Table of contents

To determine the equation of state of the universe, we propose to use a new independent variable R ≡ (H₀/c) , where H₀ and d_L(z) are the present Hubble parameter and the luminosity distance, respectively. For the flat universe suggested by the observation of the anisotropy of cosmic microwave backgrounds, the density and the pressure are expressed as ρ/ρ₀ = 4(df/dR)²/f⁶ and p/ρ₀ = -4/3(d²f/dR²)/f⁵, where ρ₀ is the present density and f(R) = 1/ ^1/2. In the (R, f) plane the sign as well as the strength of the pressure is in proportion to the curvature of the curve f(R). We propose to adopt a Pade-like expression of f(R) = 1/u^1/2 with u ≡ 1 + u_nRⁿ. For the flat Λ models, the expansion up to N = 7 has at most an error less than 0.2% for z < 1.7 and any value of Λ. We also propose a general method to determine the equation of state of the universe that has N - 1 free parameters. If the number of parameters are smaller than N - 1, there is a consistency check of the equation of state so that we may confirm or refute each model.

The large-scale structure of high-redshift galaxies produces correlated anisotropy in the far-infrared background (FIRB). In regions of the sky where the thermal emission from Galactic dust is well below average, these high-redshift correlations may be the most significant source of angular fluctuation power over a wide range of angular scales, from ~7' to ~3°, and frequencies, from ~400 to ~1000 GHz. The strength of this signal should allow detailed studies of the statistics of the FIRB fluctuations, including the shape of the angular power spectrum at a given frequency and the degree of coherence between FIRB maps at different frequencies. The FIRB correlations depend on and hence constrain the redshift-dependent spectral energy distributions, number counts, and clustering bias of the galaxies and active nuclei that contribute to the background. We quantify the accuracy to which Planck and a newly proposed balloon-borne mission, Explorer of Diffuse Galactic Emissions, could constrain models of the high-redshift universe through the measurement of FIRB fluctuations. We conclude that the average bias of high-redshift galaxies could be measured to an accuracy of ≲1% or, for example, separated into four redshift bins with ~10% accuracy.

We present a method for constraining the evolution of the galaxy luminosity-velocity (LV) relation in hierarchical scenarios of structure formation. The comoving number density of dark matter halos with circular velocities of 200 km s^-1 is predicted in observationally favored cold dark matter (CDM) cosmologies to be nearly constant over the redshift range 0 ≲ z ≲ 5. Any observed evolution in the density of bright galaxies implies in turn a corresponding evolution in the LV relation. We consider several possible forms of evolution for the zero point of the LV relation and predict the corresponding evolution in galaxy number density. The Hubble Deep Field suggests a large deficit of bright (M_V < -19) galaxies at 1.4 ≲ z ≲ 2. If taken at face value, this implies a dimming of the LV zero point by roughly 2 mag. Deep, wide-field, near-IR selected surveys will provide more secure measurements to compare with our predictions.

We derive the distribution of neutral and ionized gas in high-redshift clouds, which are optically thick to hydrogen ionizing radiation, using published data on Lyman limit and damped Lyα absorption systems in the redshift range 1.75 ≤ z < 3.25. We assume that the distribution of the hydrogen total (H I+H II) column density in the absorbers, N_H, follows a power law KN, whereas the observed H I column density distribution deviates from a pure power law as a result of ionization from a background radiation field. We use an accurate radiative transfer code for computing the rapidly varying ratio N_H/N as a function of N_H. Comparison of the models and observations gave excellent fits with maximum likelihood solutions for the exponent α and X, the value of log when the Lyman limit optical depth along the line of sight is τ_LL = 1. The slope of the total gas column density distribution with its relative 3 σ errors is α = 2.7 and X = 2.75 ± 0.35. This value of X is much lower than what would be obtained for a gaseous distribution in equilibrium under its own gravity. The ratio η₀ of dark matter to gas density, however, is not well constrained since log (η₀) = 1.1 ± 0.8. An extrapolation of our derived power-law distribution toward systems of lower column density, the Lyα forest, tends to favor models with log(η₀) ≲ 1.1 and α ~ 2.7-3.3. With α appreciably larger than 2, Lyman limit systems contain more gas than damped Lyα systems and Lyα forest clouds even more. Estimates of the cosmological gas and dark matter density due to absorbers of different column density at z ~ 2.5 are also given.

A new powerful numerical method is developed for solving the time-dependent kinetic equation describing the anisotropic pitch-angle scattering of charged particles. The model includes the effects of adiabatic focusing in a radial magnetic field, adiabatic deceleration, anisotropic pitch-angle scattering, and convection in a magnetized plasma and significantly generalizes a model introduced by Kóta in 1994. The pitch-angle scattering is assumed to scatter slowly through 90°. By applying Legendre polynomial expansions to the particle transport equation, an infinite series of first-order differential equations for the harmonics of the distribution function is obtained. By means of a characteristic method (together with operator splitting), the solution of distributions at certain harmonics is computed. Solutions exhibiting coherent pulses are obtained, and these are identical to the exact analytic results obtained by Kóta. However, the approach presented here allows for arbitrarily anisotropic initial data to be prescribed, and it can also be used to study the dependence of particle distribution on pitch angle. It is shown that the presence of adiabatic focusing results in highly asymmetric particle propagation in opposite directions with typically more particles in the sunward hemisphere than in the antisunward hemisphere, although two oppositely propagating initial beams are introduced symmetrically. An abrupt transition can be found between hemispheres, and the distribution within each hemisphere is quasi-isotropic. The model and approach discussed here lend themselves to the study of the propagation and transport of charged particles and pickup ions.

The combined CfA2/SSRS redshift catalog has recently been improved and made public. We compute the redshift-space power spectrum of this catalog using the regions at -2.5° ≤ δ₁₉₅₀ ≤ 50° north (8^h ≤ α₁₉₅₀ ≤ 17^h) and south (20^h ≤ α₁₉₅₀ ≤ 3^h,b ≤ -13°) of the Galactic plane, where it is 98% complete down to Zwicky magnitude 15.5 and contains 13,681 galaxies. Our analysis uses Heavens-Taylor mode expansion, Karhunen-Loève data compression, and the Fisher matrix technique to compute quadratic band power estimates. This allows an exact calculation of window functions, including the integral constraint, in addition to the production of a power spectrum with uncorrelated error bars. Our results with this larger data set agree well with those of previous studies. We analyze 101 h^-1 Mpc and 130 h^-1 Mpc volume-limited subsets in addition to the full magnitude-limited sample, and our results are well fitted by, e.g., ΛCDM models with bias b = 1.2, 1.4, and 1.4, respectively. We estimate the effect of extinction using the Schlegel, Finkbeiner, & Davis dust map. Our results are exclusively for the redshift space galaxy power spectrum, so they can only be compared with theoretical predictions if appropriate corrections are made for biasing and redshift space distortions.

We present spatially extended gas kinematics at parsec-scale resolution for the nuclear regions of four nearby disk galaxies and model them as rotation of a gas disk in the joint potential of the stellar bulge and a putative central black hole. The targets were selected from a larger set of long-slit spectra obtained with the Hubble Space Telescope as part of the Survey of Nearby Nuclei with STIS. They represent the four galaxies (of 24) that display symmetric gas velocity curves consistent with a rotating disk. We derive the stellar mass distribution from the STIS acquisition images adopting the stellar mass-to-light ratio normalized to match ground-based velocity dispersion measurements over a large aperture. Subsequently, we constrain the mass of a putative black hole by matching the gas rotation curve, following two distinct approaches. In the most general case we explore all the possible disk orientations; alternatively, we constrain the gas disk orientation from the dust-lane morphology at similar radii. In the latter case the kinematic data indicate the presence of a central black hole for three of the four objects, with masses of 10⁷-10⁸M_☉, representing up to ~0.025% of the host bulge mass. For one object (NGC 2787) the kinematic data alone provide clear evidence for the presence of a central black hole even without external constraints on the disk orientation. These results illustrate directly the need to determine black hole masses by differing methods for a large number of objects, demonstrate that the variance in black hole/bulge mass is much larger than previously claimed, and reinforce the recent finding that the black hole mass is tightly correlated with the bulge stellar velocity dispersion σ.

We analyze the nuclear stellar dynamics of the SB0 galaxy NGC 1023, utilizing observational data both from the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope and from the ground. The stellar kinematics measured from these long-slit spectra show rapid rotation (V ≈ 70 km s^-1 at a distance of 01 = 4.9 pc from the nucleus) and increasing velocity dispersion toward the nucleus (where σ = 295 ± 30 km s^-1). We model the observed stellar kinematics assuming an axisymmetric mass distribution with both two and three integrals of motion. Both modeling techniques point to the presence of a central dark compact mass (which presumably is a supermassive black hole) with confidence greater than 99%. The isotropic two-integral models yield a best-fitting black hole mass of (6.0 ± 1.4) × 10⁷M_☉ and mass-to-light ratio (M/L_V) of 5.38 ± 0.08, and the goodness of fit (χ²) is insensitive to reasonable values for the galaxy's inclination. The three-integral models, which nonparametrically fit the observed line-of-sight velocity distribution as a function of position in the galaxy, suggest a black hole mass of (3.9 ± 0.4) × 10⁷M_☉ and M/L_V of 5.56 ± 0.02 (internal errors), and the edge-on models are vastly superior fits over models at other inclinations. The internal dynamics in NGC 1023 as suggested by our best-fit three-integral model shows that the velocity distribution function at the nucleus is tangentially anisotropic, suggesting the presence of a nuclear stellar disk. The nuclear line-of-sight velocity distribution has enhanced wings at velocities ≥600 km s^-1 from systemic, suggesting that perhaps we have detected a group of stars very close to the central dark mass.

In this paper, we apply the perturbative least-action method to model the Local Group of galaxies for various cosmological models. Our models suggest that the classical timing argument overestimates the true mass of the Local Group system. We further show that although the galaxy masses are theoretically good discriminators of Ω_M given some observed Milky Way and M31 separation and radial velocity, current estimates of the masses are insufficient to make any cosmological claims.

We present models of interacting galaxies in order to study the development of spatial offsets between the gaseous and stellar components in tidal tails. Observationally, such large-scale offsets are observed in several systems—most notably NGC 3690—suggesting an interaction between the tidal gas and some (unseen) hot interstellar medium (ISM). Instead, our models show that these offsets are a natural consequence of the radially extended H I spatial distribution in disk galaxies, coupled with internal dissipation in the gaseous component driven by the interaction. This mechanism is most effective in systems involved in very prograde interactions and explains the observed gas/star offsets in interacting galaxies without invoking interactions with a hot ISM, starburst ionization, or dust obscuration within the tails.

We have mapped the 2.6 mm CO J = 1 → 0 emission in three optically selected "Toomre sequence" mergers (NGC 520, NGC 3921, NGC 4676). The molecular gas distribution is well resolved by the observations. For NGC 520 and NGC 4676A, the nuclear gas concentrations form a disklike or a ringlike structure, and the gas kinematics are regular and consistent with simple rotation. Discrete molecular gas complexes are found along the stellar bar in NGC 4676B, and the gas kinematics is consistent with the disk rotation traced in Hα. The molecular gas distribution in NGC 3921 is asymmetric about the stellar remnant, and both the distribution and kinematics suggest that the molecular gas has not settled into the center of the remnant. Molecular gas clouds are detected outside the central regions of NGC 3921 and NGC 4676, and they may be associated with the tidal tails and bridges mapped in H I. Departures from the canonical scenario for a merger involving two large spiral galaxies are found in all three Toomre sequence mergers studied. Our data suggest that one of the progenitor disks in NGC 520 and NGC 3921 was relatively gas-poor. A detailed comparison of these optically selected mergers and more luminous IR-selected mergers is deferred to a separate paper.

We have made a deep NICMOS imaging study of a sample of five z ~ 2-3 radio-quiet quasars with low absolute nuclear luminosities, and we have detected apparent host galaxies in all of these. Most of the hosts have luminosities approximately equal to present-day L_*, with a range from 0.2L_* to about 4L_*. These host galaxies have magnitudes and sizes consistent with those of the Lyman break galaxies at similar redshifts and at similar rest wavelengths but are about 2 mag fainter than high-z powerful radio galaxies. The hosts of our high-z sample are comparable to or less luminous than the hosts of the low-z radio-quiet quasars with similar nuclear absolute magnitudes. However, the high-z galaxies are more compact than the hosts of the low-z quasars and probably have only 10%-20% of the stellar mass of their low-z counterparts. In one host, we find a residual component that is not centered on the quasar nucleus, and several hosts have apparent companions within a projected distance of ~10 kpc, indications that these systems are possibly in some phase of a merger process. Application of the M_bulge/M_BH relation found for present-day spheroids to the stellar masses implied for the high-z host galaxies would indicate that they contain black holes with masses around 10⁸M_☉. Comparison to their nuclear magnitudes implies accretion rates that are near or at the Eddington limit. Although these high-z hosts already contain supermassive black holes, the galaxies will need to grow significantly to evolve into present-day L_* galaxies. These results are basically consistent with theoretical predictions for the hierarchical buildup of the galaxy host and its relation to the central supermassive black hole.

Associated absorption lines (AALs) are valuable probes of the gaseous environments near quasars. Here we discuss high-resolution (6.7 km s^-1) spectra of the AALs in the radio-loud quasar 3C 191 (redshift z = 1.956). The measured AALs have ionizations ranging from Mg I to N V and multicomponent profiles that are blueshifted by ~400 to ~1400 km s^-1 relative to the quasar's broad emission lines. These data yield the following new results: (1) The strengths of excited-state Si II* AALs indicate a density of ~300 cm^-3 in the Si⁺ gas. (2) If the gas is photoionized, this density implies a distance of ~28 kpc from the quasar. Several arguments suggest that all of the lines form at approximately this distance. (3) The characteristic flow time from the quasar is thus ~3 × 10⁷ yr. (4) Strong Mg I AALs identify neutral gas with very low ionization parameter and high density. We estimate n_H ≳ 5 × 10⁴ cm^-3 in this region, compared to ~15 cm^-3 where the N V lines form. (5) The total column density is N_H ≲ 4 × 10¹⁸ cm^-2 in the neutral gas and N_H ~ 2 × 10²⁰ cm^-2 in the moderately ionized regions. These column densities are consistent with 3C 191's strong soft X-ray flux and the implied absence of soft X-ray absorption. (6) The total mass in the AAL outflow is M ~ 2 × 10⁹M_☉, assuming a global covering factor (as viewed from the quasar) of ~10%. (7) The absorbing gas only partially covers the background light source(s) along our line(s) of sight, requiring absorption in small clouds or filaments less than 0.01 pc across. The ratio N_H/n_H implies that the clouds have radial (line-of-sight) thicknesses ≲0.2 pc. These properties might characterize a subclass of AALs that are physically related to quasars but form at large distances. We propose a model for the absorber in which pockets of dense neutral gas are surrounded by larger clouds of generally lower density and higher ionization. This outflowing material might be leftover from a blowout associated with a nuclear starburst, the onset of quasar activity, or a past broad absorption line (BAL) wind phase.

We have analyzed the observed image separations of the gravitationally lensed images of quasars (QSOs) for a possible correlation with the source redshift. Contrary to the previously noted anticorrelation based on a smaller data set, no correlation is found for the currently available data. We have calculated the average image separations of the lensed QSOs as a function of source redshifts for isothermal spheres with cores in a flat universe, taking into account the amplification bias caused by lensing. The shape of the distribution of the average image separation as a function of redshift is very robust and is insensitive to most model parameters. Observations are found to be roughly consistent with the theoretical results for models that assume the lens distribution to be (1) a Schechter luminosity function that, however, cannot produce images with large separation and (2) mass condensations in a cold dark matter universe, as given by the Press-Schechter theory if an upper limit of 1 × 10¹³-7 × 10¹³M_☉ is assumed on the mass of the condensations.

We have observed the canonical gigahertz-peaked spectrum source 0108+388 with the Very Long Baseline Array at a range of frequencies above and below the spectral peak. The activity that dominates the radio emission from 0108+388, which is also classified as a compact symmetric object, is thought to be less than 1000 years old. We present strong evidence that the spectral turnover in 0108+388 results from free-free absorption by nonuniform gas, possibly in the form of a disk in the central tens of parsecs.

We present a new model for low-luminosity X-ray sources in globular clusters, with L_X ≲ 10³⁴ ergs s^-1. The model we propose is that of a single neutron star accreting from cluster gas that has accumulated as a natural product of stellar evolution. An analytic luminosity function is derived under the assumption that the speed distribution of neutron stars in the central region of a cluster is described by a Maxwellian, and that the density and temperature of the gas are uniform. Predictions of the model and implications for the gas content of globular clusters are discussed.

We report on the clustering properties of Lyman break galaxies (LBGs) at z ~ 3. The correlation length of flux-limited samples of LBGs depends on their rest-frame ultraviolet (UV) luminosity at λ ~ 1700 Å, with fainter galaxies being less strongly clustered in space. We have used three samples with progressively fainter flux limits: two extracted from our ground-based survey and one from the Hubble Deep Fields (both North and South). The correlation length decreases by a factor of ≈3 over the range of limiting magnitudes that we have probed, namely, 25 ≲ ≲ 27, suggesting that samples with a fainter UV luminosity limit include galaxies with smaller mass. We have compared the observed scaling properties of the clustering strength with those predicted for cold dark matter (CDM) halos and found that (1) the clustering strength of LBGs follows, within the errors, the same scaling law with the volume density as the halos; and (2) the scaling law predicted for the galaxies using the halo mass spectrum and a number of models for the relationship that maps the halos' mass into the galaxies' UV luminosity depends only on how tightly mass and UV luminosity correlate but is otherwise insensitive to the details of the models. We interpret these results as additional evidence that the strong spatial clustering of LBGs is due to galaxy biasing, supporting the theory of biased galaxy formation and gravitational instability as the primary physical mechanism for the formation of structure. We have also fitted models of the mass-UV luminosity relationship to the data to reproduce simultaneously from the CDM halo mass spectrum the dependence of the correlation length with the UV luminosity and the luminosity function. We have found that (1) a scale invariant relationship between mass and UV luminosity (e.g., a power law) is not supported by the observations, suggesting that the properties of star formation of galaxies change along the mass spectrum of the observed LBGs; (2) the scatter of the UV luminosity of LBGs of given mass must be relatively small for massive LBGs, suggesting that the mass is an important parameter in regulating the activity of star formation in these systems; and (3) the fraction of massive halos at z ~ 3 that are not observed in UV-selected surveys is not large. From the fits, for a given choice of the cosmology, one can assign a scale of mass to the LBGs. For example, if Ω = 0.3 and Ω_Λ = 0.7, the average mass of galaxies with luminosity = 23, 25.5, and 27.0 is = 2.5, 0.9, and 0.4 × 10¹²M_☉, respectively. The numbers are ≈2 times larger and ≈10 times smaller in an open universe with Ω = 0.2 and Ω_Λ = 0 and in the Einstein-de Sitter cosmologies, respectively.

We investigate the star formation and dust extinction properties of very luminous infrared galaxies whose spectra display a strong Hδ line in absorption and a moderate [O II] emission [e(a) spectrum]. This spectral combination has been suggested to be a useful method to identify dusty starburst galaxies at any redshift on the basis of optical data alone. We compare the average e(a) optical spectrum with synthetic spectra that include both the stellar and the nebular contribution, allowing dust extinction to affect differentially the stellar populations of different ages. We find that reproducing the e(a) spectrum requires the youngest stellar generations to be significantly more extinguished by dust than older stellar populations and implies a strong ongoing star formation activity at a level higher than in quiescent spirals. A model fitting the optical spectrum does not necessarily produce the observed FIR luminosity, and this can be explained by the existence of stellar populations which are practically obscured at optical wavelengths. Models in which dust and stars are uniformly mixed yield a reddening of the emerging emission lines which is too low compared to observations: additional foreground reddening is required.

A search for star-forming objects belonging to tidal tails has been carried out in a sample of deep Hα images of 16 compact groups of galaxies. A total of 36 objects with Hα luminosities greater than 10³⁸ ergs s^-1 has been detected in five groups. The fraction of the total Hα luminosity of their respective parent galaxies shown by the tidal objects is always below 5% except for the tidal features of Hickson compact group 95, whose Hα luminosity amounts to 65% of the total luminosity. Out of these 36 objects, nine star-forming tidal dwarf galaxy candidates finally have been identified on the basis of their projected distances to the nuclei of the parent galaxies and their total Hα luminosities. Overall, the observed properties of the candidates resemble those previously reported for the so-called tidal dwarf galaxies.

We have used a suite of simplified spectrophotometric spiral galaxy evolution models to argue that there are substantial variations in stellar mass-to-light (M/L) ratios within and among galaxies, amounting to factors of between 3 and 7 in the optical and factors of 2 in the near-infrared. Our models show a strong correlation between stellar M/L and the optical colors of the integrated stellar populations. Under the assumption of a universal spiral galaxy initial mass function (IMF), relative trends in model stellar M/L with color are robust to uncertainties in stellar population and galaxy evolution modeling, including the effects of modest bursts of star formation. Errors in the dust-reddening estimates do not strongly affect the final derived stellar masses of a stellar population. We examine the observed maximum disk stellar M/L ratios of a sample of spiral galaxies with accurate rotation curves and optical and near-infrared luminosity profiles. From these observed maximum disk M/L ratios we conclude that a Salpeter IMF has too many low-mass stars per unit luminosity but that an IMF similar to the Salpeter IMF at the high-mass end with less low-mass stars (giving stellar M/L ratios 30% lower than the Salpeter value) is consistent with the maximum disk constraints. Trends in observed maximum disk stellar M/L ratios with color provide a good match to the predicted model relation, suggesting that the spiral galaxy stellar IMF is universal and that a fraction of (particularly high surface brightness) spiral galaxies may be close to maximum disk. We apply the model trends in stellar M/L ratio with color to the Tully-Fisher (T-F) relation. We find that the stellar mass T-F relation is relatively steep, has modest scatter, and is independent of the passband and color used to derive the stellar masses, again lending support for a universal IMF. The difference in slope between the optical (especially blue) and near-infrared T-F relations is due to the combined effects of dust attenuation and stellar M/L variations with galaxy mass. Assuming the Hubble Space Telescope Key Project distance to the Ursa Major Cluster and neglecting the (uncertain) molecular gas fraction, we find that the baryonic T-F relation takes the form M_baryon ∝ V^3.5 (with random and systematic 1 σ slope errors of ~0.2 each) when using a bisector fit and rotation velocities derived from the flat part of the rotation curve. Since we have normalized the stellar M/L ratios to be as high as can possibly be allowed by maximum disk constraints, the slope of the baryonic T-F relation will be somewhat shallower than 3.5 if all disks are substantially submaximal.

We present the results of X-ray imaging and spectroscopic analysis of a sample of Seyfert 2 galaxies that contain starbursts, based on their optical and UV characteristics. These composite galaxies exhibit extended, soft, thermal X-ray emission, which we attribute to their starburst components. Comparing their X-ray and far-infrared properties with ordinary Seyfert and starburst galaxies, we identify the spectral characteristics of their various intrinsic emission sources. The observed far-infrared emission of the composite galaxies may be associated almost exclusively with star formation, rather than the active nucleus. The ratio of the hard X-ray luminosity to the far-infrared and [O III] λ5007 luminosity distinguishes most of these composite galaxies from "pure" Seyfert 2 galaxies, while their total observed hard X-ray luminosity distinguishes them from "pure" starbursts. The hard nuclear X-ray source is generally heavily absorbed (N_H > 10²³ cm^-2) in the composite galaxies. Based on these results, we suggest that the interstellar medium of the nuclear starburst is a significant source of absorption. The majority of the sample are located in groups or are interacting with other galaxies, which may trigger the starburst or allow rapid mass infall to the central black hole or both. We conclude that starbursts are energetically important in a significant fraction of active galaxies and that starbursts and active galactic nuclei may be part of a common evolutionary sequence.

The division of galaxies into "barred" (SB) and "normal" (S) spirals is a fundamental aspect of the Hubble galaxy classification system. This "tuning fork" view was revised by de Vaucouleurs, whose classification volume recognized apparent "bar strength" (SA, SAB, SB) as a continuous property of galaxies called the "family." However, the SA, SAB, and SB families are purely visual judgments that can have little bearing on the actual bar strength in a given galaxy. Until very recently, published bar judgments were based exclusively on blue light images, where internal extinction or star formation can either mask a bar completely or give the false impression of a bar in a nonbarred galaxy. Near-infrared camera arrays, which principally trace the old stellar population in both normal and barred galaxies, now facilitate a quantification of bar strength in terms of their gravitational potentials and force fields. In this paper, we show that the maximum value, Q_b, of the ratio of the tangential force to the mean axisymmetric radial force in a barred disk galaxy is a quantitative measure of the strength of a bar. Q_b does not measure bar ellipticity or bar shape but rather depends on the actual forcing due to the bar embedded in its disk. We show that a wide range of true bar strengths characterizes the category "SB," while the de Vaucouleurs category "SAB" corresponds to a narrower range of bar strengths. We present Q_b values for 36 galaxies, and we incorporate our bar classes into a dust-penetrated classification system for spiral galaxies.

Here we report the detection, in Hα emission, of a radial corrugation in the velocity field of the spiral galaxy NGC 5427. The central velocity of the Hα line displays coherent, wavy-like variations in the vicinity of the spiral arms. The spectra along three different arm segments show that the maximum amplitude of the sinusoidal line variations are displaced some 500 pc from the central part of the spiral arms. The peak blueshifted velocities appear some 500 pc upstream of the arm, whereas the peak redshifted velocities are located some 500 pc downstream of the arm. This kinematical behavior is similar to the one expected in a galactic bore generated by the interaction of a spiral density wave with a thick gaseous disk, as recently modeled by Martos & Cox.

We find that variability of the iron Kα line is common in Seyfert 1 galaxies. Using data from the ASCA archive for objects that have been observed more than once during the mission, we study the time-averaged spectra from individual observations, thereby probing variability on timescales that range from days to years. Since the statistics of the data do not warrant searches for line variability in terms of a complex physical model, we use a simple Gaussian to model the gross shape of the line and then use the centroid energy, intensity, and equivalent width as robust indicators of changes in the line profile. We find that ~70% of Seyfert 1 galaxies (10 out of 15) show variability in at least one of these parameters: the centroid energy, intensity, and equivalent width vary in six, four, and eight sources, respectively. Because of the low signal-to-noise ratio, limited sampling, and time averaging, we consider these results to represent lower limits to the rate of incidence of variability. In most cases changes in the line do not appear to track changes in the continuum. In particular, we find no evidence for variability of the line intensity in NGC 4151, suggesting an origin in a region larger than the putative accretion disk, where most of the iron line has been thought to originate. Mrk 279 is investigated on short timescales. The time-averaged effective line energy (as measured by the Gaussian center energy, which is weighted by emission in the entire line profile) is 6.5 keV in the galaxy rest frame. As the continuum flux increases by 20% in a few hours, the Fe K line responds within ~10,000 s, with the effective line energy increasing by 0.22 keV (~10,500 km s^-1). We also examine the ROSAT PSPC spectrum of Mrk 279 but find inconsistencies with ASCA. Problems with the ASCA and ROSAT calibration that affect simultaneous spectral fits at low energies are discussed in an appendix.

Integral field spectroscopy in the K band (1.9-2.4 μm) was performed on four IR-bright star clusters and the two nuclei in NGC 4038/4039 ("the Antennae"). Two of the clusters are located in the overlap region of the two galaxies and together comprise ≈25% of the total 15 μm emission of the total 4.8 GHz emission from this pair of merging galaxies. The other two clusters, each of them spatially resolved into two components, are located in the northern galaxy, one in the western and one in the eastern loop of blue clusters. Comparing our analysis of Brγ, CO band heads, He I 2.058 μm, Hα (from archival Hubble Space Telescope data), and V-K colors to stellar population synthesis models indicates that the clusters are extincted (A_V ≈ 0.7-4.3 mag) and young, displaying a significant age spread (4-13 Myr). The starbursts in the nuclei are much older (65 Myr), with the nucleus of NGC 4038 displaying a region of recent star formation northward of its K-band peak. Using our derived age estimates and assuming the parameters of the initial mass function (Salpeter slope, upper mass cutoff of 100 M_☉, Miller-Scalo between 1 and 0.1 M_☉), we find that the clusters have masses between 0.4 × 10⁶ and 3 × 10⁶M_☉.

Results are presented from a long-term astrometry program on PSR B0919+06 using the NRAO Very Long Baseline Array. With 10 observations (seven epochs) between 1994 and 2000, we measure a proper motion μ_α = 18.35 ± 0.06 mas yr^-1, μ_δ = 86.56 ± 0.12 mas yr^-1 and a parallax π = 0.83 ± 0.13 mas (68% confidence intervals). This yields a pulsar distance of 1.21 ± 0.19 kpc, making PSR B0919+06 the farthest pulsar for which a trigonometric parallax has been obtained, and the implied pulsar transverse speed is 505 ± 80 km s^-1. Combining the distance estimate with interstellar scintillation data spanning 20 yr, we infer the existence of a patchy or clumpy scattering screen along the line of sight in addition to the distributed electron density predicted by models for the Galaxy and constrain the location of this scattering region to within ~250 pc of the Sun. Comparison with the lines of sight toward other pulsars in the same quadrant of the Galaxy permits refinement of our knowledge of the local interstellar medium in this direction.

We present an ASCA discovery of diffuse hard X-ray emission from the Sgr C complex with its peak in the vicinity of the molecular cloud core. The X-ray spectrum is characterized by a strong 6.4 keV line and large absorption. These properties suggest that Sgr C is a new X-ray reflection nebula that emits fluorescent and scattered X-rays via irradiation from an external X-ray source. We found no adequately bright source in the immediate Sgr C vicinity to fully account for the fluorescence. The irradiating source may be the Galactic nucleus Sgr A*, which was brighter in the past than it is now, as is suggested from observations of the first X-ray reflection nebula Sgr B2.

We present mosaics of six Very Large Array images made in the NH₃ (1, 1) and (2, 2) lines toward NGC 7538. Both the NH₃ (1, 1) and (2, 2) emissions show clumpy and filamentary structures. The dense clumps are found near water maser emission or near the youngest member of groups of infrared sources. At the edges of clumps, temperature enhancements indicate external heating possibly by the nearby H II regions. The outflows in this region are conspicuously located in the voids between the clumps. Because there is more mass in the dense clumps than in the outflows, we suggest that the dense medium shapes the outflows and channels the swept-up material to the low-density regions.

We present the results of the first numerical calculations to model magnetic decoupling in a collapsing molecular cloud core. Magnetic decoupling is the stage during which the motion of the neutrals ceases to significantly affect the magnetic field strength and the magnetic field ceases to significantly affect this motion. We have analyzed the resistivity of a weakly ionized, magnetic gas, and we have separated the contributions of ohmic dissipation and ambipolar diffusion. The chemical model used to determine the abundances of ionized particles accounts for, among other things, a distribution of grain radii. The evolution of an axisymmetric, magnetic molecular cloud core is followed from central densities of 300 to 2 × 10¹² cm^-3. Typically, magnetic decoupling begins at a central density of 3 × 10¹⁰ cm^-3 and is complete by a density of 2 × 10¹² cm^-3. We find that the mechanism responsible for magnetic decoupling is ambipolar diffusion, not ohmic dissipation, and that decoupling precedes the formation of a central stellar object. When the central density is a few times 10¹² cm^-3, a nearly uniform magnetic field of B_dec ≈ 0.1 G extends over a region ≈20 AU in radius that contains a mass ≈0.01 M_☉. This value of B_dec is consistent with measurements of remanent magnetization in meteorites. Magnetic decoupling at this stage is not accompanied by hydromagnetic shocks. We estimate that the "magnetic flux problem" of star formation is resolved by ambipolar diffusion before the magnetic field is refrozen in the matter because of thermal ionization.

Supernova remnants may exhibit both thermal and nonthermal X-ray emission. Such remnants can be distinguished by the weakness of their X-ray lines because of the presence of a strong nonthermal X-ray continuum. RCW 86 is a remnant with weak lines, resulting in low and peculiar abundances when thermal models alone are used to interpret its X-ray spectrum. This indicates the presence of a strong nonthermal synchrotron continuum. We analyze ASCA X-ray spectra of RCW 86 with the help of both nonequilibrium ionization thermal models and nonthermal synchrotron models. A two-temperature thermal model and a simple nonthermal model with an exponential cutoff (plus interstellar absorption) give reasonable results. We obtain a blast-wave velocity of 800 km s^-1, a shock ionization age of 1 × 10¹¹-3 × 10¹¹ cm^-3 s, and the break in nonthermal spectra at 2 × 10¹⁶-4 × 10¹⁶ Hz. The strength of the nonthermal continuum correlates well with the radio brightness in the bright southwest section of the remnant. This is convincing evidence for X-ray synchrotron emission in RCW 86.

It is known that most water in the interstellar medium is formed in the OH + H₂ reaction. It is also known that the newly formed water molecules from this reaction are born in highly excited vibrational and rotational states. Even at densities of 10⁹ cm^-3 in the interstellar medium these states relax in a fast IR radiation cascade to the ground state(s) of water emitting IR, far-infrared (FIR), and microwave light. Because this effect has not been included in the astrophysical literature before, we analyze here the intensity of this spontaneous emission at 6 μm, in the FIR and the microwave region. We calculate stationary rotational state distributions using a quantitative model that includes the nascent formation, the subsequent spontaneous emission, photolysis, and collisional redistribution. The radiation cascade yields FIR radiation and an alternative pump mechanism for water masers. For regions of massive water formation it is shown that the stationary rotational population is much less affected by collisions than by the nascent water formation. It is speculated that nascent reactive water might be responsible for the near-IR spectra at 6 μm, for the pure rotational transitions in the FIR radiation, and for the H₂O maser radiation.

There is growing observational evidence that gamma-ray bursts (GRBs) are powered by black holes accreting rapidly through a disk. The supernova-like outburst that accompanies some gamma-ray bursts suggest that some long-duration GRBs may be driven by the accretion of a rotating stellar core onto a central black hole. Such a system can be produced when a compact remnant spirals into the core of its binary companion. During the inspiral, orbital angular momentum is injected into the helium core. By the time the compact remnant reaches the center of the helium core, it too has gained angular momentum as well as mass, producing a rapidly accreting black hole (or neutron star) at the center of a rotating stellar core. Whether or not such a merger (termed He-merger) can produce a GRB depends upon the initial mass and spin of the central black hole, as well as the angular momentum in the stellar core. In this paper, we use a three-dimensional smooth particle hydrodynamics code to follow the He-merger process and make quantitative estimates of the initial mass and spin of the central compact remnant, as well as the angular momentum in the accreting helium core. During the inspiral, a 2 M_☉ compact remnant gains ~0.5-3.5 M_☉ depending on the mass of the helium core (more massive cores provide more accretion). The accretion rates on the central remnant are initially very high, 10⁵M_☉ yr^-1 up to 10⁶M_☉ yr^-1 (the accretion rate increases with increasing helium star mass), and the central remnant quickly becomes a black hole if it was not one already. From these accretion rates, we estimate GRB explosion energies. In all mergers, magnetically driven jets are expected to produce GRB explosions with energies above 10⁵¹ ergs. For neutrino-annihilation-driven explosions, the GRB energy increases dramatically with helium star mass: the merger of a 2 M_☉ compact remnant with a 4 M_☉ helium star only produces a 10⁴⁷ ergs explosion in ~500 s, whereas the merger of a 2 M_☉ compact remnant with a 16 M_☉ helium star produces a greater than 10⁵² ergs explosion in ~65 s.

Recently G. M. Fuller and X. Shi proposed that the gravitational collapse of supermassive objects (M ≳ 10⁴M_☉) could be a cosmological source of γ-ray bursts (GRBs). The major advantage of their model is that supermassive object collapses are far more energetic than solar mass-scale compact mergers. Also, in their proposal the seeds of supermassive black holes (SMBHs) thus formed could give rise to the SMBHs observed at the center of many galaxies. We argue here that, besides the generation of GRBs, there could well occur a strong generation of gravitational waves (GWs) during the formation of SMBHs. As a result, the rate of such GW bursts could be as high as the rate of GRBs in the model by G. M. Fuller and X. Shi. In this case, the detection of GRBs and bursts of GWs should occur with a small time difference. We also argue that the GWs produced by the SMBHs studied here could be detected when the Laser Interferometric Space Antenna becomes operative.

Growing evidence suggests that the first generation of stars may have been quite massive (~100-300 M_☉). If they retain their high mass until death, such stars will, after about 3 Myr, make pair-instability supernovae. Models for these explosions have been discussed in the literature for four decades, but very few included the effects of rotation and none employed a realistic model for neutrino trapping and transport. Both turn out to be very important, especially for those stars whose cores collapse into black holes (helium cores above about 133 M_☉). We consider the complete evolution of two zero-metallicity stars of 250 and 300 M_☉. Despite their large masses, we argue that the low metallicities of these stars imply negligible mass loss. Evolving the stars with no mass loss, but including angular momentum transport and rotationally induced mixing, the two stars produce helium cores of 130 and 180 M_☉. Products of central helium burning (e.g., primary nitrogen) are mixed into the hydrogen envelope with dramatic effects on the radius, especially in the case of the 300 M_☉ model. Explosive oxygen and silicon burning cause the 130 M_☉ helium core (250 M_☉ star) to explode, but explosive burning is unable to drive an explosion in the 180 M_☉ helium core, and it collapses to a black hole. For this star, the calculated angular momentum in the presupernova model is sufficient to delay black hole formation, and the star initially forms an ~50 M_☉, 1000 km core within which neutrinos are trapped. The calculated growth time for secular rotational instabilities in this core is shorter than the black hole formation time, and they may develop. If so, the estimated gravitational wave energy and wave amplitude are E_GW ≈ 10^-3M_☉c² and h₊ ≈ 10^-21/d(Gpc), but these estimates are very rough and depend sensitively on the nonlinear nature of the instabilities. After the black hole forms, accretion continues through a disk. The mass of the disk depends on the adopted viscosity but may be quite large, up to 30 M_☉ when the black hole mass is 140 M_☉. The accretion rate through the disk can be as large as 1-10 M_☉ s^-1. Although the disk is far too large and cool to transport energy efficiently to the rotational axis by neutrino annihilation, it has ample potential energy to produce a 10⁵⁴ erg jet driven by magnetic fields. The interaction of this jet with surrounding circumstellar gas may produce an energetic gamma-ray transient, but given the probable redshift and the consequent timescale and spectrum, this model may have difficulty explaining typical gamma-ray bursts.

In this paper we revisit the radio pulsar death line problem within the framework of curvature radiation and/or the inverse Compton scattering-induced vacuum gap model above neutron star polar caps. Our special interest is in the recently detected pulsar PSR J2144-3933 with extremal period 8.5 s, which lies far beyond conventional death lines. We argue that the formation of vacuum gaps requires a complicated multipolar surface magnetic field, with a strength B_s typically much higher than the surface dipolar component B_d and a radii of curvature much smaller than the neutron star radius R = 10⁶ cm. Such a multipolar surface field is also consistent with death lines including the extremal pulsar PSR J2144-3933. Since vacuum gap models produce sparks, our paper naturally supports the spark related models of subpulse drift phenomenon as well as the spark-associated models of coherent pulsar radio emission.

We model the formation of magnetospheric components of millisecond pulsar light curves, deriving an approximate model for the curved space, "swept-back" dipole field and following photon emission, propagation, and scattering. Magnetospheric pulse components are strongly affected by rapid rotation and Schwarzschild effects.

Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are young and radio-quiet X-ray pulsars that have been rapidly spun-down to slow spin periods clustered in the range 5-12 s. Most of these unusual pulsars also appear to be associated with supernova shell remnants (SNRs) with typical ages less than 30 kyr. By examining the sizes of these remnants versus their ages, we demonstrate that the interstellar media that surrounded the SGR and AXP progenitors and their SNRs were unusually dense compared to the environments around most young radio pulsars and SNRs. We explore the implications of this evidence on magnetar and propeller-based models for the rapid spin-down of SGRs and AXPs. We find that evidence of dense environments is not consistent with the magnetar model unless a causal link can be shown between the development of magnetars and the external interstellar medium. Propeller-driven spin-down by fossil accretion disks for SGRs and AXPs appears to be consistent with dense environments since the environment can facilitate the formation of such a disk. This may occur in two ways: (1) formation of a "pushback" disk from the innermost ejecta pushed back by prompt reverse shocks from supernova remnant interactions with massive progenitor wind material stalled in dense surrounding gas or (2) acquisition of disks by a high-velocity neutron stars, which may be able to capture sufficient amounts of comoving outflowing ejecta slowed by the prompt reverse shocks in dense environments.

We continue our study of the possible production of supernovae and a variety of high-energy transients by black hole formation in massive stars endowed with rotation: the "collapsar model." The black hole may form either promptly, since a successful outgoing shock fails to be launched by the collapsed iron core (collapsar Type I), or, in a mild explosion, by fallback (collapsar Type II). In the latter case, the inner layers of the star initially move outward but lack adequate momentum to eject all the matter exterior to the young neutron star. Over a period of minutes to hours, ~0.1-5 M_☉ falls back onto the collapsed remnant, turning it into a black hole and establishing an accretion disk. The accretion rate, ~0.001-0.01 M_☉ s^-1, is inadequate to produce a jet mediated by neutrino annihilation but is similar to what has been invoked in magnetohydrodynamic (MHD) models for gamma-ray bursts (GRBs). This fallback is modeled in detail for two 25 M_☉ progenitors using two different one-dimensional hydrodynamics codes, one Lagrangian and one Eulerian. The production and consequences of jets are then explored in both sorts of collapsars. Justification is given for assuming that the jet power is a constant times the mass accretion rate, c², and the consequences of = 0.001 and 0.01 are explored. Adopting an initial collimation half-angle of 10°, the opening of the jet as it propagates through the exploding star is strongly influenced not only by the jet's kinetic energy but also by its initial pressure and the stellar structure. Cold jets tend to stay collimated and become even more so, sometimes having an angle of only a few degrees when they reach the surface. Jets having higher internal pressure than the stellar material through which they pass, or less initial collimation, spread out and tend to make energetic, asymmetric supernovae accompanied, in helium stars, by weak GRBs. SN 1998bw may have been such an event, and other events having energies between that of ordinary GRBs and GRB 980425 await discovery. In supergiant stars, shock breakout also produces bright X-ray transients that might be a diagnostic of the model, but even the most powerful jets (equivalent isotropic energy 10⁵⁴ ergs) will not produce a GRB in a red supergiant. For such Type II supernovae the limiting Lorentz factor is Γ ≈ 2. Type II collapsars should be more frequent than Type I and may power the most common form of gamma-ray transient in the universe. However, the GRBs seen by BATSE are, for the most part, too brief to be Type II collapsars. Those are still attributed to prompt black hole formation. Even there though, the diverse energies and time structure reflect chiefly the viewing angle and the variable collimation of the jet inside the star, not a highly variable "central engine." Indeed, collapsar-induced transients may all have a common total energy in the range 10⁵¹-10⁵² ergs.

The structure of neutron stars is considered from theoretical and observational perspectives. We demonstrate an important aspect of neutron star structure: the neutron star radius is primarily determined by the behavior of the pressure of matter in the vicinity of nuclear matter equilibrium density. In the event that extreme softening does not occur at these densities, the radius is virtually independent of the mass and is determined by the magnitude of the pressure. For equations of state with extreme softening or those that are self-bound, the radius is more sensitive to the mass. Our results show that in the absence of extreme softening, a measurement of the radius of a neutron star more accurate than about 1 km will usefully constrain the equation of state. We also show that the pressure near nuclear matter density is primarily a function of the density dependence of the nuclear symmetry energy, while the nuclear incompressibility and skewness parameters play secondary roles. In addition, we show that the moment of inertia and the binding energy of neutron stars, for a large class of equations of state, are nearly universal functions of the star's compactness. These features can be understood by considering two analytic, yet realistic, solutions of Einstein's equations, by, respectively, Buchdahl and Tolman. We deduce useful approximations for the fraction of the moment of inertia residing in the crust, which is a function of the stellar compactness and, in addition, the pressure at the core-crust interface.

Recently, eigenmodes of rotating fluids, namely, inertial modes, have received much attention in relation to their destabilization when coupled to gravitational radiation within neutron stars. However, these modes have been known for a long time in fluid dynamics. We give a short account of their history and review our present understanding of their properties. Considering the case of a spherical container, we then give the exact solution of the boundary (Ekman) layer flow associated with inertial r-modes and show that previous estimations all underestimated the dissipation by these layers. We also show that the presence of an inner core has little influence on this dissipation. As a conclusion, we compute the window of instability in the temperature/rotation plane for a crusted neutron star when it is modeled by an incompressible fluid.

We present new spectroscopic and photometric measurements of the white dwarfs LHS 3250 and WD 0346+246. Along with F351-50, these white dwarfs are the coolest ones known, all with effective temperatures below 4000 K. Their membership in the Galactic halo population is discussed, and detailed comparisons of all three objects with new atmosphere models are presented. The new models consider the effects of mixed H/He atmospheres and indicate that WD 0346+246 and F351-50 have predominantly helium atmospheres with only traces of hydrogen. LHS 3250 may be a double degenerate whose average radiative temperature is between 2000 and 4000 K, but the new models fail to explain this object.

With the Hubble Space Telescope (HST) we have observed Hyades F stars, using the Space Telescope Imaging Spectrograph (STIS), in order to get more information about the heating mechanism(s) for the chromospheres and transition layers and their dependence on rotation and age. In this paper we study the Mg II lines at 2800 Å. We include earlier observations with the International Ultraviolet Explorer (IUE) satellite. The Mg II emission lines become observable for B - V > 0.3. The emission line fluxes increase steeply until B - V ~ 0.40. For single stars there is a steep decrease in flux between B - V = 0.41 and B - V = 0.44, similar to the behavior of the Ca II emission line cores. For larger B - V the Mg II emission line fluxes again increase, but much more slowly than for the Ca II lines. Generally, the low point of the emission is reached between B - V = 0.43 and B - V = 0.45, i.e., similar to the Ca II emission cores. For the Hyades F stars there appears to be a difference between the emissions for single stars and those for binaries. We find that for Hyades stars with surface line fluxes larger than 10⁶ ergs cm^-2 s^-1 the emission line fluxes decrease with increasing v sin i. For smaller fluxes they may increase with increasing v sin i. We have only three stars that perhaps show this. We study the flux ratios of the Mg II k and h lines at 2795.7 and 2802.5 Å in order to determine where the lines fall on the curve of growth. For the earliest F stars studied here the ratio is close to 2, as expected for optically thin lines. Generally, it seems that the optical depths in the line centers are less than 10. There remain problems in understanding the size of the line widths. We discuss the interpretation of the Wilson-Bappu effect. For the Hyades F stars there is a strong dependence of the line width on the effective temperature.

We present new results on the nature of moving magnetic features (MMFs) deduced from Big Bear Solar Observatory observations of the longitudinal magnetic fields of two large solar spots. MMFs are small magnetic bipoles that move outward across the moat of an eroding sunspot. We find that MMFs are not randomly oriented. To wit, in 21 out of 28 (75%) MMF pairs, the magnetic element having the polarity of the sunspot was located farther from the sunspot. Furthermore, there is a correlation between the orientation of the bipole and that of the twist in a sunspot. For the two nearly round sunpots we studied, we found that the bipoles were rotated counterclockwise in the case of a clockwise twisted sunspot and clockwise for a spot with counterclockwise twist. We also found a correlation between the orientation of MMF bipole and the amount of twist in the spot. The MMF bipoles around the highly twisted sunspot are oriented nearly tangential to the edge of the sunspot, while in the slightly twisted sunspot the bipoles are oriented nearly radially so that they point back to the spot.

We analyze long-exposure and off-pointing Yohkoh/SXT data of the solar corona observed on 1992 August 26. We develop a new (temperature) tomography method that is based on a forward-fitting method of a four-parameter model to the observed soft X-ray fluxes F₁(h) and F₂(h) of two SXT wavelength filters as a function of height h. The model is defined in terms of a differential emission measure (DEM) distribution dEM(h, T)/dT, which includes also a temperature dependence of density scale heights λ_n(T) = q_λλ_T and allows us to quantify deviations (q_λ ≠ 1) from hydrostatic equilibrium (q_λ = 1). This parametrization facilitates a proper line-of-sight integration and relates the widely used filter ratio temperature T_FR to the peak of the DEM distribution. A direct consequence of the multi-scale height atmosphere is that the filter ratio temperature T_FR(h) is predicted to increase with height, even if all magnetic field lines are isothermal. Our model fitting reveals that coronal holes and quiet-Sun regions are in perfect hydrostatic equilibrium but that coronal streamers have a scale height that exceeds the hydrostatic scale height by a factor of up to q_λ ≲ 2.3, which underscores the dynamic nature of coronal streamers. Our density measurements in coronal holes are slightly lower than most of the white-light polarized brightness inversions and seem to come closer to the requirements of solar wind models. Our DEM model provides also a physical framework for the semiempirical Baumbach-Allen formula and quantifies the temperature ranges and degree of hydrostaticity of the K, L, and F coronae.

We examine the effects of the Grevesse & Sauval solar mixture and the European Nuclear Astrophysics Compilation of Reaction Rates (NACRE) nuclear reaction rates on the calculated neutrino fluxes and the p-mode oscillation spectrum of calibrated standard solar models. Calibrated solar models with the NACRE reaction rates are in good agreement with the helioseismic constraints, i.e., the low-degree p-mode oscillation spectrum, the small frequency separations, the depth of the convection zone, and the helium abundance in the convection zone. The use of the Grevesse & Sauval solar mixture slightly worsens the agreement by producing a shallower convection zone. We exploit the uncertainties in abundance determinations, the nuclear reaction rates, and the neutrino capture rates to reduce the predicted neutrino fluxes: the predicted ⁸B flux can be brought well within the observational error bars of the Kamiokande experiment. The predicted chlorine and gallium event rates, though still higher than the observational values, are reduced by 47% and 24%, respectively. The models with the neutrino flux reductions do about as well as the reference model in satisfying the current constraints of helioseismology. We also perform g-mode calculations: these modes are much more sensitive to the core structure than the p-modes. Confirmed detection and identification of only one g-mode, with an accuracy of within only 1/10 of 1% on its period, would be enough to discriminate between our different models and assess the extent to which standard physics is able to give a partial answer to the neutrino problems.

Unusual patterns of recombination emission from gas ionized by metal-free stars may distinguish early star-forming galaxies from their present-day counterparts. This pattern arises from the harder ionizing spectrum expected from metal-free stars, which strongly enhances the strength of He II recombination lines. Our calculations indicate that line fluxes of He II λ1640 and λ4686 are sufficiently large to be detected by narrowband and spectroscopic searches for high-redshift emission-line sources at z ≲ 5 using current instruments. Beyond z ~ 5, potentially low stellar mass loss and observational limitations may hinder the detectability of Population III stars with this technique. An unknown fraction of Lyα emitters may harbor low-metallicity or metal-free stars. As the predicted He II λ1640 flux is comparable to and may exceed hydrogen Lyα, searches for high-redshift galaxies should consider He II recombination lines as possible identifications for single emission lines in observed spectra. Spectra of metal-free stars may show both H I and He II emission lines, improving the constraints on their redshift and identification. We assess the considerable uncertainties that affect our expectations for the detection and identification of true first-generation stars with present search techniques, including the role of stellar mass loss in spectral evolution and the confusion of ionization by primordial stellar sources and active galactic nuclei in the early universe.

In order to find the conditions that determine whether X-ray hot gas in galaxy groups (intragroup gas [IGG]) is heated externally or internally, we investigate the evolution of blast waves in galaxy groups growing on a hierarchical clustering scenario. We find that the blast waves driven by quasars are confined in groups and heat the IGG internally at z ≲ 1. However, at z ≳ 1, they expel the IGG from groups; the expelled gas may fall back into the groups later as externally heated gas. Moreover, this may explain the observed low metal abundance of IGG. For blast waves driven by strong starbursts, the shift of the fate of blast waves occurs at z ~ 3. On the other hand, although blast waves driven by weak starbursts do not expel IGG from groups, the heating efficiency decreases at z ≳ 3 because of radiative cooling. It will be useful to compare these results with XMM-Newton observations.

The quasar 0917+624 has been one of the best-studied intraday variable radio sources. However, debate continues as to whether the underlying cause is intrinsic or extrinsic. Much previous work has assumed the intraday variability (IDV) to be intrinsic, which implies an extraordinarily compact source for the radio emission; in contrast, an extrinsic variation due to interstellar scintillation (ISS) implies a relatively larger source diameter, although at the smaller end of the range expected for relativistic jet models. Kraus et al. reported a marked slowing of the IDV at a wavelength of 6 cm in 1998 September and suggested that a change in the source was responsible. However, here we show that the slowing is consistent with the annual modulation in the scintillation timescale expected for ISS, under the assumption that the scattering medium moves with the local standard of rest (LSR). The ISS timescale is governed by the ISS spatial scale divided by the Earth's velocity relative to the scattering plasma. It happens that in the direction of 0917+624, the transverse velocity of the Earth with respect to the LSR varies widely, with a deep minimum in the months of September-November. Hence, the slowing of the IDV in 1998 September strongly suggests that ISS rather than intrinsic variation of the source is the dominant cause of the IDV.

Beginning in 1999 January, the bright, strongly variable narrow-line Seyfert 1 (NLS1) galaxy Akn 564 has been observed by RXTE once every ~4.3 days. It was also monitored every ~3.2 hr throughout 2000 July. These evenly sampled observations have allowed the first quantitative comparison of long- and short-timescale X-ray variability in an NLS1 galaxy and the derivation of an X-ray power density spectrum (PDS). The variability amplitude in the short-timescale light curve is very similar to that in the long-timescale light curve, in marked contrast to the stronger variability on longer timescales, which is characteristic of "normal" broad-line Seyfert 1 (BLS1) galaxies. Furthermore, the Akn 564 PDS power law cuts off at a frequency of 8.7 × 10^-7 Hz, corresponding to a timescale of ~13 days, significantly shorter than that seen in the PDS of NGC 3516, a BLS1 galaxy of comparable luminosity. This result is consistent with NLS1 galaxies showing faster (as opposed to larger amplitude) variations than BLS1 galaxies, providing further evidence that NLS1 galaxies harbor lower mass black holes than BLS1 galaxies of similar luminosity, accreting at a correspondingly higher relative rate.

Chandra observations of low-luminosity supermassive black holes in nearby elliptical galaxies provide tight limits on both their nuclear luminosities and their Bondi accretion rates. We examine Chandra constraints on NGC 6166, the dominant galaxy in the cluster Abell 2199, which hosts an ~10⁹M_☉ black hole. We measure a nuclear X-ray luminosity of L_{X, 1 keV} ~ 10⁴⁰ ergs s^-1 and show that the density and temperature profiles of the hot interstellar medium imply a Bondi accretion rate of _Bondi ≲ 3 × 10^-2M_☉ yr^-1. This accretion rate predicts a nuclear luminosity of ~10⁴⁴ ergs s^-1 for a canonical radiative efficiency of 10%. Unless the Bondi estimate is inappropriate and/or the accretion rate onto the black hole is significantly reduced, the observed nuclear flux constrains the radiative efficiency of the accretion flow to be η ~ 10^-5. We show that low radiative efficiency accretion flows can explain the observed nuclear X-ray luminosity but that the power output from the jets in NGC 6166 is also important to the energetics of the system.

We have obtained far-ultraviolet spectra of two clusters of galaxies with the Far Ultraviolet Spectroscopic Explorer. The Coma Cluster was observed for a total of 28.6 ks, the Virgo Cluster for 10.9 ks. Neither spectrum shows significant O VI λλ1032, 1038 emission at the cluster redshift. Such emission would be expected from the warm [(5-10) × 10⁵ K] component of the intracluster medium that has been proposed to explain the excess EUV and soft X-ray flux present in EUVE and ROSAT observations of these clusters. Our 2 σ upper limits on the O VI λ1032 flux from Coma and Virgo exclude all published warm gas models of the EUV excess in these clusters.

Recent XMM-Newton observations of the cooling flow in M87 indicate sharply decreasing oxygen, iron, and silicon abundances within ~5 kpc of the galactic center. This result is unexpected since stellar mass loss and Type Ia supernovae are expected to produce pronounced central abundance maxima for all three elements. However, it has been suggested that many of the strong X-ray lines are optically thick and diffuse to larger radii in the cooling flow before escaping, falsifying the central abundances. We verify with radiation transfer calculations that this effect does indeed occur in the M87 cooling flow but that it is insufficient to account for the M87 observations. We suggest that some source of continuous opacity is required to reduce the central X-ray line emission, perhaps by warm gas at T ~ 10⁵-10⁶ K. The radial surface brightness profiles of X-ray resonance lines are also sensitive to turbulence in cooling flows, which reduces the line center optical depths considerably. Turbulence may provide sufficient energy to continuously heat the warm absorbing gas.

We identify nearby disk galaxies with optical structural parameters similar to those of intermediate-redshift compact blue galaxies. By comparing H I and optical emission line widths, we show that the optical widths substantially underestimate the true kinematic widths of the local galaxies. By analogy, optical emission line widths may underrepresent the masses of intermediate-z compact objects. For the nearby galaxies, the compact blue morphology is the result of tidally triggered central star formation; we argue that interactions and minor mergers may cause apparently compact morphology at higher redshift.

We compare the metallicity distribution of Galactic halo stars with three-dimensional realizations of hierarchical galaxy formation. Outflows from dwarf galaxies enrich the intergalactic medium inhomogeneously, at a rate depending on the local galaxy density. Consequently, the first stars created in small early-forming galaxies are less metal-rich than the first stars formed in more massive galaxies, which typically form later. As most halo stars are likely to originate in accreted dwarfs, while disk stars formed out of outflow-enriched gas, this scenario naturally generates a "metallicity floor" for old disk stars, which we find to be roughly coincident with the higher end of our predicted metallicity distribution of halo stars, in agreement with observations. The broad and centrally peaked distribution of halo star metallicities is well reproduced in our models, with a natural dispersion depending on the exact accretion history. Our modeling includes the important "baryonic stripping" effect of early outflows, which brush away the tenuously held gas in neighboring previrialized density perturbations. This stripping process does not significantly modify the predicted shape of the halo star metal distribution but inhibits star formation and hence the number of accreted stars, helping to reconcile our model with the observed total Galactic halo luminosity and the lack of low-luminosity local dwarf galaxies relative to N-body predictions.

We consider models for the generation of the emission line recently discovered in the X-ray afterglow spectrum of several bursts and, especially, of GRB 991216 observed by Chandra. These observations suggest the presence of 0.1-1 M_☉ of iron in the vicinity of the bursts. We show that there are strong geometric and kinematic constraints on the line-emitting material. We discuss several classes of models, favoring one where the line photons are produced by reflection off the walls of a wide funnel of semiopening angle ϑ ≈ 45° excavated in a young (a few months old) supernova remnant made of ≈10 M_☉ with radius 6 × 10¹⁵ cm and ≈1 M_☉ of iron, providing strong support for the "supranova" model.

We report on the prompt X- and γ-ray observations of GRB 990712 with the BeppoSAX Gamma-Ray Burst Monitor and Wide Field Camera 2. As a result of Sun constraints, we could not perform a follow-up observation with the BeppoSAX narrow-field instruments. The light curve of the prompt emission shows two pulses and a total duration of about 40 s in X-rays. In γ-rays, the event is even shorter. The 2-700 keV spectral emission with time shows a discontinuity in the peak energy E_p of the EF(E) spectrum: E_p is above our energy passband during the first pulse and goes down to ~10 keV during the second pulse. Another peculiarity is noted in this event for the first time: the evidence of a 2 s duration emission feature during the tail of the first pulse. The feature is consistent with either a Gaussian profile with centroid energy of 4.5 keV or a blackbody spectrum with kT_bb ~ 1.3 keV. We discuss the possible origin of the feature. The most attractive possibility is that we are observing the thermal emission of a baryon-loaded expanding fireball when it becomes optically thin.

The nature of the binary systems giving rise to Type Ia supernovae (SNe Ia) remains an unsolved problem. In this Letter, we calculate, from the statistics of initial conditions (masses and binary separations), the mass, luminosity, and velocity distributions of the possible binary companions (main-sequence star, subgiant, and red giant) following the explosion of the white dwarf that gives rise to the SNe Ia. Those companions could be detected from either their proper or their radial motions by means of high-precision astrometric and radial velocity measurements in young, nearby supernova remnants. Peculiar velocities typically ranging from 100 to 450 km s^-1 should be expected; these velocities place proper-motion measurements within reach of the Hubble Space Telescope instruments. Detections would solve the long-standing problem of which kinds of binaries produce SNe Ia and would clear the way for an accurate physical modeling of the explosions.

Infrared spectroscopic observations of LL Andromedae and EF Eridani are presented. Our K-band spectrum of LL And reveals the presence of methane absorption in the secondary star indicating an effective temperature ≲1300 K, similar to a "T"-type methane brown dwarf. The secondary star in EF Eri is seen to be warmer with an effective temperature of ~1650 K and a spectroscopic classification consistent with an L4-5 star. Both secondaries have theoretical mass estimates of 0.04-0.055 M_☉ (40-55 M_Jup). Our IR spectroscopic observations of LL And and EF Eri provide the first direct proof of the theoretical prediction that post-period minimum cataclysmic variables contain brown dwarf-like secondary stars.

A spectroscopic survey of unstudied Luyten half-second proper-motion stars has resulted in the discovery of an unusual new magnetic white dwarf. LHS 2534 proves to be the first magnetic DZ, showing Zeeman-split Na I and Mg I components, as well as Ca I and Ca II lines for which Zeeman components are blended. The Na I splittings result in a mean surface field strength estimate of 1.92 MG. Apart from the magnetic field, LHS 2534 is one of the most heavily blanketed and coolest DZ white dwarfs at T_eff ~ 6000 K.

Fully evolutionary models have been built to follow the phases of asymptotic giant branch evolution with mass loss for metal mass fractions from Z = 2 × 10^-4 to Z = 4 × 10^-3. The hot bottom burning at the base of the convective envelope is followed by fully coupled nuclear burning and noninstantaneous mixing. The models also show the occurrence of a spontaneous (i.e., not induced by overshooting) third dredge-up. For the first time, we find that temperatures close to or even larger than 10⁸ K are achieved at low Z; the full CNO cycle operates at the base of the envelope, the ¹⁶O abundance for the most metal-poor models of 4 and 5 M_☉ is drastically reduced, and sodium and aluminum production by proton capture on neon and magnesium can occur. Lithium is first largely produced in the envelope and then burned completely, so the average lithium abundance in the expelled envelope is a factor of up to 5 times smaller than the initial one, but it is never completely depleted. These results may be relevant for the evolution of primordial massive globular clusters; we suggest that the low-mass stars may have been polluted at the surface by accretion from the gas that was lost from the evolving intermediate-mass stars at early ages [(1-2) × 10⁸ yr]. In this hypothesis, we should expect that the polluted stars show smaller abundances of oxygen, larger abundances of products of advanced nucleosynthesis (as Na and Al), and lower, but never negligible, abundances of lithium. The abundance spreads should be smaller in clusters of higher metallicities, where the lithium in the polluted stars could be larger than in the nonpolluted stars.

We have detected continuum emission at 450 μm, 850 μm, and 1.35 mm from SS Lep (or 17 Lep), 3 Pup, and probably BM Gem, respectively, which are likely or confirmed binary systems consisting of at least one evolved giant. The observed submillimeter fluxes are probably emitted by grains rather than ionized gas. The fluxes are larger than those expected from a "normal" dusty wind; the dust temperature is ≤70 K within 6'' of the stars. To explain why grains are so cold near the star, we suggest that the emission at λ ≥ 450 μm is produced by particles as large as 0.1 mm in radius and that these large particles probably have grown by coagulation in circumbinary orbiting disks with masses ≥5 × 10²⁸ g.

Identifying the processes that determine strength, duration, and variability of protostellar mass growth is a fundamental ingredient of any theory of star formation. I discuss protostellar mass accretion rates from numerical models that follow molecular cloud evolution from turbulent fragmentation toward the formation of stellar clusters. In a dense cluster environment, is strongly time varying and influenced by the mutual interaction of protostellar cores and their competition for accretion from the common cluster gas reservoir. Even for protostars with similar final mass, the accretion histories may differ dramatically. High-mass stars build up in the central parts of dense, cluster-forming cloud regions. They begin to form in the early phases of cluster evolution and continue to grow at a high rate until the available gas is exhausted or expelled by feedback. Lower mass stars tend to form at later phases, and declines rapidly after a short initial phase of strong growth. I present a simple fit formula for the time evolution of the average for protostars of different masses in a dense cluster environment.

We present subarcsecond resolution observations of the star-forming region W49N made with the Berkeley-Illinois-Maryland Association array at 1.4 mm wavelength in the continuum and CH₃CN J = 12-11 lines. The continuum image, at a resolution of 018 (2000 AU), shows many of the previously identified ultracompact H II regions and at least one new source whose steep spectrum to short wavelengths indicates dust emission. This compact dust concentration also shows the strongest CH₃CN emission in the region. Additional peaks of CH₃CN emission likely mark hot cores produced by other deeply embedded young massive stars that may be precursors to O-type stars like those that power the ultracompact H II regions. The number ratio of hot cores to ultracompact H II regions found in W49N provides a measure of the relative timescales of these early evolutionary phases. Although the samples are small, the source counts suggest that the hot core lifetime is most likely shorter than, but comparable to, the ultracompact H II region lifetime.

The recently discovered protostellar jet known as HH 212 is beautifully symmetric, with a series of paired shock knots and bow shocks on either side of the exciting source region IRAS 05413-0104. We present VLA ammonia maps of the IRAS 05413-0104 molecular gas envelope in which the protostellar jet source is embedded. We find that the envelope, with a mass of 0.2 M_☉ detected by the interferometer, is flattened perpendicular to the jet axis with an FWHM diameter of 12,000 AU and an axis ratio of 2 : 1, as seen in NH₃ (1, 1) emission. There is a velocity gradient of about 4-5 km s^-1 pc^-1 across the flattened disklike core, suggestive of rotation around an axis aligned with the jet. Flux-weighted mean velocities increase smoothly with a roughly constant velocity gradient. In young (class 0) systems such as HH 212, a significant amount of material is still distributed in a large surrounding envelope, and thus the observable kinematics of the system may reflect the less centrally condensed, youthful state of the source and obscuration of central dynamics. The angular momentum of this envelope material may be released from infalling gas through rotation in the HH 212 jet, as recent observations suggest. A blueshifted wisp or bowl of emitting gas appears to be swept up along the blue side of the outflow, possibly lining the cavity of a wider angle wind around the more collimated shock jet axis. Our ammonia (2, 2)/(1, 1) ratio map indicates that this very cold core is heated to 14 K in a centrally condensed area surrounding the jet source. This edge-on core and jet system appears to be young and deeply embedded. This environment, however, is apparently not disrupting the pristine symmetry and collimation of the jet.

We here present the first results of fully three-dimensional MHD simulations of radiative cooling pulsed (time-variable) jets for a set of parameters that are suitable for protostellar outflows. Considering different initial magnetic field topologies in approximate equipartition with the thermal gas, i.e., (1) a longitudinal and (2) a helical field, both of which permeate the jet and the ambient medium, and (3) a purely toroidal field permeating only the jet, we find that the overall morphology of the pulsed jet is not very much affected by the presence of the different magnetic field geometries in comparison with a nonmagnetic calculation. Instead, the magnetic fields tend to affect essentially the detailed structure and emission properties behind the shocks at the head and at the pulse-induced internal knots, particularly for the helical and toroidal geometries. In these cases, we find, for example, that the Hα emissivity behind the internal knots can be about 3-4 times larger than that of the purely hydrodynamical jet. We also find that some features, like the nose cones that often develop at the jet head in two-dimensional calculations involving toroidal magnetic fields, are smoothed out or absent in the three-dimensional calculations.

Organic compounds observed in the interstellar medium and in solar system bodies are of particular importance for revealing the chemistry that may have led to life's origin. Among these compounds, amino acids may have played a crucial role since they are basic components of proteins, which are the essential constituents of all organisms. We present laboratory studies testing the stability of amino acids against ultraviolet (UV) photolysis. Two biological and two nonbiological amino acids have been irradiated in frozen Ar, N₂, and H₂O to simulate conditions in the interstellar gas and on interstellar grains. The experimental results can be interpreted to indicate that amino acids in the gas phase will likely be destroyed during the lifetime of a typical interstellar cloud. In regions with relatively low UV radiation, amino acids might be present as transient gas-phase species. Their survival in interstellar icy grain mantles and the surface layers of comets and planets is strongly limited in the presence of UV irradiation. The rate of destruction is rather insensitive to the amino acid structure and to the ice matrix. We consider the implications of these results for the survival and transfer of amino acids in space environments, and thus their possible availability for prebiotic chemistry.

If the solar neutrino flux were constant, as is widely assumed, the histogram of flux measurements would be unimodal. On the other hand, sinusoidal or square-wave modulation (either periodic or stochastic) may lead to a bimodal histogram. We here present evidence that the neutrino flux histogram is in fact bimodal. We analyze all available data from gallium experiments, coordinating results from the GALLium EXperiment and the Gallium Neutrino Observatory experiment into one data set and adopting results from the Soviet-American Gallium Experiment as another data set. The two histograms, from the two data sets, are consistent in showing peaks in the ranges 45-75 and 90-120 SNU, with a valley in between. By combining the data into one data set, we may form more detailed histograms; these strengthen the case that the flux is bimodal. A preliminary statistical analysis indicates that the bimodal character of the solar neutrino flux is highly significant. Since the upper peak is close to the expected flux (120-140 SNU), we may infer that the neutrino deficit is due to time-varying attenuation of the flux produced in the core. We estimate the timescale of this variation to be in the range 10-60 days. Attenuation that varies on such a timescale is suggestive of the influence of solar rotation and points toward a process involving the solar magnetic field in conjunction with a nonzero neutrino magnetic moment.

High-resolution observations of a large solar flare on 2000 July 14 ("Bastille Day Flare") from the Michelson Doppler Imager instrument on the SOHO spacecraft reveal rapid variations of the magnetic field in the lower solar atmosphere during the flare. Some of these variations were irreversible, occurred in the vicinity of magnetic neutral lines, and likely were related to magnetic energy release in the flare. A surprising result is that these variations happened very rapidly on the scale of 10-15 minutes in a large area of ~50 Mm² at the beginning of the flare. Other, more localized and impulsive magnetic field variations somewhat similar to "magnetic transients" observed by Zirin and coworkers were accompanied by impulses in continuum intensity and Doppler velocity. These impulses have dynamic characteristics similar to Ellerman's "bombs" and Severny's "mustaches" and were probably caused by high-energy particles bombarding the solar surface.

Dimensional considerations are used to analyze the distribution of times between solar flares (the flare waiting-time distribution). An analytical estimate for the mean flaring rate λ₀ is obtained, based on the idea that the rate reflects a balance between the processes of energy input into the corona and energy dissipation by flaring. The estimate is shown to be in good agreement with observations of flares by Geostationary Operational Environmental Satellite detectors. The analytic result is confirmed using the master equation (probability balance equation) formalism. Analysis of time variation leads to a differential equation describing how the free energy in the corona changes when the rates of driving of the system and of flaring vary. This equation may be used to describe the global energy balance in the corona over the solar cycle.

We study the relation between thermal and magnetic properties of active regions in the corona observed with the soft X-ray telescope aboard Yohkoh. We derive the mean temperature and pressure of 64 mature active regions using the filter ratio technique, and examine the relationship of region size with temperature and pressure. We find that the temperature T of active regions increases with increasing region size L as T ∝ L^0.28, while the pressure P slightly decreases with the region size as P ∝ L^-0.16. We confirm the scaling law T ∝ (PL)^1/3 for mature active regions found by R. Rosner, W. H. Tucker, & G. S. Vaiana. We examined the magnetic properties of active regions by analyzing 31 active regions observed with the Solar and Heliospheric Observatory/Michelson Doppler Imager and find the following empirical scaling law between thermal and magnetic properties, where U_th, Φ, and B are the total thermal energy content, total magnetic flux, and average magnetic flux density of active regions, respectively. The former is consistent with the results of L. Golub et al., but the latter is not. Implications of our findings for coronal heating mechanisms are discussed.

We present the Δn = 0 (n = 3 to n = 3) EUV spectra of M-shell iron ions from a tokamak plasma obtained during the "ramp-down" phase of the discharge. The fractional ion abundances for K-like Fe⁺⁷ to Si-like Fe⁺¹² are found from fitting model spectra to the measured spectrum. The plasma temperature is measured as a function of position by Thomson scattering of laser light and Langmuir probes. Collisional-radiative simulations have been computed for the above ions, using atomic data generated with the HULLAC suite of codes. The blended M-shell spectrum is strongly sensitive to temperature; this makes it a potentially useful diagnostic for observations of stellar coronae where the differential emission measure is dominated by temperatures around 1 MK.