Table of contents

The radial component of the peculiar velocities of galaxies causes displacements in the galaxies' positions in redshift space. We study the effect of the peculiar velocities on the linear redshift space two-point correlation function. Our analysis takes into account the radial nature of the redshift space distortions, and it highlights the limitations of the plane-parallel approximation. We consider the problem of determining the value of β and the real space two-point correlation function from the linear redshift space two-point correlation function. The inversion method proposed here takes into account the radial nature of the redshift space distortions and can be applied to magnitude-limited redshift surveys that have only partial sky coverage.

We propose a method for recovering the shape of the mass power spectrum on large scales from the transmission fluctuations of the Lyα forest that directly takes into account redshift-space distortions. The procedure, in discretized form, involves the inversion of a triangular matrix that projects the mass power spectrum in three-dimensional real space to the transmission power spectrum in one-dimensional redshift space. We illustrate the method by performing a linear calculation relating the two. A method that does not take into account redshift-space anisotropy tends to underestimate the steepness of the mass power spectrum in the case of linear distortions. The issue of the effective bias factor for the linear distortion kernel is discussed.

The rms fluctuation (variance) σ of a cosmic field α(x) is an important measure to quantify the initial fluctuation of the universe and is usually determined by the formula σ²=⟨α(x)²⟩. We investigate the necessity of using this specific formula, under the assumption that the initial fluctuation is random-Gaussian-distributed. We calculate the expected finite-volume effect on σ obtained from a general formula ⟨|α|^m⟩. We find that although the finite-volume effect is minimal at the conventional choice m=2, it is almost insensitive to m around m=1 ~3. Therefore we can reduce the relative contribution of tail parts, which might be considerably contaminated by other effects (such as measurement errors), at a very small sacrifice of the finite-volume effect.

We present analysis of the evolution of dark matter halos in dense environments of groups and clusters in dissipationless cosmological simulations. The premature destruction of halos in such environments, known as "the overmerging," reduces the predictive power of N-body simulations and makes difficult any comparison between models and observations. We analyze the possible processes that cause the overmerging and assess the extent to which this problem can be cured with current computer resources and codes. Using both analytic estimates and high-resolution numerical simulations, we argue that the overmerging is mainly due to the lack of numerical resolution. We find that the force and mass resolution required for a simulated halo to survive in galaxy groups and clusters is extremely high and was almost never reached before: ~1-3 kpc and 10⁸-10⁹M_☉, respectively. We use the high-resolution Adaptive Refinement Tree (ART) N-body code to run cosmological simulations with particle mass ≈2 × 10⁸h^-1M_☉ and spatial resolution ≈1-2 h^-1 kpc and show that in these simulations the halos do survive in regions that would appear overmerged with lower force resolution. Nevertheless, the halo identification in very dense environments remains a challenge even with resolution this high. We present two new halo-finding algorithms developed to identify both isolated and satellite halos that are stable (existed at previous moments) and gravitationally bound. To illustrate the use of the satellite halos that survive the overmerging, we present a series of halo statistics, which can be compared with those of observed galaxies. Particularly, we find that, on average, halos in groups have the same velocity dispersion as the dark matter particles; i.e., they do not exhibit significant velocity bias. The small-scale (100 kpc to 1 Mpc) halo correlation function in both models is well described by the power law ξ ∝ r^-1.7 and is in good agreement with observations. It is slightly antibiased (b≈0.7-0.9) relative to the dark matter. To test other galaxy statistics, we use the maximum of the halo rotation velocity and the Tully-Fisher relation to assign luminosity to the halos. For two cosmological models, a flat model with the cosmological constant and Ω₀=1-Ω_Λ=0.3,h=0.7 and a model with a mixture of cold and hot dark matter and Ω₀=1.0,Ω_ν=0.2,h=0.5, we construct luminosity functions and evaluate mass-to-light ratios in groups. Both models produce luminosity functions and mass-to-light ratios ( ~200-400) that are in reasonable agreement with observations. The latter implies that the mass-to-light ratio in galaxy groups (at least for M_vir ≲ 3 × 10¹³h^-1M_☉ analyzed here) is not a good indicator of Ω₀.

The Canadian Network for Observational Cosmology cluster data are used to constrain the Ω_M-Ω_Λ pair to the region Ω_M ≃ 0.24e^{± 0.3}(1-0.4Ω_Λ) for 0≤Ω_Λ ≤ 1. The constraint is based on estimating the apparent mass density of the universe, Ω_e(z), as the product of cluster mass-to-light ratios, M/L, with the field luminosity density at the same redshift. The luminosity density contains a volume element, which for measurements at z > 0 causes Ω_e(z) to depend on both the density parameter Ω_M and the cosmological constant, Ω_Λ. The Ω_Λ-dependence of the Ω_e(z) measurement is about 25% less than the volume-redshift relation but about 50% greater than the luminosity-redshift relation. Most usefully this constraint is approximately orthogonal to the luminosity-redshift relation in the Ω_M-Ω_Λ plane. The practical application to measuring cosmological parameters has the considerable benefit that all quantities are used in a differential sense, so that common selection effects and galaxy evolution effects will cancel. The residual differential galaxy evolution between field, and the clustered galaxies can be estimated from the sample data. The inferred Ω_M has an inverse correlation with Ω_Λ, giving a constraint complementary to both the cosmic microwave background and the supernovae distances. Monte Carlo simulations, calibrated with observational data, show that 100 clusters spread over the 0-1 redshift range, each having M/L values of about 25% accuracy, will measure Ω_Λ to about 7% statistical error.

Recent optical afterglow observations of gamma-ray bursts (GRBs) indicate a setting and distance scale that many relate to star formation regions. In this paper, we use ⟨V/V_max⟩ and a set of artificial trigger thresholds to probe several potential GRB source density evolutionary scenarios. In particular, we compare a uniform subset of BATSE 4B data to cosmological scenarios where GRBs evolve as the comoving density, the star formation rate, the QSO rate, and the SN Type Ic rate. Standard candle bursts with power-law spectra and a universe without vacuum energy were assumed. Our results significantly favor a comoving density model, which implies that GRB source density evolution is weaker than expected in these evolutionary scenarios. GRB density might still follow star formation rates given proper concurrent GRB luminosity evolution, significant beaming, significant error in standard candle assumptions, or if a significant modification of star formation rate estimates were to occur.

Studies of the distribution and evolution of galaxies are of fundamental importance to modern cosmology; these studies, however, are hampered by the complexity of the competing effects of spectral and density evolution. Constructing a spectroscopic sample that is able to unambiguously disentangle these processes is currently excessively prohibitive due to the observational requirements. This paper extends and applies an alternative approach that relies on statistical estimates for both distance (z) and spectral type to a deep multiband data set that was obtained for this exact purpose. These statistical estimates are extracted directly from the photometric data by capitalizing on the inherent relationships between flux, redshift, and spectral type. These relationships are encapsulated in the empirical photometric-redshift relation that we extend to z≈1.2, with an intrinsic dispersion of δz~0.06. We also develop realistic estimates for the photometric-redshift error for individual objects and introduce the use of the galaxy ensemble as a tool for quantifying both a cosmological parameter and its measured error. We present deep, multiband, optical number counts as a demonstration of the integrity of our sample. Using the photometric redshift and the corresponding redshift error, we can divide our data into different redshift intervals and spectral types. As an example application, we present the number-redshift distribution as a function of spectral type.

This paper reports the X-ray spectral analysis of five high-redshift (z≥2), radio-quiet quasars observed by ASCA. A simple power-law continuum plus cold Galactic absorption model well fits all the spectra (typically between ~2 and 30 keV in the sources frame). Neither the X-ray spectral hardening, which is attributed to a reflection component and observed in Seyfert galaxies, nor the excess absorption previously detected in high-redshift, radio-loud quasars have been revealed. Only marginal evidence of a neutral or mildly ionized FeKα line is found in one of the quasars. The average spectral slope in the observed 0.7-10 keV energy range, ⟨Γ⟩=1.67 ± 0.11 (dispersion σ ~0.07), appears to be flatter than that of low-z, radio-quiet quasars (Γ ≃ 1.9-2) and slightly steeper but consistent with Γ=1.61 ± 0.04 (σ ~0.10) of high-z, radio-loud quasars.

We investigate effects of time evolution of a rich cluster of galaxies on its member galactic halos in the standard cold dark matter (SCDM) universe using high resolution N-body simulations. We identify several hundred galactic halos within the virial radius of our simulated cluster. We also find that a large number of halos have been tidally disrupted at z=0. Therefore we improve a method of deriving merging history trees of galaxies taking account of tidally stripped galaxies. The main results are as follows: (1) At high redshift (z≃2), the mass function of the galactic halos that are in the cluster at z=0 is very similar to that obtained in the field region and agrees well with the Press-Schechter mass function. (2) The mass function of cluster galaxies that consist of both galactic halos and tidally stripped galaxies has hardly evolved since z≃2. This mass function at z=0 is well represented by the Press-Schechter mass function at z=2. (3) At high redshift (z>3), in the region that becomes the cluster, the fraction of galaxies that have undergone recent merger is larger than that in the field. After z~3, however, it rapidly decreases and becomes smaller than that in the field. (4) The fraction of strongly stripped galaxies among the cluster galaxies begins to increase from z≃0.5. At z=0, a clear correlation appears between this fraction and the distance from the center of the cluster. (5) Tidally truncated halos have steeper outer profiles than those of the model of Navarro, Frenk, & White.

We present ASCA X-ray observations of a sample of five groups selected from a cross-correlation of the ROSAT All-Sky Survey with the White et al. optical catalog of groups. These X-ray-bright groups significantly increase the number of known systems with temperatures between 2 and 3 keV. They have element abundances of roughly 0.3☉, which are typical of clusters, but their favored ratio of Si-to-Fe abundance is lower than the cluster value. Combining the ASCA results with ROSAT imaging data, we calculate total masses of a few to several times 10¹³M_☉, gas mass fractions of ~10%, and baryonic mass fractions of at least 15%-20% within a radius of 0.5 Mpc. Upper limits for the ratios of gas to galaxy mass and of the iron mass to galaxy luminosity overlap with the range observed in rich clusters and extend to lower values, but not to such low values as seen in much poorer groups. These results support the idea that groups, unlike clusters, are subject to the loss of their primordial and processed gas and show that this transition occurs at the mass scale of the 2-3 keV groups. A discussion of ASCA calibration issues and a comparison of ROSAT and ASCA temperatures are included in an Appendix.

Using a simple model of molecular cloud evolution, we have quantitatively estimated the change of star formation rate (SFR) of a disk galaxy falling radially into the potential well of a cluster of galaxies. The SFR is affected by the ram pressure from the intracluster medium (ICM). As the galaxy approaches the cluster center, the SFR increases to twice the initial value, at most, in a cluster with high gas density and a deep potential well, or with a central pressure of ~10^-2 cm^-3 keV, because the ram pressure compresses the molecular gas of the galaxy. However, this increase does not affect the color of the galaxy significantly. Further into the central region of the cluster (≲1 Mpc from the center), the SFR of the disk component drops rapidly owing to the effect of ram-pressure stripping. This makes the color of the galaxy redder and makes the disk dark. These effects may explain the observed color, morphology distribution, and evolution of galaxies in high-redshift clusters. By contrast, in a cluster with low gas density and a shallow potential well, or with the central pressure of ~10^-3 cm^-3 keV, the SFR of a radially infalling galaxy changes less significantly, because neither ram-pressure compression nor stripping is effective. Therefore, the color of galaxies in poor clusters is as blue as that of field galaxies if other environmental effects such as galaxy-galaxy interaction are not effective. The predictions of the model are compared with observations.

We report on the discovery and properties of Cepheid variable stars in the barred spiral galaxy NGC 4548, which is a member of the Virgo cluster of galaxies. This is one of the galaxies being observed as part of the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale, which aims to determine the Hubble constant to 10% accuracy. Our analysis is based on observations made with the Wide Field and Planetary Camera 2 during 1996 and 1997. We identify 24 probable Cepheids with periods between 16 and 38 days. They were observed over 13 epochs with the F555W filter and eight epochs with the F814W filter. The HST F555W and F814W data have been transformed to the Johnson V and Cousins I magnitude systems, respectively. Photometry has principally been carried out using the DAOPHOT/ALLFRAME package. A comparison is made with parallel measurements using the DoPHOT package. Apparent period-luminosity relations for V and I have been constructed. Assuming values of μ=18.50±0.10 mag and E(B-V)=0.10 mag for the distance modulus and reddening of the Large Magellanic Cloud, a true distance modulus of 31.01±0.28 mag is derived corresponding to a distance of 15.9±2.0 Mpc. HST Cepheid distances of other spiral galaxies in the Virgo Cluster are discussed. Their good agreement with the new NGC 4548 distance strengthens the evidence that this galaxy lies within the Virgo cluster core.

Using PSPC ROSAT data, we measure the X-ray surface brightness profiles, size, and luminosity of the Butcher-Oemler (BO) sample of clusters of galaxies. The cluster X-ray size, as measured by the Petrosian r_{η = 2} radius, does not change with redshift and is independent of X-ray luminosity. On the other hand, the X-ray luminosity increases with redshift. Considering that fair samples show no evolution, or negative luminosity evolution, we conclude that the BO sample is not formed from the same class of objects observed at different look-back times. This is in conflict with the usual interpretation of the Butcher-Oemler as an evolutionary (or redshift dependent) effect, based on the assumption that we are comparing the same class of objects at different redshifts. Other trends present in the BO sample reflect selection criteria rather than differences in look-back time, as independently confirmed by the fact that trends lose strength when we enlarge the sample with an X-ray-selected sample of clusters. The variety of optical sizes and shapes of the clusters in the Butcher-Oemler sample and the Malmquist-like bias are the reasons for these selection effects that mimic the trends usually interpreted as changes due to evolution.

We have compared the morphological characteristics of the 891 galaxies in the Extended 12 μm Galaxy Sample (E12GS) and assessed the effect of the 12 μm selection criterion on galaxy properties. The normal spirals in the E12GS have the same axial ratios, morphological types, and bar and ring fractions as other normal spirals. The H II/starburst galaxies have a higher incidence of bars and more than twice the normal rate of "peculiar" morphologies, both of which are attributable to relatively recent disturbances. The 12 μm Seyfert galaxies show a small (10%) deficiency of edge-on disks. This is caused by extinction but is a much less severe effect than in optically selected samples. There is a similar modest deficit of highly inclined H II/starburst galaxies in the 12 μm sample. The galaxies with active nuclei (Seyfert galaxies and LINERs) have the same incidence of bars as normal spirals but show rings significantly more often than normal galaxies or starbursts. The LINERs have elevated rates of inner rings, while the Seyfert galaxies have outer ring fractions several times those in normal galaxies. The different formation times of bars and rings suggest an interpretation of these differences. Bars form relatively quickly and indicate that material is recently being transported (by redistribution of angular momentum) to the center of the galaxy, where it is likely to trigger a short (e.g., ≲10⁸ yr) burst of star formation. Outer rings may result from similar disturbances but require much more time to form. They would then be associated with more intense nuclear activity if it takes 10⁹ yr or more for the mass transfer to reach the center and raise the black hole accretion rate, by which time the bar has dissolved or has begun to do so. Inner rings form before outer ones, with a formation time more comparable to bars. Thus, it may be that after an interaction or instability triggers an infall of gas, the galaxy in the earliest stage is likely to show enhanced star formation in its center, while later it is more likely to show LINER activity, and still later it is likely to be a Seyfert galaxy. The trends we find with morphology and nuclear activity are not biased either by the distances of the galaxies or by the slightly elevated recent star formation rates shown by the 12 μm galaxies in general.

As a step toward elucidating the physical conditions in nearby active galaxies, this paper presents spectral energy distributions (SEDs) of a sample of seven low-luminosity active galactic nuclei (AGNs). SEDs for four objects are presented for the first time (NGC 4261, NGC 4579, NGC 6251, and M84); the data for the remaining three (M81, M87, and NGC 4594) have been substantially updated compared to previous studies. The nuclear fluxes were carefully selected so as to avoid contamination by emission from the host galaxy, which can be substantial for very weak nuclei. The present sample of low-luminosity nuclei exhibits SEDs that look markedly different from the canonical broadband continuum spectrum of luminous AGNs. The most striking difference is that the low-luminosity objects lack an ultraviolet excess (the "big blue bump"), a feature normally associated with emission from a standard optically thick, geometrically thin accretion disk. The weakness of the ultraviolet band leads to an unusually steep optical-ultraviolet continuum shape and a more pronounced contribution from the X-rays to the ionizing spectrum. It is argued that the absence of the big blue bump is a property intrinsic to the SEDs and not an artifact of strong dust extinction. Another notable property of the SEDs is the prominence of the compact, flat-spectrum radio component relative to the emission in other energy bands. All seven nuclei in the sample, including three hosted by spiral galaxies, technically qualify as "radio-loud" objects according to conventional criteria. Finally, the integrated spectra confirm the exceptional weakness of the nuclei: the bolometric luminosities range from 2 × 10⁴¹ to 8 × 10⁴² ergs s^-1, or ~10^-6-10^-3 times the Eddington rate for the black hole masses previously reported for these galaxies.

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made with the Hubble Space Telescope (HST) and the Keck I telescope. The gamma-ray burst (GRB) was observed approximately 6 months after outburst, on 1997 September 4, using the HST/STIS CCD, and approximately 1 year after outburst, on 1998 February 24, using HST/NICMOS, and on 1998 April 4 using the NIRC on Keck. The unresolved counterpart is detected by STIS at V=28.0 ± 0.25, consistent with a continued power-law decline with exponent -1.10 ± 0.05. The counterpart is located within, but near the edge of, a faint extended source with diameter ~08 and integrated magnitude V=25.8 ± 0.25. A reanalysis of HST and New Technology Telescope observations performed shortly after the burst shows no evidence of proper motion of the point source or fading of the extended emission. Although the optical transient is not detected in the NICMOS images (H≥25.3), the extended source is visible and has a total magnitude H=23.3 ± 0.1. The Keck observations find K=22.8 ± 0.3. Comparison with observations obtained shortly after outburst suggests that the nebular luminosity has also been stable in the infrared. We find that several distinct and independent means of deriving the foreground extinction in the direction of GRB 970228 all agree with A_V=0.75 ± 0.2. After adjusting for this Galactic extinction, we find that the size of the observed extended emission is consistent with that of galaxies of comparable magnitude found in the Hubble Deep Field (HDF) and other deep HST images. Only 2% of the sky is covered by galaxies of similar or greater surface brightness. We therefore conclude that the extended source observed about GRB 970228 is almost certainly its host galaxy. Additionally, we find that independent of assumed redshift, the host is significantly bluer than typical nearby blue dwarf irregulars. With the caveat that the presently available infrared observations of the HDF are only fully complete to a limit about one-half magnitude brighter than the host, we find that the extinction-corrected V-H and V-K colors of the host are as blue as any galaxy of comparable or brighter magnitude in the HDF. Taken in concert with recent observations of GRB 970508, GRB 971214, and GRB 980703 our work suggests that all four GRBs with spectroscopic identification or deep multicolor broadband imaging of the host lie in rapidly star-forming galaxies.

We report on the mass-to-light ratio determination based on a newly selected binary galaxy sample, which includes a large number of pairs whose separations exceed a few hundred kpc. The probability distributions of the projected separation and the velocity difference have been calculated considering the contamination of optical pairs, and the mass-to-light (M/L) ratio has been determined based on the maximum likelihood method. The best estimate of the M/L in the B band for 57 pairs is found to be 28-36 depending on the orbital parameters and the distribution of optical pairs (solar unit: H₀=50 km s^-1 Mpc^-1). The best estimate of the M/L for 30 pure spiral pairs is found to be 12-16. These results are relatively smaller than those obtained in previous studies but are consistent with each other within the errors. Although the number of pairs with large separation is significantly increased compared with previous samples, the M/L does not show any tendency of increase but is found to be almost independent of the separation of pairs beyond 100 kpc. The constancy of the M/L beyond 100 kpc may indicate that the typical halo size of spiral galaxies is less than ~100 kpc.

We present high spectral resolution (λ/δλ≅10³) Paβ(1.28 μm) and Brγ(2.17 μm), 3.29 μm dust feature, and near-infrared broadband images of the Arp 299=NGC 3690/IC 694 galaxy system. The emission is found to be concentrated at the positions of three active regions, known as sources A, B, and C. From our Brγ/Paβ flux ratio we find the visual extinction toward the ionized gas in A, B, and C is ~6, 6, and 2 magnitudes, respectively, assuming case B recombination and a foreground screen geometry for the obscuring material. Our observations can be explained entirely by a starburst model for the Arp 299 system. The putative active galactic nucleus (AGN) in source A does not dominate the properties of this source. We see no evidence of broad recombination lines. In addition, the ratio of the 3.29 μm dust feature to total luminosity, a tracer of starburst activity, is consistent in each source with that seen in M82 and other starburst galaxies. Also, our imaging observations reveal that the dust feature emission is concentrated in the nucleus of source A, contrary to the extended annular distribution of the feature emission seen surrounding the nucleus of the more distant Seyfert galaxy NGC 7469. In this galaxy, the absence of the feature emission in the nucleus has been attributed to the destruction of the dust carriers in the hard radiation field surrounding the AGN. Our observations suggest that all of the active regions in Arp 299 are characterized by starburst episodes. The observed CO indices and Brγ equivalent widths imply that source B is older than source A and source C is the youngest star-forming region. Although these regions are probably complex physical systems not necessarily characterized by a single coeval population of stars, we have compared our observations with an instantaneous starburst model with a Salpeter initial mass function. Using this simple model, we find starburst ages of ~6 × 10⁶, 8 × 10⁶, and 4 × 10⁶ for sources A, B, and C, respectively.

We add 20, 6, and 3.6 cm VLA observations of two wide-angle tails (WATs), 1231+674 and 1433+553, to existing VLA data at 6 and 20 cm in order to study the variations of spectral index as a function of position. We apply the spectral tomography process that we introduced in our analysis of 3C 67, 3C 190, and 3C 449 (Katz-Stone; Katz-Stone & Rudnick). Both spectral tomography and polarization maps indicate that there are two distinct extended components in each source. As in the case of 3C 449, we find that each source has a flat spectrum jet surrounded by a steeper spectrum sheath. The steep components tend to be more highly polarized than the flat components. We discuss a number of possibilities for the dynamics of the jet/sheath systems and the evolution of their relativistic electron populations. Although the exact nature of these two coaxial components is still uncertain, their existence requires new models of jets in FR I sources and may also have implications for the dichotomy between FR I and FR II galaxies.

We compare results from a relativistic and a nonrelativistic set of two-dimensional axisymmetric jet simulations. For a set of five relativistic simulations that either increase the Lorentz factor or decrease the adiabatic index, we compute nonrelativistic simulations with equally useful power or thrust. We examine these simulations for morphological and dynamical differences, focusing on the velocity field, the width of the cocoon, the age of the jets, and the internal structure of the jet itself. The primary result of these comparisons is that the velocity field of nonrelativistic jet simulations cannot be scaled up to give the spatial distribution of Lorentz factors seen in relativistic simulations. Since the local Lorentz factor plays a major role in determining the total intensity for parsec-scale extragalactic jets, this suggests that a nonrelativistic simulation cannot yield the proper intensity distribution for a relativistic jet. Another general result is that each relativistic jet and its nonrelativistic equivalents have similar ages (in dynamical time units, ≡R/a_a, where R is the initial radius of a cylindrical jet and a_a is the sound speed in the ambient medium). Also, jets with a larger Lorentz factor have a smaller cocoon size. In addition to these comparisons, we have completed four new relativistic simulations to investigate the effect of varying thermal pressure on relativistic jets. The simulations confirm that faster (larger Lorentz factor) and colder jets are more stable, with smaller amplitude and longer wavelength internal variations. However, an exception to this occurs for the hottest jets, which appear the most stable. The apparent stability of these jets does not follow from linear normal mode analysis, which suggests that there are available growing Kelvin-Helmholtz modes. However, these modes are not excited because of a lack of perturbations able to couple to them. As an example of how these simulations can be applied to the interpretation of observations, we use our results to estimate some parameters of Cygnus A. Although none of these estimates alone can determine if the jets in Cyg A are relativistic or nonrelativistic, estimates for the age and the jet to ambient density ratio confirm values for these parameters estimated by other means.

We present a search for high-energy gamma-ray emission from nine nearby starburst galaxies and M31 with the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory (CGRO). Although the diffuse gamma-ray emission from starburst galaxies was suspected to be detectable, we find no emission from NGC 253, from M82, or from the average of all nine galaxies. The 2 σ upper limit for the EGRET flux above 100 MeV for the averaged survey observations is 1.8 × 10^-8 photons cm^-2 s^-1. From a model of the expected radio and gamma-ray emission, we find that the magnetic field in the nuclei of these galaxies is greater than 25 μG, and the ratio of proton and electron densities is less than 400. The EGRET limits indicate that the rate of massive star formation in the survey galaxies is only about an order of magnitude higher than in the Milky Way. The upper limit to the gamma-ray flux above 100 MeV for M31 is 1.6 × 10^-8 photons cm^-2 s^-1. At the distance of M31, the Milky Way flux would be over twice this value, indicating higher gamma-ray emissivities in our Galaxy. Therefore, since the supernova rate of the Milky Way is higher than that in M31, our null detection of M31 supports the theory of the supernova origin of cosmic rays in galaxies.

We present a study of the intrinsic absorption lines in the ultraviolet spectra of Seyfert 1 galaxies. The study is based on spectra from the Hubble Space Telescope and includes the Seyfert 1 galaxies observed with the Faint Object Spectrograph and Goddard High-Resolution Spectrograph at spectral resolutions of λ/Δλ ≈1000-20,000 with good signal-to-noise ratios. We find that the fraction of Seyfert 1 galaxies that show intrinsic absorption associated with their active nuclei is more than one-half (10 of 17), which is much higher than previous estimates (3%-10%) based on IUE data. There is a one-to-one correspondence between Seyfert galaxies that show intrinsic UV absorption and X-ray "warm absorbers," indicating that these two phenomena are related. Although our sample is not complete, we conclude that intrinsic absorption represents an important component that needs to be integrated into our overall physical picture of active galaxies. The intrinsic UV absorption is generally characterized by high ionization: C IV and N V are seen in all 10 Seyfert galaxies with detected absorption (in addition to Lyα), whereas Si IV is present in only four of these Seyfert galaxies, and Mg II absorption is detected only in NGC 4151. The absorption lines are blueshifted (or in a few cases at rest) with respect to the narrow emission lines, indicating that the absorbing gas is undergoing net radial outflow. At high resolution, the absorption often splits into distinct kinematic components that show a wide range in widths (20-400 km s^-1 FWHM), indicating macroscopic motions (e.g., radial velocity subcomponents or turbulence) within a component. The strong absorption components have cores that are much deeper than the continuum flux levels, indicating that the regions responsible for these components lie completely outside of the broad emission-line regions. Additional information on the covering factors and column densities can be derived from the absorption profiles in the high-resolution spectra. The covering factor of the absorbing gas in the line of sight, relative to the total underlying emission, is C_los ≥ 0.86, on average. The global covering factor, which is the fraction of emission intercepted by the absorber averaged over all lines of sight, is C_global ≥ 0.5. Thus, structures covering large solid angles as seen by the central continuum source (e.g., spherical shells, sheets, or cones with large opening angles) are required. The individual absorptioncomponents show a wide range in C IV column densities (0.1-14 × 10¹⁴ cm^-2), and the ratio of N V to C IV column density varies significantly from one absorption component to the next, even in the same Seyfert galaxy. Thus, the intrinsic absorption in a Seyfert 1 galaxy is typically comprised of distinct kinematic components that are characterized by a range in physical conditions (e.g., ionization parameter and hydrogen column density). Finally, we show evidence for extreme variability in the intrinsic absorption lines of NGC 3783. In addition to our earlier report of the appearance of a C IV absorption doublet at -560 km s^-1 (relative to the emission lines) over 11 months, we have detected the appearance of another C IV doublet at -1420 km s^-1 over 15 months. On the other hand, the C IV absorption lines of NGC 3516 and NGC 4151 were very stable over periods of 6 months and 4 years, respectively. Monitoring observations of individual Seyfert galaxies at higher time resolution are needed to distinguish between different sources of variability (variable ionization, motion of gas across the line of sight) and to determine the densities and radial locations of the absorption components.

The location of the gas responsible for the absorption-line system toward the nucleus of Centaurus A is a puzzle. It is generally accepted that the line features close to the systemic velocity originate in the disk. The redshifted line features in particular, however, are usually thought to be due to gas close to the nucleus or even falling toward it. We present new ¹²CO (1-0), HCO⁺ (1-0) and HCN (1-0) absorption-line measurements, as well as an alternative interpretation of the line system. Previous papers have demonstrated that the distribution of line emission of the inner molecular and outer atomic hydrogen disk can be accounted for by a system of tilted rings with varying inclination. Using the same model, but assuming that corotating absorbing gas is located at high altitudes above the disk, one can account for all major features of the absorption-line system. In this model, the absorption takes place in high-altitude clouds that are up to about 160 pc above the molecular disk of Centaurus A at radii between 1.7 and 1.9 kpc, accounting for the two strongest central line features. In our model, the redshifted line features are due to gas associated with disk material up to about 300 pc above the disk at radii of 0.4-0.6 kpc orbiting in the non-spherically symmetric potential of the Centaurus A galaxy. In this model, the systemic velocity is at 546 km s^-1, which is the velocity of the sharp spectral feature about 6 km s^-1 to the blue of the deepest absorption line. Our new model provides a natural explanation for the general structure of the complex absorption-line system, based on a tilted-ring model that already explains the disk line emission. In this model, no significant absorbing gas component closer than 200 pc to the nonthermal radio continuum nucleus is required, and the peculiar velocity structure of the absorption-line system is due to the kinematics of the molecular gas disk.

The metal-deficient (Z=Z_☉/41) blue compact dwarf galaxy SBS 0335-052 was observed with ISOCAM between 5 and 17 μm. With an L_12μm/L_B ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared for such a low-metallicity object. The mid-infrared spectrum shows no sign of the unidentified infrared bands, which we interpret as an effect of the destruction of their carriers by the very high UV energy density in SBS 0335-052. The spectral energy distribution (SED) is dominated by a very strong continuum, which makes the ionic lines of [S IV] and [Ne III] very weak. From 5 to 17 μm, the SED can be fitted with a graybody spectrum, modified by an extinction law similar to that observed toward the Galactic center, with an optical depth of A_V ~19-21 mag. Such a large optical depth implies that a large fraction (as much as ~75%) of the current star formation activity in SBS 0335-052 is hidden by dust with a mass between 3 × 10³ and 5 × 10⁵M_☉. Silicate grains that are present as silicate extinction bands at 9.7 and 18 μm can account for the unusual shape of the MIR spectrum of SBS 0335-052. It is remarkable that such a nearly primordial environment contains as much dust as galaxies that are 10 times more metal-rich. If the hidden star formation in SBS 0335-052 is typical of young galaxies at high redshifts, then the cosmic star formation rate derived from UV/optical fluxes would be underestimated.

Recently a Type Ic supernova, SN 1998bw, was discovered coincident with a gamma-ray burst, GRB 980425. The supernova had unusual radio, optical, and spectroscopic properties. Among other things, it was especially bright for a Type Ic both optically and in the radio, and it rose quickly to maximum. We explore here models based upon helium stars in the range 9-14 M_☉ and carbon-oxygen stars 6-11 M_☉, which experience unusually energetic explosions (kinetic energy 0.5-2.8 × 10⁵² ergs). Bolometric light curves and multiband photometry are calculated and compared favorably with observations. No spectroscopic data are available at this time, but both LTE and non-LTE spectra are calculated for the model that agrees best with the light curve, a carbon-oxygen core of 6 M_☉ exploded with a kinetic energy of 2.2 × 10⁵² ergs. We also examine potential mechanisms for producing the observed gamma-ray burst (GRB)—shock breakout and relativistic shock deceleration in circumstellar material. For spherically symmetric models, both fail to produce a GRB of even the low luminosity inferred for GRB 980425. However, the high explosion energies required to understand the supernova are in contrast to what is expected for such massive stars and indicate that a new sort of explosion may have been identified, possibly the consequence of a collapsar. Indeed a more likely explanation for what was seen is a highly asymmetric explosion in which the GRB was produced by mildly relativistic matter (Γ≈5) running into circumstellar matter along the line of sight to the Earth. The explosion itself was powered by black hole accretion and jets, but unlike "ordinary" gamma-ray bursts, the jets were not of sufficient energy and duration to effectively reach large values of Γ. They may also not have been oriented in our direction. The ejected mass (but not the ⁵⁶Ni mass) and explosion energy are then smaller. Other associations between luminous Type Ic supernovae and GRBs may exist and should be sought, but most Type Ib and Type Ic supernovae do not make GRBs.

Because of the roughly linear correlation between Be/H and Fe/H in low-metallicity halo stars, it has been argued that a "primary" component in the nucleosynthesis of Be must be present in addition to the "secondary" component from standard Galactic cosmic-ray nucleosynthesis. In this paper we critically reevaluate the evidence for the primary versus secondary character of Li, Be, and B (LiBeB) evolution, analyzing both the observations and Galactic chemical evolution models. Although it appears that [Be/H] versus [Fe/H] has a logarithmic slope near 1, it is rather the Be-O trend that directly arises from the physics of spallation production. Using new abundances for oxygen in halo stars based on UV OH lines, we find that in Population II stars for which O has been measured, the Be-O slope has a large uncertainty due to systematic effects. Namely, the Be-O logarithmic slope lies in the range 1.3-1.8, rendering it difficult to distinguish from the data between the secondary slope of 2 and the primary slope of 1. The possible difference between the Be-Fe and Be-O slopes is a consequence of the variation in O/Fe versus Fe: recent data suggest that the best-fit O/Fe-Fe slope for Population II is in the range -0.5 to -0.2, rather than zero (i.e., Fe∝O) as is often assumed. In addition to this phenomenological analysis of Be and B evolution, we have also examined the predicted LiBeB, O, and Fe trends in Galactic chemical evolution models that include outflow. Based on our results, it is possible that a good fit to the LiBeB evolution requires only the traditional Galactic cosmic-ray spallation and the (primary) neutrino-process contribution to ¹¹B. We thus suggest that these two processes might be sufficient to explain ⁶Li, Be, and B evolution in the Galaxy, without the need for an additional primary source of Be and B. However, the uncertainties in the data at this time prevent one from reaching a definitive conclusion. Fortunately, several observational tests of this "neoclassical" scenario are available; we note in particular the importance of further observations to secure the O/Fe Population II trend, as well as accurate measurements of B/Be, ⁶Li/Be, and ¹¹B/¹⁰B in halo stars.

We present the analysis of archival data from the Advanced Satellite for Cosmology and Astrophysics of the supernova remnant (SNR) G39.2-0.3. G39.2-0.3 has been sometimes characterized as a shell-like remnant in the literature, but our high-energy imaging and spectral analysis show the unambiguous composite nature of the remnant. We find that part of the contribution to the X-ray emission of G39.2-0.3 is distinctly nonthermal, best described by a power law with a photon index (2.53). The region of emission is consistent with a point source (extension consistent with the point-spread function of the detector at the off-axis angle of the observation) and is roughly defined by a circle of radius ~4'. A second contribution comes from a thermal component, which contributes flux primarily at low energies. Despite the absence of any pulsed emission detected from the compact source, we argue that the emission is most probably due to a rotating compact object that is powering the detected synchrotron nebula within the SNR.

To investigate their dust properties, we have imaged three sites of massive star formation in the giant H II region/star-forming cloud NGC 6334 with the MIRAC2 instrument. We obtained high-resolution (1'') continuum images at 12.5 and 20.6 μm toward each region, which were compared with observations of the radio and near-infrared (near-IR) continuum emission. Both compact sources and extended emission were found at all three star-forming sites. The detected sources span a wide range of evolutionary states in this highly complex star-forming cloud. The infrared sources near NGC 6334 I were resolved into at least four subsources. One such source is substantially colder, denser, and more optically thick than the other mid-IR sources in the region and may be at the earliest stages of stellar formation. Another may be a torus or disk of dust and gas surrounding an embedded B star. NGC 6334 I was also imaged at additional wavelengths (8.8, 9.8, and 11.7 μm) to search for silicate absorption. Only at the H II region is there a deep silicate absorption feature from foreground dust. Toward the NGC 6334 IV, warm dust is associated with both the inner portions of the massive molecular torus or disk and with the bipolar continuum lobes. A compact mid-IR source, associated with the near-IR and radio source [HHS87] IRS 20, is cooler and more optically thick than the dust emission associated with the H II region. Toward NGC 6334 V, four embedded sources were found, including one previously unidentified object. This newly identified compact object, associated with a dust temperature peak and a radio source, is probably in a more advanced stage of star formation than the other compact mid-IR sources in NGC 6334.

We present data on the polarization of the thermal emission from Galactic Clouds at 60 μm, 100 μm, and 350 μm. There are examples of rising polarization spectra in dense cloud cores [P(350 μm/P(100 μm)≈2], and falling spectra in cloud envelopes [P(350)/P(100 μm)≈0.6]. We also present data showing that the relationship, P(τ), between polarization and optical depth in cloud cores is different from that in cloud envelopes. We review the principles governing the far-infrared polarization spectrum and discuss applications to the data on P(λ) and P(τ). We conclude that the cloud envelopes we have observed must contain two populations of grains that differ in their polarization efficiencies and in their emission spectra. We propose a model for cloud envelopes in which the contrasting populations reside in domains of different mean temperatures where the warmer domains contain the aligned grains.

Recent observations have revealed a superbubble associated with the young stellar cluster OCl 352 near the W4 H II region: a void in H I emission, reported by Normandeau and coworkers, and a bright shell in Hα emission, reported by Dennison and coworkers. We investigate the hypothesis that the bubble is blown by stellar winds from the O-type stars in the association. The Kompaneets approximation is adapted to model a wind-blown bubble in a stratified interstellar medium. We describe some general principles necessary for understanding the dynamics of an expanding bubble and the associated ionization structure in a stratified atmosphere. The Kompaneets model can be used to determine the mean scale height of the ambient medium as well as the age of the bubble. The ionization structure also places constraints on the ambient density near the cluster. We also estimate the surface brightness of the shell and the fraction of ionizing photons that escape the bubble. The prescription we use can be applied to any observed bubble that is blown by the effectively continuous energy output of stellar winds or multiple supernovae. Application to the W4 superbubble shows that the mean scale height of the ambient gas around the cluster is remarkably small, 25 pc for a cluster distance of 2.35 kpc. The age of the bubble is estimated to be about 2.5 Myr, consistent with the notion that the bubble is blown by stellar winds from a very young cluster in which no supernovae have yet occurred.

We consider the contribution of neutron stars and black holes to the dynamical mass of galactic halos. In particular, we show that if these compact objects were produced by an early generation of stars with initial metallicity ≲10^-4Z_☉, they can contribute at most 30%-40% of the Galactic halo mass without creating supersolar levels of enrichment. We show that the case for halo neutron stars and black holes cannot be rejected on metal overproduction arguments alone because of the critical factor of the choice of progenitor metallicity in determining the yields. We show that this scenario satisfies observational constraints, similar to but no more severe than those faced by halo white dwarfs. We also discuss the recent results on halo microlensing, the presence of enriched hot gas in clusters and groups of galaxies, and other observations. If there are halo neutron stars and black holes, they will be detected in the future as longer timescale events by microlensing experiments.

Empirical ionization fractions of C IV, N V, Si IV, and empirical ionization plus excitation fractions of C III* and N ^* in the winds of 34 O stars and one B star have been derived. We combine the mass-loss rates derived from radio measurements and Hα with the line fitting of ultraviolet resonance lines and subordinate lines using the Sobolev plus exact integration (SEI) method. The dependence of the empirical ionization fractions, ⟨q⟩, on the stellar effective temperature and on the mean wind density is discussed. This sets constraints for the models of ionization in the winds of hot stars. The ionization and excitation fractions can be expressed in terms of an empirical radiation temperature. This radiation temperature scales with T_eff, and we derive empirical relations for T_rad as a function of T_eff. The radiation temperatures are on the order of 0.5-0.9 T_eff, with significant differences between the ions. The derived relations between the ionization fractions and the stellar parameters have an uncertainty of 0.2 dex forSi IV, N V, and C III^*, and about 0.26 dex for N IV^*. For C IV, we can only derive an expression for the mean ionization fraction in the wind if the mass-loss rate is small, < 10^-6M_☉ yr^-1, because the C IV lines are usually saturated for higher mass-loss rates. The resulting expressions for T_rad can be used to derive the mass-loss rates from studies of ultraviolet P Cygni profiles in the range of stellar parameters studied here: 30,000 K ≲ T_eff ≲ 50,500 K, 5.2 ≲ log L/L_* ≲ 6.4, and -7.5 M_☉ ≲ log ≲ -4.6 M_☉ yr^-1. An accuracy of about a factor of 2 or better can be reached, depending on the lines that are used and the accuracy of the line fits and the stellar parameters. The Si IV lines give the most reliable mass-loss rates, because the abundance is about the same for all O stars, the lines saturate only for high mass-loss rates, the doublet lines only partly overlap, and the mass-loss rate is proportional to the square root of the column density. The radiation temperature of N V shows a surprisingly strict relation with T_eff, with a scatter of only ΔT_rad/T_eff=0.01. The mass-loss rate cannot be derived from the N V lines, because the column density of the N V ions in the wind is independent of . A consistency check and a test of the method for the stars HD 14749 and HD 190429 show that the mass-loss rate derived from the UV lines with the ionization fractions of this paper agree very well with the mass-loss rate derived from new radio flux measurements.

We have secured optical spectra for the eight currently known variable DB, or V777 Her, stars. With the help of a new generation of synthetic spectra, spectroscopic effective temperatures are derived for these objects, as well as for 15 other DB or DBA stars above 20,000 K. We find that the location of the boundaries of the instability strip is sensitive to the atmospheric hydrogen abundance assumed for DB stars: the strip covers the range 22,400-27,800 K if atmospheres of pure helium are used and the range 21,800-24,700 K if undetectable traces of hydrogen are allowed for in the DB models. These determinations provide independent constraints for current seismological analyses of the V777 Her stars. More sensitive searches for weak hydrogen features in hot DB stars should help decide between the two temperature scales.

The accretion-induced collapse (AIC) of a white dwarf into a neutron star has been invoked to explain gamma-ray bursts, Type Ia supernovae, and a number of problematic neutron star populations and specific binary systems. The ejecta from this collapse has also been claimed as a source of r-process nucleosynthesis. So far, most AIC studies have focused on determining the event rates from binary evolution models and less attention has been directed toward understanding the collapse itself. However, the collapse of a white dwarf into a neutron star is followed by the ejection of rare neutron-rich isotopes. The observed abundance of these chemical elements may set a more reliable limit on the rate at which AICs have taken place over the history of the Galaxy. In this paper, we present a thorough study of the collapse of a massive white dwarf in one- and two-dimensions and determine the amount and composition of the ejected material. We discuss the importance of the input physics (equation of state, neutrino transport, rotation) in determining these quantities. These simulations affirm that AICs are too baryon rich to produce gamma-ray bursts and do not eject enough nickel to explain Type Ia supernovae (with the possible exception of a small subclass of extremely low-luminosity Type Ias). Although nucleosynthesis constraints limit the number of neutron stars formed via AICs to ≲0.1% of the total Galactic neutron star population, AICs remain a viable scenario for forming systems of neutron stars that are difficult to explain with Type II core-collapse supernovae.

We examine several theories that describe how stellar magnetic fields on classical T Tauri stars (CTTSs) interact with their surrounding accretion disks. We demonstrate that these theories require magnetic field strengths ranging from a few hundred to several thousand gauss, depending on which model is used and more importantly on the properties of individual systems. For example, the CTTS BP Tau is predicted to have a relatively strong magnetic field (1.4-4.1 kG), which should be detectable. We present infrared (IR) and optical echelle spectra of BP Tau and several reference stars of similar spectral class. Using detailed spectrum synthesis and the latest model atmospheres, we fitted 12 absorption features in the optical spectrum, including the strong titanium oxide (TiO) band head at 7055 Å. For BP Tau we determine key stellar parameters: effective temperature (T_eff=4055 ± 112 K), gravity (log g=3.67 ± 0.50), metallicity ([M/H]=0.18 ± 0.11), projected rotational velocity (v sin i=10.2 ± 1.8 km s^-1), and optical veiling (r=0.00-0.15). A similar analysis of 61 Cyg B (K7 V) is used to validate the methodology. We then use the IR spectra to look for Zeeman broadening, which has a more pronounced effect at longer wavelengths. A Zeeman sensitive Ti I line at 2.2233 μm appears significantly broadened in BP Tau, relative to several rotationally broadened standard stars. The observed line is also significantly broader than predictions based on our optical analysis. Interpreting this excess broadening as Zeeman splitting of the Ti I line, we fitted the spectrum and find a distribution of field strengths whose surface averaged mean is =2.6 ± 0.3 kG. We did not use the Zeeman sensitive Fe I line at 8468.4 Å when determining stellar or magnetic parameters for BP Tau, so this line provides a test of our results. The observed line profile is indeed broader than the nonmagnetic prediction, but the 8468.4 Å line gives a magnetic flux lower than what was obtained in the IR, perhaps indicating that strong fields are concentrated into cool spots. Finally, we investigate an ad hoc model in which the IR line is assumed to form in the accretion disk itself. We discuss several reasons why the magnetic model is preferred, but the disk atmosphere example illustrates that our magnetic field measurement must still be tested using several IR lines with a range of Zeeman sensitivities.

We report on time-series photometric observations of the dM4.5e flare star YZ Canis Minoris obtained in 1993 November with the High Speed Photometer on board the Hubble Space Telescope. The data consist of five 30 minute time sequences with a sampling rate of 0.01 s that were taken through the F240W filter (centered at 240 nm with an 80 nm width). At these wavelengths the stellar photospheric background is small, so relatively small flares can be detected. The observations show a stellar background of 120 counts s^-1 on which are superposed 54 flare events ranging in integrated flux from 2.0 × 10²⁸ to 3.0 × 10³⁰ ergs, as well as longer term variations with an amplitude of up to 50% of the average continuum intensity and timescales ranging from several minutes to hours. A statistical analysis of this background suggests that it may be composed of unresolved microflaring activity that has an energy distribution considerably steeper than that deduced for the larger flare events. This is consistent with previous observations as well as the self-organized criticality and reconnecting current sheet flare theories. These results are compared with data from the dM8e flare star CN Leonis, which was obtained earlier with the same experimental setup. CN Leo has both a smaller stellar background and a lower flare occurrence rate than YZ CMi. The fact that CN Leo also has a quiescent X-ray flux that is less than 10% of the YZ CMi emission suggests a link between chromospheric and coronal heating.

The planar magnetic reconnection problem for viscous, resistive plasmas is addressed. We show that solutions can be developed by superposing transient nonlinear disturbances onto quiescent "background" fields. The disturbance fields are unrestricted in form, but the spatial part of the background field must satisfy ▽²K=-λK. This decomposition allows previous analytic reconnection solutions, based on one-dimensional disturbance fields of "plane wave" form, to be recovered as special cases. However, we point out that planar disturbance fields must be fully two-dimensional to avoid the pressure problem associated with analytic merging models, that is, to avoid unbounded current sheet pressures in the limit of small plasma resistivities. The details of the reconnection problem are then illustrated using cellular background field simulations in doubly periodic geometries. The flux pile-up rate is shown to saturate when the pressure of the current sheet exceeds the hydromagnetic pressure of the background field. Although the presaturation regime is well described by one-dimensional current sheet theory, the nonlinear postsaturation regime remains poorly understood. Preliminary evidence suggests that, although after saturation the early evolution of the field can be described by slow Sweet-Parker scalings, the first implosion no longer provides the bulk of the energy release.

We examine the occurrence rate of microflares (transient brightenings) in an X-ray bright point (XBP) during the lifetime of the XBP observed with the Yohkoh soft X-ray telescope (SXT). The XBP with a size of ~30'' appeared near a preceding spot of NOAA 7270 on 1992 September 1 and disappeared on 1992 September 4. The XBP produced 92 microflares during the observation time of the SXT. We found that the occurrence rate (number of events per hour) did not change much during the lifetime of the XBP, although the magnetic flux of the XBP changed. We also found that the frequency distribution of microflares as a function of the soft X-ray peak intensity shows a power law with index 1.7 ± 0.4, which is consistent with the previously known index for flares and microflares based on the data for the whole Sun or a whole active region. This result suggests that the power-law distribution of flares holds not only for larger areas but also for smaller areas.

We discuss the properties of the solar background signal as observed in high-quality, l-ν power and phase difference spectra of the continuum (C), velocity (V), and line intensity (I) fluctuations of the Ni I 6768 Å line. These spectra were generated from high-resolution images acquired by the Michelson Doppler Imager on board SOHO.

We confirm that the background signal in the velocity power spectra can be reproduced by a composite model with two quasi-stationary components, describing large-scale and small-scale convective motions, and a periodic component. The line and continuum intensity power spectra require additional quasi-stationary and periodic components. The extra quasi-stationary component dominates the intensity and continuum background signals over the spectral region where the I-V phase difference spectra show essentially constant negative phase difference: i.e., below and in between the p-mode ridges (called the plateau-interridge regime by Deubner et al.). Since the I-V phase between the p-mode ridges is not random, the solar background beneath the p-modes must be considered as coherent. We thus speculate that the negative phase regime may be the manifestation of a correlated background. Such a background has been proposed to explain the opposite sense of the asymmetries of the p-mode line profiles in velocity and brightness oscillations.

A new determination of the upper limit to the cosmic diffuse background radiation, at ~110 nm, of 300 photons s^-1 cm^-2 sr^-1 nm^-1 is placed in the context of diffuse background measurements across the entire electromagnetic spectrum, including new optical, infrared, visible, and gamma-ray background measurements. The possibility that observed excess diffuse visible radiation is due to redshifted cosmological Lyα recombination radiation is explored. Also, a new standard of units for the display of spectra is advocated.

In this Letter, we present a new determination of the local (z≤0.09) X-ray luminosity function (XLF) using a large, statistical sample of 294 Abell clusters and the ROSAT All-Sky Survey. Despite the optical selection of this catalog, we find excellent agreement with other recent determinations of the local XLF. Given our large sample size, we have reduced errors by nearly a factor of 2 for L_{X(0.5−2.0 keV)} ≥ 10⁴³h⁻²₅₀ ergs s^-1. We combine our data with previous work to produce the most tightly constrained local determination of the XLF (over 3 orders of magnitude in L_X) in order to explore possible constraints imposed by the shape of the XLF on cosmological models. A set of currently viable cosmologies is used to construct theoretical XLFs assuming L∝M^p and a σ₈-Ω₀ constraint based on the local X-ray temperature function. We fit these models to our observed XLF and verify that the simplest adiabatic, analytic scaling relation disagrees strongly with observations. If we assume that clusters can be described by the preheated, constant core entropy models of Evrard & Henry, then the observed XLF is consistent only with 0.1<Ω₀<0.4 if the energy per unit mass in galaxies is roughly equal to the gas energy (i.e., if β~1).

The prompt X-ray emission from gamma-ray burst (GRB) 980519, as measured with the Wide Field Cameras on board BeppoSAX, is characterized by a strong soft-to-hard-to-soft evolution. An analysis of the evolution of the X-ray spectrum in terms of a single power-law model shows that the photon index evolved from -2.0 to -1.1 to -2.4. The onset of the burst has such a soft spectrum that the 2-27 keV emission appears to precede the ≳107 keV emission of GRB 980519 (as measured with the Burst and Transient Source Experiment) by about 70 s. Nevertheless, we show that this early spectral variation is part of a smooth evolution over the whole burst and that there is no convincing evidence that the early X-rays originate from a physical process that is different from that giving rise to the remainder of the burst.

We present de Vaucouleurs's effective radii in B and R bands for a sample of Molonglo Reference Catalogue radio galaxies and a control sample of normal galaxies. We use the ratio of the scale lengths in the two bands as an indicator in order to show that the radio galaxies tend to have an excess of blue color in their inner region much more frequently than the control galaxies do. We show that the scale length ratio is a useful indicator of radial color variation, even when the conventional color gradient is too noisy to serve the purpose.

We present a detailed analysis on the variability of the Fe K emission line in NGC 4051 using ASCA data. Through simple Gaussian line fits, we find not only obvious Fe K line variability with no significant difference in the X-ray continuum flux between two ASCA observations that were separated by ~ 440 days, but also rapid variability of the Fe K line on timescales ~10⁴ s within the second observation. During the second observation, the line is strong (EW=733⁺²⁰⁶₋₂₁₉ eV) and broad (σ=0.96^+0.49_−0.35 keV) when the source is brightest and becomes weaker (EW=165⁺⁸⁷₋₈₆ eV) and narrower (σ<0.09 keV) while the source is weakest. The equivalent width of the Fe K line correlates positively with the continuum flux, which shows an opposite trend with another Seyfert type 1 galaxy, MCG -6-30-15.

We present observations of Hα emission of Stephan's Quintet obtained with a scanning Fabry-Perot interferometer and samplings of ~11 km s^-1 and ~1''. Our observations show an intragroup arclike feature formed of bright Hα knots and diffuse gas distributed along tidal tails associated with NGC 7318B and possibly also NGC 7318A. New velocities for 23 emitting regions detected along the Hα structure are presented. South of NGC 7318B, along a path covering ~12 h⁻¹₇₅ kpc, H II regions with velocities ranging from 5540 to 6700 km s^-1 are detected, suggesting that two or more overlapping tidal tails and/or a bow-shock region may have been formed through several interaction events within the group.

A surprisingly strong Li I λ6707 feature has been identified in a red giant member of the globular cluster NGC 362. This giant, V2, is located near the tip of the red giant branch and could be either a first-ascent red giant or on the asymptotic giant branch. An abundance analysis finds the lithium abundance to be log(Li)=1.2±0.2. Since almost all low-mass giants have destroyed and diluted their photospheric lithium abundances to much lower levels, the source of lithium in star V2 is a mystery. The solution to this mystery is suggested to reside in two possible mechanisms. The proposed deep-mixing mechanism called "cool bottom processing" can account for the observed lithium at this giant's luminosity [log(L/L_☉)=3.3] if the parameterized mixing speed, measured as log(dM/dt), is ~-5.5 (in units of M_☉ yr^-1). An alternative source of the excess lithium could be the ingestion, into the red giant's convective envelope, of an ~0.04 M_☉ object with an assumed undepleted, typical halo lithium abundance of log(Li)=2.2.

A cluster of young, X-ray-emitting stars is found in the vicinity of η Chamaeleontis from a deep ROSAT high-resolution imager observation. The 12 X-ray sources have prominent (R=5-14) stellar counterparts, including two early-type stars (η Cha and RS Cha) and 10 Li-rich, Hα emission-line, late-type stars (K3-M5). Hipparcos astrometry reveals that η Cha, RS Cha, and nearby HD 75505 are comoving at d≃97 pc. The late-type stars have all the properties of pre-main-sequence weak-lined T Tauri stars: high magnetic activity, high Li abundance, and (assuming d=97 pc) bolometric luminosities 1-2 mag above the main sequence, with ages ranging from 2 to 18 Myr. As with the TW Hya association, the η Cha cluster is far from any significant molecular cloud and thus has mysterious origins. The cluster appears to share proper motions with other young stars in the Chamaeleon region and may be a compact extension of the Sco-Cen OB association.

We investigate the time dependence of the frequency of X-ray brightness oscillations during thermonuclear X-ray bursts from several neutron star low-mass X-ray binaries. We find that the oscillation frequency in the cooling tails of X-ray bursts from 4U 1702-429 and 4U 1728-34 is well described by an exponential "chirp" model. With this model, we demonstrate that the pulse trains in the cooling tails of many bursts are highly phase coherent. We measure oscillation quality factors for the bursts from 4U 1728-34 and 4U 1702-429 as high as Q≡ν₀/Δν_FWHM~4000. We use this model of the frequency evolution to search sensitively for significant power at the harmonics and first subharmonic of the 330 and 363 Hz signal in the bursts from 4U 1702-429 and 4U 1728-23, respectively, but we find no strong evidence of significant power at any harmonic or the subharmonic. We argue that the high coherence of the oscillations favors stellar rotation as the source of the oscillations. The lack of a subharmonic in the bursts from both 4U 1728-34 and 4U 1702-429 suggests that in these sources, the burst oscillation frequency is indeed the stellar spin frequency. We briefly discuss the frequency evolution in terms of the rotational motion of an angular momentum-conserving thermonuclear shell. We discuss how the limits on harmonic content can be used to infer properties of the neutron star.

The accretion-powered, X-ray pulsar 4U 1626-67 has recently shown an abrupt torque reversal accompanied by a dramatic spectral transition and a relatively small luminosity change. The time-averaged X-ray spectrum during spin-down is considerably harder than during spin-up. The observed torque reversal can be explained by an accretion flow transition triggered by a gradual change in the mass accretion rate. The sudden transition to spin-down is caused by a change in the accretion flow rotation from Keplerian to sub-Keplerian. The X-ray pulsar 4U 1626-67 is estimated to be near spin equilibrium with a mass accretion rate ~2×10¹⁶ g s^-1, decreasing at a rate ~-6×10¹⁴ g s^-1 yr^-1, and a polar surface magnetic field of ~2b^−½_p×10¹²G, where b_p is the magnetic pitch. During spin-up, the Keplerian flow remains geometrically thin and cool. During spin-down, the sub-Keplerian flow becomes geometrically thick and hot. Soft photons from near the stellar surface are Compton up-scattered by the hot accretion flow during spin-down, while during spin-up such scattering is unlikely because of the small scale height and low temperature of the flow. This mechanism accounts for the observed spectral hardening and small luminosity change. The scattering occurs in a hot radially falling column of material with a scattering depth ~0.3 and a temperature ~10⁹ K. The X-ray luminosity at energies greater than 5 keV could be a poor indicator of the mass accretion rate. We briefly discuss the possible application of this mechanism to GX 1+4, although there are indications that this system is significantly different from other torque-reversal systems.

We report the discovery of a quasi-periodic oscillation (QPO) in data obtained with the Rossi X-Ray Timing Explorer of the dipping and eclipsing low-mass X-ray binary EXO 0748-676. The QPO had a frequency between 0.58 and 2.44 Hz changing on timescales of a few days and an rms amplitude between 8% and 12%, and it was detected in the persistent emission during dips and during type I X-ray bursts. During one observation, when the count rate was a factor of 2-3 higher than otherwise, the QPO was not detected. The strength of the QPO did not significantly depend on photon energy and is consistent with being the same in the persistent emission, both during and outside the dips and during type I X-ray bursts. Frequency shifts were observed during three of the four X-ray bursts. We argue that the QPO is produced by the same mechanism as the QPO recently found by Jonker et al. in 4U 1323-62. Although the exact mechanism is not clear, it is most likely related to the high inclination of both systems. An orbiting structure in the accretion disk that modulates the radiation from the central source seems the most promising mechanism.

We report Hubble Space Telescope observations of variability within the reflection nebulosity of HH 30, a compact bipolar nebula that is a nearly edge-on accretion disk system. A dramatic lateral asymmetry appeared in the upper reflection nebula in the spring of 1998, but was largely absent in 1994 and 1995 images. The variability timescale is much shorter than disk dynamical timescales at the projected radius of the asymmetry, which indicates that its origin is a variable illumination pattern projected onto the outer disk by changes in the inner disk or the central star. Orbital motion of coherent clumps or voids in the inner disk at radii of a few AU might produce such an effect. Another possibility recently proposed is accretion hot spots near the star's magnetic poles which produce broad beams of light sweeping across the disk as the star rotates. Simulated images of a disk illuminated by such a central "lighthouse" are a reasonable match to the bright lateral asymmetry in the upper nebula of HH 30. However, a model with identical opposed hot spots is excluded by the absence of a prominent asymmetry in the lower reflection nebula. Further temporal monitoring of the system is needed to better characterize the nebular variability and establish its physical origin.

We present Infrared Space Observatory SWS06 mode observations of the 21 μm feature in eight sources, including a first detection of the feature in IRAS Z02229+6208. The observed feature peak-to-continuum ratios range from 0.13 in IRAS Z02229+6208 to 1.30 in IRAS 07134+1005. The normalized spectra, obtained by the removal of the underlying continua and by scaling the features to the same peak flux value, show that all features have the same intrinsic profile and peak wavelength. There is no evidence for any discrete substructure due to molecular bands in the observed spectra, suggesting that the 21 μm feature is due to either a solid substance or a mixture of many similarly structured large molecules.

Seven rotational transitions of SiC₃, a planar, highly polar ring molecule, have been detected in the millimeter-wave band in the expanding envelope of the evolved carbon star IRC +10216. On the assumption that SiC₃ has the same 40'' diameter shell as SiCC, the mean column density of SiC₃ is 4.3×10¹² cm⁻². The rotational excitation of SiC₃ is similar to that of SiCC, with a low rotational temperature within the K-stacks of 10-20 K and a high rotational temperature across the K-stacks of roughly 50 K, which is probably close to the kinetic temperature of the shell.

Charge transfer can affect both the ionization and thermal balance of astrophysical plasmas. Using the most recent rate coefficients and energy defects, we calculate the heating/cooling rates for charge transfer reactions between hydrogen and elements up to Z=30. We incorporate these values into the photoionization code CLOUDY. Results from models approximating a wide range of astrophysical objects and conditions suggest that charge transfer can make a significant contribution to the heating near the H ionization front, particularly in objects with a hard ionizing continuum or enhanced abundances. Charge transfer heating can also be important in regimes in which the usual heating/cooling agents are suppressed, such as the emission-line clouds near quasars. We list those reactions that are most important for determining the thermal balance, in the hopes of facilitating improved atomic data.

In a laboratory study, the lifetimes of the ²P levels producing the coronal transitions of Fe X and Fe XIV have been measured. The fluorescence from the metastable levels, which were populated when the ions were produced in a source of multiply charged ions, was studied after the selected ions were injected into an electrostatic ion trap. The results are τ(Fe X, 3s²3p⁵²P⁰_½) = 13.64±0.25 ms and τ(Fe XIV, 3s²3p²P⁰_3/2) = 17.52±0.29 ms. The data significantly reduce the uncertainty of the lifetimes when compared with existing theory.

We discuss a new technology that promises large, inexpensive mirrors. We argue that it should be possible to tilt a rotating viscous liquid by perhaps as much as a few tens of degrees. The tilted liquid parabolic surface is used as the support for a thin reflecting metallic film. It may also be possible to use it to support an ultrathin glass mirror. We demonstrate two critical steps: that a viscous liquid mirror can be tilted and that an optical-quality metallic film can be deposited on a liquid. The advent of astronomically useful tilted floating-mirror telescopes is contingent on the development of high-viscosity, high-reflectivity liquids. It is a good omen that we already have identified two classes of such liquids; however, there remain technical challenges to overcome before such liquids can be used in viable telescopes.