Table of contents

We propose that it should be possible to use the cosmic microwave background (CMB) to discriminate between dark energy models with different equations of state, which includes distinguishing a cosmological constant from many models of quintessence. The separation of peaks in the CMB anisotropies can be parametrized by three quantities: the amount of quintessence today, the amount at last scattering, and the averaged equation of state of quintessence. In particular, we show that CMB peaks can be used to measure the amount of dark energy present before last scattering.

Straightforward physical arguments are used to derive the properties of Lyα forest absorbers. It is shown that many aspects of the current physical picture of the forest, in particular the fact that the absorption arises in extended structures of moderate overdensities that contain a large fraction of the baryons in the universe, can be derived directly from the observations without making any specific assumptions about the presence and distribution of dark matter, the values of the cosmological parameters, or the mechanism for structure formation. The key argument is that along any line of sight intersecting a gravitationally confined gas cloud, the size of the region over which the density is of order the maximum density, is typically of order the local Jeans length. This is true for overdense absorbers, regardless of the shape of the cloud and regardless of whether the cloud as a whole is in dynamical equilibrium. The simple analytic model is used to derive the mass distribution of the photoionized gas directly from the observed column density distribution. It is demonstrated that the shape of the column density distribution, in particular the observed deviations from a single power law, and its evolution with redshift, reflect the shape of the matter distribution and can be understood in terms of the growth of structure via gravitational instability in an expanding universe.

Using high-resolution cosmological N-body simulations, we study how the density profiles of dark matter halos are affected by the filtering of the density power spectrum below a given scale length and by the introduction of a thermal velocity dispersion. In the warm dark matter (WDM) scenario, both the free-streaming scale, R_f, and the velocity dispersion, v, are determined by the mass, m_W, of the WDM particle. We found that v is too small to affect the density profiles of WDM halos. Down to the resolution attained in our simulations (~0.01 virial radii), there is not any significant difference in the density profiles and concentrations of halos obtained in simulations with and without the inclusion of v. Resolved soft cores appear only when we artificially increase the thermal velocity dispersion to a value that is much higher than v. We show that the size of soft cores in a monolithic collapse is related to the tangential velocity dispersion. The density profiles of the studied halos with masses down to ~0.01 the filtering mass M_f can be described by the Navarro-Frenk-White shape; soft cores are not formed. Nevertheless, the concentrations of these halos are lower than those of the CDM counterparts and are approximately independent of mass. The cosmogony of halos with masses ≲M_f is not hierarchical: they form through monolithic collapse and by fragmentation of larger structures. The formation epoch of these halos is slightly later than that of halos with masses ≈M_f. The lower concentrations of WDM halos with respect to their CDM counterparts can be accounted for by their late formation epoch. Overall, our results point to a series of advantages of a WDM model over the CDM one. In addition to solving the substructure problem, a WDM model with R_f ~ 0.16 Mpc (m_W ≈ 0.75 keV; flat cosmology with Ω_Λ = h = 0.7) also predicts concentrations, a Tully-Fisher relation, and formation epochs for small halos, which seems to be in better agreement with observations than CDM predictions.

The halo mass function, dn/dM, predicted by hierarchical clustering models can be measured indirectly using dynamical probes like the distribution of gravitational lens image separations, dn/dΔθ, or halo circular velocities, dn/dv_c. These dynamical variables depend on the halo structure as well as the halo mass. Since baryonic physics, particularly cooling, significantly modifies the central density structure of dark matter halos, both observational distributions show a feature corresponding to the mass scale below which the baryons in the halo can cool (i.e., galaxies versus clusters). We use simplified but self-consistent models to show that the structural changes to the halos produced by the cooling baryons explain both distributions. Given a fixed halo mass function, matching the observed image separation distribution or local velocity function depends largely on Ω_b through its effects on the cooling timescales. These baryonic effects on the halo structure also affect the evolution of the velocity function of galaxies with redshift.

We show that the number ratio of radial arcs to tangential arcs in a cluster of galaxies depends sensitively on the central density profile of galaxy clusters but depends little on the lensing parameter and the threshold ellipticity of the lensed images. Based on these results, we propose a method to constrain the central density profile of clusters using radial arc statistics. A tentative comparison with the observations shows that radial arc statistics can be a powerful tool for constraining the cluster central density profile. Our results show that a systematic survey of radial and tangential arcs for an unbiased uniform cluster sample can be a promising test of the fundamental characteristics of dark matter.

Large-scale correlations in the orientations of galaxies can result from alignments in their angular momentum vectors. These alignments arise from the tidal torques exerted on neighboring protogalaxies by the smoothly varying shear field. We compute the predicted amplitude of such ellipticity correlations using the Zeldovich approximation for a realistic distribution of galaxy shapes. Weak gravitational lensing can also induce ellipticity correlations, since the images of neighboring galaxies will be distorted coherently. On comparing these two effects that induce shape correlations, we find that for current weak-lensing surveys with a median redshift of z_m = 1, the intrinsic signal is of the order of 1%-10% of the measured signal. However, for shallower surveys with z_m ≤ 0.3, the intrinsic correlations dominate over the lensing signal. The distortions induced by lensing are curl-free, whereas those resulting from intrinsic alignments are not. This difference can be used to disentangle these two sources of ellipticity correlations.

We study the arc statistics of gravitational lensing generated by dark matter halos in order to probe their density profile. We characterize the halo profile by two parameters, the inner slope of the central cusp α and the median amplitude of the concentration parameter, c_norm, for a halo of mass 10¹⁴h^-1M_☉ at z = 0 and compute the numbers of tangential and radial arcs produced by gravitational lensing of galaxy clusters. We find that the number of arcs divided by the number of halos is a good statistic that is sensitive to both c_norm and α with very weak dependence on the cosmological parameters. If arc samples with well-defined selection criteria for the clusters become available, one can strongly constrain both c_norm and α. While our tentative comparison with the existing observational data indicates that the inner density profile of dark halos is indeed as steep as predicted by recent simulations (α ~ 1.5), homogeneous samples of tangential and radial arcs are required for more quantitative discussions.

We present Hubble Space Telescope measurements of surface brightness fluctuation (SBF) distances to early-type galaxies that have hosted Type Ia supernovae (SNe Ia). The agreement in the relative SBF and SN Ia multicolor light-curve shape and delta-m₁₅ distances is excellent. There is no systematic scale error with distance, and previous work has shown that SBFs and SNe Ia give consistent ties to the Hubble flow. However, we confirm a systematic offset of ~0.25 mag in the distance zero points of the two methods, and we trace this offset to their respective Cepheid calibrations. SBFs have in the past been calibrated with Cepheid distances from the H₀ Key Project team, while SNe Ia have been calibrated with Cepheid distances from the team composed of Sandage, Saha, and collaborators. When the two methods are calibrated in a consistent way, their distances are in superb agreement. Until the conflict over the "long" and "short" extragalactic Cepheid distances among many galaxies is resolved, we cannot definitively constrain the Hubble constant to better than ~10%, even leaving aside the additional uncertainty in the distance to the Large Magellanic Cloud, common to both Cepheid scales. However, recent theoretical SBF predictions from stellar population models favor the Key Project Cepheid scale, while the theoretical SN Ia calibration lies between the long and short scales. In addition, while the current SBF distance to M31/M32 is in good agreement with the RR Lyrae and red giant branch distances, calibrating SBFs with the longer Cepheid scale would introduce a 0.3 mag offset with respect to the RR Lyrae scale.

Galaxy counts in the K band, (J-K) colors, and apparent size distributions of faint galaxies in the Subaru Deep Field (SDF) down to K ~ 24.5 were studied in detail. Special attention has been paid to take into account various selection effects, including the cosmological dimming of surface brightness, to avoid any systematic bias that may be the origin of controversy in previously published results. We also tried to be very careful about systematic model uncertainties; we present a comprehensive survey of these systematic uncertainties and dependence on various parameters, and we have shown that the dominant factors to determine galaxy counts in this band are cosmology and number evolution. We found that the pure luminosity evolution (PLE) model is very consistent with all the SDF data down to K ~ 22.5, without any evidence for number or size evolution in a low-density, Λ-dominated flat universe, which is now favored by various cosmological observations. On the other hand, a number evolution of galaxies with η ~ 2, when invoked as the luminosity conserving mergers as ϕ* ∝ (1 + z)^η and L* ∝ (1 + z)^-η for all types of galaxies, is necessary to explain the data in the Einstein-de Sitter universe. If the popular Λ-dominated universe is taken for granted, our result then gives a strong constraint on the number evolution of giant elliptical or early-type galaxies to z ~ 1-2 that must be met by any models in the hierarchically clustering universe, since such galaxies are the dominant population in this magnitude range (K ≲ 22.5). A number evolution with η ~ 1 is already difficult to reconcile with the data in this universe. On the other hand, number evolution of late-type galaxies and/or dwarf galaxies, which has been suggested by previous studies of optical galaxies, is allowed from the data. In the fainter magnitude range of K ≳ 22.5, we found a slight excess of observed counts over the prediction of the PLE model when elliptical galaxies are treated as a single population. We suggest that this discrepancy reflects some number evolution of dwarf galaxies and/or the distinct populations of giant and dwarf elliptical galaxies which have been known for local elliptical galaxies.

The 15R-North galaxy redshift survey is a uniform spectroscopic survey (S/N ~ 10) covering the range 3650-7400 Å for 3149 galaxies with median redshift 0.05. The sample is 90% complete to R = 15.4. The median slit covering fraction is 24% of the galaxy, apparently sufficient to minimize the effects of aperture bias on the EW(Hα). Forty-nine percent of the galaxies in the survey have one or more emission lines detected at ≥2 σ. In agreement with previous surveys, the fraction of absorption-line galaxies increases steeply with galaxy luminosity. We use Hβ, [O III], Hα, and [N II] to discriminate between star-forming galaxies and AGNs. At least 20% of the galaxies are star-forming, at least 17% have AGN-like emission, and 12% have unclassifiable emission. The unclassified 12% may include a "hybrid" population of galaxies with both star formation and AGN activity. The AGN fraction increases steeply with luminosity; the fraction of star-forming galaxies decreases. We use the EW(Hα+[N II]) to estimate the Scalo birthrate parameter, b, the ratio of the current star formation rate to the time averaged star formation rate. The median birthrate parameter is inversely correlated with luminosity in agreement with the conclusions based on smaller samples (Kennicutt, Tamblyn, & Congdon). Because our survey is large, we identify 33 vigorously star-forming galaxies with b > 3. We confirm the conclusion of Jansen, Franx, & Fabricant that EW([O II]) must be used with caution as a measure of current star formation. Finally, we examine the way galaxies of different spectroscopic type trace the large-scale galaxy distribution. As expected the absorption-line fraction decreases and the star-forming emission-line fraction increases as the galaxy density decreases. The AGN fraction is insensitive to the surrounding galaxy density; the unclassified fraction declines slowly as the density increases. For the star-forming galaxies, the EW(Hα) increases very slowly as the galaxy number density decreases. Whether a galaxy forms stars or not is strongly correlated with the surrounding galaxy density averaged over a scale of a few Mpc. This dependence reflects, in large part, the morphology-density relation. However, for galaxies forming stars, the stellar birthrate parameter is remarkably insensitive to the galaxy density. This conclusion suggests that the triggering of star formation occurs on a smaller spatial scale.

Using deep near-infrared and optical observations of the Hubble Deep Field-North from the Hubble Space Telescope NICMOS and WFPC2 instruments and from the ground, we examine the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at 2.0 ≲ z ≲ 3.5 in order to investigate their stellar population properties. The ultraviolet-optical rest-frame SEDs of the galaxies are much bluer than those of present-day spiral and elliptical galaxies and are generally similar to those of local starburst galaxies with modest amounts of reddening. We use stellar population synthesis models to study the properties of the stars that dominate the light from LBGs. Under the assumption that the star formation rate is continuous or decreasing with time, the best-fitting models provide a lower bound on the LBG mass estimates. LBGs with "L*" UV luminosities are estimated to have minimum stellar masses ~10¹⁰ _☉, or roughly 1/10 that of a present-day L* galaxy, similar to the mass of the Milky Way bulge. By considering the photometric effects of a second stellar population component of maximally old stars, we set an upper bound on the stellar masses that is ~3-8 times the minimum mass estimate. The stellar masses derived for bright LBGs are similar to published estimates of their dynamical masses based on nebular emission line widths, suggesting that such kinematic measurements may substantially underestimate the total masses of the dark matter halos. We find only loose constraints on the individual galaxy ages, extinction, metallicities, initial mass functions, and prior star formation histories. Most LBGs are well fitted by models with population ages that range from 30 Myr to ~1 Gyr, although for models with subsolar metallicities a significant minority of galaxies are well fitted by very young (≲10 Myr), very dusty stellar populations, A(1700 Å) > 2.5 mag. We find no galaxies whose SEDs are consistent with young (≲10⁸ yr), dust-free objects, which suggests that LBGs are not dominated by "first-generation" stars and that such objects are rare at these redshifts. We also find that the typical ages for the observed star formation events are significantly younger than the time interval covered by this redshift range (~1.5 Gyr). From this, and from the relative absence of candidates for quiescent, non-star-forming galaxies at these redshifts in the NICMOS data that might correspond to the fading remnants of galaxies formed at higher redshift, we suggest that star formation in LBGs may be recurrent, with short duty cycles and a timescale between star formation events of ≲1 Gyr.

We present results of a program to obtain and analyze HST WFPC2 images and ground-based images of galaxies identified in an imaging and spectroscopic survey of faint galaxies in fields of HST spectroscopic target QSOs. Considering a sample of physically correlated galaxy and absorber pairs with galaxy-absorber cross-correlation amplitude ξ_ga(v,ρ) > 1 and with galaxy impact parameter ρ < 200 h^-1 kpc, we confirm and improve the results presented by Lanzetta et al. and Chen et al. that (1) extended gaseous envelopes are a common and generic feature of galaxies of a wide range of luminosity and morphological type, (2) the extent of tenuous gas [N(H ) ≳ 10¹⁴ cm^-2] around galaxies scales with galaxy B-band luminosity as r ∝ L, and (3) galaxy interactions do not play an important role in distributing tenuous gas around galaxies in most cases. We further demonstrate that (4) the gaseous extent of galaxies scales with galaxy K-band luminosity as r ∝ L, and (5) tenuous gas around typical L_* galaxies is likely to be distributed in spherical halos of radius ≈180 h^-1 kpc of covering factor of nearly unity. The sample consists of 34 galaxy and absorber pairs and 13 galaxies that do not produce Lyα absorption lines to within sensitive upper limits. Redshifts of the galaxy and absorber pairs range from z = 0.0752 to 0.8920 with a median of z = 0.3567; impact parameter separations of the galaxy and absorber pairs range from ρ = 12.4 to 175.2 h^-1 kpc with a median of ρ = 62.2 h^-1 kpc. Of the galaxies, 15 (32%) are of B-band luminosity L_B < 0.25 L and six (13%) are of low surface brightness. The galaxy sample is therefore representative of the galaxy population over a large fraction of the Hubble time. Because galaxies of all morphological types possess extended gaseous halos and because the extent of tenuous gas around galaxies scales with galaxy K-band luminosity, we argue that galaxy mass—rather than recent star formation activity—is likely to be the dominant factor that determines the extent of tenuous gas around galaxies. Nevertheless, applying the scaling relationship between the extent of Lyα-absorbing gas around galaxies and galaxy B-band luminosity, the results of our analysis also suggest that the number density evolution of Lyα absorption systems may serve to constrain the evolution of the comoving galaxy B-band luminosity density (at least for the redshift interval between z ~ 0 and z ~ 1 that has been studied in our survey).

The presence of a "warm absorber" was first suggested to explain spectral variability in an X-ray spectrum of the radio-quiet quasi-stellar object (QSO) MR 2251-178. A unified picture, in which X-ray warm absorbers and "intrinsic" UV absorbers are the same, offers the opportunity to probe the nuclear environment of active galactic nuclei. To test this scenario and understand the physical properties of the absorber, we obtained a UV spectrum of MR 2251-178 with the Faint Object Spectrograph on board the Hubble Space Telescope (HST). The HST spectrum clearly shows absorption due to Lyα, N V, and C IV, blueshifted by 300 km s^-1 from the emission redshift of the QSO. The rarity of both X-ray and UV absorbers in radio-quiet QSOs suggests these absorbers are physically related, if not identical. Assuming the unified scenario, we place constraints on the physical parameters of the absorber and conclude the mass outflow rate is essentially the same as the accretion rate in MR 2251-178.

We extend our models of the vertical structure and emergent radiation field of accretion disks around supermassive black holes described in previous papers of this series. Our models now include both a self-consistent treatment of Compton scattering and the effects of continuum opacities of the most important metal species (C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni). With these new effects incorporated, we compute the predicted spectrum from black holes accreting at nearly the Eddington luminosity (L/L_Edd ≈ 0.3) and central masses of 10⁶, 10⁷, and 10⁸M_☉. We also consider two values of the Shakura-Sunyaev α parameter, 0.1 and 0.01, but in contrast to our previous papers, we consider a kinematic viscosity that is independent of depth. Although it has little effect when M > 10⁸M_☉, Comptonization grows in importance as the central mass decreases and the central temperature rises. It generally produces an increase in temperature with height in the uppermost layers of hot atmospheres. Compared to models with coherent electron scattering, Comptonized models have enhanced extreme ultraviolet/soft X-ray emission, but they also have a more sharply declining spectrum at very high frequencies. Comptonization also smears the hydrogen and the He II Lyman edges. The effects of metals on the overall spectral energy distribution are smaller than the effects of Comptonization for these parameters. Compared to pure hydrogen-helium models, models with metal-continuum opacities have reduced flux in the high-frequency tail, except at the highest frequencies, where the flux is very low. Metal photoionization edges are not present in the overall disk-integrated model spectra. The viscosity parameter α has a more dramatic effect on the emergent spectrum than do metal-continuum opacities. As α increases (and therefore the disk column density decreases), the flux at both the high- and low-frequency extremes of the spectrum increases, while the flux near the peak decreases. Multitemperature blackbodies are a very poor approximation to accretion disk spectra in the soft X-ray region, and such crude modeling may greatly overestimate the accretion luminosity required to explain observed soft X-ray excesses in active galactic nuclei. In addition to our new grid of models, we also present a simple analytic prescription for the vertical temperature structure of the disk in the presence of Comptonization, and show under what conditions a hot outer layer (a corona) is formed.

We present sensitive, high dynamic range, Very Long Baseline Array (VLBA) polarimetric observations of the cores of three powerful radio galaxies: 3C 166, 3C 236, and 3C 390.3. Significant polarization is detected in one source (3C 166) allowing us to map out the Faraday rotation measure (RM) distribution and projected magnetic field direction. The inner jet of 3C 166 is found to have a rest frame RM of -2300 rad m^-2, similar to those found in quasar cores. No polarized flux was detected from the other two sources, but in both, counterjets are seen. The counterjet in 3C 236 was previously known, but the detection in 3C 390.3 is a new discovery. We suggest that the low fractional polarization in radio galaxy cores is the result of Faraday depolarization by ionized gas associated with the accretion disk. The lower polarization of radio galaxy cores compared to quasars is then naturally explained by unified models as a result of the viewing angle.

We analyze the BeppoSAX measurements of the prompt and afterglow emission of the γ-ray burst GRB 010222. Among 45 GRBs detected with the Wide Field Cameras on BeppoSAX, the 40-700 keV fluence of (9.3 ± 0.3) × 10^-5 ergs cm^-2 is only surpassed by GRB 990123. In terms of the isotropic 20-2000 keV energy output of 7.8 × 10⁵³ ergs, it ranks third of all GRBs with measured distances. Since this burst is so bright, the data provide complete and valuable coverage up to 65 hr after the event, except for a gap between 3.5 and 8.0 hr. The 2-10 keV flux history shows clear signs of a break, which is consistent with a break seen in the optical, and provides supporting evidence for the achromatic nature of the break. An explanation for the break in the context of a collimated expansion is not straightforward. Rather, a model is favored whereby the fireball is braked to the nonrelativistic regime quickly (within a fraction of day) by a dense (~10⁶ cm^-3) circumburst medium. This implies that, after a mild beaming correction, GRB 010222 may be the most energetic burst observed thus far. The X-ray decay index after the break is 1.33 ± 0.04, the spectral index 0.97 ± 0.05. The decay is, with unprecedented accuracy, identical to that observed in the optical.

We have developed a simple yet surprisingly accurate analytic scheme for tracking the dynamical evolution of substructure within dark matter halos. The scheme incorporates the effects of dynamical friction, tidal mass loss, and tidal heating via physically motivated approximations. Using our scheme, we can predict the orbital evolution and mass-loss history of individual subhalos in detail. We are also able to determine the impact and importance of the different physical processes on the dynamical evolution of the subhalos. To test and calibrate this model, we compare it with a set of recent high-resolution numerical simulations of mergers between galaxies and small companions. We find that we can reproduce the orbits and mass-loss rates seen in all of these simulations with considerable accuracy, using a single set of values for the three free parameters in our model. Computationally, our scheme is more than 1000 times faster than the simplest of the high-resolution numerical simulations. This means that we can carry out detailed and statistically meaningful investigations into the characteristics of the subhalo population in different cosmologies, the stripping and disruption of the subhalos, and the interactions of the subhalos with other dynamical structures such as a thin disk. This last point is of particular interest given the ubiquity of minor mergers in hierarchical models. In this regard, our method's simplicity and speed makes it particularly attractive for incorporation into semianalytic models of galaxy formation.

In an effort to more fully understand the variety of stellar distribution functions that can be used to construct models of realistic galaxies, the correlation integral method for orbit characterization introduced previously in 1983 by Grassberger & Procaccia and in 1984 by Carnevali & Santangelo is examined in considerable detail. The broad utility of the method is validated and demonstrated by applying it to orbits in a number of different, previously studied test cases (one-, two-, and three-dimensional; nonrotating and rotating). At the same time, the correlation integral method is compared and contrasted with other more traditional characterization tools, such as Lyapunov exponents and surfaces of section. The method is then extended to orbits in a previously unexamined rotating, three-dimensional bar potential. The correlation integral method is found to be a simple and reliable way to quantitatively categorize orbits in virtually any potential. It is recommended that it be broadly adopted as a tool for characterizing the properties of orbits and, by extension, stellar distribution functions, in all Hamiltonian dynamical systems.

We present results from high-resolution N-body/SPH (smoothed particle hydrodynamic) simulations of rotationally supported dwarf irregular galaxies moving on bound orbits in the massive dark matter halo of the Milky Way. The dwarf models span a range in disk surface density and the masses and sizes of their dark halos are consistent with the predictions of cold dark matter cosmogonies. We show that the strong tidal field of the Milky Way determines severe mass loss in their halos and disks and induces bar and bending instabilities that transform low surface brightness dwarfs (LSBs) into dwarf spheroidals (dSphs) and high surface brightness dwarfs (HSBs) into dwarf ellipticals (dEs) in less than 10 Gyr. The final central velocity dispersions of the remnants are in the range 8-30 km s^-1 and their final v/σ falls to values less than 0.5, matching well the kinematics of early-type dwarfs. The transformation requires the orbital time of the dwarf to be ≲3-4 Gyr, which implies a halo as massive and extended as predicted by hierarchical models of galaxy formation to explain the origin of even the farthest dSph satellites of the Milky Way, Leo I, and Leo II. We show that only dwarfs with central dark matter densities as high as those of Draco and Ursa Minor can survive for 10 Gyr in the proximity of the Milky Way. A correlation between the central density and the distance of the dwarfs from the primary galaxy is indeed expected in hierarchical models, in which the densest objects should have small orbital times because of their early formation epochs. Part of the gas is stripped and part is funneled to the center because of the bar, generating one strong burst of star formation in HSBs and smaller, multiple bursts in LSBs. Therefore, the large variety of star formation histories observed in Local Group dSphs arises because different types of dIrr progenitors respond differently to the external perturbation of the Milky Way. Our evolutionary model naturally explains the morphology-density relation observed in the Local Group and in other nearby loose groups. Extended low surface brightness stellar and gaseous streams originate from LSBs and follow the orbit of the dwarfs for several gigayears. Because of their high velocities, unbound stars projected along the line of sight can lead to overestimating the mass-to-light ratio of the bound remnant by a factor ≲2, but this does not eliminate the need of extremely high dark matter contents in some of the dSphs.

We present 74 and 330 MHz VLA observations of Abell 754. Diffuse, halo-like emission is detected from the center of the cluster at both frequencies. At 330 MHz, the resolution of 90'' distinguishes this extended emission from previously known point sources. In addition to the halo, and at a much lower level, outlying steep-spectrum emission regions straddle the cluster center and are seen only at 74 MHz. The location, morphology, and spectrum of this emission are all highly suggestive of at least one, and possibly two, cluster radio relics. Easily obtained higher resolution, higher sensitivity VLA observations at both frequencies are required to confirm the extended nature of the halo-like emission and the 74 MHz relic detections. However, since there is prior evidence that this cluster is or has recently been in the process of a major merger event, the possible discovery of relics in this system is of great interest in light of recent observational and theoretical evidence in favor of a merger-relic connection. We discuss the possible role that the merger shock waves, which are seen in the X-ray emission, may have played in the formation of the halo and radio relics in A754.

Early-type dwarfs are the most common galaxy in the local universe, yet their origin and evolution remain a mystery. Various cosmological scenarios predict that dwarf-like galaxies in dense areas are the first to form and hence should be the oldest stellar systems in clusters. By using radial velocities of early-type dwarfs in the Virgo cluster we demonstrate that these galaxies are not an old cluster population but have signatures of production from the infall of field galaxies. Evidence of this includes the combined large dispersions and substructure in spatial and kinematic distributions for Virgo early-type dwarfs and a velocity dispersion ratio with giant ellipticals expected for virialized and accreted populations. We also argue that these galaxies cannot originate from accreted field dwarfs, but must have physically evolved from a precursor population, of different morphology, that fell into Virgo some time in the past.

We present a comprehensive database of kinematic, photometric, and positional information for 352 objects in the field of M87 (NGC 4486), the central giant elliptical galaxy in the Virgo Cluster; the majority of the tracers are globular clusters associated with that galaxy. New kinematic information comes from multislit observations with the Multi-Object Spectrograph (MOS) of the Canada-France-Hawaii Telescope (CFHT), an investigation that has added 96 new velocities to and confirmed many of the earlier values in a preexisting data set of 256 velocities published elsewhere. The photometry, consisting of magnitudes and colors in the Washington (T₁, C-T₁) system, is based on CCD observations made at the Cerro Tololo Inter-American Observatory (CTIO) and the Kitt Peak National Observatory (KPNO). The composite database represents the largest compilation of pure Population II dynamical tracers yet identified in any external galaxy; moreover, it extends to larger spatial scales than have earlier investigations. The inclusion of photometric information allows independent study of the distinct red and blue subpopulations of the bimodal globular cluster system of M87. In a companion paper we use this powerful data set to analyze the present dynamical state of the M87 globular cluster system and consider the question of its interaction and formation history.

We present a dynamical analysis of the globular cluster system associated with M87 (=NGC 4486), the cD galaxy near the dynamical center of the Virgo Cluster. The analysis utilizes a new spectroscopic and photometric database, which is described in a companion paper. Using a sample of 278 globular clusters with measured radial velocities and metallicities and new surface density profiles based on wide-field Washington photometry, we study the dynamics of the M87 globular cluster system both globally (for the entire cluster sample) and separately (for the metal-rich and metal-poor globular cluster samples). This constitutes the largest sample of radial velocities for pure Population II tracers yet assembled for any external galaxy. Our principal findings are summarized as follows:

We present a method for finding clusters of young stars in M31 using broadband WFPC2 data from the HST data archive. Applying our identification method to 13 WFPC2 fields, covering an area of ~60 arcmin², has revealed 79 new candidate young star clusters in these portions of the M31 disk. Most of these clusters are small (≲5 pc), young (~10-200 Myr) star groups located within large OB associations. We have estimated the reddening values and the ages of each candidate individually by fitting isochrones to the stellar photometry. We provide a catalog of the candidates including rough approximations of their reddenings and ages. We also look for patterns of cluster formation with galactocentric distance, but our rough estimates are not precise enough to reveal any clear patterns.

We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young embedded star birth regions that are likely to be clusters of ultracompact H II regions. All 35 of the sources presented in this article have positive radio spectral indices (α > 0 for S_ν ∝ ν^α), suggesting an optically thick thermal bremsstrahlung origin from the H II region surrounding the hot stars. The estimated emission measures for these sources are EM_{6 cm} ≳ 10⁸ cm^-6 pc, and energy requirements indicate that the sources in our sample have a range of a few to ~560 O7 V star equivalents powering their H II regions. Assuming a Salpeter initial mass function with lower and upper mass cutoffs of 1 and 100 M_☉, respectively, this range in N_Lyc corresponds to integrated stellar masses of 0.1-60 × 10³M_☉. For roughly half of the sources in our sample there is no obvious optical counterpart, which gives further support for their deeply embedded nature; for most of the remaining sources, the correspondence to an optical source is insecure owing to relative astrometric uncertainty. Their luminosities and radio spectral energy distributions are consistent with H II regions modeled as spheres of plasma with electron densities from n_e ~ 1.5 × 10³ to ~1.5 × 10⁴ cm^-3 and radii of ~1-7 pc. Because of the high densities required to fit the data, we suggest that the less luminous of these sources are extragalactic ultracompact H II region complexes, those of intermediate luminosity are similar to W49 in the Galaxy, and the brightest will be counterparts to 30 Doradus when they emerge from their birth material. These objects constitute the lower mass range of extragalactic "ultradense H II regions," which we argue are the youngest stages of massive star cluster formation yet observed. The sample presented in this paper is beginning to fill in the continuum of objects between small associations of ultracompact H II regions and the analogous massive extragalactic clusters that may evolve into globular clusters.

We have compared Hα and far-ultraviolet (FUV) images of 10 nearby spirals, with the goal of understanding the contribution of field OB stars to the ionization of the diffuse ionized gas (DIG) in spiral galaxies. The FUV images were obtained by the Ultraviolet Imaging Telescope (UIT), and the Hα images were obtained using various ground-based telescopes. In all of the galaxies, the F_Hα/F_UIT flux ratio is lower in the DIG than in the H II regions. This is likely an indication that the mean spectral type for OB stars in the field is later than that in H II regions. Comparison of the N_Lyc/L_UIT ratio with models of evolving stellar populations shows that the stellar population in the DIG is consistent with either an older single-burst population or a steady state model with constant star formation and an initial mass function (IMF) slope steeper than α = 2.35. The steady state model is probably a more realistic representation of the stellar population outside of H II regions. The steep IMF slope simulates the steep present-day mass function slope expected for field OB stars and does not necessarily indicate that the IMF slope is actually steeper than α = 2.35. We compared the F_Hα/F_UIT ratio in the DIG of these galaxies with that in M33, in which the field OB stellar population has previously been investigated using Hubble Space Telescope images. If the mean spectral types of stars in H II regions and in the DIG are the same as in M33 and the difference in extinction between DIG and H II regions is constant among galaxies, then the analysis suggests that field stars are important sources of ionization in most galaxies and may be the dominant source in some galaxies. The F_Hα/F_UIT ratio is correlated with Hα surface brightness in both DIG and H II regions, although there is a large scatter in faint H II regions, which may be due to undersampling the IMF in regions with a low total mass of stars formed. The F_Hα/F_UIT ratio is often highest in the centers of galaxies and in the spiral arms, which is also where the DIG is brightest. This can be explained if the extinction is greater in these regions or if the fraction of DIG ionized by leakage is lower in the interarm regions.

A simple model for a hot Local Group halo is constructed using the standard β-model for the halo density and by choosing model parameters based on all available observations of X-ray emission in other groups of galaxies and on optical data of Local Group morphology. From the predicted X-ray intensities, total Local Group mass, and central cooling time of the halo, we derive very conservative upper limits on the central halo density N₀ and global temperature T of N₀ = 5 × 10^-4 cm^-3 and kT = 0.5 keV, irrespective of realistic values of the density profile parameters r_c and β. A typical poor group value of β = 0.5 requires kT < 0.15 keV and N₀ < 10^-4 cm^-3, from which it is concluded that the Local Group is very unlikely to possess a significant X-ray halo. The prospects for further constraining halo parameters from UV absorption-line observations are considered. We explicitly calculate the ability of the halo to distort the cosmic microwave background (CMB) in terms of the resulting CMB temperature variations and multipole anisotropies.

Some supernova remnant X-ray spectra show evidence for synchrotron emission from the extension of the electron spectrum that produces radio synchrotron emission. For any remnant, if the extrapolated radio flux exceeds the observed X-ray flux, thermal or nonthermal, a roll-off of the relativistic electron energy distribution must occur below X-ray-emitting energies. We have studied the X-ray emission from 11 remnants in the Large Magellanic Cloud to constrain this roll-off energy. We assume that the electron distribution is a power law with an exponential cutoff at some E_max and radiates in a uniform magnetic field. If the radio flux and spectral index are known, this simple model for the synchrotron contribution depends on only one parameter that relates directly to E_max. Here we have modeled the X-ray spectra by adding a component for thermal radiation of a Sedov blast wave to the synchrotron model. For all 11 supernova remnants in this sample, the limits for E_max range between 10 and 80 TeV (for a mean magnetic field of 10 μG). This result is similar to a study of Galactic remnants in which 13 out of 14 objects had limits between 20 and 80 TeV. We interpret E_max in the context of shock acceleration theories. Better data and models should allow either firm detections of nonthermal components or more restrictive limits on E_max.

In this paper we study the evolution of ⁷Li in the Galaxy considering the contributions of various stellar sources: Type II supernovae, novae, red giant stars, and asymptotic giant branch (AGB) stars. We present new results for the production of ⁷Li in AGB stars via the hot bottom burning process, based on stellar evolutionary models. In the light of recent observations of dense circumstellar shells around evolved stars in the Galaxy and in the Magellanic Clouds, we also consider the impact of a very high mass-loss rate episode (superwind) before the evolution off the AGB phase on the ⁷Li enrichment in the interstellar medium. We compare the Galactic evolution of ⁷Li obtained with these new ⁷Li yields (complemented with a critical reanalysis of the role of supernovae, novae and giant stars) with a selected compilation of spectroscopic observations including halo and disk field stars as well as young stellar clusters. We conclude that even allowing for the large uncertainties in the theoretical calculation of mass-loss rates at the end of the AGB phase, the superwind phase has a significant effect on the ⁷Li enrichment of the Galaxy.

A model is presented that seeks to explain quantitatively the stellar abundances of r-process elements and other elements associated with the r-process sites. It is argued that the abundances of all these elements in stars with -3 ≲ [Fe/H] < -1 can be explained by the contributions of three sources. The sources are the first generations of very massive (≳100 M_☉) stars that are formed from big bang debris and are distinct from Type II supernovae (SNe II) and two types of SNe II, the H and L events, which can occur only at [Fe/H] ≳ -3. The H events are of high frequency and produce dominantly heavy (A > 130) r-elements but no Fe (presumably leaving behind black holes). The L events are of low frequency and produce Fe and dominantly light (A ≲ 130) r-elements (essentially none above Ba). By using the observed abundances in two ultra-metal-poor stars and the solar r-abundances, the initial or prompt inventory of elements produced by the first generations of very massive stars and the yields of H and L events can be determined. The abundances of a large number of elements in a star can then be calculated from the model by using only the observed Eu and Fe abundances. To match the model results and the observational data for stars with -3 < [Fe/H] < -1 requires that the solar r-abundances for Sr, Y, Zr, and Ba must be significantly increased from the standard values. No such changes appear to be required for all other elements. If the changes in the solar r-abundances for Sr, Y, Zr, and Ba are not permitted, the model fails at -3 < [Fe/H] < -1 but still works at [Fe/H] ≈ -3 for these four elements. By using the corrected solar r-abundances for these elements, good agreement is obtained between the model results and data over the range -3 < [Fe/H] < -1. No evidence of s-process contributions is found in this region, but all the observational data in this region now show regular increases of Ba/Eu above the standard solar r-process value. Whether the solar r-components of Sr, Y, Zr, and Ba used here to obtain a fit to the stellar data can be reconciled with those obtained from solar abundances by subtracting the s-components calculated from models is not clear.

We present an alternative interpretation of the nature of the extremely cool, high-velocity white dwarfs identified by Oppenheimer et al. in a high-latitude astrometric survey. We argue that the velocities of the majority of the sample are more consistent with the high-velocity tail of a rotating population, probably the thick disk, rather than with a pressure-supported halo system. Indeed, the observed numbers are well matched by predictions based on the kinematics of a complete sample of nearby M dwarfs. Analyzing only stars showing retrograde motion gives a local density close to that expected for white dwarfs in the stellar (R^-3.5) halo. Under our interpretation, none of the white dwarfs need be assigned to the dark matter heavy halo. However, luminosity functions derived from observations of these stars can set important constraints on the age of the oldest stars in the Galactic disk.

With Hubble Space Telescope Wide-Field Planetary Camera 2 observations of the Cygnus Loop supernova remnant, we examine the interaction of an interstellar cloud with the blast wave on physical scales of 10¹⁵ cm. The shock front is distorted, revealing both edge-on and face-on views of filaments and diffuse emission, similar to those observed on larger scales at lower resolution. We identify individual shocks in the cloud of density n ≈ 15 cm^-3 having velocity v_s ≈ 170 km s^-1. We also find the morphologically unusual diffuse Balmer-dominated emission of faster shocks in a lower density region. The obstacle diffracts these shocks, so they propagate at oblique angles with respect to the primary blast wave. The intricate network of diffuse and filamentary Hα emission arises during the early stage of interaction between the cloud and blast wave, demonstrating that complex shock propagation and emission morphology occur before the onset of instabilities that destroy clouds completely.

We present subarcminute resolution imaging of the Galactic supernova remnant W49B at 74 MHz (25'') and 327 MHz (6''), the former being the lowest frequency at which the source has been resolved. While the 327 MHz image shows a shell-like morphology similar to that seen at higher frequencies, the 74 MHz image is considerably different, with the southwest region of the remnant almost completely attenuated. The implied 74 MHz optical depth (≈1.6) is much higher than the intrinsic absorption levels seen inside two other relatively young remnants, Cas A and the Crab Nebula, nor are natural variations in the relativistic electron energy spectra expected at such levels. The geometry of the absorption is also inconsistent with intrinsic absorption. We attribute the absorption to extrinsic free-free absorption by an intervening cloud of thermal electrons. Its presence has already been inferred from the low-frequency turnover in the integrated continuum spectrum and from the detection of radio recombination lines toward the remnant. Our observations confirm the basic conclusions of those measurements, and our observations have resolved the absorber into a complex of classical H II regions surrounded either partially or fully by low-density H II gas. We identify this low-density gas as an extended H II region envelope (EHE), whose statistical properties were inferred from low-resolution meter- and centimeter-wavelength recombination line observations. Comparison of our radio images with H I and H₂CO observations shows that the intervening thermal gas is likely associated with neutral and molecular material as well. This EHE may be responsible for the enhanced radio-wave scattering seen in the general direction of the W49 complex.

Important constraints on the properties of the anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) can be provided by their associations with supernova remnants (SNRs). We have made a radio search for SNRs toward the AXPs RX J170849-400910 and 4U 0142+61. We find that the former lies near a possible new SNR with which it is unlikely to be physically associated, but we see no SNR in the vicinity of the latter. We review all claimed pairings between AXPs and SNRs; the three convincing associations imply that AXPs are young (<10 000 yr) neutron stars with low projected space velocities (<500 km s^-1). Contrary to previous claims, we find no evidence that the density of the ambient medium around AXPs is higher than that in the vicinity of radio pulsars. Furthermore, the nondetection of radio emission from AXPs does not imply that these sources are radio-silent. We also review claimed associations between SGRs and SNRs. We find none of these associations to be convincing, consistent with a scenario in which SGRs and AXPs are both populations of high-field neutron stars ("magnetars") but the SGRs are an older or longer-lived group of objects than are the AXPs. If the SGR-SNR associations are shown to be valid, then SGRs must be high-velocity objects and most likely represent a different class of source to the AXPs.

Cosmic-ray electrons have been observed in the energy range from 12 to ~100 GeV with a new balloon-borne payload, the Balloon-borne Electron Telescope with Scintillating Fibers (BETS). This is the first publication of the absolute energy spectrum of electrons measured with a highly granulated fiber calorimeter. The calorimeter makes it possible to select electrons against the background protons by detailed observation of both the longitudinal and the lateral shower development. The performance of the detector was calibrated by the CERN-SPS accelerator beams: electrons from 5 to 100 GeV, protons from 60 to 250 GeV. The balloon observations were carried out twice, in 1997 and 1998, at the Sanriku Balloon Center (Institute of Space and Astronautical Science) in Japan. The observation time was ~13 hr in all at an altitude above 34 km. A total of 1349 electron candidates were collected, and the 628 events with energies above 12.5 GeV, well above the geomagnetic rigidity cutoff of ~10 GV, have been used to compose a differential absolute energy spectrum at the top of the atmosphere. The energy spectrum is described by a power-law index of 3.00 ± 0.09, and the absolute differential intensity at 10 GeV is 0.199 ± 0.015 m^-2 s^-1 sr^-1 GeV^-1. The overall shape of the energy spectrum in 10 ~ 100 GeV can be explained by a diffusion model, in which we assume an energy-dependent diffusion coefficient (∝ E^0.3) for an injection spectrum, E^-2.4.

We present Heinrich Hertz Telescope observations in the N = 3 → 2 rotational transition of the CN radical toward selected positions of the Trapezium region and of the molecular ridge in the Orion molecular cloud. Two of the positions in the ridge were also observed in the N = 2 → 1 line of CN and ¹³CN. The N = 3 → 2 CN lines have been combined with observations of the N = 2 → 1 and N = 1 → 0 transitions of CN and observations of the N = 2 → 1 of ¹³CN in order to estimate the physical conditions and CN abundances in the molecular gas. We analyze in detail the excitation of the CN lines and find that the hyperfine ratios of the N = 3 → 2 line are always close to the LTE optically thin values even in the case of optically thick emission. This is due to different excitation temperatures for the different hyperfine lines. From the line-intensity ratios between the different CN transitions we derive H₂ densities of ~10⁵ cm^-3 for the molecular ridge and densities of ~ 3 × 10⁶ cm^-3 for the Trapezium region. The CN column densities are 1 order of magnitude larger in the ridge than in the Trapezium region, but the CN to H₂ ratio is similar in both the Trapezium and the ridge. The combination of the low CN column densities, high H₂ densities, and relatively high CN abundances toward the Trapezium region requires that the CN emission arise from a thin layer with a depth along the line of sight of only ~ 5 × 10¹⁵ cm. This high-density thin layer of molecular gas seems to be related to material that confines the rear side of the H II region Orion A. However, the molecular layer is not moving as expected from the expansion of the H II region but is "static" with respect to the gas in the molecular cloud. We discuss the implication of a high-density "static" layer in the evolution of an H II region.

We model cosmic dust grains as aggregates (clusters) of spheres of appropriate geometry, whose optical properties we calculate in the framework of the transition matrix method. The calculation is performed without resorting to any approximation and with a computational effort that is noticeably lighter than the one required by other methods. Whatever the geometry chosen to model the cosmic grains, the orientational averages that are necessary to describe the propagation of the electromagnetic radiation through a dispersion of clusters are easily handled by exploiting the transformation properties of the transition matrix elements under rotation of the coordinate frame. In this paper we focus on the potentialities of the cluster model by comparing the extinction spectrum of a sphere of astronomical silicates with those of aggregates containing the same mass of silicates and composed of up to 12 spheres. Our main result is that, when a given mass of silicates is subdivided into clustering spheres, the extinction increases in regions of the spectrum determined by the degree of subdivision. We also show to what extent the substitution of the material of some of the clustering spheres with the same volume of carbon changes the extinction signature of the clusters. Finally we show to what extent modification of the geometry of the clusters produces detectable changes in their optical signatures. Detailed analysis of our results leads us to the conclusion that modeling the dust grains as clusters of a single morphology is not sufficient to describe the extinction in the whole wavelength range of astrophysical interest. The cluster model may help emphasize the decisive role of morphology in the identification of sustainable structures for dust grains in the typical physical and chemical conditions of the diffuse interstellar medium.

We model the polarized thermal dust emission from protostellar cores that are assembled by supersonic turbulent flows in molecular clouds. Self-gravitating cores are selected from a three-dimensional simulation of supersonic and super-Alfvénic magnetohydrodynamic (MHD) turbulence. The polarization is computed in two ways. In model A it is assumed that dust properties and grain alignment efficiency are uniform; in model B it is assumed that grains are not aligned at visual extinction larger than A_V,0 = 3 mag, consistent with theoretical expectations for grain alignment mechanisms. Instead of using a specific set of grain properties, we adopt a maximum degree of polarization P_max = 15%. Results are therefore sensitive mainly to the topology of the magnetic field (model A) and to the gas distribution that determines the distribution of A_V (model B). Furthermore, the radiative transfer in the MHD model is solved with a non-LTE Monte Carlo method, to compute spectral maps of the J = 1-0 transition of CS. The CS spectral maps are used to estimate the turbulent velocity, as in the observations. The main results of this work are the following: (1) Values of P between 1% and 10% (up to almost P_max) are typical, despite the super-Alfvénic nature of the turbulence. (2) A steep decrease of P with increasing values of the submillimeter dust continuum intensity I is always found in self-gravitating cores selected from the MHD simulations if grains are not aligned above a certain value of visual extinction A_V,0 (model B). (3) The same behavior is hard to reproduce if grains are aligned independently of A_V (model A). (4) The Chandrasekhar-Fermi formula, corrected by a factor f ≈ 0.4, provides an approximate estimate of the average magnetic field strength in the cores. Submillimeter dust continuum polarization maps of quiescent protostellar cores and Bok globules have recently been obtained. They always show a decrease in P with increasing value of I consistent with the predictions of our model B. We therefore conclude that submillimeter polarization maps of quiescent cores do not map the magnetic field inside the cores at visual extinction larger than A_V,0 ≈ 3 mag. The use of such maps to constrain models of protostellar core formation and evolution is questionable. This conclusion suggests that there is no inconsistency between the results from optical and near-IR polarized absorption of background stars and the observed polarization of submillimeter dust continuum from quiescent cores. In both cases, grains at large visual extinction appear to be virtually unaligned.

We extend earlier efforts to determine whether the late (t ≥ 60 days) light curves of Type Ia SNe are better explained by the escape of positrons from the ejecta or by the complete deposition of positron kinetic energy in a trapping magnetic field. We refine our selection of Ia SNe, using those that have extensive BVRI photometry 35 days or more after maximum light. Assuming that all SNe within a given Δm₁₅(B) range form a distinct subclass, we fit a combined light curve for all class members with a variety of models. We improve our previous calculations of energy deposition rates by including the transport of the Comptonized electrons. Their nonlocal and time-dependent energy deposition produces a correction of as much as 0.10 mag for Chandrasekhar-mass models and 0.18 mag for sub-Chandrasekhar-mass models.

We produce bolometric corrections, derived from measured spectra, to B, V, R, and I light curves after day 50. Comparisons of the resulting bolometric light curves with simulated energy deposition rates demonstrate that the energy deposition from the photons and positrons created in ⁵⁶Co → ⁵⁶Fe decays are consistent with the observations if positron escape is assumed. This implies that there is no evidence of additional sources of energy deposition or of a shift of emission into unobserved wavelength ranges between days 60 and 900. The V band is shown to be an accurate indicator of total emission in the 3500-9700 Å range, with a constant fraction (~25%) appearing in the V band after day 50. This suggests that the V band scales with the bolometric luminosity and that the deposited energy is instantaneously recycled into optical emission during this epoch. We see significant evolution of the colors of SNe Ia between days 50 and 170. We suggest that this may be due to the transition from spectra dominated by emission lines from the radioactive nucleus, ⁵⁶Co, to those from the stable daughter nucleus, ⁵⁶Fe.

We show that thin accretion disks made of carbon or oxygen are subject to the same thermal ionization instability as hydrogen and helium disks. We argue that the instability applies to disks of any metal content. The relevance of the instability to supernova fallback disks probably means that their power-law evolution breaks down when they first become neutral. We construct simple analytical models for the viscous evolution of fallback disks to show that it is possible for these disks to become neutral when they are still young (ages of a few 10³ to 10⁴ yr), compact in size (a few 10⁹ to 10¹¹ cm) and generally accreting at sub-Eddington rates ( ~ a few 10¹⁴-10¹⁸ g s^-1). Based on recent results on the nature of viscosity in the disks of close binaries, we argue that this time may also correspond to the end of the disk activity period. Indeed, in the absence of a significant source of viscosity in the neutral phase, the entire disk will likely turn to dust and become passive. We discuss various applications of the evolutionary model, including anomalous X-ray pulsars and young radio pulsars. Our analysis indicates that metal-rich fallback disks around newly born neutron stars and black holes become neutral generally inside the tidal truncation radius (Roche limit) for planets at ≈10¹¹ cm. Consequently, the efficiency of the planetary formation process in this context will mostly depend on the ability of the resulting disk of rocks to spread via collisions beyond the Roche limit. It appears easier for the merger product of a doubly degenerate binary, whether it is a massive white dwarf or a neutron star, to harbor planets because its remnant disk has a rather large initial angular momentum, which allows it to spread beyond the Roche limit before becoming neutral. The early super-Eddington phase of accretion is a source of uncertainty for the disk evolution models presented here.

The nebular spectra of the energetic Type Ic supernova SN 1998bw (hypernova) are studied. The transition to the nebular phase occurred at an epoch of about 100 days after outburst, which is assumed to coincide with GRB 980425. Early in the nebular epoch the spectra show the characteristics of a typical supernova (SN) Ic spectrum, with strong lines of [O I], Ca II, and Mg I], and lines of [Fe II]. However, the [Fe II] lines are unusually strong for an SN Ic. Also, lines of different elements have different widths, indicating different expansion velocities. In particular, iron appears to expand more rapidly than oxygen. Furthermore, the [O I] nebular lines decline more slowly than the [Fe II] ones, signaling deposition of γ-rays in a slowly moving O-dominated region. These facts suggest that the explosion was aspherical. The absence of [Fe III] nebular lines can be understood if the ejecta are significantly clumped. A schematic picture of what this very unusual stellar explosion may have looked like is presented.

We report on the quiescent spectrum measured with Chandra ACIS-S of the transient, type I, X-ray-bursting neutron star Aql X-1, immediately following an accretion outburst. The neutron star radius, assuming a pure hydrogen atmosphere and a hard power-law spectrum, is R_∞ = 13.4(d/5 kpc) km. Based on the historical outburst record of the Rossi X-Ray Timing Explorer All-Sky Monitor, the quiescent luminosity is consistent with that predicted by Brown, Bildsten, and Rutledge from deep crustal heating, lending support to this theory for providing a minimum quiescent luminosity of transient neutron stars. While not required by the data, the hard power-law component can account for 18% ± 8% of the 0.5-10 keV thermal flux. Short-timescale intensity variability during this observation is less than 15% rms (3 σ; 0.0001-1 Hz, 0.2-8 keV). Comparison between the Chandra spectrum and three X-ray spectral observations made between 1992 October and 1996 October find all spectra consistent with a pure H atmosphere, but with temperatures ranging from 145 to 168 eV, spanning a factor of 1.87 ± 0.21 in observed flux. The source of variability in the quiescent luminosity on long timescales (greater than years) remains a puzzle. If from accretion, then it remains to be explained why the quiescent accretion rate provides a luminosity so nearly equal to that from deep crustal heating.

Differential time series photometry has been derived for 46,422 main-sequence stars in the core of 47 Tucanae. The observations consisted of near-continuous 160 s exposures alternating between the F555W and F814W filters for 8.3 days in 1999 July with the Wide Field Planetary Camera 2 on the Hubble Space Telescope. Using Fourier and other search methods, 11 detached eclipsing binaries and 15 W Ursa Majoris stars have been discovered plus an additional 10 contact or near-contact noneclipsing systems. After correction for nonuniform area coverage of the survey, the observed frequencies of detached eclipsing binaries and W UMa stars within 90'' of the cluster center are 0.022% and 0.031%, respectively. The observed detached eclipsing binary frequency, the assumptions of a flat binary distribution with log period, and assuming that the eclipsing binaries with periods longer than about 4 days have essentially their primordial periods imply an overall binary frequency of 13% ± 6%. The observed W UMa frequency and the additional assumptions that W UMa stars have evolved to contact according to tidal circularization and angular momentum loss theory and that the contact binary lifetime is 10⁹ yr imply an overall binary frequency of 14% ± 4%. An additional 71 variables with periods from 0.4 to 10 days have been found, which are likely to be BY Draconis stars in binary systems. The radial distribution of these stars is the same as that of the eclipsing binaries and W UMa stars and is more centrally concentrated than average stars but less so than the blue straggler stars. A distinct subset of six of these stars falls in an unexpected domain of the color-magnitude diagram, comprising what we propose to call red stragglers.

We investigate the evolution of initially metal-free, low-mass red giant stars through the He core flash at the tip of the red giant branch (RGB). The low entropy barrier between the helium- and hydrogen-rich layers enables a penetration of the helium-flash-driven convective zone into the inner tail of the extinguishing H-burning shell. As a consequence, protons are mixed into high-temperature regions triggering an H-burning runaway. The subsequent dredge-up of matter processed by He and H burning enriches the stellar surface with large amounts of helium, carbon, and nitrogen. Extending previous results by Hollowell et al. and Fujimoto, Ikeda, & Iben, who claimed that the H-burning runaway is an intrinsic property of extremely metal-poor low-mass stars, we found that its occurrence depends on additional parameters like the initial composition and the treatment of various physical processes. We perform some comparisons between predicted surface chemical abundances and observational measurements for extremely metal-deficient stars. As in previous investigations, our results disclose that although the described scenario provides a good qualitative agreement with observations, considerable discrepancies still remain. They may be due to a more complex evolutionary path of "real" stars and/or some shortcomings in current evolutionary models. In addition, we analyze the evolutionary properties after the He core flash, during both the central and shell He-burning phases, allowing us to deduce some interesting differences between models whose RGB progenitor has experienced the H flash and canonical models. In particular, the asymptotic giant branch evolution of extremely metal-deficient stars and the occurrence of thermal pulses are strongly affected by the previous RGB evolution.

Motivated by mounting evidence for the presence of magnetic fields in the atmospheres of "normal" early-type main-sequence stars, we investigate the various possible modes of dynamo action in their convective core. Working within the framework of mean field electrodynamics, we compute α² and α²Ω dynamo models and demonstrate that the transition from the former class to the latter occurs smoothly as internal differential rotation is increased. Our models also include a magnetic diffusivity contrast between the core and radiative envelope. The primary challenge facing such models is to somehow bring the magnetic field generated in the deep interior to the stellar surface. We investigate the degree to which thermally driven meridional circulation can act as a suitable transport agent. In all models with strong core-to-envelope magnetic diffusivity contrast—presumably closest to reality— whenever circulation is strong enough to carry a significant magnetic flux, it is also strong enough to prevent dynamo action. Estimates of typical meridional circulation speeds indicate that this regime is likely not attained in the interior of early-type main-sequence stars. Dynamo action then remains highly probable, but an alternate mechanism must be sought to carry the magnetic field to the surface.

Motivated by recent detections by the XMM and Chandra satellites of X-ray line emission from hot, luminous stars, we present synthetic line profiles for X-rays emitted within parameterized models of a hot-star wind. The X-ray line emission is taken to occur at a sharply defined comoving-frame resonance wavelength, which is Doppler-shifted by a stellar wind outflow parameterized by a "β" velocity law, v(r) = v_∞(1 - R_*/r)^β. Above some initial onset radius R_o for X-ray emission, the radial variation of the emission filling factor is assumed to decline as a power law in radius, f(r) ~ r^-q. The computed emission profiles also account for continuum absorption within the wind, with the overall strength characterized by a cumulative optical depth τ_*. In terms of a wavelength shift from line center scaled in units of the wind terminal speed v_∞, we present normalized X-ray line profiles for various combinations of the parameters β, τ_*, q, and R_o and also including the effect of instrumental and/or macroturbulent broadening as characterized by a Gaussian with a parameterized width σ. We discuss the implications for interpreting observed hot-star X-ray spectra, with emphasis on signatures for discriminating between "coronal" and "wind-shock" scenarios. In particular, we note that in profiles observed so far the substantial amount of emission longward of line center will be difficult to reconcile with the expected attenuation by the wind and stellar core in either a wind-shock or coronal model.

We present new spectroscopic observations for a sample of C(N)-type red giants. These objects belong to the class of asymptotic giant branch stars, experiencing thermal instabilities in the He-burning shell (thermal pulses). Mixing episodes called third dredge-up enrich the photosphere with newly synthesized ¹²C in the He-rich zone, and this is the source of the high observed ratio between carbon and oxygen (C/O ≥ 1 by number). Our spectroscopic abundance estimates confirm that, in agreement with the general understanding of the late evolutionary stages of low- and intermediate-mass stars, carbon enrichment is accompanied by the appearance of s-process elements in the photosphere. We discuss the details of the observations and of the derived abundances, focusing in particular on rubidium, a neutron density sensitive element, and on the s-elements Sr, Y, and Zr belonging to the first s-peak. The critical reaction branching at ⁸⁵Kr, which determines the relative enrichment of the studied species, is discussed. Subsequently, we compare our data with recent models for s-processing in thermally pulsing asymptotic giant branch stars, at metallicities relevant for our sample. A remarkable agreement between model predictions and observations is found. Thanks to the different neutron density prevailing in low- and intermediate-mass stars, comparison with the models allows us to conclude that most C(N) stars are of low mass (M ≲ 3 M_☉). We also analyze the ¹²C/¹³C ratios measured, showing that most of them cannot be explained by canonical stellar models. We discuss how this fact would require the operation of an ad hoc additional mixing, currently called cool bottom process, operating only in low-mass stars during the first ascent of the red giant branch and, perhaps, also during the asymptotic giant branch.

We have obtained high-resolution X-ray spectra of the coronally active binary II Pegasi (HD 224085), covering the wavelength range of 1.5-25 Å. For the first half of our 44 ks observation, the source was in a quiescent state with constant X-ray flux, after which it flared, reaching twice the quiescent flux in 12 ks, then decreased. We analyze the emission-line spectrum and continuum during quiescent and flaring states. The differential emission measure derived from lines fluxes shows a hot corona with a continuous distribution in temperature. During the nonflare state, the distribution peaks near log T = 7.2, and when flaring, it peaks near 7.6. High-temperature lines are enhanced slightly during the flare, but most of the change occurs in the continuum. Coronal abundance anomalies are apparent, with iron very deficient relative to oxygen and significantly weaker than expected from photospheric measurements, while neon is enhanced relative to oxygen. We find no evidence of appreciable resonant scattering optical depth in line ratios of iron and oxygen. The flare light curve is consistent with solar two-ribbon flare models but with a very long reconnection time constant of about 65 ks. We infer loop lengths of about 0.05 to about 0.25 stellar radii in the flare, if the flare emission originated from a single, low-density loop.

Near-infrared (2.2 μm) long baseline interferometric observations of Vega are presented. The stellar disk of the star has been resolved, and the data have been fitted with a limb-darkened stellar disk of diameter Θ_LD = 3.28 ± 0.01 mas. The derived effective temperature is T_eff = 9553 ± 111 K. However, the residuals resulting from the stellar disk model appear to be significant and display organized structure. Instrumental artifacts, stellar surface structure, stellar atmosphere structure, and extended emission/scattering from the debris disk are discussed as possible sources of the residuals. While the current data set cannot uniquely determine the origin of the residuals, the debris disk is found to be the most likely source. A simple debris disk model, with 3%-6% of Vega's flux emanating from the disk at r ≲ 4 AU, can explain the residuals.

We present infrared interferometric angular size measurements for the A7IV-V star Altair that indicate a noncircular projected disk brightness distribution. Given the known rapid rotation of this star, we model the data as arising from an elongated rigid atmosphere. To first order, an ellipse may be fitted to our interferometric diameter measurements, with major and minor axes of 2a = 3.461 ± 0.038 mas and 2b = 3.037 ± 0.069 mas, respectively, for a difference of 424 ± 79 μas between 2a and 2b, and with an axial ratio of a/b = 1.140 ± 0.029. Assuming that the apparent oblateness of the photosphere is due to the star's rapid rotation, a more rigorous evaluation of the observation data in the context of a rigidly rotating Roche model shows that an estimate of v sin i = 210 ± 13 km s^-1 can be derived that is independent of spectroscopic techniques. Also derived are values for the mean effective temperature and the mean linear radius, and an observational constraint upon the relationship between rotation velocity and stellar inclination is established. Altair is the first main-sequence star for which direct observations of an oblate photosphere have been reported and the first star for which v sin i has been established from observations of the star's photospheric geometry.

Using data from the Michelson Doppler Imager and the Global Oscillation Network Group project, we demonstrate that there exists an excess of anticorrelated power variations of pairs of solar p-modes. We find a possible explanation for this behavior in the influence of poloidal velocity fields, which may lead to coupling of p-modes that results in an energy redistribution among the solar oscillations.

We have analyzed high-resolution Hα full disk data from Big Bear Solar Observatory (BBSO); magnetograph and EUV data from the Michelson Doppler Imager, Large Angle and Spectrometric Coronagraph, and Extreme Ultraviolet Imaging Telescope on board SOHO; and Yohkoh soft X-ray data of 2000 February 17. Two sympathetic M-class solar flares erupted in succession in NOAA Active Region 8869 and 8872, respectively. The eruption from AR 8872 was followed by an extremely symmetric halo coronal mass ejection (CME). We demonstrate the loop activation, which appears to be the consequence of the first flare in AR 8869 and the cause of the second flare in AR 8872. The activation started in the form of a surge just after a filament eruption and its associated flare in AR 8869. The surge quickly turned into a set of disturbances that propagated at a speed of about 80 km s^-1 toward the other active region AR 8872. The second flare followed in less than an hour after the arrival of the disturbances at AR 8872. The moving disturbances appeared in absorption in both Hα and EUV 195 Å images. The disturbances may represent mass transfer, which had a significant velocity component perpendicular to the field lines and, hence, caused the transport of field lines. In this case, the disturbances may be considered to be a special kind of surge, which we may call a "sweeping closed-loop surge." We also demonstrated large area dimmings associated with the CME in three active regions. The dimming started from AR 8869 and AR 8872 and was extended to AR 8870, which was on the opposite side of the solar equator. We believe that both the activation of inter-active region loops and the large-scale dimming are the signatures of large-scale restructuring associated with the CME.

We report the results of our investigation of interplanetary effects caused by the large solar flare (X5.7/3B) that occurred on 2000 July 14. In association with this event a bright, fast, halo coronal mass ejection (CME) was observed. The analysis of multiwavelength, high-cadence images obtained from the Nançay Radioheliograph shows the on-disk signatures of the initiation of the CME at low-coronal heights, ≤2 R_☉. The formation of the CME inferred from the radio data indicates a nearly developed halo at the low corona. The white-light images and CME follow-up measurements in the interplanetary medium also show, in agreement with the radio data, the propagation of the fully developed halo CME. The inference on the consequences of the CME in the inner heliosphere is from the interplanetary scintillation (IPS) observations obtained with the Ooty Radio Telescope and multiantenna system at the Solar-Terrestrial Environment Laboratory. Scintillation measurements at Ooty on a grid of a large number of radio sources provided an opportunity to image the disturbance associated with the CME at different distances from the Sun before its arrival at the near-Earth space. The scintillation data in particular also played a crucial role in understanding the radial evolution of the speed of the CME in the inner heliosphere. The "speed-distance" plot indicates a two-level deceleration: (1) a low decline in speed at distances within or about 100 solar radii and (2) a rapid decrease at larger distances from the Sun. The linear increase in the size of the CME with radial distance is also briefly discussed. The expansion of the CME, formation of the halo in the low corona, and its speed history in the interplanetary medium suggest a driving energy, which is likely supplied by the twisted magnetic flux rope system associated with the CME.

The pure rotational spectrum of KH (X¹Σ⁺) has been recorded using millimeter/submillimeter direct absorption techniques. The species was created by the reaction of potassium vapor and H₂ in a d.c. discharge. The J = 0 → 1 and 1 → 2 transitions were measured near 202 and 404 GHz, and electric quadrupole splittings were resolved in the J = 0 → 1 line and partly resolved in the other transition. For comparison, the quadrupole structure in the J = 0 → 1 transition of NaH (X¹Σ⁺) was also recorded. Very accurate rotational and quadrupole hyperfine constants were determined for KH, and eqQ was established for NaH as well. Diatomic hydride species are the building blocks for interstellar chemistry in both dense and diffuse gas. Knowledge of the rest frequencies of KH will enable its detection in such objects.

We present first results from high-resolution tree + smoothed particle hydrodynamics simulations of galaxy clusters and groups that are aimed at studying the effect of nongravitational heating on the entropy of the intracluster medium (ICM). We simulate three systems having emission-weighted temperatures T_ew ≃ 0.6, 1, and 3 keV with spatial resolutions better than 1% of the virial radius. We consider the effect of different prescriptions for nongravitational ICM heating, such as supernova energy feedback, as predicted by semianalytical models of galaxy formation, and two different minimum entropy floors, S_fl = 50 and 100 keV cm², imposed at z = 3. Simulations with only gravitational heating nicely reproduce predictions from self-similar ICM models, while extra heating is shown to break the self-similarity, by a degree that depends on the total injected energy and on the cluster mass. We use the observational results on the excess entropy in central regions of galaxy systems to constrain the amount of extra heating required. We find that by setting the entropy floor S_fl = 50 keV cm², which corresponds to an extra heating energy of about 1 keV per particle, we are able to reproduce the observed excess of ICM entropy.

We argue that there has been substantial evolution in the ellipticity of rich galaxy clusters between 0 < z < 0.1 and suggest that this is additional evidence for a low matter density universe.

Using the Euclidean value of ⟨V/V_max⟩ as a cosmological distance indicator, we derive the isotropic-equivalent characteristic peak luminosity of gamma-ray bursts both longer and shorter than 2 s. The short bursts have essentially the same characteristic peak luminosity of 0.6 × 10⁵¹ ergs (0.064 s)^-1 as do the long bursts. This may apply also to bursts with durations less than 0.25 s. The local space density of short bursts is around 3 times lower than that of long bursts.

Recent observations of several γ-ray burst (GRB) afterglows have shown evidence for a large amount of X-ray line-emitting material, possibly arising from ionized iron. A significant detection of an X-ray spectral feature, such as that found in the Chandra observation of GRB 991216, may provide important constraints on the immediate environment of the burst and hence on progenitor models. The large Fe Kα equivalent widths inferred from the X-ray observations favor models in which the line is produced when the primary X-ray emission from the source strikes Thomson-thick material and Compton scatters into our line of sight. We present such reflection spectra here, computed in a fully self-consistent manner, and discuss the range of ionization parameters that may be relevant to different models of GRBs. We argue that the presence of a strong hydrogen-like Kα line is unlikely, because Fe XXVI photons would be trapped resonantly and removed from the line core by Compton scattering. In contrast, a strong narrow emission line from He-like Fe XXV is prominent in the model spectra. We briefly discuss how these constraints may affect the line energy determination in GRB 991216.

We present simultaneous, subarcsecond (≤50 pc) resolution 5, 8.4, and 15 GHz Very Large Array observations of a well-defined sample of 16 low-luminosity active galactic nuclei. The radio emission in most of these nuclei does not show the rising spectrum (0.2 ≲ s ≲ 1.3, L_ν ∝ ν^s) expected from thermal electrons in an advection-dominated accretion flow (ADAF) with or without weak to moderately strong outflows. Rather, the flat radio spectra are indicative of either synchrotron self-absorbed emission from jets, convection-dominated accretion flows (CDAFs) with L ≳ 10^-5L_Edd, or ADAFs with strong (p ≳ 0.6) outflows. The jet interpretation is favored by three factors: (1) the detection of parsec-scale radio extensions, morphologically reminiscent of jets, in the five nuclei with the highest peak radio flux density, (2) the domination of parsec-scale jet radio emission over unresolved "core" emission in the three best-studied nuclei, and (3) the lack of any clear correlation between radio spectral shape and black hole mass as would be expected from the dependence of the radio turnover frequency on black hole mass in ADAF and CDAF models. A jet domination of nuclear radio emission implies significantly lower accretion rates in ADAF-type models than earlier estimated from core radio luminosities.

We discuss the observable consequences for the detection of galaxies in the X-ray bandpass resulting from a peak in the cosmic star formation rate at a redshift greater than 1. Following N. White and P. Ghosh, we assume a large evolution in the X-ray/B luminosity ratio at z ~ 0.5-1.5 resulting from the X-ray binaries that have evolved from stars formed at z > 1-2. Using the Hubble Deep Field-North (HDF-N) redshift survey data and the locally observed X-ray/B luminosity ratio as a guide, we estimate a median X-ray flux (2-10 keV) on the order of 8 × 10^-18 ergs cm^-2 s^-1 for galaxies in the HDF-N, which is consistent with a signal derived from a recent stacking analysis of the HDF-N Chandra data by N. Brandt and coworkers. We also predict the number counts in deep X-ray surveys expected from normal galaxies at high redshift.

We discuss the imprints left by a cosmological evolution of the star formation rate (SFR) on the evolution of X-ray luminosities L_X of normal galaxies, using the scheme earlier proposed by us, wherein the evolution of L_X of a galaxy is driven by the evolution of its X-ray binary population. As indicated in our earlier work, the profile of L_X with redshift can both serve as a diagnostic probe of the SFR profile and constrain evolutionary models for X-ray binaries. We report here the first calculation of the expected evolution of X-ray luminosities of galaxies, updating our work by using a suite of more recently developed SFR profiles that span the currently plausible range. The first Chandra deep imaging results on L_X evolution are beginning to probe the SFR profile of bright spiral galaxies; the early results are consistent with predictions based on current SFR models. Using these new SFR profiles, the resolution of the "birthrate problem" of low-mass X-ray binaries and recycled, millisecond pulsars in terms of an evolving global SFR is more complete. We discuss the possible impact of the variations in the SFR profile of individual galaxies and galaxy types.

In the course of the follow-up multiwavelength study of a deep radio survey, we have discovered that the millijansky radio source PDF J011423 is a low-redshift (z = 0.65) extremely red galaxy (ERG) with K = 15.3, R-K = 5.8, and J-K = 3.1. Optical, infrared, and radio photometry, together with optical and near-infrared spectroscopy, reveal a heavily obscured galaxy (A_V = 5-6, from the observed Balmer decrement) undergoing vigorous star formation and presenting an active galactic nucleus. PDF J011423 is a representative member of the dusty ERG population, providing a local counterpart for studying more distant ERGs.

We study the evolution of the galaxy UV luminosity density as a function of redshift in the Hubble Deep Field-North (HDF-N). We estimate the amount of energy absorbed by dust and hidden from optical observations by analyzing the HDF-N photometric data with the spectral energy distribution fitting method. According to our results, at redshifts of 1 ≤ z ≤ 4.5, the global energy observed in the UV rest frame at λ = 1500 Å corresponds to only 7%-11% of the stellar energy output, the rest of it being absorbed by dust and reemitted in the far-IR. Our estimates of the comoving star formation rate density in the universe from the extinction-corrected UV emission are consistent with the recent results obtained with Submillimeter Common-User Bolometric Array (SCUBA) at faint submillimeter flux levels.

The purpose of this Letter is to show that RR Lyrae variables exist and can be detected in M31 globular clusters. We report on the first tentative identification of RR Lyrae candidates in four globular clusters of the Andromeda galaxy, i.e., G11, G33, G64, and G322. Based on Hubble Space Telescope Wide Field Planetary Camera 2 archive observations in the F555W and F814W filters spanning a total interval of about 5 consecutive hours, we find evidence for two, four, 11, and eight RR Lyrae variables of both ab and c Bailey types in G11, G33, G64, and G322, respectively. Several more candidates can be found by relaxing slightly the selection criteria. These numbers are quite consistent with the horizontal-branch (HB) morphology exhibited by the four clusters, starting from the very blue HB in G11 and progressively moving to redder HBs in G64, G33, and G322.

It is suggested that there have been at least two physically distinct epochs of massive cluster formation. The first generation of globular clusters may have formed as halo gas was compressed by shocks driven inward by ionization fronts generated during cosmic reionization at z ~ 6. On the other hand, a second generation of massive clusters might have been formed at later times by compression, and subsequent collapse, of giant molecular clouds. In some cases this compression may have been triggered by heating of the interstellar medium via collisions between gas-rich disk galaxies. It is also suggested that the present specific globular cluster frequency in galaxies was mainly determined by the peak rate of star creation, with elevated peak rates of star formation per unit area resulting in high present specific globular cluster frequencies.

We modify our combined ("deep mixing" plus "primordial") scenario explaining the star-to-star abundance variations in globular cluster red giants in such a way that it conforms with new experimental data: (1) the new and better constrained (NACRE) thermonuclear reaction rates and (2) a discovery of the O-Na anticorrelation in stars below the main-sequence turnoff in the cluster NGC 6752. For the latter, we propose that some main-sequence dwarfs in globular clusters accrete material lost by red giant primary components of close binaries during a common envelope event. As a consequence of the new reaction rates, we are drawn to the conclusion that the anomalies in [Al/Fe] in globular cluster red giants are in fact manifestations of ²⁶Al^g abundance variations (instead of the stable isotope ²⁷Al) produced by deep mixing. However, Al overabundances observed in some of the subgiants in NGC 6752 are unlikely to be due to the unstable ²⁶Al^g isotope but have to be the result of the other primordial component of our combined scenario, i.e., intermediate-mass asymptotic giant branch stars. These should also provide the source for enhanced ²⁵Mg, necessary for the ²⁶Al^g production in deep mixing events.

The spins of X-ray neutron star accretors in low-mass binaries are found to cluster at about 300 Hz with the exception of a few higher frequency objects. We find that a postulated phase transition induced by the centrifugally driven dilution in the density profile of the star can produce a similar feature. It takes from 10⁷ to 10⁹ yr, depending on the mass accretion rate, to expel the high-density phase from the core. The corresponding growth in the moment of inertia retards spin-up during this epoch. Normal mass accretion-driven spin-up resumes at its completion. A phase change triggered by the changing spin and the accompanying evolution of the moment of inertia has its analog in rotating nuclei as was discovered in the 1970s.

We measured the radial velocity curve of the companion of the neutron star X-ray transient XTE J2123-058. Its semiamplitude (K₂) of 298.5 ± 6.9 km s^-1 is the highest value that has been measured for any neutron star low-mass X-ray binary. The high value for K₂ is, in part, due to the high binary inclination of the system but may also indicate a high neutron star mass. The mass function (f₂) of 0.684 ± 0.047 M_☉, along with our constraints on the companion's spectral type (K5 V-K9 V) and previous constraints on the inclination, gives a likely range of neutron star masses from 1.2 to 1.8 M_☉. We also derive a source distance of 8.5 ± 2.5 kpc, indicating that XTE J2123-058 is unusually far, 5.0 ± 1.5 kpc, from the Galactic plane. Our measurement of the systemic radial velocity is -94.5 ± 5.5 km s^-1, which is significantly different from what would be observed if this object corotated with the disk of the Galaxy.

We show that the burning of a small mass fraction X₁₂ of ¹²C in a neutron star ocean is thermally unstable at low accumulated masses when the ocean contains heavy ashes from the hydrogen-burning rapid proton process. The key to early unstable ignition is the decreased thermal conductivity of a heavy-element ocean. The instability requires accretion rates, , in excess of 1/10 the Eddington limit when X₁₂ < 0.1. Lower 's will stably burn a small mass fraction of ¹²C. The unstable flashes release ~10⁴²-10⁴³ ergs over hours to days and are likely the cause of the recently discovered large type I bursts (the so-called superbursts) from six Galactic low-mass X-ray binaries. In addition to explaining the energetics, recurrence times, and durations of the superbursts, these mixed ¹²C flashes also have an dependence of unstable burning similar to that observed. Although the instability is present at accretion rates ≈_Edd, the flashes provide less of a contrast with the accretion luminosity there, thus explaining why detection is easier when ≈ (0.1-0.3)_Edd.

The central region of the recently discovered supernova remnant (SNR) RX J0852.0-4622 was observed with the Advanced CCD Imaging Spectrometer detector aboard the Chandra X-Ray Observatory. We found only one relatively bright source, about 4' north of the SNR center, with a flux of ~2 × 10^-12 ergs s^-1 cm^-2 in the 0.5-10 keV band. The position of this pointlike source, CXOU J085201.4-461753, rules out its association with the two bright stars in the field, HD 76060 and Wray 16-30. Observations of the field with the Cerro Tololo Inter-American Observatory 0.9 m telescope show a star (R ≈ 17, B ≈ 19) at about 24 from the nominal X-ray position. We consider association of this star with the X-ray source unlikely and estimate a limiting magnitude of the optical counterpart as B ≥ 22.5 and R ≥ 21.0. Based on the X-ray-to-optical flux ratio, we argue that the X-ray source is likely the compact remnant of the supernova explosion that created the RX J0852.0-4622 SNR. The observed X-ray spectrum of the source is softer than spectra of magnetospheric radiation of rotation-powered pulsars, but it is harder than spectra of cooling neutron stars emitting thermal radiation from the entire surface, similar to the central compact source of the Cas A SNR. We suggest that CXOU J085201.4-461753 belongs to the growing family of radio-quiet compact central sources, presumably neutron stars, recently discovered in a number of SNRs.

We study numerically the thermal emission of e⁺e^- pairs from a bare strange star heated by energy input onto its surface; heating starts at some moment and is steady afterward. The thermal luminosity in e⁺e^- pairs increases to some constant value. The rise time and the steady thermal luminosity are evaluated. Both normal and color superconducting states of the strange quark matter are considered. The results are used to test the model of soft gamma-ray repeaters in which the bursting activity is explained by the fast decay of superstrong magnetic fields and the heating of the strange star surface. It is shown that the rise times observed in typical bursts may be explained in this model only if the strange quark matter is a superconductor with an energy gap of more than 1 MeV.

Using the Very Large Array, we have detected radio emission from the site of SN 1970G in the Sc galaxy M101. These observations are 31 yr after the supernova event, making SN 1970G the longest monitored radio supernova. With flux densities of 0.12 ± 0.020 mJy at 6 cm and 0.16 ± 0.015 mJy at 20 cm, the spectral index of -0.24 ± 0.20 appears to have flattened somewhat when compared with the previously reported value of -0.56 ± 0.11, taken in 1990. The radio emission at 20 cm has decayed since the 1990 observations with a power-law index of β_{20 cm} = -0.28 ± 0.13. We discuss the radio properties of this source and compare them to those of other Type II radio supernovae.

We present the results from a multiepoch H₂O maser survey toward low-mass young stellar objects using the Nobeyama 45 m telescope and the Very Large Array. Our Nobeyama survey is the first complete H₂O maser survey toward known Class 0 sources in the northern sky (δ > -35^°). During the series of the monitoring observations, we detected the maser emission toward none of the 31 pre-protostellar cores, 15 of 30 Class 0, two of 32 Class I, and zero of nine Class II sources. From this, we conclude that Class 0 sources are favorable sites to harbor the masers: the detection rates are derived to be 39.7% for Class 0, 4.0% for Class I, and 0.0% for Class II sources taking time variation into account. In addition, we found that the H₂O maser luminosities in low-mass stars are more closely related to the luminosities of 100 AU scale radio jets rather than the mechanical luminosities of large-scale CO outflows. This fact suggests that the masers are associated with the shocked regions that are impacted by neutral protostellar jets emanating from the central stars. The drastic decrease of the maser detection rate in Class I sources is likely to be caused by the dissipation of dense gas around the central objects. We base this on the fact that the radio jets are found to have similar luminosities in Class 0 and Class I. It seems difficult even for active protostellar jets to excite masers in the remaining tenuous gas around Class I sources.

The mass function of molecular cloud cores is described in terms of the power spectrum of the density fluctuation field in a filamentary molecular cloud by using Press-Schechter formalism. In contrast to the cosmological applications, the shape of the resultant mass function is time-dependent owing to the wavenumber-dependent growth rate of the perturbation in the filamentary configuration. The validity of the description can be directly examined by the observational determination of both the mass function of molecular cloud cores in the filament and the power spectrum of the mass distribution in the longitudinal direction of the filament.

Using the Chandra X-Ray Observatory, we have discovered a diffuse, center-filled region of hard X-ray emission within the young, oxygen-rich supernova remnant (SNR) G292.0+1.8. Near the peak of this diffuse emission lies a pointlike source of X-ray emission that is well described by an absorbed power-law spectrum with photon index 1.72 ± 0.09. This source appears to be marginally extended; its extent of 13 (FWHM) is greater than that of a nearby serendipitous X-ray source with FWHM = 11. This is strong evidence for the presence within SNR G292.0+1.8 of a young rapidly rotating pulsar and its associated pulsar wind nebula. Both the X-ray and radio luminosity of the pulsar wind nebula imply a spin-down energy-loss rate of ~ 10³⁷ ergs s^-1 for the still undetected pulsar. The pulsar candidate is 09 from the geometric center of the SNR, which implies a transverse velocity of ~770(D/4.8 kpc)(t/1600 yr)^-1 km s^-1 assuming currently accepted values for the distance and age of G292.0+1.8.

New Hα and [S II] images of the HH 111 jet taken with the Hubble Space Telescope reveal marked proper motions and morphological changes when compared with similar images obtained 4 years earlier. Knots in the jet, which are dominated by emission from nested bow shocks, generally move ballistically, with no evidence for turbulent motions even in regions where the emission has a complex morphology. These bow shocks sometimes overtake one another; the new images show this occurred in knot L about 80 years ago. Photometric variability, clearly visible for the first time at subarcsecond scales, can confuse ground-based measurements that require many years between epochs to detect reliable proper motions. With the exception of the bow shock L, whose wings expand laterally, the jet moves mainly along its long axis. Because HH 111 lies nearly in the plane of the sky, the proper motions translate accurately to space velocities, which range from 220 to 330 km s^-1 with a typical uncertainty of ±5 km s^-1. The fastest knots are associated with object E at the base of the visible jet, where a cooling layer is in the process of forming behind one of the shocks. Velocity differences between adjacent knots within the optically bright part of the jet are typically 40 km s^-1, in line with predictions of nonmagnetic shock models based on emission-line fluxes. This agreement limits the component of the magnetic field perpendicular to the axis of the jet to be ≲1 mG.

The pure rotational spectra of two metal carbide species, NiC (X¹Σ⁺) and CoC (X²Σ⁺), have been measured in the laboratory using millimeter/submillimeter direct absorption methods. The molecules were created by reacting metal vapor with CH₄ in a DC discharge. Four rotational transitions of CoC were recorded, each consisting of 16 hyperfine components arising from the cobalt nuclear spin of I = 7/2. Multiple transitions of the two most abundant nickel isotopomers, ⁵⁸NiC and ⁶⁰NiC, were additionally measured. The spectra were analyzed using the appropriate Hamiltonian and spectroscopic constants accurately determined. For CoC, this study included a complex hyperfine analysis of the magnetic, electric quadrupole, and nuclear spin-rotation terms. Cobalt and nickel are iron peak elements that may be produced via neutron capture processes in asymptotic giant branch stars. Dredge-up events mix these elements to the stellar surface, where they are incorporated into the expanding stellar envelope. CoC and NiC offer a means to trace these elements in carbon-rich circumstellar shells.

Theoretical time-dependent and two-component chromospheric models for late-type dwarfs are constructed based on acoustic and magnetic wave heating mechanisms. The models are used to predict the theoretical range of chromospheric activity for these stars. Comparison of this range with the one established observationally shows that the wave heating alone can explain most but not all of the observed range of stellar activity.

Results on the coronal heating function seem to strongly depend on the employed type of multitemperature modeling along the line of sight. Instruments with broadband temperature filters cause more temperature confusion than those with narrowband temperature filters. A possible bias of broadband filters is the hydrostatic weighting of multitemperature loop systems, which mimic a temperature increase with altitude and thus yield a preference for looptop heating. In this Letter we revisit a loop system previously analyzed by Priest et al., for which they found that the coronal heating is likely to be uniform in the temperature range between 1.6 and 2.2 MK. As an alternative scenario, we use standard hydrostatic solutions here (with vanishing conductive flux in the transition region). We show that hydrostatic solutions with a uniform heating function throughout the corona and transition region lead to unphysical solutions for the column depth and the altitude of the loop footpoints, while a footpoint-heating model yields acceptable physical solutions for a heating scale height of s_H ≈ 13 ± 1 Mm. The positive temperature gradient with height (which is also found in filter-ratio temperatures of other Yohkoh data) can be explained by the hydrostatic weighting bias resulting from hot loops (T_max ≈ 2.6 MK) embedded in a cooler (T_max ≈ 1.0 MK) background corona.

We study the subsurface meridional flow in the Sun as a function of latitude and depth in the period from 1994 to 2000, covering the solar minimum in 1996 and the maximum in 2000, with the technique of time-distance helioseismology. It is found that the velocity of meridional flow increased when solar activity decreased from 1994 to 1997. As solar activity increased from 1997 to 2000, a new component of meridional flow, centered at about 20° latitude, was created in each hemisphere. It moves away from its center. The new flow changes the observed meridional flow from poleward at solar minimum to equatorward at solar maximum at low latitudes. The velocity of the new component increases with depth at least down to a depth of 70,000 km.

Cross sections for photoabsorption of HCl and DCl are determined in the spectral region of 135-232 nm using radiation from a synchrotron light source. At wavelengths near the onset of absorption (λ > 200 nm), cross sections of HCl are approximately 5-10 times larger than those of DCl. These data are used to calculate rates of photodissociation of HCl and DCl in the Venusian atmosphere. For the entire wavelength region measured, the rate of photodissociation of DCl is only 16% that of HCl. The difference in rates of photodissociation contributes to the exceptionally large [D]/[H] ratio of the Venusian atmosphere.