Table of contents

In this paper, we present a redshift-space reconstruction scheme that is analogous to and extends the perturbative least action (PLA) method described by Goldberg & Spergel. We first show that this scheme is effective in reconstructing even nonlinear observations. We then suggest that by varying the cosmology to minimize the quadrupole moment of a reconstructed density field, it may be possible to lower the error bars on the redshift distortion parameter, β, as well as to break the degeneracy between the linear bias parameter, b, and Ω_M. Finally, we discuss how PLA might be applied to realistic redshift surveys.

Galaxy groups likely to be virialized are identified within the CNOC2 intermediate-redshift galaxy survey. The resulting groups have a median velocity dispersion, σ₁ ≃ 200 km s^-1. The virial mass-to-light ratios, using k-corrected and evolution-compensated luminosities, have medians in the range of 150-250 h M_☉/L_☉, depending on group definition details. The number-velocity dispersion relation at σ₁ ≳ 200 km s^-1 is in agreement with the low-mass extrapolation of the cluster-normalized Press-Schechter model. Lower velocity dispersion groups are deficient relative to the Press-Schechter model. The two-point group-group autocorrelation function has r₀ = 6.8 ± 0.3 h^-1 Mpc, which is much larger than the correlations of individual galaxies, but about as expected from biased clustering. The mean number density of galaxies around group centers falls nearly as a power law with r^-2.5 and has no well-defined core. The projected velocity dispersion of galaxies around group centers is either flat or slowly rising outward. The combination of a steeper than isothermal density profile and the outward rising velocity dispersion implies that the mass-to-light ratio of groups rises with radius if the velocity ellipsoid is isotropic but could be nearly constant if the galaxy orbits are nearly circular. Such strong tangential anisotropy is not supported by other evidence. Although the implication of a rising M/L must be viewed with caution, it could naturally arise through dynamical friction acting on the galaxies in a background of "classical" collisionless dark matter.

Observations of high-redshift supernovae imply an accelerating universe that can only be explained by an unusual energy component such as vacuum energy or quintessence. To assess the ability of current and future supernova data to constrain the properties of the dark energy, we allow its density to have arbitrary time dependence, ρ_X(z). This leads to an equation of state for the dark energy, w_X(z) = p_X(z)/ρ_X(z), which is a free function of redshift z. We find that current data of type Ia supernovae (SNe Ia) are consistent with a cosmological constant, with large uncertainties at z ≳ 0.5. We show that ρ_X(z)/ρ_X(z = 0) can be measured reasonably well to about z = 1.5 using SN Ia data from realistic future SN Ia pencil beam surveys, provided that the weak energy condition (energy density of matter is nonnegative for any observer) is imposed. While it is only possible to differentiate between different models (say, quintessence and k-essence) at z ≲ 1.5 using realistic data, the correct trend in the time dependence of the dark energy density can be clearly detected out to z = 2, even in the presence of plausible systematic effects. This would allow us to determine whether the dark energy is a cosmological constant or some exotic form of energy with a time-dependent density.

We present a comparison of the Sunyaev-Zeldovich (SZ) cluster counts predicted by the Press-Schechter (PS) mass function (MF) and the X-ray luminosity function (XLF) of clusters. The employment of the cluster XLF, together with the observationally determined X-ray luminosity-temperature (L_X-T) relation, may allow us to estimate the SZ cluster counts in a more realistic manner, although such an empirical approach depends sensitively on our current knowledge of the dynamical properties of intracluster gas and its cosmic evolution. Using both the nonevolving and evolving XLFs of clusters suggested by X-ray observations, we calculate the expectations for SZ surveys of clusters with X-ray luminosity L_X ≥ 3 × 10⁴⁴ ergs s^-1 and L_X ≥ 1 × 10⁴³ ergs s^-1 in the 0.5-2.0 band, respectively. The nonevolving XLF results in a significant excess of SZ cluster counts at high redshifts as compared with the evolving XLF, while a slightly steeper L_X-T relation than the observed one is needed to reproduce the distributions of SZ clusters predicted by the standard PS formalism. It is pointed out that uncertainties in the cosmological application of future SZ cluster surveys via the standard PS formalism should be carefully studied.

The recent discoveries of luminous quasars at high redshifts imply that black holes more massive than a few billion solar masses were already assembled when the universe was less than a billion years old. We show that the existence of these black holes is not surprising in popular hierarchical models of structure formation. For example, the black hole needed to power the quasar SDSS 1044-0125 at z = 5.8 could arise naturally from the growth of stellar-mass seeds forming at z > 10, when typical values are assumed for the radiative accretion efficiency (~0.1) and the bolometric accretion luminosity in Eddington units (~1). Nevertheless, SDSS 1044-0125 yields a nontrivial constraint on a combination of these parameters. Extrapolating our model to future surveys, we derive the highest plausible redshift for quasars that are not lensed or beamed, as a function of their apparent magnitude. We find that at a limiting magnitude of K ~ 20, quasar surveys can yield strong constraints on the growth of supermassive black holes out to z ~ 10.

We calculate the generic spectral signature of an early population of massive stars at high redshifts. For metal-free stars with mass above 300 M_☉, we find that the combined spectral luminosity per unit stellar mass is almost independent of the mass distribution of these stars. To zeroth order, the generic spectrum resembles a blackbody with an effective temperature of ~10⁵ K, making these stars highly efficient at ionizing hydrogen and helium. The production rate of ionizing radiation per stellar mass by stars more massive than ~300 M_☉ is larger by ~1 order of magnitude for hydrogen and He I and by ~2 orders of magnitude for He II than the emission from a standard initial mass function. This would result in unusually strong hydrogen and helium recombination lines from the surrounding interstellar medium. It could also alleviate the current difficulty of ionizing the intergalactic medium at z ≳ 6 with the cosmic star formation rate inferred at somewhat lower redshifts.

Approximately 30%-40% of all baryons in the present-day universe reside in a warm-hot intergalactic medium (WHIM), with temperatures in the range 10⁵ < T < 10⁷ K. This is a generic prediction from six hydrodynamic simulations of currently favored structure formation models having a wide variety of numerical methods, input physics, volumes, and spatial resolutions. Most of these warm-hot baryons reside in diffuse large-scale structures with a median overdensity around 10-30, not in virialized objects such as galaxy groups or galactic halos. The evolution of the WHIM is primarily driven by shock heating from gravitational perturbations breaking on mildly nonlinear, nonequilibrium structures such as filaments. Supernova feedback energy and radiative cooling play lesser roles in its evolution. WHIM gas may be consistent with observations of the 0.25 keV X-ray background without being significantly heated by nongravitational processes because the emitting gas is very diffuse. Our results confirm and extend previous work by Cen & Ostriker and Davé et al.

This paper introduces the use of pseudofilters that optimize the detection/extraction of sources on a background. We assume as a first approach that such sources are described by a spherical (central) profile and that the background is represented by a homogeneous and isotropic random field. We make an n-dimensional treatment, placing emphasis on astrophysical applications for spectra, images, and volumes, for the cases of exponential and Gaussian source profiles and scale-free power spectra to represent the background.

We present an analysis of the mass distribution in the core of A383 (z = 0.188), one of 12 X-ray luminous galaxy clusters at z ~ 0.2 selected for a comprehensive and unbiased study of the mass distribution in massive galaxy clusters. Deep optical imaging performed by the Hubble Space Telescope (HST) reveals a wide variety of gravitationally lensed features in the core of A383, including a giant arc, two radial arcs in the halo of the central cluster galaxy, several multiply imaged arcs, and numerous arclets. Based upon the constraints from the various lensed features, we construct a detailed model of the mass distribution in the central regions of the cluster, taking into account both a cluster-scale potential and perturbations from individual cluster galaxies. Keck spectroscopy of one component of the giant arc identifies it as an image of a star-forming galaxy at z = 1.01 and provides an accurate measurement of the mass of the cluster within the projected radius of the giant arc (65 kpc) of (3.5 ± 0.1) × 10¹³M_☉. Using the weak shear measured from our HST observations, we extend our mass model to larger scales and determine a mass of (1.8 ± 0.2) × 10¹⁴M_☉ within a radius of 250 kpc. On smaller scales we use the radial arcs to probe the shape of the total mass distribution in the cluster core (r ≲ 20 kpc) and find that the density profile is more peaked than a single Navarro, Frenk, & White (NFW) dark matter profile. Our findings imply that the dark matter in A383 may be more steeply peaked than NFW predict and that the cD galaxy measurably perturbs the cluster potential well. The optical and X-ray properties of A383 indicate the presence of a central cooling flow, for which we derive a mass deposition rate of ≳200 M_☉ yr^-1. We also use the X-ray emission from A383 to obtain independent estimates of the total mass within projected radii of 65 and 250 kpc: (4.0) × 10¹³ and (1.2 ± 0.5) × 10¹⁴M_☉, which are consistent with the lensing measurements.

The Chandra X-Ray Observatory was used to obtain a 190 ks image of three high-redshift galaxy clusters in one observation. The results of our analysis of these data are reported for the two z > 1 clusters in this Lynx field, which are the most distant known X-ray luminous clusters. Spatially extended X-ray emission was detected from both these clusters, indicating the presence of hot gas in their intracluster media. A fit to the X-ray spectrum of RX J0849+4452 at z = 1.26 yields a temperature of kT = 5.8 keV. Using this temperature and the assumption of an isothermal sphere, the total mass of RX J0849+4452 is found to be 4.0 × 10¹⁴hM_☉ within r = 1 h Mpc. The T_X for RX J0849+4452 approximately agrees with the expectation based on its L_bol = 3.3 × 10⁴⁴ ergs s^-1 according to the low-redshift L_X-T_X relation. The very different distributions of X-ray-emitting gas and of the red member galaxies in the two z > 1 clusters, in contrast to the similarity of the optical/IR colors of those galaxies, suggests that the early-type galaxies mostly formed before their host clusters.

A complete sample of 27 radio galaxies was selected from the B2 and 3CR catalogs in order to study their properties on the milliarcsecond scale. In the Appendix of this paper we present new radio images for 12 of them. Thanks to the present data, all the sources in this sample have been imaged at milliarcsecond resolution. We discuss the general results. In particular we stress the evidence for high-velocity jets in low-power radio galaxies, compare high- and low-power sources, and discuss the source properties in light of the unified scheme models. We conclude that the properties of parsec-scale jets are similar in sources with different total radio power and kiloparsec-scale morphology. From the core-total radio power correlation, we estimate that relativistic jets with Lorentz factor γ in the range 3-10 are present in high- and low-power radio sources. We discuss also the possible existence of a two-velocity structure (fast spine and lower velocity external shear layer).

We present low-resolution mid-infrared (MIR) spectra of 16 ultraluminous infrared galaxies (ULIRGs) obtained with the circular variable filter (CVF) spectroscopy mode of ISOCAM on board the Infrared Space Observatory (ISO). Our sample completes previous ISO spectroscopy of ultra- and hyperluminous infrared galaxies toward higher luminosities. The combined samples cover an infrared luminosity range of ~10¹²-10^13.1L_☉. To discriminate active galactic nucleus (AGN) and starburst activity, we use the AGN-related MIR continuum and the starburst-related 6.2, 7.7, 8.6, and 11.3 μm MIR emission bands attributed to aromatic carbonaceous material. For about half of the high-luminosity ULIRGs studied here, strong aromatic emission bands suggest starburst dominance. Other spectra are dominated by a strong AGN-related continuum with weak superposed emission features of uncertain nature. Our sample contains one unusual example, IRAS F00183-7111, of an AGN that is highly obscured even in the MIR. An improved method to characterize quantitatively the relative contribution of star formation and AGN activity to the MIR emission of ULIRGs is presented. The ULIRG spectra are fitted by a superposition of a starburst and an AGN spectrum, both of which may be obscured at different levels. Models in which starburst and AGN obscuration differ are significantly more successful than models with a single extinction. Previous results based on a simpler line-to-continuum measure of aromatic emission strength are confirmed, further supporting the robustness of the aromatic emission feature as a diagnostic of ULIRG power sources. As dominant sources of the bolometric luminosity, starbursts prevail at the lower end and AGNs at the higher end of this range. The transition between mostly starburst and mostly AGN powered occurs at ~10^12.4-10^12.5L_☉, and individual luminous starbursts are found up to ~10^12.65L_☉.

We present new infrared observations of the central regions of the starburst galaxy M82. The observations consist of near-infrared integral field spectroscopy in the H and K bands obtained with the MPE 3D instrument and of λ = 2.4-45 μm spectroscopy from the Short Wavelength Spectrometer (SWS) onboard the Infrared Space Observatory. These measurements are used, together with data from the literature, to (1) reexamine the controversial issue of extinction, (2) determine the physical conditions of the interstellar medium (ISM) within the star-forming regions, and (3) characterize the composition of the stellar populations. Our results provide a set of constraints for detailed starburst modeling, which we present in a companion paper. We find that purely foreground extinction cannot reproduce the global relative intensities of H recombination lines from optical to radio wavelengths. A good fit is provided by a homogeneous mixture of dust and sources, and with a visual extinction of A_V = 52 mag. The SWS data provide evidence for deviations from commonly assumed extinction laws between 3 and 10 μm. The fine-structure lines of Ne, Ar, and S detected with SWS imply an electron density of ≈300 cm^-3, and abundance ratios Ne/H and Ar/H nearly solar and S/H about one-fourth solar. The excitation of the ionized gas indicates an average effective temperature for the OB stars of 37,400 K, with little spatial variation across the starburst regions. We find that a random distribution of closely packed gas clouds and ionizing clusters and an ionization parameter of ≈10^-2.3 represent well the star-forming regions on spatial scales ranging from a few tens to a few hundreds of parsecs. From detailed population synthesis and the mass-to-K-light ratio, we conclude that the near-infrared continuum emission across the starburst regions is dominated by red supergiants with average effective temperatures ranging from 3600 to 4500 K and roughly solar metallicity. Our data rule out significant contributions from older, metal-rich giants in the central few tens of parsecs of M82.

The dynamical friction timescale for globular clusters to sink to the center of a dwarf elliptical galaxy (dE) is significantly less than a Hubble time if the halos have King-model or isothermal profiles and the globular clusters formed with the same radial density profile as the underlying stellar population. We examine the summed radial distribution of the entire globular cluster systems and the bright globular cluster candidates in 51 Virgo and Fornax Cluster dE's for evidence of dynamical friction processes. We find that the summed distribution of the entire globular cluster population closely follows the exponential profile of the underlying stellar population. However, there is a deficit of bright clusters within the central regions of dE's (excluding the nuclei), perhaps due to the orbital decay of these massive clusters into the dE cores. We also predict the nuclear magnitude of each dE assuming that the nuclei form via dynamical friction. The observed trend of decreasing nuclear luminosity with decreasing dE luminosity is much stronger than predicted if the nuclei formed via simple dynamical friction processes. We find that the bright dE nuclei could have been formed from the merger of orbitally decayed massive clusters, but the faint nuclei are several magnitudes fainter than expected. These faint nuclei are found primarily in M_V > -14 dE's, which have high globular cluster specific frequencies and extended globular cluster systems. In these galaxies, supernova-driven winds, high central dark matter densities, extended dark matter halos, the formation of new star clusters, or tidal interactions may act to prevent dynamical friction from collapsing the entire globular cluster population into a single bright nucleus.

We attempt to determine whether the MACHO microlensing source stars are drawn from the average population of the LMC or from a population behind the LMC by examining the Hubble Space Telescope (HST) color-magnitude diagram (CMD) of microlensing source stars. We present WFPC2 HST photometry of eight MACHO microlensing source stars and the surrounding fields in the LMC. The microlensing source stars are identified by deriving accurate centroids in the ground-based MACHO images using difference image analysis (DIA) and then transforming the DIA coordinates to the HST frame. We consider in detail a model for the background population of source stars based on that presented by Zhao, Graff, & Guhathakurta. In this model, the source stars have an additional reddening of = 0.13 mag and a slightly larger distance modulus, ~ 0.3 mag, than the average LMC population. We also investigate a series of source star models, varying the relative fraction of source stars drawn from the average and background populations and the displacement of the background population from the LMC. Because of the small number of analyzed events, the distribution of probabilities of different models is rather flat. A shallow maximum occurs at a fraction s_LMC ~ 0.8 of the source stars in the LMC. This is consistent with the interpretation that a significant fraction of observed microlensing events are due to lenses in the Milky Way halo, but does not definitively exclude other models.

We model the star formation history (SFH) and the chemical evolution of the Galactic disk by combining an infall model and a limit-cycle model of the interstellar medium (ISM). Recent observations have shown that the SFH of the Galactic disk violently variates or oscillates. We model the oscillatory SFH based on the limit-cycle behavior of the fractional masses of three components of the ISM. The observed period of the oscillation (~1 Gyr) is reproduced within the natural parameter range. This means that we can interpret the oscillatory SFH as the limit-cycle behavior of the ISM. We then test the chemical evolution of stars and gas in the framework of the limit-cycle model since the oscillatory behavior of the SFH may cause an oscillatory evolution of the metallicity. We find, however, that the oscillatory behavior of metallicity is not prominent because the metallicity reflects the past integrated SFH. This indicates that the metallicity cannot be used to distinguish an oscillatory SFH from one without oscillations.

We present the results of 450 and 850 μm continuum mapping of the H II region KR 140 using the Submillimeter Common-User Bolometer Array (SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT). KR 140 is a small (5.7 pc diameter) H II region at a distance of 2.3 ± 0.3 kpc. Five of the six IRAS point sources near KR 140 were mapped in this study. Our analysis shows that two of these IRAS sources are embedded late B-type stars lying well outside the H II region, two are a part of the dust shell surrounding the H II region, and one is the combined emission from an ensemble of smaller sources unresolved by IRAS. We have discovered a number of relatively cold submillimeter sources not visible in the IRAS data, ranging in size from 0.2 to 0.7 pc and in mass from 0.5 to 130 M_☉. The distribution of masses for all sources is well characterized by a power law N(> M) ∝ M^-α with α = 0.5 ± 0.04, in agreement with the typical mass function for clumped structures of this scale in molecular clouds. Several of the submillimeter sources are found at the H II molecular gas interface and have probably been formed as the result of the expansion of the H II region. Many of the submillimeter sources we detect are gravitationally bound and most of these follow a mass-size relationship expected for objects in virial equilibrium with nonthermal pressure support. Upon the loss of nonthermal support, they could be sites of star formation. Along with the two B stars that we have identified as possible cluster members along with VES 735, we argue that five nearby highly reddened stars are in a pre-main-sequence stage of evolution.

G272.2-3.2 is a supernova remnant (SNR) characterized by an apparent centrally brightened X-ray morphology and thermally dominated X-ray emission. Because of this combination of Sedov-type (thermal emission) and non-Sedov-type (non-shell-like morphology) features, the remnant is classified as a "thermal composite" SNR. This class of remnant is still poorly understood, in part because of the difficulties in modeling accurately all the physical conditions which shape the emission morphology. This paper presents a combined analysis of data from the ASCA and ROSAT satellites coupled with previous results at other wavelengths. We find that the X-ray emission from G272.2-3.2 is best described by a nonequilibrium ionization (NEI) model with a temperature around 0.70 keV, an ionization timescale of 3200 cm^-3 yr, and a relatively high column density (N_H ~ 10²² atoms cm^-2). We look into the possible explanations for the apparent morphology of G272.2-3.2 using several models (among which are both cloud evaporation and thermal conduction models). For each of the models considered, we examine all the implications on the evolution of G272.2-3.2.

We performed spectroscopic X-ray observations of the eastern and northern regions of the Cygnus Loop with the ASCA observatory. The X-ray surface brightness of these regions shows a complex structure in the ROSAT all-sky survey image. We carried out a spatially resolved analysis for both regions and found that kT_e did not increase toward the center region, but showed inhomogeneous structures. Such variation cannot be explained by a blast-wave model propagating into a homogeneous interstellar medium. We thus investigated the interaction between a blast wave and an interstellar cloud. Two major emission mechanisms are plausible: a cloud evaporation model and a reflection-shock model. In both regions, only a reflection-shock model qualitatively explains our results. Our results suggest the existence of a large-scale interstellar cloud. We suppose that such a large-scale structure would be produced by a precursor.

We have investigated the evolution and distribution of molecules in collapsing prestellar cores via numerical chemical models, adopting the Larson-Penston solution and its delayed analogs to study collapse. Molecular abundances and distributions in a collapsing core are determined by the balance among the dynamical, chemical, and adsorption timescales. When the central density n_H of a prestellar core with the Larson-Penston flow rises to 3 × 10⁶ cm^-3, the CCS and CO column densities are calculated to show central holes of radius 7000 and 4000 AU, respectively, while the column density of N₂H⁺ is centrally peaked. These predictions are consistent with observations of L1544. If the dynamical timescale of the core is larger than that of the Larson-Penston solution owing to magnetic fields, rotation, or turbulence, the column densities of CO and CCS are smaller, and their holes are larger than in the Larson-Penston core with the same central gas density. On the other hand, N₂H⁺ and NH₃ are more abundant in the more slowly collapsing core. Therefore, molecular distributions can probe the collapse timescale of prestellar cores. Deuterium fractionation has also been studied via numerical calculations. The deuterium fraction in molecules increases as a core evolves and molecular depletion onto grains proceeds. When the central density of the core is n_H = 3 × 10⁶ cm^-3, the ratio DCO⁺/HCO⁺ at the center is in the range 0.06-0.27, depending on the collapse timescale and adsorption energy; this range is in reasonable agreement with the observed value in L1544.

We present BIMA Array observations of formic acid (HCOOH) in Galactic hot molecular cores. It has been found that among nearly 120 interstellar and circumstellar molecular species identified to date, the more complex and saturated organic species are usually observed in hot molecular cores—dense and warm molecular condensations associated with active star formation regions inside molecular clouds. Formic acid, one of the molecules in this category, shares common structural elements with both methyl formate (HCOOCH₃) and acetic acid (CH₃COOH). In this study, we successfully mapped HCOOH emission in three regions: Orion KL, Sgr B2, and W51. Column densities of HCOOH are above 10¹⁵ cm^-2 in these sources. The derived HCOOH column density in Sgr B2(N-LMH) is comparable to the CH₃COOH column density found by Mehringer et al. in 1997. Ethyl cyanide (C₂H₅CN) and HCOOCH₃ emission spectra were also detected in several sources. The distribution of HCOOH emission is consistent with a surface chemistry origin for the species. The abundance ratios of HCOOH to C₂H₅CN and to HCOOCH₃ vary by nearly 2 orders of magnitude from source to source.

We undertake a comparison of observed Algol-type binaries with a library of computed Case A binary evolution tracks. The library consists of 5500 binary tracks with various values of initial primary mass M₁₀, mass ratio q₀, and period P₀, designed to sample the phase-space of Case A binaries in the range -0.10 ≤ log M₁₀ ≤ 1.7. Each binary is evolved using a standard code with the assumption that both total mass and orbital angular momentum are conserved. This code follows the evolution of both stars to the point where contact or reverse mass transfer occurs. The resulting binary tracks show a rich variety of behavior that we sort into several subclasses of case A and case B. We present the results of this classification, the final mass ratio, and the fraction of time spent in Roche Lobe overflow for each binary system. The conservative assumption under which we created this library is expected to hold for a broad range of binaries, where both components have spectra in the range G0 to B1 and luminosity classes III to V. We gather a list of relatively well-determined, observed hot Algol-type binaries meeting this criterion, as well as a list of cooler Algol-type binaries, for which we expect significant dynamo-driven mass loss and angular momentum loss. We fit each observed binary to our library of tracks using a χ²-minimizing procedure. We find that the hot Algols display overall acceptable χ², confirming the conservative assumption, while the cool Algols show much less acceptable χ², suggesting the need for more free parameters, such as mass and angular momentum loss.

We present spectroscopic observations of object CE 315 revealing a blue continuum with strong emission lines. Most of the detected lines are identified with He I or He II in emission, with a handful of faint lines of nitrogen. Notable is the complete absence of hydrogen lines. The He lines exhibit triple-peaked profiles with remarkably broad widths of ~2000 km s^-1 (FWZP). The observations show that CE 315 is an interacting binary system with an orbital period of 65.1 ± 0.7 minutes and a mass ratio of 0.022. We conclude that the most likely scenario for this object is that of an accreting ~0.77 M_☉ white dwarf with a ~0.017 M_☉ helium white dwarf as mass donor.

We present a model for the structure, temporal behavior, and evolutionary status of the bipolar nebula M2-9. According to this model, the system consists of an asymptotic giant branch (AGB) or post-AGB star and a hot white dwarf companion, with an orbital period of about 120 yr. The white dwarf has undergone a symbiotic nova eruption about 1200 yr ago, followed by a supersoft X-ray source phase. The positional shift of the bright knots in the inner nebular lobes is explained in terms of a revolving ionizing source. We show that the interaction between the slow, AGB star's wind and a collimated fast wind from the white dwarf clears a path for the ionizing radiation in one direction, while the radiation is attenuated in others. This results in the mirror-symmetric (as opposed to the more common point-symmetric) shift in the knots. We show that M2-9 provides an important evolutionary link among planetary nebulae with binary central stars, symbiotic systems, and supersoft X-ray sources.

Progress in understanding the embedded stars in LkHα 225 has been hampered by their variability, making it hard to compare data taken at different times, and by the limited resolution of the available data, which cannot probe the small scales between the two stars. In an attempt to overcome these difficulties, we present new near-infrared data on this object taken using the adaptive optics with a laser for astronomy adaptive optics system with the MPE 3D integral field spectrometer and the near-infrared camera Omega-Cass. The stars themselves have K-band spectra which are dominated by warm dust emission, analogous to classes I-II for low-mass young stellar objects (YSOs), suggesting that the stars are in a phase where they are still accreting matter. On the other hand, the ridge of continuum emission between them is rather bluer, suggestive of extinct and/or scattered stellar light rather than direct dust emission. The compactness of the CO emission seen toward each star argues for accretion disks (which can also account for much of the K-band veiling) rather than a neutral wind. In contrast to other YSOs with CO emission, LkHα 225 has no detectable Brγ emission. In addition, there is no H₂ detected on the northern star, although we do confirm that the strongest H₂ emission is on the southern star, where we find it is excited primarily by thermal mechanisms. A second knot of H₂ is observed to its northeast, with a velocity shift of -75 km s^-1 and a higher fraction of nonthermal emission. This is discussed with reference to the H₂O maser, the molecular outflow, and [S II] emission observed between the stars.

We have observed four transits of the planet of HD 209458 using the STIS spectrograph on the Hubble Space Telescope (HST). Summing the recorded counts over wavelength between 582 and 638 nm yields a photometric time series with 80 s time sampling and relative precision of about 1.1 × 10^-4 per sample. The folded light curve can be fitted within observational errors using a model consisting of an opaque circular planet transiting a limb-darkened stellar disk. In this way we estimate the planetary radius R_p = 1.347 ± 0.060 R_Jup, the orbital inclination i = 866 ± 014, the stellar radius R_* = 1.146 ± 0.050 R_☉, and one parameter describing the stellar limb darkening. Our estimated radius is smaller than those from earlier studies but is consistent within measurement errors and also with theoretical estimates of the radii of irradiated Jupiter-like planets. Satellites or rings orbiting the planet would, if large enough, be apparent from distortions of the light curve or from irregularities in the transit timings. We find no evidence for either satellites or rings, with upper limits on satellite radius and mass of 1.2 R_⊕ and 3 M_⊕, respectively. Opaque rings, if present, must be smaller than 1.8 planetary radii in radial extent. The high level of photometric precision attained in this experiment confirms the feasibility of photometric detection of Earth-sized planets circling Sun-like stars.

We construct a simple model for stationary, axisymmetric black hole magnetospheres, in which the poloidal magnetic field is generated by a toroidal electric current in a thin disk with an inner edge, by solving the vacuum Maxwell equations in the Schwarzschild background. In this work, to obtain a concise analytical form of the magnetic stream function, we use the approximation that the inner edge is far distant from the event horizon. The global magnetospheric structure with a closed-loop and open field lines threading the inner and outer parts of the disk is explicitly shown, claiming that the model is useful as a starting point to study astrophysical problems involving inward disk-driven winds to a black hole and outward ones to infinity. The asymptotic shape of the field lines at the event horizon becomes nearly cylindrical, while at infinity it becomes conical. The magnetic spot in the disk connected with the black hole through the loop field lines occupies a very narrow region with the ring area roughly equal to the horizon area. By taking account of the existence of a uniform (external) magnetic field, we also obtain a model for collimated open field lines. Then, it is found that the magnetic connection between the black hole and the disk breaks down if the uniform field is strong enough. Considering slow rotation of the magnetosphere and angular momentum transfer by inward winds from the disk, the final discussion is devoted to gradual disruption of the closed loops due to radial accretion of disk plasma toward the black hole.

We report the measurement of the primordial D/H abundance ratio toward QSO HS 0105+1619. The column density of the neutral hydrogen in the z ≃ 2.536 Lyman limit system is high, log N = 19.422 ± 0.009 cm^-2, allowing for the deuterium to be seen in five Lyman series transitions. The measured value of the D/H ratio toward QSO HS 0105+1619 is found to be D/H = 2.54 ± 0.23 × 10^-5. The metallicity of the system showing D/H is found to be ≃0.01 solar, indicating that the measured D/H is the primordial D/H within the measurement errors. The gas that shows D/H is neutral, unlike previous D/H systems that were more highly ionized. Thus, the determination of the D/H ratio becomes more secure since we are measuring it in different astrophysical environments, but the error is larger because we now see more dispersion between measurements. Combined with prior measurements of D/H, the best D/H ratio is now D/H = 3.0 ± 0.4 × 10^-5, which is 10% lower than the previous value. The new values for the baryon-to-photon ratio and baryonic matter density derived from D/H are η = 5.6 ± 0.5 × 10^-10 and Ω_bh² = 0.0205 ± 0.0018, respectively.

We present the photometry and theoretical models for a Galactic bulge microlensing event, OGLE-2000-BUL-43. The event is very bright, with I = 13.54 mag, and has a very long timescale, t_E = 156 days. The long timescale and its light curve deviation from the standard shape strongly suggest that it may be affected by the parallax effect. We show that OGLE-2000-BUL-43 is the first discovered microlensing event, in which the parallax distortion is observed over a period of 2 yr. Difference image analysis (DIA) using the PSF matching algorithm of Alard & Lupton enabled photometry accurate to 0.5%. All photometry obtained with DIA is available electronically. Our analysis indicates that the viewing condition from a location near Jupiter will be optimal and could lead to magnifications of ~50 around 2001 January 31. These features offer a great promise for resolving the source (a K giant) and breaking the degeneracy between the lens parameters, including the mass of the lens, if the event is observed with the imaging camera on the Cassini space probe.

We have discovered a ~29 keV cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of 4U 0352+309 (X Per) using observations taken with the Rossi X-Ray Timing Explorer. The source 4U 0352+309 is a persistent low-luminosity (L_X = 4.2 × 10³⁴ ergs s^-1) X-ray pulsar with a 837 s period, which accretes material from the Be star X Per. The X-ray spectrum, unusual when compared to brighter accreting pulsars, may be due to the low mass accretion rate and could be typical of the new class of persistent low-luminosity Be/X-ray binary pulsars. We attempted spectral fits with continuum models used historically for 4U 0352+309 and found that all were improved by the addition of a CRSF at ~29 keV. The model that best fitted the observations is a combination of a 1.45 ± 0.02 keV blackbody with a 5.4 × 10⁸ cm² area and a power law with a 1.83 ± 0.03 photon index modified by the CRSF. In these fits the CRSF energy is 28.6 keV, implying a magnetic field strength of 2.5(1 + z) × 10¹² G in the scattering region (where z is the gravitational redshift). Phase-resolved analysis shows that the blackbody and cyclotron line energies are consistent with being constant through the pulse.

We report the detection of large flux changes in the persistent X-ray flux of soft gamma repeater (SGR) 1900+14 during its burst active episode in 1998. Most notably, we find a factor of ~700 increase in the nonburst X-ray flux following the August 27 flare, which decayed in time as a power law. Our measurements indicate that the pulse fraction remains constant throughout this decay. This suggests a global flux enhancement as a consequence of the August 27 flare rather than localized heating. While the persistent flux has since recovered to the preoutburst level, the pulse profile has not. The pulse shape changed to a near sinusoidal profile within the tail of the August 27 flare (in γ-rays), and this effect has persisted for more than 1.5 years (in X-rays). The results presented here suggest that the magnetic field of the neutron star in SGR 1900+14 was significantly altered (perhaps globally) during the giant flare of August 27.

Since SN 1987A, many observations have indicated that supernova explosions are not spherical. The cause of the asymmetric explosion is still controversial (e.g., asymmetry in the envelope, the convective engine in the central core or in the proto-neutron star). In our previous study, anisotropic neutrino radiation has been proposed as an explanation for this asymmetry. In this paper we carried out a series of systematic multidimensional numerical simulations in order to investigate the effect of anisotropic neutrino radiation itself on the supernova explosion energy. The neutrino luminosity and the degree of anisotropy in neutrino radiation were assumed as input parameters, and the numerical results for various parameters were compared with each other. It was found that only a few percent of anisotropy in the neutrino emission distribution is sufficient to increase the explosion energy by a large factor. The explosion energy calculated so far in many supernova models has tended to be too short to explain the observation. Anisotropy of 10% in neutrino radiation roughly corresponds to an enhancement of 4% in total neutrino luminosity as far as the explosion energy is concerned. The increase in the explosion energy due to anisotropic neutrino radiation can be explained as follows. Anisotropically emitted neutrinos locally heat the supernova matter and revive a stalled shock wave in the direction of enhanced radiation. The expansion of the gas by the shock propagation results in a decrease in the neutrino cooling (emission) rate that rapidly decreases with the matter temperature. It is this suppression of energy loss that contributes largely to the increase in explosion energy. The efficiency of neutrino heating (absorption) itself is almost unchanged between anisotropic and spherical models with available energy fixed for neutrinos. In order for a stalled shock wave to revive, enhancement of the local intensity in the neutrino flux is of great importance, rather than that of the total neutrino luminosity over all the solid angle. It is first pointed out that such local neutrino heating is capable of triggering a supernova explosion. Anisotropic neutrino radiation is considered to be a plausible mechanism for a "successful" explosion other than the so far suggested "convective trigger."

We present our extended polarimetric observations of a fast nova from the premaximum to transition stages. The observations were made for V1494 Aquilae (=Nova Aql 1999 No. 2) at the Dodaira Observatory of the National Astronomical Observatory of Japan between 1999 December 2 and 2000 March 29. We discovered that the light from the nova had already been intrinsically polarized in the premaximum stage and that the intrinsic polarization took a local maximum at visual maximum light. For the next 6 days a nearly orthogonal polarization component gradually increased. This component was later accompanied and finally replaced by rapidly oscillating components. These observations provide direct evidence that an asymmetric geometry was present even prior to maximum brightness. The principal geometry of the wind projected onto the sky appears nearly constant, at least from shortly after visual maximum to transition stages, but clumping of the ejecta appears to become significant in the later stages.

For about 11,000 stars observed in the Hipparcos Survey and detected by IRAS, we calculate bolometric luminosities by integrating their spectral energy distributions from the B band to far-IR wavelengths. We present an analysis of the dependence of dust emission on spectral type and of the correlations between luminosity and dust emission for about 1000 sources with the best data (parallax error less than 30%; luminosity error ~50% or better). This subsample includes stars of all spectral types and is dominated by K and M giants. We use the IRAS [25]-[12] color to select stars with emission from circumstellar dust and show that they are found throughout the Hertzsprung-Russell diagram, including on the main sequence. Clear evidence is found that M giants with dust emission have luminosities about 3 times larger (~3000 L_☉) than their counterparts without dust and that mass loss on the asymptotic giant branch for both M and C stars requires a minimum luminosity of order 2000 L_☉. Above this threshold the mass-loss rate seems to be independent of, or only weakly dependent on, luminosity. We also show that the mass-loss rate for these stars is larger than the core-mass growth rate, indicating that their evolution is dominated by mass loss.

We revisit the idea that density wave wakes of planets drive accretion in protostellar disks. The effects of many small planets can be represented as a viscosity if the wakes damp locally but the viscosity is proportional to the damping length. Damping occurs mainly because of shocks even for Earth-mass planets. The excitation of the wake follows from standard linear theory including the torque cutoff. We use this as input to an approximate but quantitative nonlinear theory based on Burger's equation for the subsequent propagation and shock. Shock damping is indeed local, but weakly so. If all metals in a minimum-mass solar nebula are invested in planets of a few Earth masses each, dimensionless viscosities (α) of the order of -4 dex to -3 dex result. We compare this with observational constraints. Such small planets would have escaped detection in radial velocity surveys and could be ubiquitous. If so, then the similarity of the observed lifetime of T Tauri disks to the theoretical timescale for assembling a rocky planet may be fate rather than coincidence.

We generalize the derivation of the dynamo coefficient α of Field et al. to include the following two aspects: first, the decorrelation times of velocity field and magnetic field are different; second, the magnetic Prandtl number can be arbitrary. We find that the contributions of velocity field and magnetic field to the α-effect are not equal, but affected by their different statistical properties. In the limit of large kinetic Reynolds number and large magnetic Reynolds number, α-coefficient may not be small if the decorrelation times of velocity field and magnetic field are shorter than the eddy turnover time of the MHD turbulence. We also show that under certain circumstances, for example if the kinetic helicity and current helicity are comparable, α depends insensitively on magnetic Prandtl number, while if either the kinetic helicity or the current helicity is dominated by the other one, a different magnetic Prandtl number will significantly change the dynamo α-effect.

We have analyzed Yohkoh soft and hard X-ray images of 36 flares, primarily to study the loop-top source that often prevails in these wavelengths during and following the impulsive phase. There are typically two patterns for the location of the low-energy (15-30 keV) hard X-ray (HXR) source with respect to the soft X-ray (SXR) loop. In a quarter of the flares, the HXR source lies in an extended structure separate from the brightest SXR loop. In other flares, the HXR source appears to be part of the same bipolar structure as the SXR loop, but its centroid is often displaced from the SXR loop-top source. The fact that the HXR source is not cospatial with the SXR source may reflect the presence of a distinct hotter structure. According to Yohkoh X-ray emission-line spectroscopy, the ~20 MK plasma accounts for only a fraction of the HXR counts. The temperature maps obtained from the SXR broadband photometry occasionally reveal high-temperature areas outside the bright loop, but they also tend to be displaced from the HXR source, indicating that they do not represent the superhot (≳30 MK) plasma. We discuss possible distributions of plasma of different temperatures that could be consistent with the data.

We present observations of the magnetic field configuration and its transformation in six solar eruptive events that show good agreement with the standard bipolar model for eruptive flares. The observations are X-ray images from the Yohkoh soft X-ray telescope (SXT) and magnetograms from Kitt Peak National Solar Observatory, interpreted together with the 1-8 Å X-ray flux observed by GOES. The observations yield the following interpretation. (1) Each event is a magnetic explosion that occurs in an initially closed single bipole in which the core field is sheared and twisted in the shape of a sigmoid, having an oppositely curved elbow on each end. The arms of the opposite elbows are sheared past each other so that they overlap and are crossed low above the neutral line in the middle of the bipole. The elbows and arms seen in the SXT images are illuminated strands of the sigmoidal core field, which is a continuum of sheared/twisted field that fills these strands as well as the space between and around them. (2) Although four of the explosions are ejective (appearing to blow open the bipole) and two are confined (appearing to be arrested within the closed bipole), all six begin the same way. In the SXT images, the explosion begins with brightening and expansion of the two elbows together with the appearance of short bright sheared loops low over the neutral line under the crossed arms and, rising up from the crossed arms, long strands connecting the far ends of the elbows. (3) All six events are single-bipole events in that during the onset and early development of the explosion they show no evidence for reconnection between the exploding bipole and any surrounding magnetic fields. We conclude that in each of our events the magnetic explosion was unleashed by runaway tether-cutting via implosive/explosive reconnection in the middle of the sigmoid, as in the standard model. The similarity of the onsets of the two confined explosions to the onsets of the four ejective explosions and their agreement with the model indicate that runaway reconnection inside a sheared core field can begin whether or not a separate system of overlying fields, or the structure of the bipole itself, allows the explosion to be ejective. Because this internal reconnection apparently begins at the very start of the sigmoid eruption and grows in step with the explosion, we infer that this reconnection is essential for the onset and growth of the magnetic explosion in eruptive flares and coronal mass ejections.

Fast upflows observed in the late gradual phase of an M6.8 two-ribbon flare by the Solar and Heliospheric Observatory/Coronal Diagnostic Spectrometer have provided evidence for the presence of chromospheric evaporation more than an hour after the impulsive phase of the flare. The chromospheric heating necessary to generate these upflows requires the continued injection and deposition of energy, which we presume to be provided by magnetic reconnection in the flaring corona. We investigate the nature of the transport of this energy from the reconnection site to the chromosphere by comparing the observed upflow velocities with those expected from different chromospheric heating models. A nonthermal beam of energetic electrons (≳ 15 keV) that is capable of generating the observed velocities would also generate significant hard X-ray emission that is not observed at this stage of the flare. We conclude, therefore, that the most likely energy transport mechanism is thermal conduction.

Within a deka-keV energy range, the power-law electron beams interacting with the solar atmosphere also result in the power-law bremsstrahlung of hard X-rays. The energy spectrum of electrons can thus be deduced from the observed hard X-ray spectrum, and the total energy carried by accelerated electrons can then be estimated. For quite a long time, one has always assumed the lower energy cutoff (E_c) of the power-law electron beams to be around 20 keV, an assumption that constitutes a main ingredient of the so-called standard picture of a solar flare, since the nonthermal electrons are substantial in powering a solar flare. However, there is in fact no solid observational basis for E_c = 20 keV. Here we present a quantitative method to determine E_c and its application to 14 BATSE/Compton Gamma Ray Observatory hard X-ray events. We find that E_c, varying from 47 to 141 keV in our samples, is on average 76.4 keV. The total energy carried by nonthermal electrons is therefore shown to be at least 1 order of magnitude lower than that derived by taking E_c = 20 keV. This energy shortage of nonthermal electrons in our sample hard X-ray events conflicts with the widely accepted scenario of a solar flare.

Using both the Electron, Proton, and Alpha Monitor and Ultra Low Energy Isotope Spectrometer instruments on board the Advanced Composition Explorer, we investigated from 1997 September to 1999 December a total of 27 ³He-rich solar energetic particle (SEP) ion events. The majority of these events (16 out of 27) have a clear association with impulsive injections of energetic electrons (38-53 keV). However, we find that the maximum electron intensity is statistically uncorrelated with the ³He/⁴He ratio (at 0.4-2.0 MeV nucleon^-1). We therefore examined the electron associations in the general class of impulsive ⁴He ion SEP events over the same time period. The general class is statistically indistinguishable from the ³He-enhanced subclass: 43 out of 97 impulsive ⁴He ion events were associated with impulsive electron events. The weak correlations between maximum electron intensities and maximum helium intensities (either ³He or ⁴He) are statistically indistinguishable between the general class of impulsive He events and the special class that was ³He enriched. Consequently, we have found no evidence (at the energies we studied) that the energetic electron population that escapes into interplanetary space is causally involved in the preferential enrichment of ³He. However, this does not rule out the possibility that a separate (nonescaping) population of electrons in the corona participates in the preferential acceleration of ³He.

We report on Télescope Héliographique pour l'Etude du Magnétisme et des Instabilités Solaires (THEMIS) observations of linear polarization events associated with umbral flashes observed in the Ca II infrared (IR) triplet lines. The observed signals are usually delayed in time and shifted in space when compared to the intensity and circular polarization signals from the flash. The observations are compatible with a scenario whereby flashes are produced by a perturbation propagating along the magnetic field lines as they bend out toward the penumbra. Only a fraction of the resolution element appears to be emitting flashlike profiles, as if the waves were propagating only within localized magnetic field lines. This localization, however, does not impede the apparent propagation of the perturbation horizontally within the umbra.

We present a complete atomic model for Si I line synthesis. We study how the computed profiles of two blue lines of this atom are influenced by the choice of the atomic parameters and find that, although several cross sections are not known accurately, the line profiles do not depend strongly on them and are therefore useful as diagnostics of the atmospheric structure. We study which transitions need not be included in the model, in order to reduce as much as possible the computing time. We compare the profiles computed for a standard model of the quiet solar atmosphere with the observations and find very good agreement. We confirm that irradiation by UV lines originating in the transition region above sunspot umbrae or plages strongly enhances the continuum between 1300 and 1700 Å, which is due to Si I bound-free transitions. If line fluxes typical of the impulsive phase of flares are assumed, the line profiles are also affected.

The averaged probability of detecting a planetary companion of a lensing star during a microlensing event toward the Galactic center when the planet to star mass ratio q = 0.001 is shown to have a maximum exceeding 10% at an orbit semimajor axis near 1.5 AU for a uniform distribution of impact parameters. This peak value is somewhat lower than the maximum of 17% obtained by Gould & Loeb in 1992, but it is raised to more than 20% for a distribution of source-lens impact parameters that is determined by the efficiency of event detection. Although these probabilities, based on a signal-to-noise ratio (S/N) detection criterion, are model and assumption dependent, the fact that they change in predictable ways as functions of the orbit semimajor axes but remain robust for plausible variations of all the relevant Galactic parameters implies that they are representative of real values. In addition, the averaging procedures are carefully defined, and they determine the dependence of the detection probabilities on several properties of the Galaxy. The probabilities for other planet to star mass ratios can be estimated from an approximate scaling of q^1/2. A planet is assumed detectable if the perturbation of the single-lens light curve exceeds 2/(S/N) sometime during the event, where it is understood that at least 20 consecutive photometric points during the perturbation are necessary to confirm the detection. S/N is the instantaneous value for the amplified source. In addition, 2 m telescopes with 60 s integrations in I band with high time resolution photometry throughout the duration of an ongoing event are assumed. The probabilities are derived as a function of semimajor axis a of the planetary orbit, where the peak probability occurs where a is approximately the mean Einstein ring radius of the distribution of lenses along the line of sight. The probabilities remain significant for 0.6 ≲ a ≲ 10 AU. Dependence of the detection probabilities on the lens mass function, luminosity function of the source stars as modified by extinction, distribution of source-lens impact parameters, and the line of sight to the source are also determined, and the probabilities are averaged over the distribution of the projected position of the planet onto the lens plane, over the lens mass function, over the distribution of impact parameters, over the distribution of lens and sources along the line of sight, and over the I-band luminosity function of the sources adjusted for the source distance and extinction. The probability for a particular impact parameter and particular source I magnitude but averaged over remaining degenerate parameters also follows from the analysis. In the latter case, the extraction of the probability as a function of a for a particular q from the empirical data from a particular event is indicated.

Weak lensing by large-scale structure provides a unique method to directly measure matter fluctuations in the universe and has recently been detected from the ground. Here we report the first detection of this "cosmic shear" based on space-based images. The detection was derived from the Hubble Space Telescope (HST) Survey Strip (or "Groth Strip"), a 4' × 42' set of 28 contiguous Wide Field Planetary Camera 2 (WFPC2) pointings with I < 27. The small size of the HST point-spread function affords both a lower statistical noise and a much weaker sensitivity to systematic effects, a crucial limiting factor of cosmic shear measurements. Our method and treatment of systematic effects were discussed in an earlier paper. We measure an rms shear of 1.8% on the WFPC2 chip scale (127), in agreement with the predictions of cluster-normalized cold dark matter (CDM) models. Using a maximum likelihood analysis, we show that our detection is significant at the 99.5% confidence level (CL) and measure the normalization of the matter power spectrum to be σ₈Ω = 0.51, in a ΛCDM universe. These 68% CL errors include (Gaussian) cosmic variance, systematic effects, and the uncertainty in the redshift distribution of the background galaxies. The signal comes primarily from the chip scale (127) with gradually decreasing contributions up to roughly 10'. Our result is consistent with earlier lensing measurements from the ground and with the normalization derived from cluster abundance. We discuss how our measurement can be improved with the analysis of a large number of independent WFPC2 fields.

The topology of weak lensing fields is studied using the two-dimensional genus statistic. Simulated fields of the weak lensing convergence are used to study the effect of nonlinear gravitational evolution and to model the statistical errors expected in observational surveys. For large smoothing angles, the topology is in agreement with the predictions from linear theory. On smoothing angles smaller than 10', the genus curve shows the non-Gaussian signatures of gravitational clustering and differs for open and flat cold dark matter models. Forthcoming surveys with areas larger than 10 deg² should have adequate signal-to-noise ratio to measure the non-Gaussian shape and the Ω-dependence of the genus statistic.

We present a comparison of X-ray and optical luminosities and luminosity functions of cluster candidates from a joint optical/X-ray survey, the ROSAT Optical X-Ray Survey. Completely independent X-ray and optical catalogs of 23 ROSAT fields (4.8 deg²) were created by a matched-filter optical algorithm and by a wavelet technique in the X-ray. We directly compare the results of the optical and X-ray selection techniques. The matched-filter technique detected 74% (26 out of 35) of the most reliable cluster candidates in the X-ray-selected sample; the remainder could be either constellations of X-ray point sources or z > 1 clusters. The matched-filter technique identified approximately 3 times the number of candidates (152 candidates) found in the X-ray survey of nearly the same sky (57 candidates). While the estimated optical and X-ray luminosities of clusters of galaxies are correlated, the intrinsic scatter in this relationship is very large. We can reproduce the number and distribution of optical clusters with a model defined by the X-ray luminosity function and by an L_X-Λ_cl relation if H₀ = 75 km s^-1 Mpc^-1 and if the L_X-Λ_cl relation is steeper than the expected L_X ∝ Λ. On statistical grounds, a bimodal distribution of X-ray luminous and X-ray faint clusters is unnecessary to explain our observations. Follow-up work is required to confirm whether the clusters without bright X-ray counterparts are simply X-ray faint for their optical luminosity because of their low mass or youth or are a distinct population of clusters that do not, for some reason, have dense intracluster media. We suspect that these optical clusters are low-mass systems, with correspondingly low X-ray temperatures and luminosities, or that they are not yet completely virialized systems.

We report the results of a long BeppoSAX observation of Abell 3667, one of the most spectacular galaxy clusters in the southern sky. A clear detection of hard X-ray radiation up to ~35 keV is reported, while a hard excess above the thermal gas emission is present at a marginal level that should be considered as an upper limit to the presence of nonthermal X-ray radiation. The strong, hard excesses reported by BeppoSAX in Coma and A2256 and the only marginal detection of nonthermal emission in A3667 can be explained in the framework of the inverse Compton model. We argue that the nonthermal X-ray detections in the Phoswich Detection System energy range are related to the radio index structure of halos and relics present in the observed clusters of galaxies.

We present a study of the galaxy population in the IR-selected cluster RX J0848+4453 at z = 1.27, using deep Hubble Space Telescope (HST) NICMOS H_F160W and WFPC2 I_F814W images of the cluster core. We morphologically classify all galaxies to K_s = 20.6 that are covered by the HST imaging and determine photometric redshifts using deep ground-based BRIzJK_s photometry. Of 22 likely cluster members with morphological classifications, 11 (50%) are classified as early-type galaxies, nine (41%) as spiral galaxies, and two (9%) as "merger/peculiar." At HST resolution, the second brightest cluster galaxy is resolved into a spectacular merger between three red galaxies of similar luminosity, separated from each other by ≈6 h kpc, with an integrated magnitude K = 17.6 (~3L_* at z = 1.27). The two most luminous early-type galaxies also show evidence for recent or ongoing interactions. Mergers and interactions between galaxies are possible because RX J0848+4453 is not yet relaxed. The fraction of early-type galaxies in our sample is similar to that in clusters at 0.5 < z < 1 and consistent with a gradual decrease of the number of early-type galaxies in clusters from z = 0 to z ≈ 1.3. We find evidence that the color-magnitude relation of the early-type galaxies is less steep than in the nearby Coma Cluster. This may indicate that the brightest early-type galaxies have young stellar populations at z = 1.27 but is also consistent with predictions of single-age "monolithic" models with a galactic wind. The scatter in the color-magnitude relation is ≈0.04 in rest-frame U-V, similar to that in clusters at 0 < z < 1. Taken together, these results show that luminous early-type galaxies exist in clusters at z ≈ 1.3 but that their number density may be smaller than in the local universe. Additional observations are needed to determine whether the brightest early-type galaxies harbor young stellar populations.

Recent spectroscopic and morphological observational studies of galaxies around NGC 1399 in the Fornax Cluster have discovered several "ultracompact dwarf" galaxies with intrinsic sizes of ~100 pc and absolute B-band magnitudes ranging from -13 to -11 mag. In order to elucidate the origin of these enigmatic objects, we perform numerical simulations on the dynamical evolution of nucleated dwarf galaxies orbiting NGC 1399 and suffering from its strong tidal gravitational field. Adopting a plausible scaling relation for dwarf galaxies, we find that the outer stellar components of a nucleated dwarf are totally removed. This is due to them being tidally stripped over the course of several passages past the central region of NGC 1399. The nucleus, however, manages to survive. We also find that the size and luminosity of the remnant are similar to those observed for ultracompact dwarf galaxies, if the simulated precursor nucleated dwarf has a mass of ~10⁸M_☉. These results suggest that ultracompact dwarf galaxies could have previously been more luminous dwarf spheroidal or elliptical galaxies with rather compact nuclei.

We investigate models for the class of ultraluminous nonnuclear X-ray sources (i.e., ultraluminous compact X-ray sources [ULXs]) seen in a number of galaxies and probably associated with star-forming regions. Models in which the X-ray emission is assumed to be isotropic run into several difficulties. In particular, the formation of sufficient numbers of the required ultramassive black hole X-ray binaries is problematic, and the likely transient behavior of the resulting systems is not in good accord with observation. The assumption of mild X-ray beaming suggests instead that ULXs may represent a short-lived but extremely common stage in the evolution of a wide class of X-ray binaries. The best candidate for this is the phase of thermal-timescale mass transfer that is inevitable in many intermediate- and high-mass X-ray binaries. This in turn suggests a link with the Galactic microquasars. The short lifetimes of high-mass X-ray binaries would explain the association of ULXs with episodes of star formation. These considerations still allow the possibility that individual ULXs may contain extremely massive black holes.

The virial mass (M_vir)-metallicity relation among the Local Group dwarf spheroidal (dSph) galaxies is examined. Hirashita, Takeuchi, & Tamura showed that the dSph galaxies can be divided into two distinct classes with respect to the relation between their virial masses and luminosities: low-mass (M_vir ≲ 10⁸M_☉) and high-mass (M_vir ≳ 10⁸M_☉) groups. We see that both the mass-metallicity and mass-luminosity relations of the high-mass dSph galaxies are understood as a low-mass extension of giant elliptical galaxies. On the contrary, we find that the classical galactic wind model is problematic when applied to the low-mass dSph galaxies, whose low binding energy is comparable to that released by several supernova explosions. A strongly regulated star formation in their formation phase is required to reproduce their observed metallicity. Such regulation is naturally expected in a gas cloud with the primordial elemental abundance according to Nishi & Tashiro. A significant scatter in the mass-metallicity relation for the low-mass dSph galaxies is also successfully explained along with the scenario of Hirashita and coworkers. We not only propose a new picture for a chemical enrichment of the dSph galaxies but also suggest that the mass-metallicity and mass-luminosity relations be understood in a consistent context.

The dispersion and mean trends of r-process abundances in metal-poor stars are discussed based on a model of diverse supernova sources for the r-process. This model is unique in that its key parameters are inferred from solar system data independent of stellar observations at low metallicities. It is shown that this model provides a good explanation for the observed dispersion and mean trend of Eu abundances over -3 ≲ [Fe/H] ≲ -1. It is also shown that this model provides a means to discuss r-abundances in general. For example, the Ag abundance in any metal-poor star with observed Eu and Fe abundances can be calculated from the model. This approach is demonstrated with success for two stars and can be further tested by future Ag data. The dispersion and mean trend of Ag abundances in metal-poor stars are also calculated for comparison with future observations.

The optical light curve of the afterglow following the gamma-ray burst GRB 990712 is reexamined. Recently published polarization measurements of that source require a collimated outflow geometry that in turn predicts a break in the light curve. We show that the V-band light curve is consistent with such a break and that the postbreak light-curve evolution is dominated by a supernova contribution.

We report the detection of radio and X-ray pulsations at a period of 51.6 ms from the X-ray source RX/AX J2229.0+6114 in the error box of the EGRET source 3EG J2227+6122. An ephemeris derived from a single ASCA observation and multiple epochs at 1412 MHz from Jodrell Bank indicates steady spin-down with = 7.83 × 10^-14 s s^-1. From the measured P and , we derive spin-down power = 2.2 × 10³⁷ ergs s^-1, magnetic field B_p = 2.0 × 10¹² G, and characteristic age P/2 = 10,460 yr. An image from the Chandra X-Ray Observatory reveals a point source surrounded by centrally peaked diffuse emission that is contained within an incomplete radio shell. We assign the name G106.6+2.9 to this new supernova remnant, which is evidently a pulsar wind nebula. For a distance of 3 kpc estimated from X-ray absorption, the ratio of X-ray luminosity to spin-down power is ≈8 × 10^-5, smaller than that of most pulsars but similar to the Vela pulsar. If PSR J2229+6114 is the counterpart of 3EG J2227+6122, then its efficiency of γ-ray production, if isotropic, is 0.016 (d/3 kpc) ². It obeys an established trend of γ-ray efficiency among known γ-ray pulsars, which, in combination with the demonstrated absence of any other plausible counterpart for 3EG J2227+6122, makes the identification compelling. If confirmed, this identification bolsters the pulsar model for unidentified Galactic EGRET sources.

We report the results of the spectral analysis of two observations of the Vela pulsar with the Chandra X-Ray Observatory. The spectrum of the pulsar does not show statistically significant spectral lines in the observed 0.25-8.0 keV band. Similar to middle-aged pulsars with detected thermal emission, the spectrum consists of two distinct components. The softer component can be modeled as a magnetic hydrogen atmosphere spectrum—for the pulsar magnetic field B = 3 × 10¹² G and neutron star mass M = 1.4 M_☉ and radius R^∞ = 13 km, we obtain T = 0.68 ± 0.03 MK, L = (2.6 ± 0.2) × 10³² ergs s^-1, and d = 210 ± 20 pc (the effective temperature, bolometric luminosity, and radius are as measured by a distant observer). The effective temperature is lower than that predicted by standard neutron star cooling models. A standard blackbody fit gives T^∞ = 1.49 ± 0.04 MK, L = (1.5 ± 0.4) × 10³²d ergs s^-1 (d₂₅₀ is the distance in units of 250 pc); the blackbody temperature corresponds to a radius R^∞ = (2.1 ± 0.2)d₂₅₀ km, much smaller than realistic neutron star radii. The harder component can be modeled as a power-law spectrum, with parameters depending on the model adopted for the soft component: γ = 1.5 ± 0.3, L_X = (1.5 ± 0.4) × 10³¹d ergs s^-1 and γ = 2.7 ± 0.4, L_X = (4.2 ± 0.6) × 10³¹d ergs s^-1 for the hydrogen atmosphere and blackbody soft component, respectively (γ is the photon index; L_X is the luminosity in the 0.2-8 keV band). The extrapolation of the power-law component of the former fit toward lower energies matches the optical flux at γ ≃ 1.35-1.45.

We report the discovery of coupling between periodic and aperiodic variability and 12 mHz X-ray quasi-periodic oscillations (QPOs) from the X-ray binary pulsar Hercules X-1 using data from the Rossi X-Ray Timing Explorer. We found two different couplings, one during the preeclipse dips and the other during the normal state of the source, using a method that directly compares the low-frequency power-density spectra (PDSs) with those of the sidebands around the coherent pulse frequency. The preeclipse dip light curves show significant time variation of photon counts, and this variation appears in the PDSs as both strong millihertz powers and well-developed sidebands around the coherent pulse frequency. The linear correlation coefficients between the millihertz PDSs and the sideband PDSs obtained from two preeclipse dip data segments are 0.880 ± 0.003 and 0.982 ± 0.001, respectively. This very strong coupling demonstrates that the amplitudes of the coherent pulsations are almost exactly modulated by the aperiodic variabilities, suggesting that both the periodic and aperiodic variabilities are related to time variation of obscuration of X-rays from the central pulsar by an accretion disk during preeclipse dips. We also found weak coupling during the normal state of the source, together with 12 mHz QPOs. The normal state coupling seems to reconcile with the prediction that a significant fraction of the aperiodic variabilities from X-ray binary pulsars are due to time-varying accretion flows onto the pulsar's magnetic poles. We discuss the possible origin of the 12 mHz QPOs.

We present new theoretical period-radius relations for first-overtone Galactic Cepheids. Current predictions are based on several sequences of nonlinear, convective pulsation models at solar chemical composition (Y = 0.28, Z = 0.02) and stellar masses ranging from 3.0 to 5.5 M_☉. The comparison between predicted and empirical radii of four short-period Galactic Cepheids suggests that QZ Normae and EV Scuti are pulsating in the fundamental mode, whereas Polaris and SZ Tauri pulsate in the first overtone. This finding supports the mode identifications that rely on the comparison between direct and period-luminosity-based distance determinations, but it is somewhat at variance with the mode identification based on Fourier parameters. In fact, we find from our models that fundamental and first-overtone pulsators attain, for periods ranging from 2.7 to 4 days, quite similar ϕ₂₁-values, making mode discrimination from this parameter difficult. The present mode identifications for our sample of Cepheids are strengthened by the accuracy of their empirical radius estimates as well as by the evidence that predicted fundamental and first-overtone radii do not show, within the current uncertainty on the mass-luminosity relation, any degeneracy in the same period range. Accurate radius determinations are therefore an excellent tool to unambiguously determine the pulsation modes of short-period Cepheids.

We present complete near-infrared (0.85-2.45 μm), low-resolution (~100) spectra of a sample of 26 disk L dwarfs with reliable optical spectral type classification. The observations have been obtained with the Near Infrared Camera and Spectrograph at the Telescopio Nazionale Galileo using a prism-based optical element (the Amici device) that provides a complete spectrum of the source on the detector. Our observations show that low-resolution near-infrared spectroscopy can be used to determine the spectral classification of L dwarfs in a fast but accurate way. We present a library of spectra that can be used as templates for spectral classification of faint dwarfs. We also discuss a set of near-infrared spectral indices well correlated with the optical spectral types that can be used to classify accurately L dwarfs earlier than L6.

We present ground-based near-infrared (H-band) imaging of the circumstellar disk around the nearby classical T Tauri star TW Hydrae. The scattered-light image shows a face-on disk with a radius of 4'' (corresponding to 225 AU) and a morphology that agrees with recent images from the Hubble Space Telescope and the Very Large Array. The best-fit power-law for the disk's radial surface brightness profile obeys the law r^-3.3±0.3. We use our image and published continuum flux densities to derive properties of the disk with a simple model of emission from a flat disk. The best-fit values for disk mass and inner radius are 0.03 M_☉ and 0.3 AU, respectively; the best-fit values for the temperature, density, and grain opacity power-law exponents (q, p, and β) are 0.7, 1.3, and 0.9, respectively. These properties are similar to those of disks around classical T Tauri stars located in more distant molecular clouds. Because of TW Hydrae's nearby location and pole-on orientation, it is a uniquely favorable object for future studies of radial disk structure at the classical T Tauri stage.

New observations of the Magellanic Cloud luminous blue variable candidate S119 (HD 269687) show the relationship of the star to its environs. Echelle spectroscopy and high-resolution Hubble Space Telescope imagery reveal an expanding bubble centered on the star. This bubble appears in both Hα and [N II] and is noticeably brighter on the near (blueshifted) side. The systemic velocity of both the expanding bubble and the star itself (as seen by the very broad Hα emission feature in the stellar spectrum) is V_hel ~ 160 km s^-1, whereas the velocity of the superposed LMC interstellar medium (ISM) is 250-300 km s^-1. ISM absorption features seen in Far-Ultraviolet Spectroscopic Explorer spectra reveal components at both stellar and LMC velocities. Thus, we conclude that S119 is located within the LMC ISM and that the bubble is interacting strongly with the ISM in a bow shock.

The Bok globule B175, located toward a giant region of patchy extinction 10° above the Galactic plane known as the Cepheus Flare, is currently engaged in a chance encounter with a B9.5 V star, producing a prominent reflection nebula, Ced 201. Partly owing to the presence of this star, a fairly accurate distance estimate of 400 pc can be derived for B175. An imaging and spectroscopic study of B175 led to the discovery of a new Herbig-Haro (HH) object, HH 450, emerging from a deeply embedded and cold IRAS source, 22129+7000. Furthermore, we find several parsec-scale filaments of emission that trace the rim of a new supernova remnant, G110.3+11.3, which appears to be approaching the globule. This remnant is located near the edge of a large, 6° radius H I and CO cavity that coincides with a bright diffuse soft X-ray enhancement, probably due to several recent supernova explosions. At 400 pc, G110.3+11.3 is one of the closer known supernova remnants. The supernova remnant and the HH flow appear to be heading toward a frontal collision in about 1000 yr.

We report the detection of the doubly deuterated forms of ammonia (ND₂H) and formaldehyde (D₂CO) toward 16293E, a newly identified low-luminosity protostar in the ρ Ophiucus molecular complex. The abundances of ND₂H and D₂CO compared with their hydrogenated counterparts NH₃ and H₂CO are ~3% and 40% ± 20%, respectively. To date, 16293E is thus the source with the highest levels of multiple deuteration: [ND₂H]/[NH₃] is 5-6 times larger there than in the only other astronomical source where ND₂H has ever been found, the dense ammonia core of L134N; and [D₂CO]/[H₂CO] is more than 5 times higher than in the proto-binary system IRAS 16293-2422. The relative abundances of doubly deuterated molecules in low-luminosity protostars are much higher than their current gas-phase [D]/[H] ratios would suggest and, therefore, likely reflect active grain surface chemistry followed by some desorption process.

A survey for molecular outflows was carried out by mapping the CO J = 2-1 line toward a sample of 69 luminous IRAS point sources. Sixty objects have IRAS luminosities from 10³ to 10⁵L_☉ and are associated with dense gas traced by NH₃, identifying them as high-mass star-forming regions. Among 69 sources, 65 sources have data that are suitable for outflow identification. Thirty-nine regions show spatially confined high-velocity wing emission in CO, indicative of molecular outflows. Most objects without identifiable outflows lie within 0^° < l < 50^° where outflow signatures are confused by multiple cloud components along the line of sight. Excluding 26 sources with 0^° < l < 50^°, we found 35 outflows out of 39 sources, which yields an outflow detection rate of 90%. Many of the outflows contain masses of more than 10 M_☉ and have momenta of a few hundred M_☉ km s^-1, at least 2 orders of magnitude larger than those in typical low-mass outflows. This class of massive and energetic outflows is most likely driven by high-mass young stellar objects. The high detection rate indicates that molecular outflows are common toward high-mass young stars. Given the connection between outflows and accretion disks in low-mass stars, we suggest that high-mass stars may form via an accretion-outflow process, similar to their low-mass counterparts.

A strong superequipartition magnetic field strength on the order of 10 T (10⁵ G) has been inferred at the bottom of the solar convection zone. We show that the "explosion" of weak magnetic flux tubes, which is caused by a sudden loss of pressure equilibrium in the flux loop rising through the superadiabatically stratified convection zone, provides a mechanism that leads to a strong field: the flow of high-entropy material out of the exploded loop leads to a significant intensification of the magnetic field in the underlying flux sheet at the bottom. In contrast to the amplification by differential rotation, this process converts the potential energy of the stratification into magnetic energy and thus is not dynamically limited by the back-reaction on the flow field via the Lorentz force.

By using a two-dimensional, fully electromagnetic and relativistic particle-in-cell simulation, we investigate coherent emission mechanisms of electromagnetic waves from a dynamical current sheet, in which the pinching of the current sheet as well as the coalescence of the magnetic islands are taking place. In a formation phase of the current sheet, the electron pinching dominates, and subsequently the sheet becomes unstable against the tearing instability. In this pinching phase, emissions of the electron Bernstein mode propagating across the magnetic field from the Harris instability appear. The extraordinary mode (the X-mode) with a second-harmonic plasma frequency can also be excited from the electron Bernstein mode through mode coupling. On the other hand, we found that the electromagnetic waves obliquely propagating to the magnetic field and the ordinary mode (the O-mode) propagating across the magnetic field can be emitted during the coalescence of magnetic islands, which are formed in the nonlinear stage of the tearing instability. The generation mechanism of these waves may be due to the decay of the electromagnetic fields inside the O-type magnetic islands produced by the counterstreaming instability. The quasi-periodic radio bursts observed in the impulsive phase of solar flares could be explained by this coherent wave emission process.