Table of contents

Wide-field near-infrared (IR) surveys have revealed a population of galaxies with very red optical-IR colors, which have been termed "extremely red objects" (EROs). Modeling suggests that such red colors (R-K > 5) could be produced by galaxies at z ≳ 1 with either very old stellar populations or very high dust extinction. Recently, it has been discovered that EROs are strongly clustered. Are these objects the high-redshift progenitors of present-day giant ellipticals (gEs)? Are they already massive at this epoch? Are they the descendants of the z ~ 3 Lyman break galaxies (LBGs), which have also been identified as possible high-redshift progenitors of gEs? We address these questions within the framework of the cold dark matter paradigm, using an analytic model that connects the number density and clustering, or bias, of an observed population with the halo occupation function (the number of observed galaxies per halo of a given mass). We find that EROs reside in massive dark matter halos, with average mass > 10¹³hM_☉. The occupation function that we derive for EROs is very similar to the one we derive for z = 0, L > L_*, early-type galaxies, whereas the occupation function for LBGs is skewed toward much smaller host halo masses ( ≈ 10¹¹-10¹²hM_☉). We then use the derived occupation function parameters to explore the possible evolutionary connections between these three populations.

We construct an analytical model of the mass distribution function (MDF) of subhalos in a galaxy cluster in the context of the cold dark matter theory. Our model takes account of two important effects: the high density of the precluster region and the spatial correlation of initial density fluctuations. For subhalos with small masses, the MDF that our model predicts is in between the Press-Schechter (PS) MDF and a conditional MDF as an extension of the PS formalism. We compare the results of our model with those of numerical simulations. We find that our model predictions are consistent with the mass and velocity distribution functions of subhalos in a cluster obtained by numerical simulations. We estimate the probability of finding large X-ray subhalos that often have "cold fronts." The observed large X-ray subhalos and cold fronts may not always be the result of cluster mergers but may instead be internal structures in clusters.

We explore the possibility of detecting H I 21 cm absorption by the neutral intergalactic medium (IGM) toward very high redshift radio sources. The epoch considered is between the time when the first ionizing sources form and when the bulk of the neutral IGM becomes ionized. Because of the extreme Lyα opacities of the neutral IGM, objects within this "gray age" can be observed only at wavelengths longer than about 1 μm. We use the latest simulations of the evolution of the IGM in the context of Λ cold dark matter structure formation models constrained by observations of the highest redshift QSOs to predict the optical depth as a function of frequency of the neutral IGM due to the H I 21 cm line. We then simulate radio spectra assuming observational parameters for future large-area radio telescopes. These spectra show that H I 21 cm absorption studies could be a powerful probe of the rich structure of the neutral IGM prior to the epoch of reionization, including ~1% absorption by the mean neutral IGM, plus deeper, narrow lines (≥5% and a few kilometers per second). Most of the variations in transmissivity are due to mild density inhomogeneities with typical values of the cosmic overdensity δ ~ 10, precisely the structures that at later times give rise to the Lyα forest. We also consider sensitivity limits and the evolution of radio source populations and conclude that it is reasonable to hypothesize the existence of an adequate number of high-z radio sources against which such absorption studies could be performed.

We report the results of [O III] λ5007 surveys for planetary nebulae (PNe) in six galaxies: NGC 2403, NGC 3115, NGC 3351, NGC 3627, NGC 4258, and NGC 5866. Using on-band/off-band [O III] λ5007 images, as well as images taken in Hα, we identify samples of PNe in these galaxies and derive their distances using the planetary nebula luminosity function (PNLF). We then combine these measurements with previous data to compare the PNLF, Cepheid, and surface brightness fluctuation (SBF) distance scales. We use a sample of 13 galaxies to show that the absolute magnitude of the PNLF cutoff is fainter in small, low-metallicity systems, but the trend is well modeled by the theoretical relation of Dopita, Jacoby, & Vassiliadis. When this metallicity dependence is removed, the scatter between the Cepheid and PNLF distances becomes consistent with the internal errors of the methods and independent of any obvious galaxy parameter. We then use these data to recalibrate the zero point of the PNLF distance scale. We use a sample of 28 galaxies to show that the scatter between the PNLF and SBF distance measurements agrees with that predicted from the techniques' internal errors and that there is no systematic trend between the distance residuals and stellar population. However, we also find that the PNLF and SBF methods have a significant scale offset: Cepheid-calibrated PNLF distances are, on average, ~0.3 mag smaller than Cepheid-calibrated SBF distances. We discuss the possible causes of this offset and suggest that internal extinction in the bulges of the SBF calibration galaxies is the principal cause of the discrepancy. If this hypothesis is correct, then the SBF-based Hubble constant must be increased by ~7%. We also use our distance to NGC 4258 to argue that the short distance scale to the LMC is correct and that the global Hubble constant inferred from the Hubble Space Telescope Key Project should be increased by 8% ± 3% to H₀ = 78 ± 7 km s^-1 Mpc^-1.

We derive an analytic expression for the mean magnification due to strong gravitational lensing, using a simple lens model: a singular isothermal sphere embedded in an external shear field. We compute separate expressions for two-image and four-image lensing. For four-image lensing, the mean magnification takes a particularly simple form: ⟨μ₄⟩ = 3.6/ , where γ is the external shear. We compare our analytic results with a numerical evaluation of the full magnification distribution. The results can be used to understand the magnification bias that favors the discovery of four-image systems over two-image systems in flux-limited lens surveys.

We predict the effects of gravitational lensing on the color-selected flux-limited samples of z_s ~ 4.3 and z_s ≳ 5.8 quasars, recently published by the Sloan Digital Sky Survey (SDSS). Our main findings are the following: (1) The lensing probability should be 1-2 orders of magnitude higher than for conventional surveys. The expected fraction of multiply imaged quasars is highly sensitive to redshift and the uncertain slope of the bright end of the luminosity function, β_h. For β_h = 2.58 (3.43) we find that at z_s ~ 4.3 and i* < 20.0 the fraction is ~4% (13%), while at z_s ~ 6 and z* < 20.2 the fraction is ~7% (30%). (2) The distribution of magnifications is heavily skewed; sources having the redshift and luminosity of the SDSS z_s ≳ 5.8 quasars acquire median magnifications of med(μ_obs) ~ 1.1-1.3 and mean magnifications of ⟨μ_obs⟩ ~ 5-50. Estimates of the quasar luminosity density at high redshift must therefore filter out gravitationally lensed sources. (3) The flux in the Gunn-Peterson trough of the highest redshift (z_s = 6.28) quasar is known to be f_λ < 3 × 10^-19 ergs s^-1 cm^-2 Å^-1. Should this quasar be multiply imaged, we estimate a 40% chance that light from the lens galaxy would have contaminated the same part of the quasar spectrum with a higher flux. Hence, spectroscopic studies of the epoch of reionization need to account for the possibility that a lens galaxy, which boosts the quasar flux, also contaminates the Gunn-Peterson trough. (4) Microlensing by stars should result in ~ of multiply imaged quasars in the z_s ≳ 5.8 catalog varying by more than 0.5 mag over the next decade. The median emission-line equivalent width of multiply imaged quasars would be lowered by ~20% with respect to the intrinsic value because of differential magnification of the continuum and emission-line regions.

We present Very Long Baseline Array observations of the radio source J1625+4134 at 22 and 15 GHz and analyze them in concurrence with other existing VLBI data on this source. The high-resolution images at 15 and 22 GHz show a short and bending jet, which differs by about 270° in position angle with the northern jet detected at lower frequencies. The new high-resolution data, combined with the data available in the literature, allow us to estimate the spectral index of the components and to identify one of the compact components as the VLBI core based on its flat spectrum between 5 and 22 GHz. Relative to this core component, the jet appears to be bidirectional. The proper-motion measurement of the component C2 and the estimate of the Doppler boosting factor suggest that the orientation of the jet is close to the line of sight. The projection effect of an intrinsically sharply bending jet within a few milliarcseconds from the core or the erratic change in the nozzle direction of the jet may account for the uncommon bidirectional structure of the jet in J1625+4134.

Broadband spectra of the flat-spectrum radio quasars (FSRQs) detected in the high-energy γ-ray band imply that there may be two types of such objects: those with steep γ-ray spectra, hereafter called MeV blazars, and those with flat γ-ray spectra, GeV blazars. We demonstrate that this difference can be explained in the context of the external radiation Compton (ERC) model using the same electron injection function. A satisfactory unification is reachable, provided that (1) spectra of GeV blazars are produced by internal shocks formed at the distances where cooling of relativistic electrons in a jet is dominated by Comptonization of broad emission lines, whereas spectra of MeV blazars are produced at the distances where cooling of relativistic electrons is dominated by Comptonization of near-IR radiation from hot dust, and (2) electrons are accelerated via a two-step process and their injection function takes the form of a double power law, with the break corresponding to the threshold energy for the diffusive shock acceleration. Direct predictions of our model are that, on average, variability timescales of the MeV blazars should be longer than variability timescales of the GeV blazars, and that both types of the blazar phenomenon can appear in the same object.

We present Very Long Baseline Array images of 42 γ-ray bright blazars, including 36 with polarization vectors, obtained during the course of a multiepoch monitoring program. Each object was observed at either 43 or 22 GHz, with some objects observed at both frequencies and/or at 15 or 8.4 GHz. The morphologies are varied, with some of the blazars displaying long, thin jets, others short, broad jets, and still others containing cores with only very weak features that are probably knots in faint jets. The polarization of the cores ranges from less than 1% to 8.6%, with electric vector position angles (EVPAs) that are split between nearly parallel and nearly transverse to the jet axis. The polarization of knots in the jets covers a much broader range, from less than 2% to tens of percent. The EVPA of the brightest compact feature in each jet ranges from 0° to 80° from the jet position angle, with roughly half measuring less than 20°. The distribution is consistent with intrinsically oblique magnetic fields whose observed directions are altered by relativistic aberration.

We present observations of the UV absorption lines in the Seyfert 1 galaxy NGC 3516, obtained at a resolution of λ/Δλ ≈ 40,000 with the Space Telescope Imaging Spectrograph (STIS) on 2000 October 1. The UV continuum was ~4 times lower than that observed during 1995 with the Goddard High Resolution Spectrograph (GHRS), and the X-ray flux from a contemporaneous Chandra X-Ray Observatory observation was a factor of ~8 below that observed with ASCA. The STIS spectra show kinematic components of absorption in Lyα, C IV, and N V at radial velocities of -376, -183, and -36 km s^-1 (components 1, 2, and 3+4, respectively), which were detected in the earlier GHRS spectra; the last of these is a blend of two GHRS components that have increased greatly in column density. Four additional absorption components have appeared in the STIS spectra at radial velocities of -692, -837, -994, and -1372 km s^-1 (components 5-8); these may also have been present in earlier low-flux states observed by the International Ultraviolet Explorer. Based on photoionization models, we suggest that the components are arranged in increasing radial distance in the order 3+4, 2, 1, followed by components 5-8. We have achieved an acceptable fit to the X-ray data using the combined X-ray opacity of the UV components 1, 2, and 3+4. By increasing the UV and X-ray fluxes of these models to match the previous high states, we are able to match the GHRS C IV column densities, the absence of detectable C IV absorption in components 5-8, and the 1994 ASCA spectrum. We conclude that variability of the UV and X-ray absorption in NGC 3516 is primarily due to changes in the ionizing flux.

We describe results from our initial study of the diffuse X-ray emission structures in NGC 5128 (Centaurus A) using the Chandra X-ray Observatory observations. The high angular resolution Chandra images reveal multiscale X-ray structures with unprecedented detail and clarity. The large-scale structures suggest complex symmetry, including a component possibly associated with the inner radio lobes, and a separate component with an orthogonal symmetry that may be associated with the galaxy as a whole. We detect arclike soft X-ray structures, extending to ~8 kpc in the direction perpendicular to the jet. These arcs appear to be tracing an ellipse, or a ring seen in projection, and are enclosed between the dust and H I-emitting gas in the central region of the galaxy and the optical and H I shell fragments at a distance of about 8-10 kpc. The diffuse X-ray and the optical emission in the arcs could originate in a region of interaction (possibly a shock) between the infalling material from the outer regions of the galaxy and the cool dust and H I-emitting material in the center or from an equatorial outflow resulting from an outburst of nuclear activity ~10⁷ yr ago.

We present a measurement of the rate of distant Type Ia supernovae derived using four large subsets of data from the Supernova Cosmology Project. Within this fiducial sample, which surveyed about 12 deg², 38 supernovae were detected at redshifts 0.25-0.85. In a spatially flat cosmological model consistent with the results obtained by the Supernova Cosmology Project, we derive a rest-frame Type Ia supernova rate at a mean redshift z ≃ 0.55 of 1.53 × 10^-4h³ Mpc^-3 yr^-1 or 0.58 h² SNu (1 SNu = 1 supernova per century per 10¹⁰L_B☉), where the first uncertainty is statistical and the second includes systematic effects. The dependence of the rate on the assumed cosmological parameters is studied and the redshift dependence of the rate per unit comoving volume is contrasted with local estimates in the context of possible cosmic star formation histories and progenitor models.

We present imaging and spectroscopic observations of the gravitationally lensed arcs in the field of RX J1347.5-1145, the most X-ray luminous galaxy cluster known. Based on the detection of the [O II] λ3727 emission line, we confirm that the redshift of one of the arcs is z = 0.806. Its color and [O II] line strength are consistent with those of distant, actively star-forming galaxies. In a second arc, we tentatively identify a pair of absorption lines superposed on a red continuum; the lines are consistent with Ca II λ3933 (K) and Ca II λ3968 (H) at z = 0.785. We detected a faint blue continuum in two additional arcs, but no spectral line features could be measured. We establish lower limits to their redshifts based on the absence of [O II] emission, which we argue should be present and detectable in these objects. Redshifts are also given for a number of galaxies in the field of the cluster.

A 40 ks Chandra ACIS-S observation of the dynamically young cluster A1367 yields new insights on X-ray emission from cluster member galaxies. We detect 59 pointlike sources in the ACIS field, of which eight are identified with known cluster member galaxies. Thus, in total 10 member galaxies are detected in X-rays when three galaxies discussed in Paper I (NGC 3860 is discussed in both papers) are included. The superior spatial resolution and good spectroscopy capability of Chandra allow us to constrain the emission nature of these galaxies. Central nuclei, thermal halos, and stellar components are revealed in their spectra. Two new low-luminosity active galactic nuclei (LLAGNs) are found, including an absorbed one (NGC 3861). Besides these two for sure, two new LLAGN candidates are also found. This discovery makes the LLAGN/AGN content in this part of A1367 very high (≳12%). Thermal halos with temperatures around 0.5-0.8 keV are revealed in the spectra of NGC 3842 and NGC 3837, which suggests that galactic coronae can survive in clusters and that heat conduction must be suppressed. The X-ray spectrum of NGC 3862 (3C 264) resembles a BL Lac object with a photon index of ~2.5. We also present an analysis of other point sources in the field and discuss the apparent source excess (~2.5 σ) in the central field.

Rest-frame far-ultraviolet (FUV) luminosities form the "backbone" of our understanding of star formation (SF) at all cosmic epochs. These luminosities are typically corrected for dust by assuming that the tight relationship between the UV spectral slopes (β) and the FUV attenuations (A_FUV) of starburst galaxies applies to all star-forming galaxies. Data from seven independent UV experiments demonstrate that quiescent, "normal" star-forming galaxies deviate substantially from the starburst galaxy β-A_FUV correlation in the sense that normal galaxies are redder than starbursts. Spatially resolved data for the Large Magellanic Cloud suggest that dust geometry and properties, coupled with a small contribution from older stellar populations, cause deviations from the starburst galaxy β-A_FUV correlation. Folding in data for starbursts and ultraluminous infrared galaxies, it is clear that neither rest-frame UV/optical colors nor UV/Hα colors help significantly in constraining the UV attenuation. These results argue that the estimation of SF rates from rest-frame UV and optical data alone is subject to large (factors of at least a few) systematic uncertainties because of dust, which cannot be reliably corrected for using only UV/optical diagnostics.

We present radio observations of the afterglow of the bright γ-ray burst GRB 980329 made between 1 month and several years after the burst, a reanalysis of previously published submillimeter data, and late-time optical and near-infrared (NIR) observations of the host galaxy. From the absence of a spectral break in the optical/NIR colors of the host galaxy, we exclude the earlier suggestion that GRB 980329 lies at a redshift of z ≳ 5. We combine our data with the numerous multiwavelength observations of the early afterglow, fit a comprehensive afterglow model to the entire broadband data set, and derive fundamental physical parameters of the blast wave and its host environment. Models for which the ejecta expand isotropically require both a high circumburst density and extreme radiative losses from the shock. No low-density model (n ≪ 10 cm^-3) fits the data. A burst with a total energy of ~10⁵¹ ergs, with the ejecta narrowly collimated to an opening angle of a few degrees, driven into a surrounding medium with density of ~20 cm^-3, provides a satisfactory fit to the light curves over a range of redshifts.

We present results from a detailed spectrophotometric analysis of the blue compact dwarf galaxy (BCD) Mrk 370, based on deep UBVRI broadband and Hα narrowband observations, and on long-slit and two-dimensional spectroscopy of its brightest knots. The spectroscopic data are used to derive the internal extinction and to compute metallicities, electronic density, and temperature in the knots. By subtracting the contribution of the underlying older stellar population, modeled by an exponential function, removing the contribution from emission lines, and correcting for extinction, we can measure the true colors of the young star-forming knots. We show that the colors obtained this way differ significantly from those derived without the above corrections and lead to different estimates of the ages and star-forming history of the knots. Using predictions of evolutionary synthesis models, we estimate the ages of both the starburst regions and the underlying stellar component. We found that we can reproduce the colors of all the knots with an instantaneous burst of star formation and the Salpeter initial mass function (IMF) with an upper mass limit of 100 M_☉. The resulting ages range between 3 and 6 Myr. The colors of the low surface brightness component are consistent with ages larger than 5 Gyr. The kinematic results suggest ordered motion around the major axis of the galaxy.

We present an analysis of a Chandra ACIS-S observation of the elliptical galaxy NGC 720, to verify the existence of a dark matter halo and to measure its ellipticity. The ACIS-S3 image reveals over 60 point sources distributed throughout the field, most of which were undetected and therefore unaccounted for in previous X-ray studies. For semimajor axes a ≲ 150'' (18.2 h kpc), the ellipticity of the diffuse X-ray emission is consistent with a constant value, _X ≈ 0.15, which is systematically less than the values of 0.2-0.3 obtained from previous ROSAT PSPC and HRI observations because of the unresolved point sources contaminating the ROSAT values. The Chandra data confirm the magnitude of the ~20° position angle (P.A.) twist discovered by ROSAT over this region. However, the twist in the Chandra data is more gradual and occurs at smaller a, also because of the point sources contaminating the ROSAT values. For a ≳ 150'' out to a = 185'' (22.4 h kpc), which is near the edge of the S3 CCD, _X and P.A. diverge from their values at smaller a. Possible origins of this behavior at the largest a are discussed. Overall, the ellipticities and P.A. twist for a ≲ 150'' can be explained by the triaxial mass model of NGC 720 published by Romanowsky & Kochanek (which could not produce the abrupt P.A. twist in the ROSAT HRI data). Since the optical image displays no substantial isophote twisting, the X-ray P.A. twist requires a massive dark matter halo if the hot gas is in hydrostatic equilibrium. Furthermore, the values of _X obtained by Chandra are too large to be explained if the gravitating mass follows the optical light (M ∝ L_*), irrespective of the P.A. twist. The M ∝ L_* hypothesis is inconsistent with the Chandra ellipticities at the 96% confidence level, assuming oblate symmetry, and at the 98% confidence level for prolate symmetry. Thus, both the P.A. twist and the ellipticities of the Chandra image imply the existence of dark matter, independent of the temperature profile of the gas. This geometric evidence for dark matter cannot be explained by alternative gravity theories, such as the modification of Newtonian dynamics (MOND). To constrain the ellipticity of the dark matter halo, we considered both oblate and prolate spheroidal mass models to bracket the full range of (projected) ellipticities of a triaxial ellipsoid. The dark matter density model, ρ ∝ (a + a²)^-1, provides the best fit to the data and gives ellipticities and 1 σ errors of = 0.37 ± 0.03 for oblate and = 0.36 ± 0.02 for prolate models. Navarro-Frenk-White (NFW) and Hernquist models give similar ellipticities for the dark matter. These moderate ellipticities for the dark halo are inconsistent with both the nearly spherical halos predicted if the dark matter is self-interacting and the highly flattened halos predicted if the dark matter is cold molecular gas. These ellipticities may also be too large to be explained by warm dark matter, but they are consistent with galaxy-sized halos formed in the currently popular ΛCDM paradigm.

High-resolution, two-dimensional hydrodynamical simulations with a large dynamic range are performed to study the turbulent nature of the interstellar medium (ISM) in galactic disks. The simulations are global, where the self-gravity of the ISM, realistic radiative cooling, and galactic rotation are taken into account. In the analysis undertaken here, feedback processes from the stellar energy source are omitted. We find that the velocity field of the disk in a nonlinear phase shows a steady power-law energy spectrum over 3 orders of magnitude in wavenumber. This implies that the random velocity field can be modeled as fully developed, stationary turbulence. Gravitational and thermal instabilities under the influence of galactic rotation contribute to the formation of the turbulent velocity field. The Toomre effective Q-value, in the nonlinear phase, covers a wide range, and gravitationally stable and unstable regions are distributed patchily in the disk. These results suggest that large-scale galactic rotation coupled with the self-gravity of the gas can be the ultimate energy sources that maintain the turbulence in the local ISM. Our models of turbulent rotating disks are consistent with the velocity dispersion of an extended H I disk in the dwarf galaxy NGC 2915, where there is no prominent active star formation. Numerical simulations show that the stellar bar in NGC 2915 enhances the velocity dispersion, and it also drives spiral arms, as observed in the H I disk.

The origin of the empirical laws of galactic scale star formation is considered in view of the self-similar nature of interstellar gas and the observation that most local clusters are triggered by specific high-pressure events. The empirical laws suggest that galactic scale gravity is involved in the first stages of star formation, but they do not identify the actual triggering mechanisms for clusters in the final stages. Many triggering processes satisfy the empirical laws, including turbulence compression and expanding shell collapse. The self-similar nature of the gas and associated young stars suggests that turbulence is more directly involved, but the energy source for this turbulence is not clear, and the small-scale morphology of gas around most embedded clusters does not look like a random turbulent flow. Most clusters appear to be triggered by other nearby stars. Such a prominent local influence makes it difficult to understand the universality of the Kennicutt and Schmidt laws on galactic scales. A unified view of multiscale star formation avoids most of these problems. The Toomre and Kennicutt surface density thresholds, along with the large-scale gas and star formation morphology, imply that ambient self-gravity produces spiral arms and giant cloud complexes and at the same time drives much of the turbulence that leads to self-similar structures. Localized energy input from existing clusters and field supernovae drives turbulence and cloud formation too, while triggering clusters directly in preexisting clouds. The hierarchical structure in the gas made by turbulence ensures that the triggering time scales with size, thereby giving the Schmidt law over a wide range of scales and the size-duration correlation for young star fields. Reanalysis of the Schmidt law from a local point of view suggests that the efficiency of star formation is determined by the fraction of the gas above a critical density of around 10⁵m(H₂) cm^-3. Such high densities probably result from turbulence compression in a self-gravitating gas, in which case their mass fraction can be estimated from the density distribution function that results from turbulence. For Wada & Norman's lognormal function that arises in whole-galaxy simulations, the theoretically predicted mass fraction of star-forming material is the same as that observed directly from the galactic Schmidt law and is ~10^-4. The unified view explains how independent star formation processes can combine into the empirical laws while preserving the fractal nature of interstellar gas and the pressurized, wind-swept appearance of most small-scale clouds. Likely variations in the relative roles of these processes from region to region should not affect the large-scale average star formation rate. Self-regulation by spiral instabilities and star formation ensures that most regions are in a marginally stable state in which turbulence limits the mass available for star formation and the overall rate is independent of the nature of the energy sources. In this sense, star formation is saturated to its largest possible value given the fractal nature of the interstellar medium.

We report the first ensemble results from the Far Ultraviolet Spectroscopic Explorer survey of molecular hydrogen in lines of sight with A_V ≳ 1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H₂, and thus determining column densities for J = 0 and J = 1, which contain ≳99% of the total H₂. From these column densities and ancillary data we have derived the total H₂ column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of ~10²¹ cm^-2, measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H₂-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H₂ and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN abundance, the steepness of the far-UV extinction rise, and the width of the 2175 Å bump. Despite the relatively large molecular fractions, we do not see the values greater than 0.8 expected in translucent clouds. With the exception of a few lines of sight, we see little evidence for the presence of individual translucent clouds in our sample. We conclude that most of the lines of sight are actually composed of two or more diffuse clouds similar to those found toward targets like ζ Oph. We suggest a modification in terminology to distinguish between a "translucent line of sight" and a "translucent cloud."

We conducted a deep JHK_s-band imaging survey of the M17 region, using a near-infrared camera, the Simultaneous 3-color InfraRed Imager for Unbiased Survey (SIRIUS), mounted on the InfraRed Survey Facility (IRSF) 1.4 m telescope at the South African Astronomical Observatory. This survey covers an area of ~200 arcmin² with 10 σ limiting magnitudes of J ~ 18.7, H ~ 18.2, and K_s ~ 17.5. The near-infrared (NIR) images reveal an unprecedented view of the region. The NIR nebulae are highly structured, with two nebular bars corresponding to, but a little larger than, the H II region defined by Felli, Massi, & Churchwell, constructing a conical shape. Fine structures are found all over the nebular area. The central region contains a congregation of intermediate- to high-mass stars. From the slope of the K_s-band luminosity function and the frequency of young stellar objects (YSOs) we infer that the central cluster has an age less than 3 Myr. The central OB cluster provides tremendous energy that heats and ionizes its surrounding materials, triggering the star formation of second-generation in the nebular bars. The second generation stars are so numerous that could they affect the star formation efficiency in the whole region. To the southwest of the central cluster and the nebular bars, where a giant molecular cloud core is located, a large number of red stars are detected. We argue that these red stars are most probably associated YSOs with intrinsic color excesses, not normal field stars reddened by the molecular cloud in front of them. Being located beyond the photodissociation region, the star-forming process in the molecular region could be independent of the impact of the central cluster.

CH₃CN (J = 6-5) was observed with a resolution of 2'' toward W75N using the BIMA interferometer. Two continuum sources were detected at 3 mm and designated MM1 and MM2 in previous studies. Alignment of the two continuum sources with the outflow axis from MM1 suggests that these continuum sources may be the result of the outflow interacting with the interstellar medium. MM1 is coincident with compact CH₃CN emission. CH₃CN was not detected toward MM2. The distribution of optical depth (τ_L) is derived. An excitation analysis was not done because of large-line optical depths.

The reaction of H atoms with the solid CO thin film was reinvestigated in the temperature region of 10-25 K. H₂CO was found to be only the reaction product, and no other products such as CH₃OH were detected. This indicates that the tunneling reactions of H with H₂CO to form CH₃OH are even slower than the slow reaction of H with CO to form H₂CO. The CH₃OH found in the envelopes of the dark clouds may have other sources for their production in addition to reactions 2H + H₂CO → CH₃OH, e.g., reaction of O(¹D) with CH₄ trapped on the dust grains to form CH₃OH. The yield of H₂CO from the reaction H with solid CO showed a steep increase with a decrease of temperature from 25 to 10 K. This indicates that the dark clouds whose temperature is kept at as low as 10 K are the favorable place for the chemical evolution via tunneling reactions. The erosion of the solid CO film was not observed with the spray of the H atoms over the CO solid film in the temperature range of 10-25 K. This finding suggests that the contribution of the highly exothermic reaction 2H + H₂ to desorption of the grain mantle may not be as large as thought before.

The high-resolution (R = 25,000) infrared M-band spectrum of the massive protostar NGC 7538 IRS 9 shows a narrow absorption feature at 4.779 μm (2092.3 cm^-1) that we attribute to the vibrational stretching mode of the ¹³CO isotope in pure CO icy grain mantles. This is the first detection of ¹³CO in icy grain mantles in the interstellar medium. The ¹³CO band is a factor of 2.3 narrower than the apolar component of the ¹²CO band. With this in mind, we discuss the mechanisms that broaden solid-state absorption bands. It is shown that ellipsoidally shaped pure CO grains fit the bands of both isotopes at the same time. Slightly worse but still reasonable fits are also obtained by CO embedded in N₂-rich ices and thermally processed O₂-rich ices. In addition, we report new insights into the nature and evolution of interstellar CO ices by comparing the very high resolution multicomponent solid ¹²CO spectrum of NGC 7538 IRS 9 with that of the previously studied low-mass source L1489 IRS. The narrow absorption of apolar CO ices is present in both spectra but much stronger in NGC 7538 IRS 9. It is superposed on a smooth broad absorption feature well fitted by a combination of CO₂ and H₂O-rich laboratory CO ices. The abundances of the latter two ices, scaled to the total H₂O ice column, are the same in both sources. We thus suggest that thermal processing manifests itself as evaporation of apolar ices only and not the formation of CO₂ or polar ices. Finally, the decomposition of the ¹²CO band is used to derive the ¹²CO/¹³CO abundance ratio in apolar ices. A ratio of ¹²CO/¹³CO = 71 ± 15 (3 σ) is deduced, in good agreement with gas-phase CO studies (~77) and the solid ¹²CO₂/¹³CO₂ ratio of 80 ± 11 found in the same line of sight. The implications for the chemical path along which CO₂ is formed are discussed.

We present a detailed comparison of an extended set of elemental abundances observed in some of the most metal poor stars presently known and the ejecta produced by a generation of primordial core collapse supernovae. At variance with most of the analysis performed up to now (in which mainly the global trends with the overall metallicity are discussed), we think that it is important (and complementary to the other approach) to fit the (available) chemical composition of specific stars. In particular, we first discuss the differences among the five stars that form our initial database and define a "template" ultra-metal-poor star that is then compared to the theoretical predictions. Our main findings are the following: (1) The fit to [Si/Mg] and [Ca/Mg] of these very metal-poor stars seems to favor the presence of a rather large C abundance at the end of the central He burning; in a classical scenario in which the border of the convective core is strictly determined by the Schwarzschild criterion, such a large C abundance would imply a rather low ¹²C(α,γ)¹⁶O reaction rate. (2) A low C abundance left by the central He burning would imply a low [Al/Mg] (<-1.2 dex) independently on the initial mass of the exploding star, while a rather large C abundance would produce such a low [Al/Mg] only for the most massive stellar model. (3) At variance with current beliefs that it is difficult to interpret the observed overabundance of [Co/Fe], we find that a mildly large C abundance in the He exhausted core (well within the present range of uncertainty) easily and naturally allows a very good fit to [Co/Fe]. (4) Our yields allow a reasonable fit to eight of the 11 available elemental abundances. (5) Within the present grid of models it is not possible to find a good match of the remaining three elements, Ti, Cr, and Ni (even for an arbitrary choice of the mass cut). (6) The adoption of other yields available in the literature does not improve the fit. (7) Since no mass in our grid provides a satisfactory fit to these three elements, even an arbitrary choice of the initial mass function would not improve their fit.

Radiatively inefficient accretion flows onto black holes are unstable due to both an outwardly decreasing entropy ("convection") and an outwardly decreasing rotation rate (the "magnetorotational instability" [MRI]). Using a linear MHD stability analysis, we show that long-wavelength modes with λ/H ≫ β^-1/2 are primarily destabilized by the entropy gradient and that such "convective" modes transport angular momentum inward (λ is the wavelength of the mode, H is the scale height of the disk, and β is the ratio of the gas pressure to the magnetic pressure). Moreover, the stability criteria for the convective modes are the standard Høiland criteria of hydrodynamics. By contrast, shorter wavelength modes with λ/H ~ β^-1/2 are primarily destabilized by magnetic tension and differential rotation. These "MRI" modes transport angular momentum outward. The convection-dominated accretion flow (CDAF) model, which has been proposed for radiatively inefficient accretion onto a black hole, posits that inward angular momentum transport and outward energy transport by long-wavelength convective fluctuations are crucial for determining the structure of the accretion flow. Our analysis suggests that the CDAF model is applicable to an MHD accretion flow provided that the magnetic field saturates at a value sufficiently below equipartition (β ≫ 1), so that long-wavelength convective fluctuations with λ/H ≫ β^-1/2 can fit inside the accretion disk. Numerical MHD simulations are required to determine whether such a subequipartition field is in fact obtained.

Within the framework of the internal shock model, we study the luminosity and variability in gamma-ray bursts from collimated fireballs. In particular, we pay attention to the role of the e^± pair photosphere produced by the internal shock synchrotron photons. It is shown that the observed Cepheid-like relationship between the luminosity and variability can be interpreted as a correlation between the opening angle of the fireball jet and the mass included at the explosion with a standard energy output. We also show that such a correlation can be a natural consequence of the collapsar model. Narrow jets, in which the typical Lorentz factors are higher than in wide jets, can produce more variable temporal profiles because of smaller angular spreading timescales at the photosphere radius. Using a multiple-shell model, we numerically calculate the temporal profiles of gamma-ray bursts and show that our simulations reproduce the observed correlation.

It is often proposed that a massive torus with approximately solar mass surrounding a stellar-mass black hole could be a central engine of gamma-ray bursts. We study the properties of such massive accretion tori (or disks) based on the α viscosity model. For surface density exceeding about 10²⁰ g cm^-2, which is realized when ~1 M_☉ of material is contained within a disk of size ~5 × 10⁶ cm, we find that (1) the luminosity of photons is practically zero because of significant photon trapping, (2) neutrino cooling dominates over advective cooling, (3) the pressure of degenerate electrons dominates over the pressure of gas and photons, and (4) the magnetic field strength exceeds the critical value of about 4 × 10¹³ G, even if we take 0.1% of the equipartition value. The possible observable quantum electrodynamical (QED) effects arising from supercritical fields are discussed. Most interestingly, photon splitting may occur, producing a significant number of photons of energies below ~511 keV, thereby possibly suppressing e^± pair creation.

In this paper we return to the old problem of conal component pair widths and profile dimensions. Observationally, we consider a set of 10 pulsars with prominent conal component pairs for which well-measured profiles exist over the largest frequency range now possible. Apart from some tendency to narrow at high frequency, the conal components exhibit almost constant widths. We use all three profile measures, the component separation as well as the outside half-power and 10% widths, to determine conal beam radii, which are the focus of our subsequent analysis. These radii at different frequencies are well fitted by a relationship introduced by Thorsett, but the resulting parameters are highly correlated. Three different types of behavior are found: one group of stars exhibits a continuous variation of beam radius that can be extrapolated down to the stellar surface along the "last open field lines," a second group exhibits beam radii that asymptotically approach a minimum high-frequency value that is 3-5 times larger, and a third set shows almost no spectral change in beam radius at all. The first two behaviors are associated with outer-cone component pairs, whereas the constant separation appears to reflect inner-cone emission. The first group, remarkably, can be fitted by a Thorsett relation in which the constant term is constrained to be the field tangent direction at the edge of the polar cap ρ_pc, but the others cannot. The first group can also be fitted well using an index of - , but the second group cannot. We first compute heights from the conal beam radii, assuming dipolar fields and emission along the last open field lines, which we find are again well fitted by a suitable Thorsett relation. Here we find that the first group can be fitted using a constant term h_pc of 10 km and also that the first two groups are remarkably well fitted by an index of - . We then argue that physical emission heights can be estimated using the component separation along an interior annulus of field lines having their "feet" about halfway out on the polar cap—such values agree well with most existing height values based on physical criteria. Therefore, we find that "radius-to-frequency" mapping is associated with outer-cone component pairs. The near constant behavior of inner cones is thus arresting. We explore possible interrelationships between the spectral behavior of the component and profile widths produced by both the field line flaring and the changing sight line geometry. We also attempt to understand the physical implications of the parameter values resulting from the Thorsett relation fits.

We report the discovery with the Proportional Counter Array on board the Rossi X-Ray Timing Explorer of highly coherent 582 Hz pulsations during the 2001 February 22 (UT) "superburst" from 4U 1636-53. The pulsations are detected during an ≈800 s interval spanning the flux maximum of the burst. Within this interval the barycentric oscillation frequency increases in a monotonic fashion from 581.89 to 581.93 Hz. The predicted orbital motion of the neutron star during this interval is consistent with such an increase as long as optical maximum corresponds roughly with superior conjunction of V801 Arae, the optical companion to the neutron star in 4U 1636-53. We show that a range of circular orbits with velocity 90 km s^-1 < v_ns sin i < 175 km s^-1 and fractional phase 0.336 > ϕ₀ > 0.277 for the neutron star can provide an excellent description of the frequency and phase evolution. The brevity of the observed pulse train with respect to the 3.8 hr orbital period unfortunately does not allow for more precise constraints. The average pulse profile is sinusoidal, and the time-averaged pulsation amplitude, as inferred from the half amplitude of the sinusoid, is 1%, smaller than typical for burst oscillations observed in normal thermonuclear bursts. We do not detect any higher harmonics nor the putative subharmonic near 290 Hz. The 90% upper limits on signal amplitude at the subharmonic and first harmonic are 0.1% and 0.06%, respectively. The highly coherent pulsation, with a Q ≡ ν₀/Δν > 4.5 × 10⁵, provides compelling evidence for a rapidly rotating neutron star in 4U 1636-53 and further supports the connection of burst oscillation frequencies with the spin frequencies of neutron stars. Our results provide further evidence that some millisecond pulsars are spun up via accretion in low-mass X-ray binaries. We also discuss the implications of our orbital velocity constraint for the masses of the components of 4U 1636-53.

We obtained four Chandra/ACIS-S observations beginning 2 weeks after the end of the 2000 November outburst of the neutron star (NS) transient Aql X-1. Over the 5 month span in quiescence, the X-ray spectra are consistent with thermal emission from a NS with a pure hydrogen photosphere and R_∞ = 15.9 (d/5 kpc) km at the optically implied X-ray column density. We also detect a hard power-law tail during two of the four observations. The intensity of Aql X-1 first decreased by 50% ± 4% over 3 months, then increased by 35% ± 5% in 1 month, and then remained constant (<6% change) over the last month. These variations in the first two observations cannot be explained by a change in either the power-law spectral component or the X-ray column density. Presuming a pure hydrogen atmosphere and that R_∞ is not variable, the long-term changes can only be explained by variations in the NS effective temperature, from kT_eff,∞ = 130 eV, down to 113 eV, and finally increasing to 118 eV for the final two observations. During one of these observations, we observe two phenomena that were previously suggested as indicators of quiescent accretion onto the NS: short-timescale (<10⁴ s) variability (at 32% rms) and a possible absorption feature near 0.5 keV. The possible absorption feature can potentially be explained as being due to a time-variable response in the ACIS detector. Even so, such a feature has not been detected previously from a NS and, if confirmed and identified, can be exploited for simultaneous measurements of the photospheric redshift and NS radius.

On 2000 May 5, we began a large, multiwavelength campaign to study the intermediate polar (IP) EX Hydrae. The simultaneous observations from six satellites and four telescopes were centered around a 1 million second observation with Extreme Ultraviolet Explorer (EUVE). Although EX Hya has been studied previously with EUVE, our higher signal-to-noise ratio observations present new results and challenge the current IP models. Previously unseen dips in the light curve are reminiscent of the stream dips seen in polar light curves. Also of interest is the temporal extent of the bulge dip, approximately 0.5 in phase, implying that the bulge extends over half of the accretion disk. We propose that the magnetic field in EX Hya is strong enough (a few MG) to begin pulling material directly from the outer edge of the disk, thereby forming a large accretion curtain, which would produce a very broad bulge dip. This would also result in magnetically controlled accretion streams originating from the outer edge of the disk. We also present a period analysis of the photometric data, which shows numerous beat frequencies with strong power and also intermittent and wandering frequencies, an indication that physical conditions within EX Hya changed over the course of the observation. Iron spectral line ratios give a temperature of log T = 6.5-6.9 K for all spin phases and a poorly constrained density of n_e = 10¹⁰-10¹¹ cm^-3 for the emitting plasma. This paper is the first in a series detailing our results from this multiwavelength observational campaign.

The RS CVn-type binary σ Geminorum was observed during a large, long-duration flare simultaneously with XMM-Newton and the Very Large Array. The light curves show a characteristic time dependence that is compatible with the Neupert effect observed in solar flares: the time derivative of the X-ray light curve resembles the radio light curve. This observation can be interpreted in terms of a standard flare scenario in which accelerated coronal electrons reach the chromosphere, where they heat the cool plasma and induce chromospheric evaporation. Such a scenario can hold only if the amount of energy in the fast electrons is sufficient to explain the X-ray radiative losses. We present a plausibility analysis that supports the chromospheric evaporation model.

New visual spectrophotometric observations of non-supergiant solar neighborhood stars are combined with IUE Newly Extracted Spectra (INES) energy distributions in order to derive their overall metallicities, [M/H]. This fundamental parameter, together with effective temperature and apparent angular diameter, is obtained by applying the flux-fitting method while surface gravity is derived from the comparison with evolutionary tracks in the theoretical H-R diagram. Trigonometric parallaxes for the stars of the sample are taken from the Hipparcos Catalogue. The quality of the flux calibration is discussed by analyzing a test sample via comparison with external photometry. The validity of the method in providing accurate metallicities is tested on a selected sample of G-type stars with well-determined atmospheric parameters from recent high-resolution spectral analysis. The extension of the overall procedure to the determination of the chemical composition of all the INES non-supergiant G-type stars with accurate parallaxes is planned in order to investigate their atmospheric temperature structure.

Wind models of very massive stars with metallicities in a range from 10^-4 to 1.0 solar are calculated using a new treatment of radiation-driven winds with depth-dependent radiative force multipliers and a comprehensive list of more than two million spectral lines in non-LTE (NLTE). The models are tested by a comparison with observed stellar wind properties of O stars in the Galaxy and the SMC. Satisfying agreement is found. The calculations yield mass-loss rates, wind velocities, wind momenta, and wind energies as functions of metallicity and can be used to discuss the influence of stellar winds on the evolution of very massive stars in the early universe and on the interstellar medium in the early phases of galaxy formation. It is shown that the normal scaling laws, which predict stellar mass-loss rates and wind momenta to decrease as a power law with metal abundance, break down at a certain threshold. Analytical fit formulae for mass-loss rates are provided as a function of stellar parameters and metallicity. Ionizing fluxes of hot stars depend crucially on the strengths of their stellar winds, which modify the absorption edges of hydrogen and helium (neutral and ionized) and the line blocking in the far-UV. The new wind models are therefore also applied to calculate ionizing fluxes and observable spectra of very massive stars as a function of metallicity using the new hydrodynamic, NLTE line-blanketed flux constant model atmosphere code developed by Pauldrach and coworkers. Numbers of ionizing photons for the crucial ionization stages are given. For a fixed effective temperature, the He II ionizing emergent flux depends very strongly on metallicity but also on stellar luminosity. A strong dependence on metallicity is also found for the C III, Ne II, and O II ionizing photons, whereas the H I and He I ionizing flux is almost independent of metallicity. We also calculate UV spectra for all the models and discuss the behavior of significant line features as a function of metallicity.

We have reevaluated the orbital elements for each pair of the quadruple (W-R+O) + (O+O) stellar system GP Cep and propose new spectral types WN6o/WCE + O3-6, B0: I + B1: V-III. It is shown that there is only one Wolf-Rayet (W-R) star in GP Cep, contrary to a previous claim. A rate of change = 1.3 ± 0.2 s yr^-1 is determined for the W-R+O pair, which leads to a new period of 6.6887 days and to a W-R mass-loss rate of (0.8-3.0) × 10^-5M_☉ yr^-1. Masses for this pair are estimated to be M_W-R ≳ 6 M_☉ and M_O ≳ 21 M_☉. The effects of wind-wind collision in the W-R+O pair are studied. It is shown that even after allowing for dilution by the OB components of the quadruple system, these effects are not as strong as in the binary V444 Cygni (WN5+O6, P = 4.212 days). In GP Cep, the phase-dependent, relatively weak excess emission does not originate in the arms of the bow shock cone. Rather, it emerges from the extra heated portion of the W-R wind facing the hot O companion. The trailing bow shock arm is clearly seen, however, as an enhanced He I absorption component near quadrature at phase ~0.73. An anomalous blueshifted He I absorption is present at phase ~0.9, as is also seen in V444 Cyg, in the WC8+O9 I/O8 III binary γ Velorum and in the LBV-cotype binary R81 (B2.5 Iab:e). A 3.5 day orbit for the eclipsing B star pair is confirmed.

In our previous paper, we presented a theory to explain the observed universal correlation between the emission measure (EM = n²V) and temperature (T) for solar/stellar flares on the basis of the magnetic reconnection model with heat conduction and chromospheric evaporation. Here n is the electron density and V is the volume. By extending our theory to general situations, we examined the EM-T diagram in detail and found the following properties: (1) The universal correlation sequence ("main-sequence flares") with EM ∝ T^17/2 corresponds to the case of constant heating flux or, equivalently, the case of constant magnetic field strength in the reconnection model. (2) The EM-T diagram has a forbidden region, in which gas pressure of flares exceeds magnetic pressure. (3) There is a coronal branch with EM ∝ T^15/2 for T < 10⁷ K and EM ∝ T^13/2 for T > 10⁷ K. This branch is situated on the left side of the main-sequence flares in the EM-T diagram. (4) There is another forbidden region determined by the length of flare loop; the lower limit of the flare loop is 10⁷ cm. Small flares near this limit correspond to nanoflares observed by the Solar and Heliospheric Observatory EUV Imaging Telescope. (5) We can plot the flare evolution track on the EM-T diagram. A flare evolves from the coronal branch to main-sequence flares, then returns to the coronal branch eventually. These properties of the EM-T diagram are similar to those of the H-R diagram for stars, and thus we propose that the EM-T diagram is quite useful for estimating the physical quantities (loop length, heating flux, magnetic field strength, total energy, and so on) of flares and coronae when there are no spatially resolved imaging observations.

We report on the results of an I-band photometric variability survey of 18 L dwarfs. We find that seven exhibit statistically significant variations above the 95.4% confidence level with rms scatter (including photometric errors) between 0.010 and 0.083 mag. Another five targets have variability probabilities ≈80%, suggesting that these are likely variable objects. Two objects display significant peaks in a CLEAN periodogram that are several times higher than the noise. The periods found for 2MASS 0746+20AB and 2MASS 1300+19 are longer than those periods likely from rotation velocity measurements, and they do not represent periodic behavior in the light curve that persists through the entire data set. These observations suggest that we are not observing the rotation modulation of a long-lived albedo feature. Instead, rapid evolution of atmospheric features is likely causing the nonperiodic variability. The remaining variable objects show no prominent features in their light curves, suggesting even more rapid evolution of atmospheric features. We argue against the existence of magnetic spots in these atmospheres and favor the idea that nonuniform condensate coverage is responsible for these variations. The magnetic Reynolds number in the atmospheres of L dwarfs is too small to support the formation of magnetic spots. In contrast, silicate and iron clouds are expected to form in the photospheres of L dwarfs. Inhomogeneities in such cloud decks and the evolution of the inhomogeneities can plausibly produce the observed photometric variations.

As part of the long-term monitoring of Mira variables at the Palomar Testbed Interferometer, we report high-resolution narrowband angular sizes of the oxygen-rich Mira S Lac and the carbon-rich Mira RZ Peg in the near-infrared. The data set spans three pulsation cycles for S Lac and two pulsation cycles for RZ Peg (a total of 1070 25 s observations) and represents the first study to correlate multiepoch narrowband interferometric data of Mira variables. When the calibrated visibility data are fitted using a uniform disk brightness model, differences are seen in their angular diameters as a function of wavelength within the K band (2.0-2.4 μm), the source of which is believed to be molecular absorptions in or above the photospheres of the two chemically different Miras. Using visible photometric data provided by the Association Francaise des Observateurs d'Etoiles Variables (AFOEV), the continuum minimum size of RZ Peg lags this by 0.28 ± 0.02 in pulsation, similar to the phase lag found in Correlation Radial Velocities (CORAVEL) data. However, for S Lac, the continuum minimum size tracks the visual maximum brightness. Based on the mean of the continuum angular diameter cycloids, basic stellar parameters are computed for both RZ Peg and S Lac, with both showing maximum atmospheric extension with respect to the 2.0 and 2.4 μm diameters near phase 0.9. Using the mean value of the fitted cycloids, RZ Peg has a radius R_mean = 377 ± 111 R_☉ and a mean T_eff = 2706 ± 36 K; S Lac has a radius R_mean = 292 ± 73 R_☉ and a mean T_eff = 2605 ± 47 K. The dominant source of error in the radii is the large uncertainty in the distances to these two stars.

We compare listings of coronal mass ejections (CMEs) observed by LASCO on SOHO and solar microwave bursts (SMBs) recorded by the Radio Solar Telescope Network (RSTN) operated by the United States Air Force. These data sets are the product of stable and continuous observations of the whole Sun and provide suitable bases for robust statistical studies. In total, 3557 coronal ejections and nearly 1051 bursts above 50 sfu were observed from 1996 January through 2001 May. Correlated events are easily distinguished by time proximity. Correlations improve as CME launch heights are projected to the solar limb, when the rms scatter in CME-SMB delay was as little as 16 minutes, but because coronal disturbances are only visible when they emerge from behind occulting disks, timing associations depend on the assumed source and acceleration. The probability of correlation rises with burst flux, duration or temporal complexity, and ejection speed or width. For the 164 SMBs with intensities over 500 sfu, 70% ± 8% were associated with CMEs. For the 160 CMEs that were halo-like or have speeds over 1000 km s^-1 (characteristics that have been associated with geoeffective events), 60% ± 8% and 84% ± 10% were associated with SMBs, respectively.

The support of solar prominences is normally described in terms of a magnetic force on the prominence plasma that balances the solar gravitational force. Because the prominence plasma is only partially ionized, this support needs to be understood in terms of the frictional coupling between the neutral and ionized components of the prominence plasma, the efficacy of which depends directly on the ion density. More specifically, the frictional force is proportional to the relative flow of neutral and ion species, and for a plasma with a sufficiently small vertical ion column density, this flow must be relatively large to produce a frictional force that balances gravity. A large relative flow, of course, implies significant draining of neutral particles from the prominence. We evaluate the importance of this draining effect for a hydrogen-helium plasma and consider the variation of the draining with a variety of prominence parameters. Our calculations show that the loss timescale for hydrogen is much longer than that for helium, which for typical prominence parameters is about one day.

Motivated by recent Transition Region and Coronal Explorer (TRACE) observations of damped oscillations in coronal loops, we consider analytically the motion of an inhomogeneous coronal magnetic tube of radius a in a zero-β plasma. An initially perturbed tube may vibrate in its kink mode of oscillation, but those vibrations are damped. The damping is due to resonant absorption, acting in the inhomogeneous regions of the tube, which leads to a transfer of energy from the kink mode to Alfvén (azimuthal) oscillations within the inhomogeneous layer. We determine explicitly the decrement γ (decay time γ^-1) for a coronal flux tube whose plasma density varies only in a thin layer of thickness ℓ on the tube boundary. The effect of viscosity is also considered. We show that, in general, the problem involves two distinct timescales, γ^-1 and ωR^1/3, where R is the Reynolds number and ω_k is the frequency of the kink mode. Under coronal conditions (when γ^-1 ≪ ωR^1/3), the characteristic damping time of global oscillations is γ^-1. During this time, most of the energy in the initial perturbation is transferred into a resonant absorption layer of thickness of order ℓ²/a, with motions in this layer having an amplitude of order a/ℓ times the initial amplitude. We apply our results to the observations, suggesting that loop oscillations decay principally because of inhomogeneities in the loop. Our theory suggests that only those loops with density inhomogeneities on a small scale (confined to within a thin layer of order aγ/ω_k in thickness) are able to support coherent oscillations for any length of time and so be observable. Loops with a more gradual density variation, on the scale of the tube radius a, do not exhibit pronounced oscillations.

In the framework of a refined Kolmogorov hypothesis, the scaling behavior of the B_z-component of the photospheric magnetic field is analyzed and compared with flaring activity in solar active regions. We use Solar and Heliospheric Observatory Michelson Doppler Imager, Huairou (China), and Big Bear measurements of the B_z-component in the photosphere for nine active regions. We show that there is no universal behavior in the scaling of the B_z-structure functions for different active regions. Our previous study has shown that scaling for a given active region is caused by intermittency in the field, ^(B)(), describing the magnetic energy dissipation. When intermittency is weak, the B_z field behaves as a passive scalar in the turbulent flow, and the energy dissipation is largely determined by the dissipation of kinetic energy in the active regions with low flare productivity. However, when the field ^(B)() is highly intermittent, the structure functions behave as transverse structure functions of a fully developed turbulent vector field, and the scaling of the energy dissipation is mostly determined by the dissipation of the magnetic energy (active regions with strong flaring productivity). Based on this recent result, we find that the dissipation spectrum of the B_z-component is strongly related to the level of flare productivity in a solar active region. When the flare productivity is high, the corresponding spectrum is less steep. We also find that during the evolution of NOAA Active Region 9393, the B_z dissipation spectrum becomes less steep as the active region's flare activity increases. Our results suggest that the reorganization of the magnetic field at small scales is also relevant to flaring: the relative fraction of small-scale fluctuations of magnetic energy dissipation increases as an active region becomes prone to producing strong flares. Since these small-scale changes seem to begin long before the start of a solar flare, we suggest that the relation between scaling exponents, calculated by using only measurements of the B_z-component, and flare productivity of an active region can be used to monitor and forecast flare activity.

The infrared spectrum of sunspots is analyzed in the H-band region (5540-6997 cm^-1) with the aid of a new, hot (T = 1800 K) laboratory emission spectrum of water covering 4878-7552 cm^-1. There are 682 lines in the sunspot spectrum and 5589 lines in the laboratory spectrum assigned quantum numbers corresponding to transitions due to H₂¹⁶O using a combination of previously known experimental energy levels for water and variational line lists. A further 201 unassigned lines common to both spectra can also be associated with water.

We develop a new methodology that can determine magnetic helicity flux as well as Poynting flux across the photosphere based on magnetograph observation. By applying this method, we study the injection mechanism of magnetic helicity and magnetic free energy into the solar corona. In order to derive the helicity and energy fluxes, first the velocity tangential to the solar surface is constructed by applying a correlation tracking technique on the magnetic observation, and second, the velocity component normal to the photosphere is derived from the condition that the magnetic evolution must be consistent with the induction equation. Through this procedure, we can determine the helicity and energy fluxes separately for the shear motion effect and for the flux emergence effect. Based on this new method, NOAA Active Region 8100 was analyzed from 1997 November 1 to 5 using data observed by the Solar and Heliospheric Observatory Michelson Doppler Interferometer and the vector magnetograph at the National Astronomical Observatory of Japan (NAOJ) in Tokyo. The results indicate that the photospheric shear motion and the flux emergence process have equally contributed to the helicity injection and have supplied magnetic helicity of opposite signs into this active region.

This article reports nine solar energetic particle (SEP) events detected by the New Instrument for Nuclear Analysis (NINA) between 1998 October and 1999 April. NINA is a silicon-based particle detector mounted on board the Russian satellite Resurs-01-4, which has flown at an altitude of about 800 km in polar inclination since 1998 July. For every solar event, the power-law ⁴He spectrum across the energy interval 10-50 MeV nucleon^-1 was reconstructed and spectral indexes, γ, from 1.8 to 6.8 extracted. Data of ³He and ⁴He were used to determine the ³He/⁴He ratio, which for some SEP events indicated an enrichment in ³He. For the 1998 November 7 event, the ratio reached a maximum value of 0.33 ± 0.06, with spectral indexes of γ = 2.5 ± 0.6 and γ = 3.7 ± 0.3 for ³He and ⁴He, respectively. The ³He/⁴He ratio averaged over the remaining events was 0.011 ± 0.004. For all events, a deuterium-to-proton ratio was estimated. An upper limit on the average value over all events was ²H/¹H < 4 × 10^-5 across the energy interval 9-12 MeV nucleon^-1. Upper limits on the ³H/¹H counting ratio for all events were determined. For the 1998 November 14 SEP event, the high flux of heavy particles detected made it possible to reconstruct the carbon, nitrogen, and oxygen flux.

We consider the linear axisymmetric stability of a differentially rotating collisionless plasma in the presence of a weak magnetic field; we restrict our analysis to wavelengths much larger than the proton Larmor radius. This is the kinetic version of the magnetorotational instability explored extensively as a mechanism for magnetic field amplification and angular momentum transport in accretion disks. The kinetic calculation is appropriate for hot accretion flows onto compact objects and for the growth of very weak magnetic fields, where the collisional mean free path is larger than the wavelength of the unstable modes. We show that the kinetic instability criterion is the same as in MHD, namely that the angular velocity decrease outward. However, nearly every mode has a linear kinetic growth rate that differs from its MHD counterpart. The kinetic growth rates also depend explicitly on β, i.e., on the ratio of the gas pressure to the pressure of the seed magnetic field. For β ~ 1 the kinetic growth rates are similar to the MHD growth rates, while for β ≫ 1 they differ significantly. For β ≫ 1, the fastest growing mode has a growth rate ≈ Ω for a Keplerian disk, larger than its MHD counterpart; there are also many modes whose growth rates are negligible, ≲ β^-1/2Ω ≪ Ω. We provide a detailed physical interpretation of these results and show that gas pressure forces, rather than just magnetic forces, are central to the behavior of the magnetorotational instability in a collisionless plasma. We also discuss the astrophysical implications of our analysis.

The nonlinear evolution of the magnetorotational instability (MRI) in weakly ionized accretion disks, including the effect of the Hall term and ohmic dissipation, is investigated using local three-dimensional MHD simulations and various initial magnetic field geometries. When the magnetic Reynolds number, Re_M ≡ v/ηΩ (where v_A is the Alfvén speed, η is the magnetic diffusivity, and Ω is the angular frequency), is initially larger than a critical value Re_M,crit, the MRI evolves into MHD turbulence in which angular momentum is transported efficiently by the Maxwell stress. If Re_M < Re_M,crit, however, ohmic dissipation suppresses the MRI, and the stress is reduced by several orders of magnitude. The critical value is in the range of 1-30 depending on the initial field configuration. The Hall effect does not modify the critical magnetic Reynolds number by much but enhances the saturation level of the Maxwell stress by a factor of a few. We show that the saturation level of the MRI is characterized by v/ηΩ, where v_Az is the Alfvén speed in the nonlinear regime along the vertical component of the field. The condition for turbulence and significant transport is given by v/ηΩ ≳ 1, and this critical value is independent of the strength and geometry of the magnetic field or the size of the Hall term. If the magnetic field strength in an accretion disk can be estimated observationally and the magnetic Reynolds number v/ηΩ is larger than about 30, this would imply that the MRI is operating in the disk.

With a redshift of z ≈ 1.7, SN 1997ff is the most distant Type Ia supernova (SN Ia) discovered so far. This SN is close to several bright, z = 0.6-0.9 galaxies, and we consider the effects of lensing by those objects on the magnitude of SN 1997ff. We estimate their velocity dispersions using the Tully-Fisher and Faber-Jackson relations, corrected for evolution effects, and calculate, applying the multiple-plane lensing formalism, that SN 1997ff is magnified by 0.34 ± 0.12 mag. Due to the spatial configuration of the foreground galaxies, the shear from individual lenses partially cancels out, and the total distortion induced on the host galaxy is considerably smaller than that produced by a single lens having the same magnification. After correction for lensing, the revised distance to SN 1997ff is m-M = 45.49 ± 0.34 mag, which improves the agreement with the Ω_M = 0.35, Ω_Λ = 0.65 cosmology expected from lower redshift SNe Ia and which is inconsistent at the ~3 σ confidence level with a uniform gray dust model or a simple evolution model.

We report the discovery and monitoring of radio emission from the Type Ic supernova SN 2002ap ranging in frequency from 1.43 to 22.5 GHz and in time from 4 to 50 days after the SN explosion. As in most other radio SNe, the radio spectrum of SN 2002ap shows evidence for absorption at low frequencies, usually attributed to synchrotron self-absorption (SSA) or free-free absorption. While it is difficult to discriminate between these two processes based on a goodness of fit, the unabsorbed emission in the free-free model requires an unreasonably large ejecta energy. Therefore, on physical grounds we favor the SSA model. In the SSA framework, at about day 2, the shock speed is ≈0.3c, the energy in relativistic electrons and magnetic fields is ≈1.5 × 10⁴⁵ ergs, and the inferred progenitor mass-loss rate is ≈5 × 10^-7M_☉ yr^-1 (assuming a 10³ km s^-1 wind). These properties are consistent with a model in which the outer, high-velocity SN ejecta interact with the progenitor wind. The amount of relativistic ejecta in this model is small, so the presence of broad lines in the spectrum of a Type Ib/c SN as observed in SN 2002ap is not a reliable indicator of relativistic ejecta and hence γ-ray emission.

The recent detection of a 3 hr X-ray flare by the Chandra X-Ray Observatory has raised the possibility of enhanced emission over a broad range of wavelengths from Sagittarius A*, the suspected 2.6 × 10⁶M_☉ black hole at the Galactic center, during a flaring event. We have, therefore, reconstructed 3 hr data sets from 2 μm speckle and adaptive optics images (θ_core = 50-100 mas) obtained with the W. M. Keck 10 m telescopes between 1995 and 2001. In 25 separate observations, no evidence of any significant excess emission associated with Sgr A* was detected. The lowest of our detection limits gives an observed limit for the quiescent state of Sgr A* of 0.09 ± 0.005 mJy or, equivalently, a dereddened value of 2.0 ± 0.1 mJy, which is a factor of 2 lower than the best previously published quiescent value. Under the assumption that there are random 3 hr flares producing both enhanced X-ray and near-infrared emission, our highest limit constrains the variable state of Sgr A* to ≲0.8 mJy (observed) or 19 mJy (dereddened). These results suggest that the model favored by Markoff et al., in which the flare is produced through local heating of relativistic particles surrounding Sgr A* (e.g., a sudden magnetic reconnection event), is unlikely because it predicts peak 2 μm emission of ~300 mJy, well above our detection limit.

Broad Fe Kα emission lines have recently been reported in a number of Galactic black holes. Such lines are useful accretion flow diagnostics because they may be produced at the inner accretion disk and shaped by relativistic effects, but in general they have been observed only at luminosities of L_X ~ 10³⁷-10³⁸ ergs s^-1 in soft X-rays. The Galactic microquasar V4641 Sgr—widely known for its 12.2 Crab (1.5-12 keV) outburst in 1999 September—displayed low-level activity in 1999 March. BeppoSAX observed the source in this state, and Fe Kα line emission was found by in 't Zand et al. In reanalyzing these data, we find strong evidence that the Fe Kα line profile is broadened. For the most likely values of the source distance and black hole mass measured by Orosz et al., our fits to the total spectrum indicate that the source was observed at a luminosity of L_X = 1.9 × 10³⁶ ergs s^-1 (2-10 keV), or L_X/L_Edd = 1.8 × 10^-3. Advection-dominated accretion flow models predict a radially recessed disk in this regime. In contrast, fits to the observed Fe Kα emission-line profile with a relativistic line model constrain the inner disk to be consistent with the marginally stable circular orbit of a Schwarzschild black hole.

We report on a deep search for radio pulsations toward five unidentified ASCA X-ray sources coincident with EGRET γ-ray sources. This search has led to the discovery of a young and energetic pulsar using data obtained with the new Wideband Arecibo Pulsar Processor. PSR J2021+3651 is likely associated with the X-ray source AX J2021.1+3651, which in turn is likely associated with the COS B high-energy γ-ray source 2CG 075+00, also known as GeV J2020+3658 or 3EG J2021+3716. PSR J2021+3651 has a rotation period of P ~ 104 ms and ~ 9.6 × 10^-14, implying a characteristic age of τ_c ~ 17 kyr and a spin-down luminosity of ~ 3.4 ×10³⁶ ergs s^-1. The dispersion measure DM ~ 371 pc cm^-3 is by far the highest of any observed pulsar in the Galactic longitude range 55^° < l < 80^°. This DM suggests a distance d ≳ 10 kpc and a high γ-ray efficiency of ~15%, but the true distance may be closer if there is a significant contribution to the DM from excess gas in the Cygnus region. The implied luminosity of the associated X-ray source suggests the X-ray emission is dominated by a pulsar wind nebula unresolved by ASCA.

We interpret the correlation over 5 orders of magnitude between high frequency ν_high and low frequency ν_low in quasi-periodic oscillations (QPOs) found by Psaltis, Belloni, & van der Klis for black hole (BH) and neutron star (NS) systems and then extended by Mauche to white dwarf (WD) binaries. The observed correlation strongly constrains theoretical models and provides clues to understanding the nature of the QPO phenomena at large. We argue that the observed correlation is a natural consequence of the Keplerian disk flow adjustment to the innermost sub-Keplerian boundary conditions near the central object that ultimately leads to the formation of the sub-Keplerian transition layer (TL) between the adjustment radius and the innermost boundary (the star surface for the NS and WD and the horizon for the BH). In the frameworks of the TL model, ν_high is related to the Keplerian frequency at the outer (adjustment) radius ν_K, and ν_low is related to the magnetoacoustic oscillation frequency ν_MA. Using a relation between ν_MA, the magnetic and gas pressure, and the density and the hydrostatic equilibrium condition in the disk, we infer a linear correlation between ν_K and ν_MA. Identification of ν_high, ν_low with ν_K, ν_MA, respectively, leads us to the determination of H/r_out = 1.5 × 10^-2 and β = 0.1 (where H is the half-width of the disk and β is a ratio of magnetic pressure to gas pressure). We estimate the magnetic field strength near the TL outer radius for BHs, NSs, and WDs. The fact that the observed high-low frequency correlation over 5 orders of magnitude is valid for BHs, NSs, and down to WDs strongly rules out relativistic models for QPO phenomena. We come to the conclusion that the QPO observations indicate the adjustment of the geometrically thin disk to sub-Keplerian motion near the central object. This effect is a common feature for a wide class of systems, starting from WD binaries up to BH binaries.

The discovery of two accreting millisecond X-ray pulsars in binaries with ≈43 minute orbital periods allows for a new probe of the donor's structure. For XTE J1751-305, only a hot white dwarf (WD) can fill the Roche lobe. A cold He WD is a possible solution for XTE J0929-314, although I will show that evolutionary arguments make a hot WD more likely. In addition to being larger than the T = 0 models, these finite entropy, low-mass (M_c < 0.03 M_☉) WDs have a minimum mass for a fixed core temperature. If they remain hot as they lose mass and expand, they can "evaporate" to leave an isolated millisecond radio pulsar. They also adiabatically expand upon mass loss at a rate faster than the growth of the Roche radius if the angular momentum deposited in the disk is not returned to the donor. If the timescale of the resulting runaway mass transfer is shorter than the viscous timescale in the outer disk, then the mass transfer instability of Ruderman & Shaham for He WDs would be realized. However, my estimates of these timescales still make the instability unlikely for adiabatic responses. I close by noting the possible impact of finite temperature WDs on our understanding of AM CVn binaries.

Convection-dominated accretion flow (CDAF) is a promising model to explain underluminous accreting black holes in X-ray binaries and galactic nuclei. I discuss the effects of angular momentum transport in viscous hydrodynamical and MHD CDAFs. In hydrodynamical CDAFs, convection transports angular momentum inward, and this, together with the outward convection transport of thermal energy, determine the radial structure of the flow. In MHD CDAFs, convection can transport angular momentum either inward or outward, depending on the properties of turbulence in rotating magnetized plasma, which are not fully understood yet. The direction of convection angular momentum transport can affect the law of rotation of MHD CDAFs.

Many nearby stars are surrounded by a bright ring or disk of cold dust. Our calculations show that these disks and rings of dust are signposts of recent planet formation. Bright rings appear because dust associated with the formation of a planet absorbs and scatters light from the central star. The calculations explain the rings observed so far and predict that all nascent solar systems have dusty rings.

Using WIYN/Hydra observations, we report the first discovery of a super-Li-rich dwarf, the star J37 of the Hyades-aged cluster NGC 6633. This star's gargantuan surface Li abundance of A(Li) = 4.29 ± 0.07 [A(X) ≡ 12 + log(N_X/N_H)] is a factor of 10 larger than the Li abundances observed in meteorites and in minimally depleted young open cluster stars. J37 may be the smoking gun for the action of diffusion, models of which predict the existence of striking Li overabundances due to radiative acceleration (the "Li peak") in the extremely narrow T_eff range of 6900-7100 K. The low-C, high-Fe, and higher Ni abundances in J37 also support the diffusion model, although other elemental abundances do not agree as well with the model predictions. We have discounted other Li-enrichment scenarios (spots, asymptotic giant branch contamination, planetesimal accretion). While slow mixing seems to be the universally dominant Li-depleting mechanism during the main-sequence evolution of F, G, and K dwarfs, the F dwarf J37 illustrates that Li diffusion can occur in solar-type stars under the right circumstances.

We present an improved uvby-metallicity relation calibrated for F, G, and early K dwarfs and an analogous uvby-T_eff relation, both derived using a Levenberg-Marquardt minimization scheme. Our calibrations are based on 1533 stars that appear in both the Cayrel de Strobel metallicity compilation and in the Hauck-Mermilliod catalog of uvby photometry. We also examine the speculative possibility of using uvby photometry to produce a uvby-planeticity calibration. We conclude that while there is likely no strong photometric indicator of the presence or absence of short-period planets, the possibility of a spectroscopic indicator of planeticity is well worth examining.

We report the results of coordinated Hubble Space Telescope and Chandra observations of the Crab synchrotron nebula. Similar dynamical structures, including equatorial wisps moving outward at ~0.5c, are seen in both passbands. The inner X-ray ring is a variable, irregular structure from which wisps and diffuse emission emerge. The X-ray/visible jet to the southeast of the pulsar is a true jet. The data support the interpretation that the inner ring and a highly dynamical feature at the base of the jet are unstable quasi-stationary shocks in the "cold" equatorial wind and polar jet from the pulsar.

We use a magnetic flux transport model to simulate the evolution of the Sun's polar fields and open flux during solar cycles 13 through 22 (1888-1997). The flux emergence rates are assumed to scale according to the observed sunspot-number amplitudes. We find that stable polarity oscillations can be maintained if the meridional flow rate is allowed to vary from cycle to cycle, with higher poleward speeds occurring during the more active cycles. Our model is able to account for a doubling of the interplanetary field strength since 1900, as deduced by Lockwood, Stamper, & Wild from the geomagnetic aa index. We confirm our earlier conclusion that such a doubling of the open flux does not imply that the base level of the total photospheric flux has increased significantly over the last century.

We examine the angular distributions of He, O, and Fe in large solar energetic particle (SEP) events measured on the Wind spacecraft. We report, for the first time, that in a fixed velocity interval, Fe/O is often larger for particles flowing sunward along the magnetic field than for particles flowing outward from the Sun in many SEP events. This occurs because the anisotropy for O exceeds that for Fe, even though both species are streaming outward. There are no examples of events for which the outward Fe/O dominates. The behavior of Fe and O conflicts with the expectations of simple diffusion theory, that angular distributions should be independent of species. It also seems to conflict with the idea that energetic Fe scatters less than O of the same velocity. However, preliminary modeling suggests that the presence of a reflecting magnetic boundary beyond 1 AU, together with the increased scattering of O over Fe due to proton-generated Alfvén waves, can explain the direction and magnitude of the effect. These observations add a new dimension to the study of SEP transport.

Magnetic reconnection is widely believed responsible for heating the solar corona as well as for generating X-rays and energetic particles in solar flares. On astrophysical scales, reconnection in the intergalactic plasma is a prime candidate for a local source (<100 Mpc) of cosmic rays exceeding the Greisen-Zatsepin-Kúzmin cutoff (~10¹⁹ eV). In a laboratory astrophysics experiment, we have made the first observation of particles accelerated by magnetic reconnection events to energies significantly above both the thermal and the characteristic magnetohydrodynamic energies. These particles are correlated temporally and spatially with the formation of three-dimensional magnetic structures in the reconnection region.

The calcium carbide radical, CaC, has been observed in the laboratory for the first time using millimeter/submillimeter direct absorption spectroscopy. The species was created in the gas phase by the reaction of calcium vapor and methane under extreme DC discharge conditions. Eleven rotational transitions of CaC in its X³Σ^- ground state were recorded in the frequency range 247-536 GHz. Each transition of CaC was found to consist of triplets due to fine-structure interactions that indicate that the ground state is ³Σ^-, as opposed to ⁵Σ^-, both of which have been suggested by theory. The data were analyzed in a Hund's case (b) basis, and rotational, spin-spin, and spin-rotation constants have been accurately determined. CaC may be detectable in circumstellar envelopes of asymptotic giant branch stars, in particular those that are carbon-rich.

Two new silicon-bearing molecules, the closed-shell asymmetric tops cyanosilylene HSiCN and its isomer HSiNC, have been detected in a laboratory discharge by molecular beam Fourier transform microwave spectroscopy. The rotational spectra of the normal and deuterated isotopic species of both molecules have been analyzed to derive precise spectroscopic constants, which allow the astronomically most interesting transitions up to 120 GHz to be calculated to an accuracy better than 1 km s^-1 in equivalent radial velocity. Both molecules are good candidates for astronomical detection, closely related in structure and composition to known astronomical molecules, and they are highly polar, with estimated dipole moments of 3.5 D for HSiCN and 2.5 D for HSiNC.