of gas. The X-ray flux changes by a factor of 2 from period to period, but the other spectral parameters show no significant change. The photon index is flat with Γ = 0.9-1.3. The column density of hydrogen N_H is ~ 1.0 × 10²³ H cm^-2 and that of iron N_Fe is ~ 10¹⁹ Fe cm^-2. These large column densities indicate that 1E 1740.7-2942 is near the Galactic center. The column density ratio leads the iron abundance to be 2 times larger than the other elements in a unit of the solar ratio. The equivalent width of the Kα line from a neutral iron is less than 15 eV in 90% confidence. This indicates that the iron column density within several parsecs from 1E 1740.7-2942 is less than 5 × 10¹⁷ Fe cm^-2. In addition, the derived hydrogen column density is about one-sixth of that of giant molecular clouds in the line of sight. All these facts support the fact that 1E 1740.7-2942 is not in a molecular cloud, but possibly in front of it; the X-rays are not powered by accretion from a molecular cloud, but from a companion star like ordinary X-ray binaries.

We present evidence that the integrated profiles of some millisecond pulsars exhibit severe changes that are inconsistent with the moding phenomenon as known from slowly rotating pulsars. We study these profile instabilities in particular for PSR J1022+1001 and show that they occur smoothly, exhibiting longer time constants than those associated with moding. In addition, the profile changes of this pulsar seem to be associated with a relatively narrowband variation of the pulse shape. Only parts of the integrated profile participate in this process, which suggests that the origin of this phenomenon is intrinsic to the pulsar magnetosphere and unrelated to the interstellar medium. A polarization study rules out profile changes due to geometrical effects produced by any sort of precession. However, changes are observed in the circularly polarized radiation component. In total we identify four recycled pulsars that also exhibit instabilities in the total power or polarization profiles due to an unknown phenomenon (PSRs J1022+1001, J1730-2304, B1821-24, and J2145-0750). The consequences for high-precision pulsar timing are discussed in view of the standard assumption that the integrated profiles of millisecond pulsars are stable. As a result we present a new method to determine pulse times of arrival that involves an adjustment of relative component amplitudes of the template profile. Applying this method to PSR J1022+1001, we obtain an improved timing solution with a proper-motion measurement of -17 ± 2 mas yr^-1 in ecliptic longitude. Assuming a distance to the pulsar as inferred from the dispersion measure, this corresponds to a one-dimensional space velocity of 50 km s^-1.

A magnetic field diagnostic of stellar winds that uses the Hanle effect is discussed. This diagnostic pertains to the modification of resonance-line-scattering polarization in the presence of magnetic fields. The case for resolved polarized profiles of optically thin emission lines is considered, and some analytic results for an expanding equatorial disk are derived. Numerical results for a dipole magnetic field embedded in a spherical outflow are also presented. Although the considerations are somewhat simplified, the primary conclusion is that the modification or creation of line profile polarization by the Hanle effect can be used to discriminate between different magnetic field geometries and to estimate the magnetic field strength in the region of line formation.

Classical novae eject significant amounts of nuclear-processed material into the interstellar medium. Among the isotopes synthesized during such explosions, two radioactive nuclei deserve particular attention: ²²Na and ²⁶Al. In this paper, we investigate the nuclear paths leading to ²²Na and ²⁶Al production during nova outbursts by means of an implicit hydrodynamic code that follows the course of the thermonuclear runaway from the onset of accretion up to the ejection stage. New evolutionary sequences of ONe novae have been computed, using updated nuclear reaction rates relevant to ²²Na and ²⁶Al production. Special attention is focused on the role played by nuclear uncertainties within the NeNa and MgAl cycles in the synthesis of such radioactive species. From a series of hydrodynamic models, which assume upper, recommended, or lower estimates of the reaction rates, we derive limits on the production of both ²²Na and ²⁶Al. We outline a list of nuclear reactions that deserve new experimental investigations in order to reduce the wide dispersion introduced by nuclear uncertainties in the ²²Na and ²⁶Al yields.

We extend the Bayesian method of Gregory and Loredo for the detection of a periodic signal of unknown shape and period to the case where the noise sampling distribution is independent Gaussian. The analysis readily handles nonuniformly sampled data and allows for an unknown noise variance. The method is applied to the radio astronomy data for the interesting X-ray binary system LS I +61°303, which exhibits periodic radio outbursts with a period of 26.5 days. Several authors have suggested that the peak flux density of the outbursts exhibit a periodic or quasi-periodic modulation of approximately 1600 days. Our Bayesian analysis of the outburst peak flux densities provides strong support for such a modulation. We derive the posterior probability density function of the modulation period and the estimated mean shape of the modulation based on the available flux density data. The probability density for the modulation period exhibits a broad peak in the range 1599-1660 days (68% credible region) with a mean value of 1632 days. The rms flux density deviation from the mean shape, amounting to 45 mJy, is much larger than the measurement errors of ≈10 mJy, which suggests additional complexity in the source that is yet to be understood. The next maximum in the long-term flux modulation is predicted to occur near 1999 July 22 (JD 2,451,382).

A Bayesian analysis of the phase variations of the 26.5 day periodic radio outbursts from the high-mass X-ray binary LS I +61°303 demonstrates a clear periodic modulation on a timescale similar to that previously found for the long-term modulation of the outburst peak flux density. Combining the outburst phase and flux information we obtain a phase and flux modulation period of 1584⁺¹⁴_-11 days as well as a more accurate outburst period of 26.4917 ± 0.0025 days. From the shape of the phase and outburst flux modulation we find that larger outbursts occur at an earlier orbital phase, closer to periastron, probably as a result of variations in the wind from the rapidly rotating Be star primary. The phase modulation also suggests a rather sudden onset to each new cycle of mass loss by the Be star. The next maximum in long-term flux modulation is predicted to occur between 1999 February and 2000 March (JD 2,451,233-2,451,633).

Using Nobeyama 17 GHz data, we have studied the radio properties of 19 coronal jets identified in Yohkoh soft X-ray imaging telescope (SXT) X-ray observations. The radio data provide information on the physical conditions in the jets, which complements the data from the X-ray surveys. Microwave emission was associated with the majority of the X-ray jets in our sample. The radio emission typically came from the base or the base and lower part of the jets. We detected radio emission from almost all jets that showed flarelike activity at their bases. The jets that were not associated with radio emission did not show any significant increase in X-ray emission at their bases. The strongest radio emission came from two of the largest jets in our sample. Our data show a general correlation between the X-ray jet fluxes and the associated radio fluxes. The 17 GHz time profiles were gradual and unpolarized, implying that the emission was thermal. In a two-sided-loop jet (1992 July 22 event) and one anemone-type jet (1993 February 9 event), the observed microwave fluxes from the lower part of the jets were well above the fluxes calculated from the computed physical parameters of the soft X-ray-emitting material on the basis of thermal free-free emission. We interpret the large discrepancies in terms of the presence of lower temperature material, which cannot be detected by the SXT (the SXT is most sensitive to hot plasma above 2 × 10⁶ K), but which produces strong microwave free-free emission. This is the first time that such material has been observed in two-sided-loop-type jets. We also observed motion of a jet-associated microwave source with a velocity of 55 km s^-1. The microwave motion occurred after the appearance of the X-ray jet. There is clear evidence that the microwave emission of that source was associated with the jet and not with the associated small flare.

We discuss the two-dimensional structure and distribution of magnetic field outside the magnetic cavity in comets. We further compare our one-dimensional results for the distribution along the Sun-comet axis with those of Cravens, Ip, and Axford.

We detect four isolated, X-ray overluminous [L_x > 2 × 10^43-2 ergs s^-1] elliptical galaxies (OLEGs) in our 160 deg²ROSAT PSPC survey. The extent of their X-ray emission, total X-ray luminosity, total mass, and mass of the hot gas in these systems corresponds to poor clusters, and the optical luminosity of the central galaxies (M_R < -22.5 + 5 log h) is comparable to that of cluster cD galaxies. However, there are no detectable fainter galaxy concentrations around the central elliptical galaxy. The estimated mass-to-light ratio within the radius of detectable X-ray emission is in the range of 250-450 M_☉/L_☉, which is 2-3 times higher than typically found in clusters or groups. These objects can be the result of galaxy merging within a group. OLEGs must have been undisturbed for a very long time, which makes them the ultimate examples of systems in hydrostatic equilibrium. The number density of OLEGs is n = 2.4 × 10^-7 (h/0.5)^-3 Mpc^-3 at the 90% confidence level. They comprise 20% of all clusters and groups of comparable X-ray luminosity, and nearly all field galaxies brighter than M_R = -22.5. The estimated contribution of OLEGs to the total mass density in the universe is close to that of T > 7 keV clusters.

With the availability of large-scale redshift survey data, it is now becoming possible to explore correlations between large-scale structure and the properties and morphologies of galaxy clusters. We investigate the spatial distributions of a 98% complete, volume-limited sample of nearby (z < 0.1) Abell clusters with well-determined redshifts and find that cooling flow clusters with high mass accretion rates have nearest neighbors which are much closer than those of other clusters in the sample (at the 99.8% confidence level) and reside in more crowded environments out to 60 h Mpc. Several possible explanations of this effect are discussed.

Hierarchical theories of structure formation predict that clusters of galaxies should be embedded in a weblike structure, with filaments emanating from them to large distances. The amount of mass contained within such filaments near a cluster can be comparable to the collapsed mass of the cluster itself. Diffuse infalling material also contains a large amount of mass. Both of these components can contribute to the cluster weak lensing signal. This "projection bias" is maximized if a filament lies close to the line of sight to a cluster. Using large-scale numerical simulations of structure formation in a Λ-dominated cold dark matter model, we show that the projected mass typically exceeds the actual mass by several tens of percent. This effect is significant for attempts to estimate cluster masses through weak lensing observations and will affect weak lensing surveys aimed at constructing the cluster mass function.

The thermal history of the intergalactic medium (IGM) after reionization is to a large extent determined by photoheating. Here we demonstrate that calculations of the photoheating rate which neglect radiative transfer effects substantially underestimate the energy input during and after reionization. The neglect of radiative transfer effects results in temperatures of the IGM which are too low by a factor of 2 after He II reionization. We briefly discuss implications for the absorption properties of the IGM and the distribution of baryons in shallow potential wells.

I show that the cluster L_X-σ relation should be sensitive to cosmologies with a scale-free power spectrum of initial density fluctuations, P(k) ∝ kⁿ. I derive the dependence and argue that a conservative interpretation of current observations implies n < -2.0 and n < -1.1 at the one-sided 90% and 99% confidence levels, respectively. This result, which agrees with constraints on n from the X-ray cluster temperature function, should be roughly independent of the value of Ω or Λ.

We examine an ensemble of 48 simulated clusters to determine the effects of small-scale density fluctuations and large-scale substructure on X-ray measurements of the intracluster medium (ICM) mass. We measure rms density fluctuations in the ICM that can be characterized by a mean mass-weighted clumping factor C ≡ ⟨ρ²⟩/⟨ρ⟩² between 1.3 and 1.4 within a density contrast of 500. These fluctuations arise from the cluster history of accretion shocks and major mergers, and their presence enhances the cluster's luminosity relative to the smooth case. We expect, therefore, that ICM mass measurements utilizing models that assume uniform density at a given radius carry a bias of order ≈ 1.16. We verify this result by performing ICM mass measurements on X-ray images of the simulations and finding the expected level of bias. The varied cluster morphologies in our ensemble also allow us to investigate the effects of departures from spherical symmetry on our measurements. We find that the presence of large-scale substructure does not further bias the resulting gas mass unless it is pronounced enough to produce a second peak in the image of at least 1% the maximum surface brightness. We analyze the subset of images with no secondary peaks and find a bias of 9% and a Gaussian random error of 4% in the derived mass.

Numerous reports have been made of features, in either emission or absorption, in the 10-1000 keV spectra of some gamma-ray bursts. Originally interpreted in the context of Galactic neutron star models as cyclotron line emission and e⁺-e^- annihilation features, the recent demonstration that the majority of gamma-ray bursts (GRBs) lie at cosmological distances makes these explanations unlikely. In this Letter, we adopt a relativistic fireball model for cosmological GRBs in which dense, metal-rich blobs or filaments of plasma are entrained in the relativistic outflow. In the context of this model, we investigate the conditions under which broadband features, similar to those detected, can be observed. We find a limited region of parameter space capable of reproducing the observed GRB spectra. Finally, we discuss possible constraints additional high-energy spectral observations could place on fireball model parameters.

Likely progenitors for gamma-ray bursts (GRBs) are the mergers of compact objects or the explosions of massive stars. These two cases have distinctive environments for the GRB afterglow: the compact object explosions occur in the interstellar medium (ISM) and those of massive stars occur in the preburst stellar wind. We calculate the expected afterglow for a burst in a Wolf-Rayet star wind and compare the results with those for constant, interstellar density. The optical afterglow for the wind case is generally expected to decline more steeply than in the constant density case, but this effect may be masked by variations in electron spectral index, and the two cases have the same evolution in the cooling regime. Observations of the concurrent radio and optical/X-ray evolution are especially useful for distinguishing between the two cases. The different rates of decline of the optical and X-ray afterglows of GRB 990123 suggest constant density interaction for this case. We have previously found strong evidence for wind interaction in SN 1998bw/GRB 980425 and here present a wind model for GRB 980519. We thus suggest that there are both wind-type GRB afterglows with massive star progenitors and ISM-type afterglows with compact binary star progenitors. The wind-type bursts are likely to be accompanied by a supernova, but not the ISM type.

We present BV photometry of the Sculptor dwarf galaxy to V = 22. These data give evidence for a bimodality in Sculptor's metallicity distribution based on a discontinuity in the luminosities of horizontal-branch (HB) stars and by the presence of two distinct red giant branch (RGB) bumps. A consistent picture of the evolved stars in Sculptor is given by the presence of (1) a metal-poor population of [Fe/H] ~ -2.3 with an exclusively blue HB and that corresponds to the blueward side of the Sculptor RGB and the more luminous RGB bump, and (2) a less metal-poor population of [Fe/H] ~ -1.5 required to explain the less luminous red HB, the red side of the RGB, and a second, less luminous RGB bump. Best fits to the HB populations are obtained with enhanced oxygen abundances, [O/Fe] ~ +0.5. Variations in the global HB and RGB morphology of Sculptor can be explained by differences in the radial distribution of the two different [Fe/H] populations. The presence of these two populations shows that the Sculptor dwarf galaxy has an internal second-parameter problem.

The behavior of quasi-periodic oscillations (QPOs) at frequencies near 1 kHz in the X-ray emission from the neutron star X-ray binary 4U 1820-30 has been interpreted as evidence for the existence of the marginally stable orbit, a key prediction of strong-field general relativity. The signature of the marginally stable orbit is a saturation in QPO frequency (assumed to track inner disk radius) versus mass accretion rate. Previous studies of 4U 1820-30 have used X-ray count rate as an indicator of mass accretion rate. However, X-ray count rate is known to not correlate robustly with mass accretion rate or QPO frequency in other sources. Here, we examine the QPO frequency dependence on two other indicators of mass accretion rate: energy flux and X-ray spectral shape. Using either of these indicators, we find that the QPO frequency saturates at high mass accretion rates. We interpret this as strong evidence for the existence of the marginally stable orbit.

We present high-resolution mid-infrared observations of the nearby late-type young binary system Hen 3-600. The binary, at a distance of ~50 pc, could be a member of the TW Hydrae association, the nearest known group of young stars, with an age of a few million years. Our images make it possible for the first time to determine which star in the pair, separated by 14, harbors the mid-infrared excess detected by IRAS. In the near-infrared, where the radiation is primarily photospheric, Hen 3-600A (M3) and Hen 3-600B (M3.5) have a flux ratio of 1.6. At 4.8, 10.8, and 18.2 μm, the primary becomes increasingly dominant over the secondary, suggesting that most of the circumstellar dust in the system resides around Hen 3-600A. Comparison of the spectral energy distribution (SED) of Hen 3-600A to the median SED of classical T Tauri stars suggests that its disk may be truncated by the secondary and provides tentative evidence for a central disk hole. The distribution of dust in the Hen 3-600 system may provide important clues to the formation and evolution of protoplanetary disks in close binaries.

Spectral energy distributions (SEDs) from 0.4 to 4.7 μm are presented for the two principal stellar components of HD 98800, A and B. The third major component, an extensive planetary debris system (PDS), emits greater than 20% of the luminosity of star B in a blackbody SED at 164 ± 5 K extending from mid-IR to millimeter wavelengths. At 0.95 μm, a preliminary upper limit of less than 0.06 is obtained for the ratio of reflected light to the total from star B. This result limits the albedo of the PDS to less than 0.3. Values are presented for the temperature, luminosity, and radius of each major systemic component. Remarkable similarities are found between the PDS and the interplanetary debris system around the Sun as it could have appeared a few million years after its formation.

The observed giant outbursts of luminous blue variables (LBVs) may occur when these massive stars approach their Eddington limits. When this happens, they must reach a point at which the centrifugal force and the radiative acceleration cancel out gravity at the equator. We call this the Ω limit. When stars are close to the Ω limit, strong nonspherical mass loss should occur. This suggests a scenario in which a slow and very dense wind, strongly confined to the equatorial plane, is followed by a fast and almost spherical wind. We compute two-dimensional hydrodynamic models of the evolution of the nebula formed from such interacting winds, using parameters consistent with the outburst of η Carinae in the last century. This outburst gave birth to the Homunculus, the hourglass-shaped inner part of a highly structured circumstellar nebula. Assuming the star was very close to the Ω limit during outburst, our models produce gas distributions that strongly resemble the Homunculus on large and small scales. This supports the general conjecture that giant outbursts in LBVs occur when they approach the Eddington limit. Our models constrain the average mass-loss rate since the outburst to values smaller than the present-day mass-loss rate and suggest that η Car is approaching another outburst. Our models imply that the occurrence of giant LBV outbursts depends on the initial stellar rotation rate and that the initial angular momentum is as important to the evolution of very massive stars as their initial mass or metallicity.

We report the detection of three mercury-manganese stars in the Orion OB1 association. HD 37886 and BD -0°984 are in the approximately 1.7 million year old Orion OB1b. HD 37492 is in the approximately 4.6 million year old Orion OB1c. Orion OB1b is now the youngest cluster with known HgMn star members. This places an observational upper limit on the timescale needed to produce the chemical peculiarities seen in mercury-manganese stars, which should help in the search for the cause or causes of the peculiar abundances in HgMn and other chemically peculiar upper main-sequence stars.

RW Triangulum (RW Tri) is a 13th magnitude nova-like cataclysmic variable star with an orbital period of 0.2319 days (5.56 hr). Infrared observations of RW Tri indicate that its secondary is most likely a late-K dwarf (Dhillon). Past analyses predicted a distance of 270 pc, derived from a blackbody fit to the spectrum of the central part of the disk (Rutten, van Paradijs, & Tinbergen). Recently completed Hubble Space Telescope Fine Guidance Sensor interferometric observations allow us to determine the first trigonometric parallax to RW Tri. This determination puts the distance of RW Tri at 341, one of the most distant objects with a direct parallax measurement. We compare our result with methods previously employed to estimate distances to cataclysmic variables.

Maps of the whole Sun at 48 GHz made during the period 1991-1993, using the large dish of Itapetinga Radio Observatory with the multibeam system, allowed solar limb determinations with unprecedented precision. From a large sample of maps, the solar radius at 48 GHz is found to vary in time. The observations showed that there is an apparent decrease of the measured radius as the solar cycle declines. We estimated the decrease to be 8'' for half of the solar cycle. The 48 GHz solar radius variations are found to be very well correlated with the solar irradiance.

We identify a new mechanism of internal dissipation of rotational kinetic energy in spinning dust grains, arising from the reorientation of nuclear angular momentum, e.g., spins of protons. Grain rotation induces magnetization of the nuclear spin system, with net alignment of nuclear spins parallel to the grain angular velocity. When the grain does not rotate around a principal axis, the nuclear magnetization vector precesses in grain-body coordinates, resulting in dissipation of energy. The analogous process involving electron spins was discovered by E. M. Purcell and termed "Barnett relaxation." We revisit the physics of the Barnett relaxation process and correct the estimate for the Barnett relaxation rate. We show that nuclear relaxation can be orders of magnitude more important than Barnett relaxation. This finding implies that the processes of "thermal flipping" and "thermal trapping" are important for a broad range of grain sizes.

High-cadence He II λ304 images recorded with the Extreme-ultraviolet Imaging Telescope on the Solar and Heliospheric Observatory show quiescent prominences as organized systems of long-lived jets. The prominence loop arcades consist of chromospheric material that streams continually from one end of each loop to the other; the speeds are typically of order 30 km s^-1 and tend to increase along the length of the loop. We suggest that the jets are triggered by magnetic reconnection during the process of flux submergence at the photospheric neutral line.

We present the infrared (IR) absorption spectrum of the jet-cooled naphthalene-Ar van der Waals cluster cation in the 400-1600 cm^-1 range, which is expected to resemble closely the IR spectrum of the bare naphthalene cation. Cluster cations are produced in their ground states via resonance-enhanced two-color ionization. Subsequent irradiation of the ions with IR light of a free-electron laser causes cluster dissociation, if this light is resonant with IR-active transitions of the cationic cluster. Mass-selective measurement of the dissociated fraction as a function of IR wavelength yields the IR spectrum of the cationic cluster. Comparison with previously reported Ar matrix isolation spectra of naphthalene⁺ shows good agreement, with more bands observed in the present study. The IR spectrum observed is compared to a calculated spectrum of the naphthalene cation, allowing for unambiguous assignments of all the observed modes. The experimental method presented is generally applicable to obtain laboratory IR spectra of ionized polyaromatic hydrocarbons under astrophysically relevant conditions.