Table of contents

In this paper, we raise the hypothesis that the density fluctuation field, which originates the growth of large-scale structures, is a combination of two or more distributions, instead of assuming that the observed distribution of matter stems from a single Gaussian field produced in the very early universe, as is widely accepted. By applying the statistical analysis of finite-mixture distributions to a specific combination of Gaussian plus non-Gaussian random fields, we studied the case in which just a small departure from Gaussianity is allowed. Our results suggest that even a very small level of non-Gaussianity may introduce significant changes in the cluster abundance evolution rate.

We present a method for measuring the galaxy power spectrum based on multiresolution analysis of the discrete wavelet transformation (DWT). Apart from the technical advantages of the computational feasibility for data sets with a large volume and complex geometry, the DWT scale-by-scale decomposition provides a physical insight into the covariance matrix of the cosmic mass field. Since the DWT representation has a strong capability for suppressing the off-diagonal components of the covariance for self-similar clustering, the DWT covariance for all popular models of the cold dark matter cosmogony is generally diagonal, or j (scale) diagonal in the scale range in which the second or higher order scale-scale correlations are weak. In this range, the DWT covariance gives a lossless estimation of the power spectrum, which is equal to the corresponding Fourier power spectrum banded with a logarithmical scaling. This DWT estimator is optimized in the sense that the spatial resolution is automatically adaptive to the perturbation wavelength to be studied. In the scale range in which the scale-scale correlation is significant, the accuracy of a power spectrum detection depends on the scale-scale or band-band correlations. In this case, for a precision measurements of the power spectrum, or a precision comparison of the observed power spectrum with models, a measurement of the scale-scale or band-band correlations is needed. We show that the DWT covariance can be employed to measure both the band-power spectrum and second-order scale-scale correlation. We also present the DWT algorithm of the binning and Poisson sampling with real observational data. We show that the so-called alias effect appeared in usual binning schemes can exactly be eliminated by the DWT binning. Since the Poisson process possesses diagonal covariance in the DWT representation, the Poisson sampling and selection effects on the power spectrum and second order scale-scale correlation detection are suppressed into a minimum. Moreover, the effect of the non-Gaussian features of the Poisson sampling can also be calculated in this frame. The DWT method is open, i.e., one can add further DWT algorithms on the basic decomposition in order to estimate other effects on the power spectrum detection, such as non-Gaussian correlations and bias models.

We show that the cosmic star formation rate per comoving volume should exhibit a distinct drop around the reionization redshift, when the H II regions in the intergalactic medium around individual ionizing sources first overlapped. The drop results from the increase in the temperature of the intergalactic medium as it was photoionized, and the consequent suppression of the formation of low-mass galaxies. We show quantitatively that the detection of this drop, which is marked by a corresponding fall in the number counts of faint galaxies, should become feasible over the coming decade with the Next Generation Space Telescope.

We make a prediction of the cosmic supernova rate history as a composite of the supernova rates in spiral and elliptical galaxies. We include the metallicity effect on the evolution of Type Ia supernova (SN Ia) progenitors and construct detailed models for the evolutions of spiral and elliptical galaxies in clusters and the field to meet the latest observational constraints. In the cluster environment, the synthesized cosmic star formation rate (SFR) has an excess at z ≳ 3 corresponding to the early starburst in ellipticals and a shallower slope from the present to the peak at the redshift of z ~ 1.4 compared with Madau's plot. In the field environment, we assume that ellipticals form over a wide range of redshifts as 1 ≲ z ≲ 4. The synthesized cosmic SFR has a broad peak around z ~ 3, which is in good agreement with the observed one. The resultant cosmic SFRs lead to the following predictions for the cosmic SN Ia rate: (1) the SN Ia rate in spirals has a break at z ~ 2 because of the low-metallicity inhibition of SNe Ia, regardless of whether the galaxies are in clusters or in the field; (2) at high redshifts, the SN Ia rate has a strong peak around z ~ 3 in clusters, whereas in the field much lower rate is expected, reflecting the difference in the formation epochs of ellipticals.

We present interferometric measurements of the Sunyaev-Zeldovich (SZ) effect toward the galaxy cluster Abell 370. These measurements, which directly probe the pressure of the cluster's gas, show the gas distribution to be strongly aspherical, as do the X-ray and gravitational lensing observations. We calculate the cluster's gas mass fraction in two ways. We first compare the gas mass derived from the SZ measurements to the lensing-derived gravitational mass near the critical lensing radius. We also calculate the gas mass fraction from the SZ data by deprojecting the three-dimensional gas density distribution and deriving the total mass under the assumption that the gas is in hydrostatic equilibrium (HSE). We test the assumptions in the HSE method by comparing the total cluster mass implied by the two methods and find that they agree within the errors of the measurement. We discuss the possible systematic errors in the gas mass fraction measurement and the constraints it places on the matter density parameter, Ω_M.

We describe the HEMT Advanced Cosmic Microwave Explorer (HACME), a balloon-borne experiment designed to measure subdegree-scale cosmic microwave background anisotropy over hundreds of deg², using a unique two-dimensional scanning strategy. A spinning flat mirror that is canted relative to its spin axis modulates the direction of beam response in a nearly elliptical path on the sky. The experiment was successfully flown in 1996 February, achieving near laboratory performance for several hours at float altitude. A map free of instrumental systematic effects is produced for a 3.5 hr observation of 630 deg², resulting in a flat-band power upper limit of ⟨l(l + 1)C_l/(2π)⟩^0.5 < 77 μK at l = 38 (95% confidence). The experiment design, flight operations, and data, including atmospheric effects and noise performance, are discussed.

We have used the Berkeley-Illinois-Maryland Association (BIMA) millimeter array outfitted with sensitive centimeter-wave receivers to search for cosmic microwave background (CMB) anisotropies on arcminute scales. The interferometer was placed in a compact configuration that produces high brightness sensitivity, while providing discrimination against point sources. Operating at a frequency of 28.5 GHz, the FWHM primary beam of the instrument is ~66. We have made sensitive images of seven fields, four of which where chosen specifically to have low IR dust contrast and to be free of bright radio sources. Additional observations with the Owens Valley Radio Observatory (OVRO) millimeter array were used to assist in the location and removal of radio point sources. Applying a Bayesian analysis to the raw visibility data, we place limits on CMB anisotropy flat-band power of Q_flat = 5.6 μK and Q_flat < 14.1 μK at 68% and 95% confidence, respectively. The sensitivity of this experiment to flat-band power peaks at a multipole of l = 5470, which corresponds to an angular scale of ~2'. The most likely value of Q_flat is similar to the level of the expected secondary anisotropies.

The Ryle telescope has been used at a frequency of 15 GHz, to detect a flux decrement in the direction of the quasar pair PC 1643+4631A, B. This signal was interpreted as the Sunyaev-Zeldovich effect (SZE) produced by a distant cluster of galaxies. In the course of an effort to measure cosmic microwave background (CMB) anisotropies with a deep pointing of the VLA at 8.4 GHz, a second group detected a similar, but smaller, decrement. They also proposed that this signal might be explained as the SZE signal produced by a distant galaxy cluster. We report observations with the Berkeley-Illinois-Maryland Association (BIMA) interferometer operating at 28.5 GHz, in which we find no evidence for a SZE signal in the direction of either of the proposed clusters. In the case of the Ryle detection, the BIMA data are inconsistent with the SZE model proposed to explain the observed decrement at greater than 99.99% confidence. Together with published X-ray and optical searches, these results make a compelling case against the interpretation of the Ryle decrement as being due to the SZE in a massive cluster of galaxies. For the smaller VLA source, the BIMA observations are not as constraining. The BIMA data are inconsistent with the proposed SZE model at greater than 90% confidence.

Standard big bang nucleosynthesis predicts the average baryon density of the universe to be a few percent of the critical density. Only about one-tenth of the predicted baryons have been seen. A plausible repository for the missing baryons is in a diffuse ionized intergalactic medium (IGM). In an attempt to measure the IGM, we searched for Thomson-scattered halos around strong high-redshift radio sources. Observations of the radio source 1935-692 were made with the Australia Telescope Compact Array. We assumed a uniform IGM, and isotropic steady emission of 1935-692 for a duration between 10⁷ and 10⁸ yr. A model of the expected halo visibility function was used in χ² fits to place upper limits on Ω_IGM. The upper limits varied depending on the methods used to characterize systematic errors in the data. The results are 2 σ limits of Ω_IGM < 0.65. While not yet at the sensitivity level to test primordial nucleosynthesis, improvements in the technique will probably allow this in future studies.

We discuss the cosmological dependence of the star formation history of the universe, within the framework of a very simple semianalytic model, where star formation occurs within the virialized cores of dark matter halos, at a rate which is primarily governed by the rate of matter infall into the halo core. Our model is extremely simple, contains a number of uncertain features, and cannot be expected to predict detailed properties of the galaxy distribution. In spite of these great uncertainties, we find that at sufficiently high redshifts fundamental differences between cosmologies (and not uncertainties in the star formation model) are the dominant factor determining star formation history. Consequently, we argue that observations of star-forming galaxies at high redshift (z > 5) with telescopes such as the Next Generation Space Telescope (NGST) can provide a powerful probe of cosmology.

We develop a method of testing the null hypothesis of intrinsic randomness in the angular distribution of gamma-ray bursts collected in the Current BATSE Catalog. The method is a modified version of the well-known counts-in-cells test and fully eliminates the nonuniform sky-exposure function of the BATSE instrument. Applying this method to the case of all gamma-ray bursts, we found no intrinsic nonrandomness. The test also did not find intrinsic nonrandomness for the short and long gamma-ray bursts. However, using the method on the new, intermediate subclass of gamma-ray bursts, the null hypothesis of intrinsic randomness for 181 intermediate gamma-ray bursts is rejected on the 96.4% confidence level. Taking 92 dimmer bursts from this subclass, we obtain a surprising result: this "dim" subclass of the intermediate subclass has an intrinsic nonrandomness on the 99.3% confidence level. On the other hand, the 89 "bright" gamma-ray bursts show no intrinsic nonrandomness.

The time- and angle-dependent line and continuum emission from a dense torus around a cosmological gamma-ray burst source is simulated, taking into account photoionization, collisional ionization, recombination, and electron heating and cooling due to various processes. The importance of the hydrodynamical interaction between the torus and the expanding blast wave is stressed. Because of the rapid deceleration of the blast wave as it interacts with the dense torus, the material in the torus will be illuminated by a drastically different photon spectrum than that observable through a low-column-density line of sight and will be heated by the hydrodynamical interaction between the blast wave and the torus. A model calculation to reproduce the Fe Kα line emission observed in the X-ray afterglow of GRB 970508 is presented. The results indicate that ~10^-4M_☉ of iron must be concentrated in a region of R ≲ 10^-3 pc. The illumination of the torus material due to the hydrodynamical interaction of the blast wave with the torus is the dominant heating and ionization mechanism leading to the formation of the iron line. These results suggest that misaligned gamma-ray bursts may be detectable as X-ray flashes with strong emission-line features.

Far-ultraviolet spectra of the gravitational lens components Q0957+561A, B were obtained with the Hubble Space Telescope Faint Object Spectrograph (FOS) at five equally spaced epochs, one every 2 weeks. We confirm the flux variability of the quasar's Lyα and O VI λ1037 emission lines in IUE spectra reported in earlier work of Dolan et al. The fluxes in these lines vary on a timescale of weeks in the observer's rest frame, independently of each other and of the surrounding continuum.

The individual spectra of each image were co-added to investigate the properties of the Lyα forest along the two lines of sight to the quasar. Absorption lines having equivalent width W_λ ≥ 0.3 Å in the observer's frame not previously identified by Michalitsianos et al. as interstellar lines, metal lines, or higher order Lyman lines were taken to be Lyα forest lines. The existence of each line in this consistently selected set was then verified by its presence in two archival FOS spectra with ~ 1.5 times higher signal to noise than our co-added spectra. Lyα forest lines with W_λ ≥ 0.3 Å appear at 41 distinct wavelengths in the spectra of the two images. One absorption line in the spectrum of image A has no counterpart in the spectrum of image B, and one line in image B has no counterpart in image A. Based on the separation of the lines of sight over the redshift range searched for Lyα forest lines, the density of the absorbing clouds in the direction of Q0957+561 must change significantly over a distance R = 160h kpc in the simplified model where the absorbers are treated as spherical clouds and the characteristic dimension, R, is the radius. (We adopt H₀ = 50 h₅₀ km s^-1 Mpc^-1, q₀ = , and Λ = 0 throughout this paper.) The 95% confidence interval on R extends from 50 to 950 h kpc.

We show in the Appendix that the fraction of Lyα forest lines that appear in only one spectrum can be expressed as a rapidly converging power series in 1/r, where r the ratio of the radius of the cloud to the separation of the two lines of sight at the redshift of the cloud. This power series can be rewritten to give r in terms of the fraction of Lyα forest wavelengths that appear in the spectrum of only one image. A simple linear approximation to the solution that everywhere agrees with the power series solution to better than 0.8% for r ≥ 2 is derived in the Appendix.

We examine the necessity of resolving the Jeans mass in hydrodynamical simulations of the hierarchical formation of cosmological structures. We consider a standard two-component fluid and use smoothed particle hydrodynamics to model the baryonic component. It is found that resolution of the Jeans mass is not necessary to extract bulk properties of bound objects that are independent of resolution, provided that the objects have undergone a number of merger events. The degree of merging must be sufficient to populate the objects with substructure of similar total mass. After this criterion is met, neither the density profiles of structures nor their density-temperature distributions depend appreciably on the resolution of the simulation. Resolution of the Jeans mass by the imposition of a minimum temperature is not found to affect adversely the state of the matter in the final clusters. The baryon concentration profiles still indicate a deficit of gas with respect to the dark matter in the interiors of the dense halo structures. This is offset by a baryon enrichment in the filamentary and sheetlike structures as well as the voids.

The form of the galaxy luminosity function (GLF) in poor groups—regions of intermediate galaxy density that are common environments for galaxies—is not well understood. Multiobject spectroscopy and wide-field CCD imaging now allow us to measure the GLF of bound group members directly (i.e., without statistical background subtraction) and to compare the group GLF with the GLFs of the field and of rich clusters. We use R-band images in 1.5 × 1.5 degree² mosaics to obtain photometry for galaxies in the fields of six nearby (2800 < cz < 7700 km s^-1) poor groups for which we have extensive spectroscopic data, including 328 new galaxy velocities. For the five groups with luminous X-ray halos, the composite group GLF for group members with -23 + 5 log h < M_R < -16 + 5 log h and within projected radii of ≲0.4-0.6 h^-1 Mpc from the group center is fit adequately by a Schechter function with M = -21.6 ± 0.4 + 5log h and α = -1.3 ± 0.1. We also find that (1) the ratio of dwarfs (-17 + 5 log h ≥ M_R > -19 + 5 log h) to giants (M_R ≤ -19 + 5 log h) is significantly larger for the five groups with luminous X-ray halos than for the one marginally X-ray-detected group; (2) the composite GLF for the luminous X-ray groups is consistent in shape with two measures of the composite R-band GLF for rich clusters (Trentham; Driver et al.) and flatter at the faint end than another (α ≈ -1.5; Smith et al.); (3) the composite group GLF rises more steeply at the faint end than the R-band GLF of the Las Campanas Redshift Survey (LCRS; α = -0.7 from Lin et al.), a large volume survey dominated by galaxies in environments more rarefied than luminous X-ray groups; (4) the shape difference between the LCRS field and composite group GLFs results mostly from the population of non-emission line galaxies (EW [O II] < 5 Å), whose dwarf-to-giant ratio is larger in the denser group environment than in the field (cf. Ferguson & Sandage; Bromley et al.); and (5) the non-emission line dwarfs are more concentrated about the group center than the non-emission line giants, except for the central, brightest (M_R < M) group elliptical (BGG). This last result indicates that the dwarfs, giants, and BGGs occupy different orbits (i.e., have not mixed completely) and suggests that at least one of these populations formed at a different time. Our results show that the shape of the GLF varies with environment and that this variation is due primarily to an increase in the dwarf-to-giant ratio of quiescent galaxies in higher density regions, at least up to the densities characteristic of X-ray luminous poor groups. This behavior suggests that, in some environments, dwarfs are more biased than giants with respect to dark matter. This trend conflicts with the prediction of standard biased galaxy formation models.

We have found that the stellar populations of early-type galaxies are homogeneous with no significant difference in color or Mg₂ index, despite the dichotomy between X-ray-extended early-type galaxies and X-ray-compact ones. Since the X-ray properties reflect the potential gravitational structure and, hence, the process of galaxy formation, the homogeneity of the stellar populations implies that the formation of stars in early-type galaxies predates the epoch when the dichotomy of the potential structure was established.

The universal coupling relation between the baryonic matter of spiral galaxy disks and the inner part of the corresponding dark halos requires the existence of a secondary dark component in faint late-type spirals. In the present paper, this empirical relation is used to predict the amount of this secondary component within 1.5-2 optical radii of the nearby spiral M33. At such radii there is no clear need of any additional disk dark mass, but the low value of the derived stellar mass-to-light ratio suggests masses of the order 0.5-1M for this component, if any. At four optical radii, the coupling relation between detected disk baryons and dark halo cannot describe the observed rotation curve. The luminous mass (stars and gas) in M33 appears to have a higher collapse factor with respect to the halo than in galaxies where the coupling relation has been successfully tested, in bright galaxies in particular. An alternative solution is to invoke a disk dark baryonic component. The density distribution of such a component is not proportional to that of the neutral hydrogen at large radii and must have a larger scale length. A distribution obtained by scaling radially the H I distribution is a possible solution. The nature of this component cannot be easily inferred from its distribution. If it is made of cold molecular H₂, the observational constraint is the detection of a column density of N ~ 6 × 10²¹ cm^-2.

We report X-ray fluxes in the 2-10 keV band from LINERs (low-ionization nuclear emission-line regions) and low-luminosity Seyfert galaxies obtained with the ASCA satellite. Observed X-ray luminosities are in the range between 4 × 10³⁹ and 5 × 10⁴¹ ergs s^-1, which are significantly smaller than that of the "classical" low-luminosity Seyfert 1 galaxy NGC 4051. We found that X-ray luminosities in 2-10 keV of LINERs with broad Hα emission in their optical spectra (LINER 1s) are proportional to their Hα luminosities. This correlation strongly supports the hypothesis that the dominant ionizing source in LINER 1s is photoionization by hard photons from low-luminosity active galactic nuclei (AGNs). On the other hand, the X-ray luminosities of most LINERs without broad Hα emission (LINER 2s) in our sample are lower than LINER 1s at a given Hα luminosity. The observed X-ray luminosities in these objects are insufficient to power their Hα luminosities, suggesting that their primary ionizing source is other than an AGN, or that an AGN, if present, is obscured even at energies above 2 keV.

Infrared spectrophotometric observations from 0.8 to 2.5 μm are presented for the narrow-line, type 1 Seyfert galaxy I Zw 1. The data clearly reveal Fe II λ9997, λ10501, λ10863, and λ11126—the so-called "1 μm Fe II lines." These features are by far the strongest Fe II transitions in the entire 0.8-2.5 μm spectral region and, relative to the hydrogen lines, are equal or stronger in I Zw 1 than in any of the Galactic sources in which they have been detected previously. The 1 μm Fe II lines, which share a common upper term, are probably emitted by the broad line regions of many active galaxies but have escaped identification because of blending with nearby features of hydrogen and helium. A mechanism for their excitation through fluorescence by Lyα has been suggested previously in the literature, but the crucial cascade lines that feed the upper term in this process are not seen in I Zw 1. The low energy of the upper term indicates that the 1 μm Fe II lines are collisionally excited.

Previous studies of the Fe abundances in the hot gas of galaxies and groups have reported conflicting results with most studies finding very subsolar Fe abundances that disagree with standard theory. To investigate the possible role of Fe abundance gradients on these measurements we present deprojection analysis of the ROSAT PSPC data of 10 of the brightest cooling flow galaxies and groups. The PSPC allows for spatially resolved spectral analysis on a half-arcminute scale, and interesting constraints on both the temperatures and Fe abundances are possible because the ~1 keV temperatures of these systems are well matched to the bandpass of the PSPC. In nine out of 10 systems we find clear evidence that the Fe abundance decreases with increasing radius: Z_Fe ≈ 1-several Z_☉ within the central radial bin (r ≲ 10 kpc), which decreases to Z_Fe ~ 0.5 Z_☉ at the largest radii examined (r ~ 50-100 kpc). The Fe abundances (and temperatures) are consistent with the average values for these systems that we obtained in our previous analyses of the ASCA data using multitemperature models, which confirms that previous inferences of very subsolar Fe abundances from ASCA arise from the incorrect assumption of isothermal gas and not the presence of Fe abundance gradients. We discuss why this "Fe bias" affects much more seriously the measurements of Z_Fe from ASCA data than from ROSAT data. We show that the Fe abundance profiles for these galaxies and groups are consistent with a gasdynamical model where the gas is enriched by stellar ejecta and supernovae in the "solar supernova proportion," the stars formed with a Galactic initial mass function, and the gas is diluted by mixing with primordial gas at large radii.

We reexamine Voyager Ultraviolet Spectrometer (UVS) data used to establish upper limits to the 500-900 Å and 900-1100 Å cosmic diffuse background. The measurement of diffuse flux with the Voyager UVS data requires complex corrections for noise sources which are far larger than the astronomical signal. In the analyses carried out to date, the upper limits obtained on the diffuse background show statistical anomalies which indicate that substantial systematic errors are present. We detail these anomalies and identify specific problems with the analyses. We derive statistically robust 2 σ upper limits for continuum flux of 570 photons s^-1 cm^-2 sr^-1 Å^-1 and for the 1000 Å diffuse line flux of 11,790 photons s^-1 cm^-2 sr^-1. The true limits may be substantially higher because of unknown systematic uncertainties. The new statistical limits alone are insufficient to support previous conclusions based on the Voyager data, including work on the character of interstellar dust and estimates of the diffuse extragalactic far-UV background as absorbed by intergalactic dust.

The new transient X-ray pulsar XTE J0111.2-7317 was observed with Advanced Satellite for Cosmology and Astrophysics (ASCA) on 1998 November 18, a few days after its discovery with the proportional counter array on board the Rossi X-ray Timing Explorer. The source was detected at a flux level of 3.6 × 10^-10 ergs cm^-2 s^-1 in the 0.7-10.0 keV band, which corresponds to the X-ray luminosity of 1.8 × 10³⁸ ergs s^-1, if a distance of 65 kpc for this pulsar in the Small Magellanic Cloud is assumed. Nearly sinusoidal pulsations with a period of 30.9497 ± 0.0004 s were unambiguously detected during the ASCA observation. The pulsed fraction is low and slightly energy dependent with an average value of ~27%. The energy spectrum shows a large soft excess below ~2 keV when fitted to a simple power-law-type model. The soft excess is eliminated if the spectrum is fitted to an "inversely broken power-law" model, in which photon indices below and above a break energy of 1.5 keV are 2.3 and 0.8, respectively. The soft excess can also be described by a blackbody or a thermal bremsstrahlung when the spectrum above ~2 keV is modeled by a power law. In these models, however, the thermal soft component requires a very large emission zone, and hence it is difficult to explain the observed pulsations at energies below 2 keV. A bright state of the source enables us to identify a weak iron line feature at 6.4 keV with an equivalent width of 50 ± 14 eV. Pulse phase-resolved spectroscopy revealed a slight hardening of the spectrum and marginal indication of an increase in the iron line strength during the pulse maximum.

We present the first results of a study of the stellar population in a region of 30 pc radius around SN 1987A, based on an analysis of multiband Hubble Space Telescope (HST) WFPC2 images. The effective temperature, radius and, possibly, reddening of each star were determined by fitting the measured broadband magnitudes to the ones calculated with model atmospheres. In particular, we have determined effective temperatures and bolometric luminosities for 21,995 stars, and for a subsample of 2510 stars we also determined individual reddening corrections. In addition, we have identified all stars with Hα equivalent widths in excess of 8 Å, a total of 492 stars. An inspection of the H-R diagram reveals the presence of several generations of young stars, with ages between 1 and 150 Myr, superposed on a much older field population (0.6-6 Gyr). A substantial fraction of young stars with ages around 12 Myr make up the stellar generation coeval to SN 1987A progenitor. The youngest stars in the field appear to be strong-line T Tauri stars, identified on the basis of their conspicuous (W_eq > 8 Å) Hα excesses. This constitute the first positive detection of low-mass (about 1-2 M_☉) pre-main-sequence (PMS) stars outside the Milky Way. Their positions in the H-R diagram appear to require that star formation in the LMC occurs with accretion rates about 10 times higher than in the Milky Way, i.e., ~10^-4M_☉ yr^-1. SN 1987A appears to belong to a loose, young cluster 12 ± 2 Myr old, in which the slope of the present mass function is almost identical to Salpeter's, i.e., Γ = d log N/d log M ≃ -1.25 for masses above 3 M_☉, but becomes much flatter for lower masses, i.e., Γ ≃ -0.5. On a large scale, we find that the spatial distributions of massive stars and low-mass PMS stars are conclusively different, indicating that different star formation processes operate for high- and low-mass stars. This results casts doubts on the validity of an initial mass function (IMF) concept on a small scale (say, less than 10 pc). Moreover, it appears that a determination of the low-mass end IMF in the LMC requires an explicit identification of PMS stars. A preliminary analysis, done for the whole field as a single entity, shows that the IMF slope for the young population present over the entire region is steeper than Γ ≃ -1.7.

In 1998 and 1999 the Whipple Observatory 10 m telescope was used to search for diffuse γ-ray emission from the Galactic plane. At this time, the telescope was equipped with a large (48) field of view camera, well suited to detect diffuse γ-ray emission. No significant evidence of emission was found. Assuming the TeV emission profile matches EGRET observations above 1 GeV with a differential spectral index of 2.4, we derive an upper limit of 3.0 × 10^-8 cm^-2 s^-1 sr^-1 for the average diffuse emission above 500 GeV in the Galactic latitude range from -2° to +2° at Galactic longitude 40°. Comparisons with EGRET observations provide a lower limit of 2.31 for the differential spectral index of the diffuse emission, assuming there is no break in the spectrum between 30 and 500 GeV. This constrains models for diffuse emission with a significant inverse Compton contribution.

We study radiation hydrodynamical normal modes of radiation-supported accretion disks in the WKB limit. It has long been known that in the large optical depth limit the standard equilibrium is unstable to convection. We study how the growth rate depends on location within the disk, optical depth, disk rotation, and the way in which the local dissipation rate depends on density and pressure. The greatest growth rates are found near the disk surface. Rotation stabilizes vertical wavevectors, so that growing modes tend to have nearly horizontal wavevectors. Over the likely range of optical depths, the linear growth rate for convective instability has only a weak dependence on disk opacity. Perturbations to the dissipation have little effect on convective mode growth rates but can cause growth of radiation sound waves.

We present self-similar solutions for advection-dominated accretion flows with radial viscous force in the presence of outflows from the accretion flow or infall. The axisymmetric flow is treated in variables integrated over polar sections and the effects of infall and outflows on the accretion flow are parametrized for possible configurations compatible with the self-similar solution. We investigate the resulting accretion flows for three different viscosity laws and derive upper limits on the viscosity parameter α. In addition, we find a natural connection to nonrotating and spherical accretion with turbulent viscosity, which is assumed to persist even without differential rotation. Positive Bernoulli numbers for advection-dominated accretion allow a fraction of the gas to be expelled in an outflow, and the upper limit on the viscosity predicts that outflows are inevitable for equations of state close to an ideal gas.

We have studied the effect of adopting different values of the total baryonic mass surface density in the local disk at the present time, Σ(R_☉, t_Gal), on the model of the chemical evolution of the Galaxy. We have compared our model results with the G dwarf metallicity distribution, the amounts of gas, stars, and stellar remnants, the infall rate, and the supernova rate in the solar vicinity, and with the radial abundance gradients and gas distribution in the disk. This comparison strongly suggests that the value of Σ(R_☉, t_Gal) that best fits the observational properties should lie in the range 50-75 M_☉ pc^-2 and that values of the total disk mass surface density outside this range should be ruled out.

The Space Interferometry Mission (SIM) is the instrument of choice when it comes to observing astrometric microlensing events where nearby, usually high proper motion, stars ("lenses") pass in front of more distant stars ("sources"). Each such encounter produces a deflection in the source's apparent position that, when observed by SIM, can lead to a precise mass determination of the nearby lens star. We search for lens-source encounters during the 2005-2015 period using Hipparcos, ACT, and NLTT to select lenses, and USNO-A2.0 to search for the corresponding sources, and rank these by the SIM time required for a 1% mass measurement.

For Hipparcos and ACT lenses, the lens distance and lens-source impact parameter are precisely determined so that the events are well characterized. We present 32 candidates beginning with a 61 Cyg A event in 2012 that requires only a few minutes of SIM time. Proxima Centauri and Barnard's star each generate several events. For NLTT lenses, the distance is known only to a factor of 3, and the impact parameter only to 1''. Together, these produce uncertainties of a factor ~10 in the amount of SIM time required. We present a list of 146 NLTT candidates and show how single-epoch CCD photometry of the candidates could reduce the uncertainty in SIM time to a factor of ~1.5.

The evolution and appearance of protostellar disks can be significantly altered by their UV environment. We investigate numerically the photoevaporation of protostellar disks under the influence of an external radiation field with both EUV (hν > 13.6 eV) and FUV (6 eV < hν < 13.6 eV) components. Our two-dimensional axisymmetric radiation hydrodynamics calculations begin with star-disk configurations resulting from previously published collapse simulations. We follow the evolution after the external UV radiation source has been turned on. We consider the transfer of both direct (from the UV point source) as well as diffuse radiation fields simultaneously with the ionization of hydrogen and carbon. A simplified cooling function is employed which assumes that the carbon ionization front separates the molecular region from the region in atomic or ionized form. For some simulations an isotropic stellar wind has been included at the position of the disk's central star. At selected evolutionary times a frequency-dependent ray-tracing diagnostic code is used to calculate emission line spectra and emission line maps over the volume of interest. The interaction of the FUV-induced neutral flow at the disk surface with the direct and diffuse EUV radiation fields leads to the typical head-tail objects with bright emission line crescents and tails pointing away from the external radiation source. The properties of the head-tail objects are in agreement with the properties of the proplyds in the Orion Nebula, M8, NGC 2024, and—in a more extreme UV environment—of the newly discovered proplyds in NGC 3603. After losing material via photoevaporation over a time ≳10⁵ yr, our initially rather massive disks are reduced to typical observed disk masses. At this time the radius of the disk, the radius of the hydrogen ionization front, and the length of the tail are compatible to observed proplyds. Our model disks can be either silhouetted or nonsilhouetted in the emission line maps, depending on orientation. The [O III] 5007 Å emission appears more diffuse than [O II] 3726 Å, because the abundance of O III is low near the hydrogen ionization front and in the shadow regions along the tail. Monopolar and bipolar microjets emerging from the proplyds can be explained by spherically symmetric stellar winds hydrodynamically focused by the neutral evaporating flow from the disk surface.

We perform direct three-dimensional numerical simulations for magnetohydrodynamic (MHD) turbulence in a periodic box of size 2π threaded by strong uniform magnetic fields. We use a pseudospectral code with hyperviscosity and hyperdiffusivity to solve the incompressible MHD equations. We analyze the structure of the eddies as a function of scale. A straightforward calculation of anisotropy in wavevector space shows that the anisotropy is scale independent. We discuss why this is not the true scaling law and how the curvature of large-scale magnetic fields affects the power spectrum and leads to the wrong conclusion. When we correct for this effect, we find that the anisotropy of eddies depends on their size: smaller eddies are more elongated than larger ones along local magnetic field lines. The results are consistent with the scaling law_∥ ~ recently proposed by Goldreich & Sridhar. Here_∥ (and_⊥) are wavenumbers measured relative to the local magnetic field direction. However, we see some systematic deviations that may be a sign of limitations to the model or our inability to fully resolve the inertial range of turbulence in our simulations.

We have used a multiwavelength data set from the Canadian Galactic Plane Survey (CGPS) to study the Galactic H II region KR 140, both on the scale of the nebula itself and in the context of the star-forming activity in the nearby W3/W4/W5 complex of molecular clouds and H II regions. From both radio and infrared data we have found a covering factor of about 0.5 for KR 140, and we interpret the nebula as a bowl-shaped region viewed close to face on. Extinction measurements place the region on the near side of its parent molecular cloud. The nebula is kept ionized by one O8.5 V(e) star, VES 735, which is less than a few million years old. CO data show that VES 735 has disrupted much of the original molecular cloud for which the estimated mass and density are about 5000 M_☉ and 100 cm^-3, respectively. KR 140 is isolated from the nearest star-forming activity, in W3. Our data suggest that KR 140 is an example of spontaneous (i.e., nontriggered) formation of, unusually, a high-mass star.

We have monitored the radio flux density of 21 pulsars on a daily basis for five years. The 610 MHz flux density time series for these pulsars range from nearly constant for the most distant and heavily scattered pulsars to rapidly varying, saturated time series for more nearby pulsars. The measured stability of the flux density from the most distant pulsars (variations less than 5%) implies that the average radio emission from pulsars, before it has been affected by propagation through the interstellar medium, is constant in strength on timescales of a few hours to several years. The modulation index of the flux density variations never exceeds 0.5, ruling out a density inhomogeneity spectrum with a steep power-law exponent (β > 4). The flux density variations for 15 of the pulsars are consistent with a Kolmogorov turbulence spectrum over a range of more than 5 orders of magnitude in scattering strength, with no detectable presence of an inner scale. For these lines of sight we constrain the inhomogeneity slope to be in the range 3.5 ≤ β ≤ 3.7, which brackets the Kolmogorov value of β = 3.67. The flux density variations are greater than predicted by this model for six pulsars—including the Crab and Vela—but this group is consistent with a Kolmogorov spectrum and an inner scale of ≈10¹⁰ cm. The lines of sight to three of the other pulsars in this group pass through H II regions around young, hot stars. For six pulsars we have found a change in the slope of the intensity structure function, which could be connected with a change in the slope of the inhomogeneity power spectrum at a scale of ≈10¹³ cm.

The Crab Nebula has been observed by the HEGRA (High-Energy Gamma-Ray Astronomy) stereoscopic system of imaging air Cerenkov telescopes (IACTs) for a total of ~200 hr during two observational campaigns: from 1997 September to 1998 March and from 1998 August to 1999 April. The recent detailed studies of system performance give an energy threshold and an energy resolution for γ-rays of 500 GeV and ~18%, respectively. The Crab energy spectrum was measured with the HEGRA IACT system in a very broad energy range up to 20 TeV, using observations at zenith angles up to 65°. The Crab data can be fitted in the energy range from 1 to 20 TeV by a simple power law, which yields dJ_γ/dE = (2.79 ± 0.02 ± 0.5) × 10^-7(E/1 TeV)^{-2.59±0.03±0.05} photons m^-2 s^-1 TeV^-1. The Crab Nebula energy spectrum, as measured with the HEGRA IACT system, agrees within 15% in the absolute scale and within 0.1 units in the power-law index with the latest measurements by the Whipple, CANGAROO, and CAT groups, consistent within the statistical and systematic errors quoted by the experiments. The pure power-law spectrum of TeV γ-rays from the Crab Nebula constrains the physics parameters of the nebula environment as well as the models of photon emission.

71 Tauri (HD 28052; F0 IV-V) is an enigmatic object for two reasons: (1) it is the second brightest X-ray source in the Hyades, yet early F stars as a rule are not strong coronal emitters; and (2) it lies a magnitude above the cluster main sequence, but radial velocity studies and speckle imaging suggest that it is single. Recently, long-slit ultraviolet spectra of the star, obtained with the Space Telescope Imaging Spectrograph (STIS), serendipitously have revealed the presence of a stellar companion at a distance of 01 directly south of the primary. The companion is seen only in its far-UV chromospheric emission lines. The nature of this object cannot be determined from our STIS spectra alone, but its high emission levels are most readily explained if it is a close binary of coronally active dG/dK stars. The presence of the secondary can account for the striking X-ray properties of 71 Tau but not its unusual location in the cluster color-magnitude diagram. It is conceivable that the primary itself is a close double of nearly equal stars, making 71 Tau a possible quadruple system. The alternative—that 71 Tau is ~150 Myr older than other members of the Hyades, approaching the end of core hydrogen burning for a 2 M_☉ star—would challenge the presumed synchrony of star formation in the cluster.

We present the results of Monte Carlo simulations for the dynamical evolution of star clusters containing two stellar populations with individual masses m₁ and m₂ > m₁, and total masses M₁ and M₂ < M₁. We use both King and Plummer model initial conditions, and we perform simulations for a wide range of individual and total mass ratios, m₂/m₁ and M₂/M₁. We ignore the effects of binaries, stellar evolution, and the galactic tidal field. The simulations use N = 10⁵ stars and follow the evolution of the clusters until core collapse. We find that the departure from energy equipartition in the core follows approximately the theoretical predictions of Spitzer and Lightman & Fall, and we suggest a more exact condition that is based on our results. We find good agreement with previous results obtained by other methods regarding several important features of the evolution, including the precollapse distribution of heavier stars, the timescale on which equipartition is approached, and the extent to which core collapse is accelerated by a small subpopulation of heavier stars. We briefly discuss the possible implications of our results for the dynamical evolution of primordial black holes and neutron stars in globular clusters.

Statistical sampling from the stellar initial mass function (IMF) for all star-forming regions in the Galaxy would lead to the prediction of ~1000 M_☉ stars unless there is a rapid turn-down in the IMF beyond several hundred solar masses. Such a turn-down is not necessary for dense clusters because the number of stars sampled is always too small. Although no upper mass limits to star formation have ever been observed, a theory for the IMF should be able to explain the lack of ~1000 M_☉ stars in normal galaxy disks. Here we explore several mechanisms for an upper mass cutoff, including an exponential decline of the star formation probability after a turbulent crossing time. The results are in good agreement with the observed IMF over the entire stellar mass range, and they give a gradual turn-down compared to the Salpeter function above ~100 M_☉ for the normal thermal Jeans mass, M_J. However, they cannot give both the observed power-law IMF out to the high-mass sampling limit in dense clusters and the observed lack of supermassive stars in whole galaxy disks. The exponential decline is too slow for this. Either there is a sharp upper mass cutoff in the IMF, perhaps from self-limitation, or the IMF is different for dense clusters than for the majority of star formation that occurs at lower density. In the latter case, dense clusters would have to form an overabundance of massive stars relative to the average IMF in a galaxy. Evidence for a difference in the cluster and field IMFs supports this picture, but systematic effects could mimic this evidence even with a universal IMF. Within the framework of the sampling model, the upper mass turn-down should shift toward higher mass when M_J shifts upward, as might be the case in some starburst galaxies, and shift toward lower mass when M_J is lower, as might be the case in ultracold or high-pressure regions. Supermassive stars may therefore be possible in starburst galaxies, while in low surface brightness regions, where ultracold gas might exist at normal pressures, or in galactic cluster cooling flows, where cold gas could have extremely high pressures, a high fraction of the star formation could end up as brown dwarfs.

The evolutionary state of magnetic Ap stars is rediscussed using the recently released Hipparcos data. The distribution of the magnetic Ap stars of mass below 3 M_☉ in the H-R diagram differs from that of the normal stars in the same temperature range at a high level of significance. Magnetic stars are concentrated toward the center of the main-sequence band. This is shown in two forms of the H-R diagram: one where log L is plotted against log T_eff and a version more directly tied to the observed quantities, showing the astrometry-based luminosity (Arenou & Luri) against the (B2-G)₀ index of Geneva photometry. In particular, it is found that magnetic fields appear only in stars that have already completed at least approximately 30% of their main-sequence lifetime. No clear picture emerges as to the possible evolution of the magnetic field across the main sequence. Hints of some (loose) relations between magnetic field strength and other stellar parameters are found: stars with shorter periods tend to have stronger fields, as do higher temperature and higher mass stars. A marginal trend of the magnetic flux to be lower in more slowly rotating stars may possibly be seen as suggesting a dynamo origin for the field. No correlation between the rotation period and the fraction of the main-sequence lifetime completed is observed, indicating that the slow rotation in these stars must already have been achieved before they became observably magnetic.

The prediction of the fluctuation theory of stellar mass loss is shown to be compatible with the principle of minimum entropy production.

We present a grid of spherically symmetric model atmospheres for young pre-MS stars. This grid spans the parameter range 2000 K ≤ T_eff ≤ 6800 K and 2.0 ≤ log g ≤ 3.5 for M = 0.1 M_☉, appropriate for low-mass stars and brown dwarfs. A major improvement is the replacement of TiO and H₂O line lists with the newer line list, calculated by the NASA-Ames group, for TiO (about 175 million lines of five isotopes) and for H₂O (about 350 million lines in two isotopes). We provide the model structures, spectra, and broadband colors in standard filters in electronic form.

Pre-white dwarf (PWD) evolution can be driven by energy losses caused by neutrino emission in the core. Unlike the solar neutrino flux, this is not the by-product of nuclear fusion but is instead the result of electron-scattering processes in the hot, dense regions of the PWD core. We show that the observed rate of period change in cool PWD pulsators will constrain neutrino emission in their cores, and we identify appropriate targets for future observation. Such a measurement will tell us whether the theories of lepton interactions correctly describe the production rates and therefore neutrino cooling of PWD evolution. This would represent the first test of standard lepton theory in dense plasma.

We have obtained time-resolved ultraviolet spectroscopy for the pulsating DAV stars G226-29 and G185-32 and for the pulsating DBV star PG 1351+489 with the Hubble Space Telescope Faint Object Spectrograph to compare the ultraviolet to the optical pulsation amplitude and determine the pulsation indices. We find that for essentially all observed pulsation modes, the amplitude rises to the ultraviolet as the theoretical models predict for l = 1 nonradial g-modes. We do not find any pulsation mode visible only in the ultraviolet, nor any modes whose phase flips by 180° in the ultraviolet, as would be expected if high l pulsations were excited. We find one periodicity in the light curve of G185-32, at 141 s, which does not fit theoretical models for the change of amplitude with wavelength of g-mode pulsations.

Ginga and Rossi X-Ray Timing Explorer observations have allowed an unprecedented view of the recurrent systematic pulse shape changes associated with the 35 day cycle of Hercules X-1, a phenomenon currently unique among the known accretion-powered pulsars. We present observations of the pulse shape evolution. An explanation for the pulse evolution in terms of a freely precessing neutron star is reviewed and shown to have several major difficulties in explaining the observed pulse evolution pattern. Instead, we propose a phenomenological model for the pulse evolution based on an occultation of the pulse-emitting region by the tilted, inner edge of a precessing accretion disk. The systematic and repeating pulse shape changes require a resolved occultation of the pulse emission region. The observed pulse profile motivates the need for a pulsar beam consisting of a composite coaxial pencil and fan beam, but the observed evolution pattern requires the fan beam to be focused around the neutron star and beamed in the antipodal direction. The spectral hardness of the pencil beam component suggests an origin at the magnetic polar cap, with the relatively softer fan beam emission produced by backscattering from within the accretion column, qualitatively consistent with several theoretical models for X-ray emission from the accretion column of an accreting neutron star.

We have carried out a precise energy spectral analysis of the superluminal jet source GRS 1915+105 observed with ASCA six times from 1994 to 1999. The source was so bright that most SIS data suffered from event pileup. We have developed a new technique to circumvent the pileup effect, which enabled us to study the spectrum in detail and at high resolution (ΔE/E ≈ 2%). In the energy spectra of 1994 and 1995, resonant absorption lines of Ca XX Kα, Fe XXV Kα, Fe XXVI Kα, as well as blends of the absorption lines of Ni XXVII Kα + Fe XXV Kβ and Ni XXVIII Kα + Fe XXVI Kβ, were observed. Such absorption lines have not been found in other objects, except for iron absorption lines from GRO J1655-40, another superluminal jet source. We carried out a "curve of growth" analysis for the absorption lines and estimated column densities of the absorbing ions. We found that a plasma of moderate temperature (0.1-10 keV) and cosmic abundance cannot account for the observed large equivalent widths. The hydrogen column density of such plasma would be so high that the optical depth of Thomson scattering would be too thick (N_H ≳ 10²⁴ cm^-2). We require either a very high kinetic temperature of the ions (≳100 keV) or extreme overabundances (≳100 Z_☉). In the former case, the ion column densities have reasonable values of 10¹⁷-10¹⁸ cm^-2. We modeled the absorber as a photoionized disk which envelops the central X-ray source. Using a photoionization calculation code, we constrain physical parameters of the plasma disk, such as the ionization parameter, radius, and density. Estimated parameters were found to be consistent with those of a radiation-driven disk wind. These absorption line features may be peculiar to superluminal jet sources and related to the jet formation mechanism. Alternatively, they may be common characteristics of supercritical edge-on systems.

We model the linear polarization of the radiation of β Pic scattered by dust particles in the circumstellar disk. The observed spatial distribution and the wavelength dependence of the polarization together with the colors of the β Pic disk require that particles in a wide size range be present in the disk, with the grains smaller than a few microns in size being somewhat depleted but still of importance for the polarization and colors. The inferred size distribution is consistent with the production and loss mechanisms: the sources—presumably collisions and evaporation of large bodies—continuously produce dust with a power-law size distribution with the exponent ~3.5 over a broad range of sizes, but the particles smaller than a few microns are blown away by the radiation pressure, which shortens the time they spend in the disk and decreases their number densities. Compact (or slightly porous) silicates are found to give better agreement with the observations, although other materials are still not ruled out and a high fluffiness of the large particles is possible. The observed asymmetry in the polarization of two wings can be explained if more small grains (by 20%-30%) are present on the northeast side of the disk. We show that such an asymmetry in the size distributions in two wings might be caused by an influence of the interstellar medium; a required amount of small grains could be produced by destructive collisions of interstellar grains with the circumstellar dust particles.

We present new coronagraphic images of β Pictoris obtained with the Space Telescope Imaging Spectrograph (STIS) in 1997 September. The high-resolution images (01) clearly detect the circumstellar disk as close to the star as 075, corresponding to a projected radius of 15 AU. The images define the warp in the disk with greater precision and at closer radii to β Pic than do previous observations. They show that the warp can be modeled by the projection of two components: the main disk and a fainter component, which is inclined to the main component by 4°-5° and extends only as far as ≈4'' from the star. We interpret the main component as arising primarily in the outer disk and the tilted component as defining the inner region of the disk. The observed properties of the warped inner disk are inconsistent with a driving force from stellar radiation. However, warping induced by the gravitational potential of one or more planets is consistent with the data. Using models of planet-warped disks constructed by Larwood & Papaloizou, we derive possible masses of the perturbing object.

We have obtained optical spectra of the soft X-ray transient GRO J1655-40 during different X-ray spectral states (quiescence, high-soft, and hard outburst) between 1994 August and 1997 June. Characteristic features observed during the 1996-1997 high-soft state were: (1) broad absorption lines at Hα and Hβ, probably formed in the inner disk; (2) double-peaked He II λ4686 emission lines, formed in a temperature-inversion layer on the disk surface, created by the soft X-ray irradiation; and (3) double-peaked Hα emission, with a strength associated with the hard X-ray flux, suggesting that it was probably emitted from deeper layers than He II λ4686. The He II λ4686 line profile appeared approximately symmetric, as we would expect from a disk surface with an axisymmetric emissivity function. The Balmer emission, on the other hand, appeared to come only from a double-armed region on the disk, possibly the locations of tidal density waves or spiral shocks. The observed rotational velocities of all the double-peaked lines suggest that the disk was extended slightly beyond its tidal radius. Three classes of lines were identified in the spectra taken in 1994 August-September, during a period of low X-ray activity between two strong X-ray flares: broad absorption, broad (flat-topped) emission, and narrow emission. We have found that the narrow (single-peaked or double-peaked) emission lines cannot be explained by a conventional thin accretion disk model. We propose that the system was in a transient state, in which the accretion disk might have had an extended optically thin cocoon and significant matter outflow, which would also explain the systematic blueshift of the narrow emission lines and the flat-topped profiles of the broad emission lines. After the onset of a hard X-ray flare the disk signatures disappeared, and strong single-peaked Hα and Paschen emission was detected, suggesting that the cocoon became opaque to optical radiation. High-ionization lines disappeared or weakened. Two weeks after the end of the flare, the cocoon appeared to be once again optically thin.

The possible scenario of Alfvén wave transformation into other MHD waves due to inhomogeneous flow in different regions of solar wind/corona is examined. We have chosen four examples with different plasma parameters and velocity shear parameters as representative cases for the plasma conditions. Two of the cases are representative of coronal holes and streamers at a radial distance of 1.6 R_☉. The third example examines the effect of an increased velocity shear due to the vicinity of a boundary. The fourth case represents solar wind conditions at 1 AU, where the plasma parameter is about unity. To study the wave coupling in the above described cases, we make use of "nonmodal" formalism, which appears to be a useful tool for wave processes taking place in a plasma with velocity shear. Our finding is that the possibility of coupling between Alfvén waves and fast magnetosonic waves exists in coronal holes while for streamer conditions the Alfvén mode couples to the slow magnetosonic mode. In the inner corona, it seems that these two possible ways of coupling do not overlap each other in the same structures but beyond the Alfvénic point, where β is close to unity, both of them can operate at the same time and the mechanism is more sensitive to the background plasma conditions.

The dynamics of the convection occurring in the outer 30% by radius of the Sun must determine the transport of energy, angular momentum, and magnetic fields, thereby producing the observed solar differential rotation and playing a strong part in the solar cycle. Attempts to understand the nature of solar convection by analogy with laboratory experiments (e.g., Rayleigh-Bénard convection) are of limited usefulness owing to the large density contrast in the solar convection zone (which cannot be reproduced in the laboratory) and the distributed heating and cooling (produced by the divergence of the radiative flux), which contrasts with the forcing from thermal boundary layers in the laboratory experiments. Given these considerations, numerical simulations appear to provide the best chance of understanding solar convection. Such simulations have typically solved the full Navier-Stokes equation with a greatly enhanced viscosity and the heat transport equation with a constant thermal diffusivity, as well as prescribed temperatures or heat fluxes at the boundaries. Thermal and viscous boundary layers that generate small-scale turbulence tending to fill the computational domain are typically formed. We choose instead to solve the Euler equation and the total energy equation with thermal forcing diagnosed from a standard solar model (but with an extended region of near-surface cooling). We employ the anelastic approximation to filter out sound waves (which play a comparatively small role in convection-zone dynamics owing to the relatively small Mach number) and use a conservative, monotonicity-preserving numerical scheme, which is second-order accurate in smooth regions of the flow and first-order accurate (i.e., dissipative) in rapidly varying regions. We carry out a range of simulations with four different density contrasts up to 30 (which is still modest compared to the value of 10⁷ in the solar convection zone) and at two different numerical resolutions and analyze them paying particular attention to the horizontal scales of the convective structures formed. For the cases with higher density contrasts, we find a small-scale granulation pattern at the surface (whose horizontal scale is proportional to the density scale height) giving way to larger scale plumelike structures in the interior. The horizontal scales of convection are almost insensitive to the numerical resolution, suggesting a good degree of convergence in the results.

Faraday rotation observations of polarized radiation from natural radio sources are unique among remote diagnostics of the solar corona in that they provide information on the coronal magnetic field. Dual frequency radio polarization measurements yield the rotation measure, a quantity that is proportional to the integral along the line of sight of the product of the electron density and the line-of-sight component of the magnetic field. We made linear polarization observations with the NRAO Very Large Array of 13 polarized radio sources occulted by the solar corona. The observations were made at frequencies of 1465 and 1665 MHz on four days in 1997 May and cover a 20 day period, sampling elongations ranging from about 5 to 14 R_☉. The magnitudes of the rotation measures observed range from about 11 to 0 rad m^-2. The relatively low values for the rotation measures are due to the solar minimum configuration of the corona at the time of the observations, with the lines of sight to the sources generally not crossing sector boundaries. The largest rotation measure was observed for the extended radio source 3C 79 on 1997 May 11 and corresponds to a case in which the line of sight passed next to the streamer belt at small solar elongations. We have developed a three-dimensional model of the solar corona that is in excellent agreement with the observed rotation measures, as well as being completely consistent with other coronal diagnostics such as coronagraph images. In particular, our observations support the coronal magnetic field model of Pätzold et al. (1987); they would be inconsistent with coronal magnetic fields significantly weaker or stronger than this model. The plasma density distribution in the corona is successfully modeled by a dense streamer belt component and a more tenuous coronal hole component. Details of these models are given in § 3 of this paper. The principal disagreement between the model and observations occurs for three lines of sight for which the model predicts nearly zero rotation measure but for which we measure small but significant values of -1 to -2 rad m^-2. These lines of sight passed over the solar polar regions. We discuss the possibility that these residual rotation measures are due to static coronal plasma structures, not described by global coronal models, or to very long wavelength coronal Alfvén waves. Fluctuations in the rotation measure on timescales of a few hours were observed for some sources and not others. When detected, they were of order 1-2 rad m^-2 and occurred on timescales of several hours.

Under the standard model for recombination of the primeval plasma and the cold dark matter model for structure formation, recent measurements of the first peak in the angular power spectrum of the cosmic microwave background temperature indicate that the spatial geometry of the universe is nearly flat. If sources of Lyα resonance radiation, such as stars or active galactic nuclei, were present at z ~ 1000 they would delay recombination, shifting the first peak to larger angular scales and producing a positive bias in this measure of space curvature. It can be distinguished from space curvature by its suppression of the secondary peaks in the spectrum.

We present a weak lensing analysis of a region around the galaxy cluster Cl 1604+4304 (z = 0.897) on the basis of the deep observations with the Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 (WFPC2). We apply a variant of Schneider's aperture mass technique to the observed WFPC2 field and obtain the distribution of weak lensing signal-to-noise ratio (S/N) within the field. The resulting S/N map reveals a clear pronounced peak located about 17 (850 h kpc at z = 0.897) southwest of the second peak associated with the optical cluster center determined from the dynamical analysis of Postman et al. A nonlinear finite-field inversion method has been used to reconstruct the projected mass distribution from the observed shear field. The reconstructed mass map shows a supercritical feature at the location of the S/N peak as well as in the cluster central region. Assuming the redshift distribution of field galaxies, we obtain the total mass in the observed field to be 1.0 × 10¹⁵hM_☉ for ⟨z⟩ = 1.0. The estimated mass within a circular aperture of radius 280 h kpc centered on the dark clump is 2.4 × 10¹⁴hM_☉. We have confirmed the existence of the "dark" mass concentration from another deep HST observation with a slightly different (~20'') pointing.

The masses of supermassive black holes correlate almost perfectly with the velocity dispersions of their host bulges, M_bh ∝ σ^α, where α = 4.8 ± 0.5. The relation is much tighter than the relation between M_bh and bulge luminosity, with a scatter no larger than expected on the basis of measurement error alone. Black hole masses recently estimated by Magorrian et al. lie systematically above the M_bh-σ relation defined by more accurate mass estimates, some by as much as 2 orders of magnitude. The tightness of the M_bh-σ relation implies a strong link between black hole formation and the properties of the stellar bulge.

We describe a correlation between the mass M_bh of a galaxy's central black hole and the luminosity-weighted line-of-sight velocity dispersion σ_e within the half-light radius. The result is based on a sample of 26 galaxies, including 13 galaxies with new determinations of black hole masses from Hubble Space Telescope measurements of stellar kinematics. The best-fit correlation is M_bh = 1.2(±0.2) × 10⁸M_☉(σ_e/200 km s^-1)^{3.75 (±0.3)} over almost 3 orders of magnitude in M_bh; the scatter in M_bh at fixed σ_e is only 0.30 dex, and most of this is due to observational errors. The M_bh-σ_e relation is of interest not only for its strong predictive power but also because it implies that central black hole mass is constrained by and closely related to properties of the host galaxy's bulge.

The twin peaks in the nucleus of M31 have been interpreted by Tremaine as a thick, eccentric disk of stars orbiting a massive dark object; the required alignment of the apoapsides of the stellar orbits could be maintained by self-gravity, and the whole structure might be a discrete, nonlinear eigenmode. The pattern speed of this mode could, in principle, be determined by the Tremaine-Weinberg (TW) method, which requires measurements of the surface brightness and radial velocity along a strip parallel to the line of nodes. However, spectroscopic observations along the line of nodes are not available. We propose a variant of the TW method, which exploits a basic feature of the eccentric disk model, to extract estimates of the pattern speed from Hubble Space Telescope spectroscopic data, taken along the line joining the two peaks. Within limitations imposed by the data, we estimate that the pattern rotates in a prograde manner and, for an assumed disk inclination of 77°, the pattern speed |Ω_p| < 30 km s^-1 pc^-1, or the period is more than 200,000 yr.

MeV seed photons produced in shocks in a variable ultrarelativistic outflow gain energy by the Fermi mechanism, because the photons Compton scatter off relativistically colliding shells. The Fermi-modified high-energy photon spectrum has a nonuniversal slope and a universal cutoff. A significant increase in the total radiative efficiency is possible. In some gamma-ray bursts, most of the power might be emitted at the high-energy cutoff for this mechanism, which would be close to 100 MeV for outflows with a mean bulk Lorentz factor of 100.

The fraction of a fireball kinetic energy that is radiated by internal shocks is sensitive to the amplitude of initial fluctuations in the fireball. We give a simple analytical description for the dissipation of modest-amplitude fluctuations and confirm it with direct numerical simulations. At high amplitudes, the dissipation occurs in a nonlinear regime with efficiency approaching 100%. Most of the fireball energy can then be radiated away by the prompt gamma-ray burst, and only a fraction remains for the afterglow.

Recent observational studies on galaxies in distant clusters discovered a significant fraction of possible dusty starburst galaxies with the so-called e(a) spectra that are characterized by strong Hδ absorption and relatively modest [O II] emission. We numerically investigate spectroscopic and photometric evolution of dusty starburst galaxies in order to clarify the origin of the e(a) spectra. We found that if a young starburst population is preferentially obscured by dust over an old one in a dusty starburst galaxy, the galaxy shows an e(a) spectrum. It is therefore confirmed that the selective dust extinction, which was first suggested by Poggianti & Wu and means that the strongest dust extinction occurs in the youngest among stellar populations with different ages, is critically important to reproduce quantitatively the observed e(a) spectra for the first time in the present numerical study. We also discuss what physical process is closely associated with this selective dust extinction in a cluster environment.

We present a spherically symmetric, Newtonian core collapse simulation of a 15 M_☉ star with a 1.28 M_☉ iron core. The time-, energy-, and angle-dependent transport of electron neutrinos (ν_e) and antineutrinos (_e) was treated with a new code that iteratively solves the Boltzmann equation and the equations for neutrino number, energy, and momentum to order O(v/c) in the velocity v of the stellar medium. The supernova shock expands to a maximum radius of 350 km instead of only ~240 km as in a comparable calculation with multigroup flux-limited diffusion (MGFLD) by Bruenn, Mezzacappa, & Dineva. This may be explained by stronger neutrino heating due to the more accurate transport in our model. Nevertheless, after 180 ms of expansion the shock finally recedes to a radius around 250 km (compared to ~170 km in the MGFLD run). The effect of an accurate neutrino transport is helpful but not large enough to cause an explosion of the considered 15 M_☉ star. Therefore, postshock convection and/or an enhancement of the core neutrino luminosity by convection or reduced neutrino opacities in the neutron star seem necessary for neutrino-driven explosions of such stars. We find an electron fraction Y_e > 0.5 in the neutrino-heated matter, which suggests that the overproduction problem of neutron-rich nuclei with mass numbers A ≈ 90 in exploding models may be absent when a Boltzmann solver is used for the ν_e and _e transport.

We present multiwavelength observations of the newly discovered X-ray transient XTE J1118+480 obtained in the rising phase of the 2000 April outburst. This source is located at unusually high Galactic latitude and in a very low absorption line of sight. This made the first Extreme Ultraviolet Explorer (EUVE) spectroscopy of an X-ray transient outburst possible. Together with our Hubble Space Telescope, Rossi X-Ray Timing Explorer, and United Kingdom Infrared Telescope data, this gives unprecedented spectral coverage. We find the source in the low hard state. The flat IR-UV continuum appears to be a combination of optically thick disk emission and possibly synchrotron, while at higher energies (including EUV), a typical low hard state power law is seen. EUVE observations reveal no periodic modulation, suggesting an inclination low enough that no obscuration by the disk rim occurs. We discuss the nature of the source and this outburst and conclude that it may be more akin to minioutbursts seen in GRO J0422+32 than to a normal X-ray transient outburst.

High-resolution spectra of the active binary Capella (G8 III + G1 III) covering the energy range of 0.4-8.0 keV (1.5-30 Å) show a large number of emission lines, demonstrating the performance of the High-Energy Transmission Grating Spectrometer. A preliminary application of plasma diagnostics provides information on coronal temperatures and densities. Lines arising from different elements in a range of ionization states indicate that Capella has plasma with a broad range of temperatures, from log T = 6.3 to 7.2, generally consistent with recent results from observations with the Extreme-Ultraviolet Explorer and the Advanced Satellite for Cosmology and Astrophysics. The electron density is determined from He-like O VII lines, giving the value of N_e ~ 10¹⁰ cm^-3 at T_e ~ 2 × 10⁶ K; He-like lines formed at higher temperatures give only upper limits to the electron density. The density and emission measure from O VII lines together indicate that the coronal loops are significantly smaller than the stellar radius.

A one-zone, shocked wind model is developed for the X-ray luminosity of pulsar nebulae. If the electrons and positrons cool rapidly by synchrotron radiation and the initial particle spectrum is similar to that of the Crab Nebula, the X-ray luminosity is produced from the pulsar power with high efficiency and is insensitive to the model parameters. If the electrons are not in the cooling regime, the X-ray luminosity is produced with lower efficiency, as appears to be the case for the compact nebula around the Vela pulsar. The observed X-ray spectral index is an indicator for which case applies and thus plays a role in estimating the pulsar power needed to produce an observed X-ray luminosity.

We present the ultraviolet spectrum of the SW Sextantis star and nova-like variable DW Ursae Majoris in an optical low state, as observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). The data are well described by a synthetic white dwarf (WD) spectrum with T_eff = 46,000 ± 1000 K, log g = 7.60 ± 0.15, v sin i = 370 ± 100 km s^-1, and Z/Z_☉ = 0.47 ± 0.15. For this combination of T_eff and log g, WD models predict M_WD = 0.48 ± 0.06 M_☉ and R_WD = (1.27 ± 0.18) × 10⁹ cm. Combining the radius estimate with the normalization of the spectral fit, we obtain a distance estimate of d = 830 ± 150 pc. During our observations, DW UMa was approximately 3 mag fainter in V than in the high state. A comparison of our low-state HST spectrum with a high-state spectrum obtained with the International Ultraviolet Explorer shows that the former is much bluer and has a higher continuum level shortward of 1450 Å. Since DW UMa is an eclipsing system, this suggests that an optically thick accretion disk rim blocks our view of the WD primary in the high state. If self-occulting accretion disks are common among the SW Sex stars, we can account for (1) the preference for high-inclination systems within the class and (2) their V-shaped continuum eclipses. Moreover, even though the emission lines produced by a self-obscured disk are generally still double-peaked, they are weaker and narrower than those produced by an unobscured disk. This may allow a secondary line emission mechanism to dominate and produce the single-peaked, optical lines that are a distinguishing characteristic of the SW Sex stars.

We report on the R-band eclipse mapping analysis of high-speed photometry of the dwarf nova EX Dra on the rise to the maximum of the 1995 November outburst. The eclipse map shows a one-armed spiral structure of ~180° in azimuth, extending in radius from R ≃ 0.2R_L1 to 0.43R_L1 (where R_L1 is the distance from the disk center to the inner Lagrangian point), that contributes about 22% of the total flux of the eclipse map. The spiral structure is stationary in a reference frame corotating with the binary and is stable for a timescale of at least five binary orbits. The comparison of the eclipse maps on the rise and in quiescence suggests that the outbursts of EX Dra may be driven by episodes of enhanced mass transfer from the secondary star. Possible explanations for the nature of the spiral structure are discussed.

One of the primary difficulties with using transits to discover extrasolar planets is the low probability a planet has of transiting its parent star. One way of overcoming this difficulty is to search for transits in dense stellar fields, such as the Galactic bulge. Here I estimate the number of planets that might be detected from a monitoring campaign toward the bulge. A campaign lasting 10 nights on a 10 m telescope (assuming 8 hr of observations per night and a 5' × 5' field of view) would detect five to 50 planets with radius R_p = 1.5 R_J or two to 15 planets with R_p = 1.0 R_J, if the frequency of planets in the bulge is similar to that in the solar neighborhood. The precise number detected depends sensitively on the distribution of planets around their parent stars. Most of these planets will be discovered around stars just below the turnoff, i.e., slightly evolved G dwarfs. Campaigns involving 1 or 4 m class telescopes are unlikely to discover any planets, unless there exists a substantial population of companions with R_p > 1.5 R_J.

We present a new method to identify and probe planetary companions of stars in the Galactic bulge and Magellanic Clouds using gravitational microlensing. While spectroscopic study of these planets is well beyond current observational techniques, monitoring polarization fluctuations during high-magnification events induced by binary microlensing events will probe the composition of the planetary atmospheres, an observation which otherwise is currently unattainable even for nearby planetary systems.

We use time series observations from the Solar and Heliospheric Observatory and Yohkoh spacecraft to study solar polar rays. Contrary to our expectations, we find that the rays are associated with active regions on the Sun and are not features of the polar coronal holes. They are extended, hot plasma structures formed in the active regions and projected onto the plane of the sky above the polar coronal holes. We present new observations and simple projection models that match long-lived polar ray structures seen in limb synoptic maps. Individual projection patterns last for at least five solar rotations.

We discuss the behavior of the intensity of the Mg VI λ1191.64 spectral line relative to the intensity of the Ne VI λ1005.78 spectral line as a function of height above the limb in the solar north polar coronal hole. The intensities of Mg VI lines relative to Ne VI lines have been shown to be excellent indicators of element abundance variations due to the first ionization potential (FIP) effect. We find that the Mg VI/Ne VI intensity ratio increases with height above the limb by factors ranging from 1.7 to 4 over a height range extending from about 6'' above the limb to 28'' above the limb. We conclude that this intensity ratio increase is primarily due to an increase of electron temperature with height, rather than the result of an FIP effect, and therefore caution must be exercised in using any Mg VI/Ne VI line ratio as an abundance diagnostic above the limb in the polar holes. At 6'' above the limb, the Mg VI/Ne VI line ratio indicates that the solar Mg/Ne abundance ratio is probably within a factor of 2 of the photospheric abundance ratio. The spectra we use were recorded by the Solar Ultraviolet Measurements of Emitted Radiation spectrometer on the Solar and Heliospheric Observatory spacecraft.