Table of contents

Recent measurements of Type Ia supernovae, as well as other concordant observations, suggest that the expansion of our universe is accelerating. A dark energy component has usually been invoked as the most feasible mechanism for the acceleration. However, effects arising from possible extra dimensions can mimic dark energy through a modified Friedmann equation. In this work, we investigate some observational constraints on a scenario in which this modification is given by H² = (8πG/3)(ρ + Cρⁿ). We mainly focus our attention on the constraints from recent measurements of the dimensionless coordinate distances to Type Ia supernovae and Fanaroff-Riley Type IIb radio galaxies compiled by Daly & Djorgovski and the X-ray gas mass fractions in clusters of galaxies published by Allen et al. We obtain the confidence region on the power index n and the density parameter Ω_m of the universe from a combined analysis of these databases. We find that n = 0.06 and Ω_m = 0.30, at the 95.4% confidence level, which is consistent within the errors with the standard ΛCDM model. These parameter ranges give a universe whose expansion switches from deceleration to acceleration at a redshift between 0.52 to 0.73.

Under the assumption that the concordance Λ cold dark matter (CDM) model is the correct model, we test the cosmic microwave background (CMB) anisotropy data for systematic effects by examining the bandpass temperature residuals with respect to this model. Residuals are analyzed as a function of angular scale l, Galactic latitude, frequency, calibration source, instrument type, and several other variables that may be associated with potential systematic effects. Our main result is that we find no significant systematic errors associated with these variables. However, we do find marginal evidence for a trend associated with Galactic latitude indicative of Galactic contamination.

We present a theoretical model for primordial star formation. First we describe the structure of the initial gas cores as virialized, quasi-hydrostatic objects in accord with recent high-resolution numerical studies. The accretion rate can then be related to characteristic densities and temperatures that are set by the cooling properties of molecular hydrogen. We allow for rotation of the gas core, assuming angular momentum conservation inside the sonic point of the flow. In the typical case, most mass then reaches the star via an accretion disk. The structure of the inner region of this disk is described with the standard theory of viscous disks, but with allowance for the substantial energies absorbed in ionizing and dissociating the gas. The size of the protostar and its luminosity depend on the accretion rate, the energetics of the accreting gas, and the ability of the radiation to escape from the stellar accretion shock. We combine these models for the infall rate, inner disk structure, and protostellar evolution to predict the radiation field that is the basis for radiative feedback processes acting against infall (second paper in the series). For realistic initial angular momenta, the photosphere of the protostar is much smaller and hotter than in the spherical case, leading to stronger radiative feedback at earlier stages in the evolution. In particular, once the star is older than its Kelvin-Helmholtz time, contraction toward the main sequence causes a rapid increase in ionizing and far-ultraviolet luminosity at masses ~30 M_☉ in the fiducial case. Since the cores out of which the first stars formed were much more massive than 30 M_☉ and since feedback is dynamically unimportant at lower masses, we conclude that the first stars should have had masses 30 M_☉.

Are magnetic fields important in primordial star formation? Assuming that star formation occurs via an accretion disk that is turbulent, initially because of local gravitational instability, we calculate the disk structure for realistic accretion rates. We predict that local gravitational viscosity is able to drive accretion, without the disk fragmenting. We then estimate the rate of dynamo amplification of seed magnetic field. Turbulence in a stratified disk can be helical, with different signs of the helicity in each hemisphere. This provides a key ingredient for production of global-scale magnetic fields whose sign of flux is sustained over many orbit times. The resulting fields can drive collimated protostellar outflows that reduce the star formation efficiency from the initial gas cloud, especially once the protostar has contracted to the main sequence, at ~100 M_☉. We estimate that the outflows are powerful enough to eject some material from the host dark matter halo and to initiate relatively strong magnetization of the local intergalactic medium. Close to the protostar, the outflow acts to shield the disk and equatorial regions from radiative feedback, such as ionizing photons, and this may enable accretion up to relatively large stellar masses. We conclude that magnetic fields cannot be ignored in models of primordial star formation.

We present results from a blind, spectroscopic search for redshift 5.7 Lyα emission-line galaxies at Keck I. Using a band-limiting filter and custom slit mask, we cover the LRIS detector with low-resolution spectra in the 8100-8250 Å atmospheric window, which contains no bright night-sky emission lines. We find nine objects with line fluxes greater than our flux limit of 6 × 10^-18 ergs s^-1 cm^-2 in our ~5.1 arcmin² field. We rule out a Lyα identification for six of these based on the absence of the continuum break, expected at rest 1215 Å for high-z galaxies and/or the identification of additional emission lines in our follow-up spectra. We find that extremely metal-poor, foreground emission-line galaxies are the most difficult type of interloper to recognize. For the three remaining emission-line objects, we identify a plausible counterpart for each object in a deep V-band image of the field, suggesting that none of them has a continuum break in the i band. Our preliminary conclusion is that our field contains no z = 5.7 Lyα emitters brighter than 0.6L, where L ≡ 3.26 × 10⁴² ergs s^-1. Selecting a field with zero Lyα emitters is marginally consistent with the no-evolution hypothesis—i.e., we expected to recover 2-3 Lyα emitters, assuming that the Lyα luminosity function at redshift 5.7 is the same as it is at redshift 3. Our null result rules out a brightening of L by more than a factor of 1.7 from redshift 3 to 5.7, or, over the same redshift interval, an increase of more than a factor of 2.2 in the number density of Lyα emitters. The paucity of z = 5.7 Lyα emitters raises the question of whether the Lyα-selected population plays a significant role in maintaining the ionization of the intergalactic medium (IGM) at z = 5.7. We find that if the escape fraction of Lyα radiation is less than 0.4f_Lyc, where f_Lyc is the escape fraction of Lyman continuum photons, then the star formation rate in the Lyα-emitting population is high enough in the no-evolution model (our upper limit) to maintain the ionization of the IGM at z = 5.7.

With a 1999 ASCA observation, PG 2112+059 became notable as the first broad absorption line (BAL) quasar found to exhibit a typical radio-quiet quasar X-ray continuum underlying a large amount of intrinsic absorption. We present a recent Chandra ACIS-S3 observation of PG 2112+059 that demonstrates remarkable spectral and luminosity variability since that time. In addition to a decrease in the continuum normalization by a factor of ~3.5, the absorption column density has apparently increased substantially, and a strong feature in the Fe Kα region has appeared. Concurrent Hubble Space Telescope (HST) STIS data compared with archival HST data from earlier epochs show evidence for variability of the continuum (up to a factor of ~1.7 in the ultraviolet), and in some absorption features of the C IV λ1549 BAL since 1992; however, the O VI BAL structure is consistent with a 1995 observation. We also present evidence for Lyβ-O VI λ1037.62 and Lyα-N V λ1242.80 line-locked absorption systems, supporting the assumption that ultraviolet line pressure is driving the BAL outflow. Whereas ultraviolet BALs typically exhibit only modest equivalent width variability over timescales of years, the dramatic X-ray variability of PG 2112+059 suggests that X-ray spectral variability studies of BAL quasars have great potential for probing the physics of quasar winds.

Numerical simulations of the interiors of radiation-dominated accretion disks show that significant density inhomogeneities can be generated in the gas. Here, we present the first results of our study on X-ray reflection spectra from such heterogeneous density structures. We consider two cases: first, we produce a number of toy models in which a sharp increase or decrease in density, of variable width, is placed at different depths in a uniform slab. Comparing the resulting reflection spectra to those from an unaltered slab shows that the inhomogeneity can affect the emission features, in particular the Fe Kα and O VIII Lyα lines. The magnitude of any differences depends on both the parameters of the density change and the ionizing power of the illuminating radiation, but the inhomogeneity is required to be within ~2 Thomson depths of the surface to cause an effect. However, only relatively small variations in density (by factors of a few) are necessary for significant changes in the reflection features to be possible. Our second test was to compute reflection spectra from the density structure predicted by a simulation of the nonlinear outcome of the photon-bubble instability. The resulting spectra also exhibited differences from the constant-density models, caused primarily by a strong 6.7 keV iron line. Nevertheless, constant-density models can provide a good fit to simulated spectra, albeit with a low reflection fraction, between 2 and 10 keV. Below 2 keV, differences in the predicted soft X-ray line emission result in very poor fits with a constant-density ionized-disk model. The results indicate that density inhomogeneities may further complicate the relationship between the Fe Kα equivalent width and the X-ray continuum. Further calculations are needed to verify that density variations of sufficient magnitude will occur within a few Thomson depths of the disk photosphere.

We present the high-resolution X-ray spectrum of the BL Lac object PKS 2155-304 taken with the RGS units on board XMM-Newton in 2000 November. We detect an O VII Kα resonant absorption line from warm/hot local gas at 21.59 Å (~4.5 σ detection). The line profile is possibly double peaked. We do not confirm the strong 20.02 Å absorption line seen with Chandra and interpreted as z ~ 0.05 O VIII Kα. We set a 3 σ upper limit of 14 mÅ on the equivalent width. We also detect the ~23.5 Å interstellar O I 1s→2p line and derive a factor of ≤1.5 subsolar O/H ratio in the interstellar medium along the PKS 2155-304 line of sight.

We present results from a 50 ks observation of the narrow-line Seyfert 1 galaxy Ark 564 with the Chandra HETGS. The spectra above 2 keV are modeled by a power law with a photon index of 2.56 ± 0.06. We confirm the presence of the soft excess below about 1.5 keV. If we fit the excess with a blackbody model, the best-fit temperature is 0.124 keV. Ark 564 has been reported to show a peculiar emission-line-like feature at 1 keV in various observations using lower resolution detectors, and the Chandra grating spectroscopy rules out an origin of blends of several narrow emission lines. We detect an edgelike feature at 0.712 keV in the source rest frame. The preferred interpretation of this feature is a combination of the O VII K edge and a number of L-absorption lines from slightly ionized iron, which suggests a warm absorber with ionization parameter ξ ~ 1 and N_H ~ 10²¹ cm^-2. These properties are roughly consistent with those of the UV absorber. We also detect narrow absorption lines of O VII, O VIII, Ne IX, Ne X, and Mg XI at the systemic velocity. From these lines, a second warm absorber having log ξ ~ 2 and N_H ~ 10²¹ cm^-2 is required.

We have investigated the physical conditions in the narrow-line region (NLR) of M51 by using long-slit spectra obtained with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope and 3.6 cm radio continuum observations obtained with the Very Large Array. Emission-line diagnostics were employed for nine NLR clouds, which extend 25 (102 pc) from the nucleus, to examine the electron density, temperature, and ionization state of the NLR gas. The emission-line ratios are consistent with those typically found in Seyfert nuclei and indicate that within the inner near-nuclear region (r 1'') the ionization decreases with increasing radius. Upper limits to the [O III] electron temperature (T_e 11,000 K) for the inner NLR clouds indicate that photoionization is the dominant ionization mechanism close to the nucleus. The emission-line fluxes for most of the NLR clouds can be reproduced reasonably well by simple photoionization models using a central power-law continuum source and supersolar nitrogen abundances. Shock+precursor models, however, provide a better fit to the observed fluxes of an NLR cloud ~25 south of the nucleus that is identified with the extranuclear cloud. The large [O III] electron temperature of this cloud (T_e = 24,000 K) further suggests the presence of shocks. This cloud is straddled by two radio knots and lies near the location where a weak radio jet, ~25 (102 pc) in extent, connects the near-nuclear radio emission with a diffuse lobe structure spanning ~4'' (163 pc). It is plausible that this cloud represents the location where the radio jet impinges on the disk ISM.

We present deep submillimeter observations of 17 galaxies at z = 0.5 that are hosts of a Type Ia supernova. Two galaxies are detected directly, and the sample is detected statistically with a mean 850 μm flux of 1.01 ± 0.33 mJy, which is 25%-135% higher than locally. We infer that the mean value of A_V in normal galaxies at z = 0.5 is comparable to or greater than the mean A_V in local normal galaxies, in agreement with galaxy chemical evolution models and indirect observational evidence. Scaling from the local value given by Rowan-Robinson gives a mean extinction at z = 0.5 of A_V = 0.56 ± 0.17. The dust in the brightest submillimeter object in our sample is best interpreted as normal "cirrus" dust similar to that seen locally. The detection rate of our sample suggests that some sources found in blank-field submillimeter surveys may not be high-redshift starbursts, but rather cirrus galaxies at moderate redshifts and with lower star formation rates. Finally, an increase in host dust extinction with increasing redshift may impact the cosmological results from distant supernova searches. This emphasizes the need to carefully monitor dust extinction when using Type Ia supernovae to measure the cosmological parameters.

Most models of double-barred galaxies suggest that a molecular gas component is crucial for maintaining long-lived nuclear bars. We have undertaken a CO survey in an attempt to determine the gas content of these systems and to locate double-barred galaxies with strong CO emission that could be candidates for high-resolution mapping. We observed 10 galaxies in CO J = 2-1 and J = 3-2 and did not detect any galaxies that had not already been detected in previous CO surveys. We preferentially detect emission from galaxies containing some form of nuclear activity. Simulations of these galaxies require that they contain 2%-10% gas by mass in order to maintain long-lived nuclear bars. The fluxes for the galaxies for which we have detections suggest that the gas mass fraction is in agreement with these models requirements. The lack of emission in the other galaxies suggests that they contain as little as 7 × 10⁶ M_☉ of molecular material, which corresponds to 0.1% gas by mass. This result combined with the wide variety of CO distributions observed in double-barred galaxies suggests the need for models of double-barred galaxies that do not require a large, well-ordered molecular gas component.

We present multicolor photometry of bright star cluster candidates in the nearby starburst galaxies NGC 3077 and NGC 5253, observed with the Hubble Space Telescope Wide Field Planetary Camera 2 in both broadband (F300W, F547M, and F814W) and narrowband (F487N and F656N) filters. By comparing the photometry with theoretical population synthesis models, we estimate the age and mass of each star cluster, which provides constraints on the recent star formation histories of the host galaxies. We compare the star cluster populations in these dwarf starburst galaxies with those of the nuclear starburst in the barred spiral M83 and discuss the implications for our understanding of the nature and evolution of starburst events.

We present new optical and Chandra observations of the field containing the ultraluminous X-ray source NGC 1313 X-2. On an ESO 3.6 m image, the Chandra error box embraces an R = 21.6 pointlike object and excludes a previously proposed optical counterpart. The resulting X-ray/optical flux ratio of NGC 1313 X-2 is ~500. The value of f_X/f_opt, the X-ray variability history, and the spectral distribution derived from a reanalysis of the ROSAT, ASCA, and XMM-Newton data indicate a luminous X-ray binary in NGC 1313 as a likely explanation for NGC 1313 X-2. If the X-ray soft component observed in the XMM-Newton EPIC spectrum originates from an accretion disk, the inferred mass of the compact remnant is ≈100 M_☉, making it an intermediate-mass black hole. The derived optical luminosity (L ≈ 10⁵ L_☉) is consistent with that of a ≈15-20 M_☉ companion. The properties of the environment of NGC 1313 X-2 are briefly discussed.

We introduce crowded-field integral field (3D) spectrophotometry as a useful technique for the study of resolved stellar populations in nearby galaxies. The spectroscopy of individual extragalactic stars, which is now feasible with efficient instruments and large telescopes, is confronted with the observational challenge of accurately subtracting the bright, spatially and wavelength-dependent nonuniform background of the underlying galaxy. As a methodological test, we present a pilot study with selected extragalactic planetary nebulae (XPNe) in the bulge of M31, demonstrating how 3D spectroscopy is able to improve the limited accuracy of background subtraction that one would normally obtain with classical slit spectroscopy. It is shown that because of the absence of slit effects, 3D spectroscopy is a most suitable technique for spectrophometry. We present spectra and line intensities for five XPNe in M31, obtained with the MPFS instrument at the Russian 6 m Bolshoi Teleskop Azimutal'nij, INTEGRAL at the William Herschel Telescope , and PMAS at the Calar Alto 3.5 m telescope. The results for two of our targets, for which data are available in the literature, are compared with previously published emission-line intensities. The three remaining PNe have been observed spectroscopically for the first time. One object is shown to be a previously misidentified supernova remnant. Our monochromatic Hα maps are compared with direct Fabry-Pérot and narrowband filter images of the bulge of M31, verifying the presence of filamentary emission of the interstellar medium in the vicinity of our objects. We present an example of a flux-calibrated and continuum-subtracted filament spectrum and demonstrate how the interstellar medium component introduces systematic errors in the measurement of faint diagnostic PN emission lines when conventional observing techniques are employed. It is shown how these errors can be eliminated with 3D spectroscopy, using the full two-dimensional spatial information and point-spread function (PSF) fitting techniques. Using 3D spectra of bright standard stars, we demonstrate that the PSF is sampled with high accuracy, providing a centroiding precision at the milliarcsecond level. Crowded-field 3D spectrophotometry and the use of PSF fitting techniques is suggested as the method of choice for a number of similar observational problems, including luminous stars in nearby galaxies, supernovae, QSO host galaxies, gravitationally lensed QSOs, and others.

Over the last three years, the Galactic center region has been monitored with the Chandra X-Ray Observatory. Besides the X-ray emission from the target object, Sgr A*, diffuse X-ray emission was detected throughout most of the 17' × 17' field of view. With 11 Chandra observations through 2002 June, the total effective exposure reaches ~590 ks, providing significant photon statistics on much of the detailed structure within the faint, often filamentary, diffuse X-ray emission. The true-color X-ray image and the equivalent-width images for the detected elemental species of the Galactic center region demonstrate that the diffuse X-ray features have a broad range of spatiospectral properties. Enhancements of the low ionization state Fe line emission (E ~ 6.4 keV) to the northeast of Sgr A* can be interpreted as fluorescence within the dense interstellar medium resulting from irradiation by hard, external X-ray sources. They may also be explained by emission induced by the bombardments by high-energy particles on the ambient medium, such as may accompany unresolved supernova ejecta intruding into dense interstellar medium. The detection of molecular cloud counterparts to the 6.4 keV Fe line-emitting features indicates that these Fe line features are associated with dense Galactic center clouds and/or active star-forming regions, which strongly supports the proposed origins of the X-ray reflection and/or supernova ejecta for the Fe line emission. We detect highly ionized S and Si lines that are generally coincident with the low ionization state Fe line emission and the dense molecular clouds in the northeast of Sgr A*. These hot plasmas are then most likely produced by massive star-forming activities and/or supernova remnants in the Galactic center. In contrast, we find that highly ionized He-like Fe line emission (E ~ 6.7 keV) is primarily distributed along the plane instead of being concentrated in the northeast of Sgr A*. The implied high temperature and the relatively uniform, but strong, alignment along the plane are consistent with the magnetic-confinement model, suggesting that this hot gas component has been reheated by the strong interstellar magnetic fields in the Galactic center to produce the observed He-like Fe line emission.

We present a 21 cm emission-absorption study toward extragalactic sources in the Canadian Galactic Plane Survey (CGPS). We have analyzed H I spectra toward 437 sources with S_ν ≥ 150 mJy, giving us a source density of 0.6 sources per square degree at arcminute resolution. We present the results of a first analysis of the H I temperatures, densities, and feature statistics. Particular emphasis is placed on five features with observed spin temperatures below 40 K. We find most spin temperatures in the range 40-300 K. A simple H I two-component model constrains the bulk of the cold component to temperatures (T_c) between 40 and 100 K. We find that T_c peaks in the Perseus arm region, and clearly drops off with Galactocentric radius R beyond that. The H I density follows this trend, ranging from a local value of 0.4 cm^-3 to less than 0.1 cm^-3 at R = 20 kpc. We find that H I emission alone on average traces about 75% of the total H I column density, as compared to the total inferred by the emission and absorption. Comparing the neutral hydrogen absorption to CO emission, no correlation is found in general, but all strong CO emission is accompanied by a visible H I spectral feature. Finally, the number of spectral H I absorption features per kiloparsec drops off exponentially with increasing R.

The Galactic magnetic field has an energy density comparable to that of the interstellar medium turbulence and a coherence spanning the Galaxy. It is not known if this field was formed before, during, or after the Galaxy. However, it is often assumed to originate from a turbulent dynamo process. We investigate the early stages of a Galactic dynamo when the dynamics is well approximated by homogeneous turbulence. Our simulations show that homogeneous magnetized turbulence with large Prandtl number yields magnetic energy at the small, resistive scale rather than at the Galactic scale. Thus, additional phenomena—perhaps helicity generated from Galactic rotation, stratification, and the differential rotation of the disk—are needed to explain the observed field. We simulate the growth of magnetic energy in forced nonhelical turbulence from an initially weak value until it saturates with the same energy density as the turbulence. When the field is dynamically weak, the simulations agree with the kinematic theory. In the long-term saturated state, the magnetic field is strong enough to modify the turbulence. This is the magnetohydrodynamic (MHD) analog of the Kolmogorov problem for hydrodynamic turbulence. The nature of the back-reaction is to neutralize the net shear (stretching) in small-scale eddies that are less energetic than the magnetic field. Only the forcing-scale eddies remain energetic enough to shear and cascade the magnetic field. The magnetic field at all scales therefore forward-cascades at the forcing timescale through spectrally nonlocal interactions with the forcing-scale eddies. Furthermore, the magnetic field folds into a reduced-tension state where field-line curvature anticorrelates with intensity. Direct consequences of these statements are that the magnetic spectrum is largely independent of viscosity and that the magnetic energy is located at the small, resistive scale.

We have used CCD imaging polarimetry to obtain linear polarization measurements of background stars toward the filamentary Musca dark cloud. We present a catalog of 2497 objects with polarization signal-to-noise ratio larger than 5. This allows us to build polarization maps to infer the detailed geometry of the local magnetic field. We show composite polarization maps along the cloud and explore general correlations of the polarimetric data with the morphology of the region. We find the overall field to be strikingly aligned with the projected small axis of the filamentary cloud. We detect a lower limit for the polarization across the cloud of ~2%, with an enhanced polarization of 6%-7% in the central region. We find evidence that the polarization pattern is altered in the inner regions, those associated with higher extinction.

We have measured the proper motion (PM) and kinematics of the ansae in NGC 7009 using high-dispersion echelle spectra and archive narrowband Hubble Space Telescope images. Assuming that the ansae are moving at equal and opposite velocities from the central star, we obtain a system radial velocity of -53 ± 2 km s^-1, the eastern ansa approaching and the western ansa receding at v_r = 5.3 ± 1 km s^-1 with respect to this value. The PM of the eastern ansa is 28 ± 8 mas yr^-1, which, with our weighted distance to NGC 7009 of 0.86 ± 0.34 kpc, gives V_exp = 114 ± 32 km s^-1. The electron temperature and density in both ansae were determined to be T_e ~ 9000 ± 400 K and n_e ~ 2300 ± 400 cm^-3. The dynamic age of the ansae is ~910 ± 260 yr, and the implied PM of the central star is μ_CS = 1 ± 0.5 mas yr^-1. This is in qualitative but not quantitative agreement with previous work.

A by-product of experiments designed to map the cosmic microwave background is the recent detection of a new component of foreground Galactic emission. The anomalous foreground at ~10-30 GHz, unexplained by traditional emission mechanisms, correlates with 100 μm dust emission. We use planetary nebulae (PNs) as astrophysical laboratories to test known radio emission processes and report that in the Helix the emission at 31 GHz and 100 μm are well correlated and exhibit similar features on sky images, which are absent in Hβ. Upper limits on the 250 GHz continuum emission in the Helix rule out cold grains as candidates for the 31 GHz emission and provide spectroscopic evidence for an excess at 31 GHz over bremsstrahlung. We estimate that the 100 μm-correlated radio emission, presumably due to dust, accounts for at least 20% of the 31 GHz emission in the Helix. This result strengthens previous tentative interpretations of diffuse interstellar medium spectra involving a new dust emission mechanism at radio frequencies. Very small grains, thought not to survive in evolved PNs, have not been detected in the Helix, which hampers interpreting the new component in terms of electric dipole emission from spinning grains. The observed iron depletion in the Helix favors considering the identity of this new component to be magnetic dipole emission from hot ferromagnetic grains. The reduced level of free-free continuum that we report also implies an electronic temperature of T_e = 4600 ± 1200 K for the free-free emitting material, which is significantly lower than the temperature of 9500 ± 500 K inferred from collisionally excited lines.

We calculate the nuclear composition of matter in accretion disks surrounding stellar mass black holes as are thought to accompany gamma-ray bursts (GRBs). We follow a mass element in the accretion disk starting at the point of nuclear dissociation and calculate the evolution of the electron fraction due to electron, positron, electron neutrino, and electron antineutrino captures. We find that the neutronization of the disk material by electron capture can be reversed by neutrino interactions in the inner regions of disks with accretion rates of 1 M_☉ s^-1 and higher. For these cases the inner disk regions are optically thick to neutrinos, and so to estimate the emitted neutrino fluxes, we find the surface of last scattering for the neutrinos (the equivalent of the proto-neutron star neutrinosphere) for each optically thick disk model. We also estimate the influence of neutrino interactions on the neutron-to-proton ratio in outflows from GRB accretion disks and find that it can be significant even when the disk is optically thin to neutrinos.

We report initial results on the statistical properties of the dimmest gamma-ray bursts (GRBs) observed with the Burst and Transient Source Experiment (BATSE), using new ground-based methods to obtain a sample of GRBs from 502 days of BATSE data. Using the most sensitive ground-based detection of GRBs, the sample extends to GRBs much fainter than those detected by the onboard trigger, but because of the temporal resolution of the data, the sample is limited to GRBs of duration of at least 2 s. For each detected event, Bayesian probabilities are calculated for the event to belong to each of seven classes of differing physical origins. The sample of GRB candidates is defined by the requirement that the Bayesian probability for belonging to the GRB class is higher than 0.5. The intensity distribution of the GRB sample is corrected using a Monte Carlo simulation of the postflight detection efficiency. The sky distribution of the dimmest BATSE bursts is consistent with isotropy. The dimmest bursts are softer than the brightest triggered bursts.

We describe an observation of the X-ray-luminous SN 1978K in NGC 1313 using the ACIS detector on board Chandra and an archival XMM-Newton EPIC observation. The models that provided good fits to the ASCA SIS and GIS and the ROSAT PSPC spectra no longer do so for the ACIS and EPIC spectra. The best-fit models to the ACIS and EPIC spectra are dual hot plasma models (VMEKAL); one component is soft (T = 0.61 keV, 90% errors), and the other is harder (T = 3.16 keV). For the varying abundances permitted within the model, only the Si abundance of the soft component differs from solar, with a value n/n = 3.20 (90% errors). From a ratio of the low- and high-T model fits to the Chandra and XMM-Newton spectra, we infer an exponent n of the ejecta density distribution, ρ_ejecta ∝ r^-n, of ~5.2, adopting a circumstellar matter distribution exponent of s = 2 (ρ_cs ∝ r^-s). The 0.5-2 keV light curve shows essentially no decline; the 2-10 keV light curve, comprised of only the ASCA, XMM-Newton, and Chandra observations, shows a drop of 1.5 from the ASCA epoch. The hard-band decline, together with the apparently enhanced Si emission, signal the start of the X-ray decline of SN 1978K.

In this paper I study the magnetosphere of a black hole that is connected by the magnetic field to a thin conducting Keplerian disk. I consider the case of a Schwarzschild black hole only, leaving the more interesting but difficult case of a Kerr black hole to a future study. I assume that the magnetosphere is ideal, stationary, axisymmetric, and force-free. I pay special attention to the two singular surfaces present in the system, i.e., the event horizon and the inner light cylinder; I use the regularity condition at the light cylinder to determine the poloidal electric current as a function of poloidal magnetic flux. I numerically solve the Grad-Shafranov equation, which governs the structure of the magnetosphere, for two cases: the case of a nonrotating disk and the case of a Keplerian disk. I find that, in both cases, the poloidal flux function on the horizon matches a simple analytical expression corresponding to a radial magnetic field that is uniform on the horizon. Using this result, I express the poloidal current as an explicit function of the flux and find a perfect agreement between this analytical expression and my numerical results.

We apply the tidal truncation model proposed by Negueruela and Okazaki to arbitrary Be/compact star binaries to study the dependence of the truncation efficiency on the binary parameters. We find that the viscous decretion disks around the Be stars can be truncated very effectively in narrow systems. Combining this with the population synthesis results of Podsiadlowski, Rappaport, and Han which state that binary black holes are most likely to be born in systems with orbital periods less than about 30 days, we suggest that most of the Be/black hole binaries may be transient systems with a very long quiescent state. This could explain the lack of observed Be/black hole X-ray binaries. We also discuss the evolution of the Be/black hole binaries and their possible observational features.

We describe results from time-dependent numerical modeling of the collisionless reverse shock terminating the pulsar wind in the Crab Nebula. We treat the upstream relativistic wind as composed of ions and electron-positron plasma embedded in a toroidal magnetic field, flowing radially outward from the pulsar in a sector around the rotational equator. The relativistic cyclotron instability of the ion gyrational orbit downstream of the leading shock in the electron-positron pairs launches outward-propagating magnetosonic waves. Because of the fresh supply of ions crossing the shock, this time-dependent process achieves a limit cycle, in which the waves are launched with periodicity on the order of the ion Larmor time. Compressions in the magnetic field and pair density associated with these waves, as well as their propagation speed, semiquantitatively reproduce the behavior of the wisp and ring features described in recent observations obtained using the Hubble Space Telescope and the Chandra X-Ray Observatory. By selecting the parameters of the ion orbits to fit the spatial separation of the wisps, we predict the period of time variability of the wisps that is consistent with the data. When coupled with a mechanism for nonthermal acceleration of the pairs, the compressions in the magnetic field and plasma density associated with the optical wisp structure naturally account for the location of X-ray features in the Crab. We also discuss the origin of the high-energy ions and their acceleration in the equatorial current sheet of the pulsar wind.

We report the results of an extensive monitoring campaign of PSR J0537-6910, the 16 ms pulsar in the Large Magellanic Cloud, using data acquired with the Rossi X-Ray Timing Explorer. The spin evolution of this pulsar is found to experience extreme episodic discontinuities in its spin-down rate during the 2.6 yr campaign. The rate of occurrence of these timing glitches is 2.3 per year, comparable to the highest seen for any pulsar. The mean glitch amplitude produced a fractional change in the frequency of Δν/ν = 0.36 × 10^-6 and in the frequency derivative of Δ/ = 3 × 10^-4. Despite this prodigious timing activity, we are able to derive a phase-connected timing solution between glitch events with an average spin-down rate of -1.9743 × 10^-10 Hz s^-1. The integrated effect of the glitches in was so large that the apparent characteristic age of the pulsar (-ν/2) decreased significantly during the campaign. We discuss the implications of a large glitch activity and high braking index on the spin evolution of young pulsars.

Recent RXTE and Chandra discoveries of low-mass X-ray binaries with ultrashort orbital periods have initiated theoretical work on the origins of these peculiar systems. Using the StarTrack population synthesis code, we analyze the formation and evolution of X-ray ultracompact binaries (UCBs) in the Galactic field. The relative number of UCBs with a neutron star or a black hole accretor populating our Galaxy is predicted. Our results demonstrate that standard evolutionary scenarios involving primordial binaries can be sufficient to produce the UCBs in the Galactic field without requiring additional processes associated with the dense stellar environments in the cores of globular clusters. In contrast to previous studies, we find that the majority of the immediate progenitors of these systems consist of a hydrogen-exhausted donor with an ONeMg white dwarf. The evolution of these systems leads to the accretion-induced collapse of the white dwarf to a neutron star, which can play an important role in the formation of a majority of Galactic UCBs. We predict that with an increase in the number of X-ray-active UCBs hosting neutron stars by an order of magnitude, a system with a black hole accretor may be found.

Using classical model atmospheres and an LTE analysis, Mg isotope ratios ²⁴Mg:²⁵Mg:²⁶Mg are measured in 32 Hyades dwarfs covering effective temperatures 4000 K ≤ T_eff ≤ 5000 K. We find no significant trend in any isotope ratio versus T_eff, and the mean isotope ratio is in excellent agreement with the solar value. We determine stellar parameters and Fe abundances for 56 Hyades dwarfs covering 4000 K ≤ T_eff ≤ 6200 K. For stars warmer than 4700 K, we derive a cluster mean value of [Fe/H] = 0.16 ± 0.02 (σ = 0.1), in good agreement with previous studies. For stars cooler than 4700 K, we find that the abundance of Fe from ionized lines exceeds the abundance of Fe from neutral lines. At 4700 K, [Fe/H]_II - [Fe/H]_I 0.3 dex, while at 4000 K [Fe/H]_II - [Fe/H]_I 1.2 dex. This discrepancy between the Fe abundance from neutral and ionized lines likely reflects inadequacies in the model atmospheres and the presence of non-LTE or other effects. Despite the inability of the models to reproduce the ionization equilibrium for Fe, the Mg isotope ratios appear immune to these problems and remain a powerful tool for studying Galactic chemical evolution.

We report on a detailed abundance analysis of HE 0107-5240, a halo giant with [Fe/H]_NLTE = -5.3. This star was discovered in the course of follow-up medium-resolution spectroscopy of extremely metal-poor candidates selected from the digitized Hamburg/ESO objective-prism survey. On the basis of high-resolution VLT/UVES spectra, we derive abundances for eight elements (C, N, Na, Mg, Ca, Ti, Fe, and Ni) and upper limits for another 12 elements. A plane-parallel LTE model atmosphere has been specifically tailored for the chemical composition of HE 0107-5240. Scenarios of the origin of the abundance pattern observed in the star are discussed. We argue that HE 0107-5240 is most likely not a post-asymptotic giant branch star and that the extremely low abundances of the iron-peak and other elements are not due to selective dust depletion. The abundance pattern of HE 0107-5240 can be explained by preenrichment from a zero-metallicity Type II supernova (SN II) of 20-25 M_☉, plus either self-enrichment with C and N or production of these elements in the asymptotic giant branch phase of a formerly more massive companion, which is now a white dwarf. However, significant radial velocity variations have not been detected within the 52 days covered by our moderate- and high-resolution spectra. Alternatively, the abundance pattern can be explained by enrichment of the gas cloud from which HE 0107-5240 formed by a 25 M_☉ first-generation star exploding as a subluminous SN II, as proposed by Umeda & Nomoto. We discuss consequences of the existence of HE 0107-5240 for low-mass star formation in extremely metal-poor environments and for currently ongoing and future searches for the most metal-poor stars in the Galaxy.

When a main sequence star evolves into a red giant and its Kuiper Belt objects (KBOs) reach a temperature of ~170 K, the dust released during the rapid ice sublimation of these cometary bodies may lead to a detectable infrared excess at 25 μm, depending on the mass of the KBOs. Analysis of IRAS data for 66 first-ascent red giants with 200 L_☉ < L < 300 L_☉ within 150 pc of the Sun provides an upper limit to the mass in KBOs at 45 AU orbital radius that is usually less than ~0.1 M_⊕. With improved infrared data, we may detect systems of KBOs around first-ascent red giants that are analogs to our solar system.

Submillimeter and near-infrared images of cool dusty debris disks and rings suggest the existence of unseen planets. At dusty but nonimaged stars, semimajor axes of associated planets can be estimated from the dust temperature. For some young stars these semimajor axes are greater than 1'' as seen from Earth. Such stars are excellent targets for sensitive near-infrared imaging searches for warm planets. To probe the full extent of the dust and hence of potential planetary orbits, Spitzer observations should include measurements with the 160 μm filter.

A time-dependent model based on a numerical solution of Parker's transport equation is used to model the modulation of cosmic-ray protons, electrons, and helium for full 11 year and 22 year modulation cycles using a compound approach. This approach incorporates the concept of propagating diffusion barriers, increases in the heliospheric magnetic field as they propagate from the Sun throughout the heliosphere, time-dependent gradient, curvature, and current-sheet drifts, and other basic modulation mechanisms. The model results are compared with those of the observed 11 year and 22 year cycles for 1.2 GV electrons and 1.2 GV helium at Earth for the period of 1975-1998. The model solutions are also compared with the observed charge-sign-dependent modulation along the Ulysses trajectory for the period of 1990-1998. This compound approach to long-term modulation, especially charge-sign-dependent modulation, is found to be remarkably successful. It is shown that the model can account for the latitude dependence of cosmic-ray protons and electrons by assuming a large perpendicular diffusion in the polar direction. This approach contributes to an improved understanding of how diffusion and drifts vary from solar minimum to maximum modulation and what the time dependence of the heliospheric diffusion coefficients may be. It is found that less than 10% of the available drifts are needed at solar maximum, when the solar magnetic field reverses, to explain, e.g., the observed electron-to-proton ratio along the Ulysses trajectory.

Resonant absorption, first proposed by Ionson, was extensively studied as one of the major candidates for coronal heating, and it is believed that the Alfvénic fluctuation with large amplitudes and gradients established around the resonant layer is essential to this mechanism. On the basis of this point, we investigate the effect of nonlinear coupling in the dissipative layer of a coronal loop driven by azimuthal footpoint motions with two frequencies. With a three-wave interaction model, we find that the pump mode (ω₂, k₂) could be intensively modulated by the large-amplitude resonant fluctuation v₁ accumulated localizedly around the dissipative layer. The derived modified wavenumber (k_m > 10²k₂) implies strong phase mixing and the consequent phase modulation dissipation, and the resulting damping length L_d is smaller than the loop length L in the active regions, which may be instructive to explain the recent observation on the localized heating at the footpoints of coronal loops.

From 2001 September 14 to 16 the Ultraviolet Coronal Spectrometer aboard the Solar and Heliospheric Observatory conducted a series of observations designed to study the absolute elemental abundances of an active region streamer. The absolute elemental abundances for O, Si, Fe, Mg, N, S, and Ar were calculated utilizing a constant electron temperature technique and an alternative approach to the emission measure technique, both producing complementary results. The manifestation of the first ionization potential (FIP) effect was observed here as in previous streamers. Two aspects of this streamer make it unusual and worth special attention. The active region streamer observed possessed an abundance-depleted core typically, although not exclusively, seen in quiescent streamers. A coronal mass ejection (CME) occurred at the active region during the observations. This event resulted in the second unexpected aspect of this streamer, its quick return to the same general abundance characteristics that existed before the CME occurred. The re-formation of a depleted core and the presence of the FIP effect immediately after a highly disruptive event are surprising given the 1 day timescale predicted by streamer theories.

By applying time-distance helioseismology measurements and inversions to Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) dynamics campaign data, we obtain synoptic maps of subsurface plasma-flow fields at a depth of 0-12 Mm for seven solar Carrington rotations, covering the years 1996-2002, from solar-activity minimum to maximum. Vorticity distribution and both zonal and meridional flows are derived from such synoptic flow maps, which contain an enormous amount of information about solar dynamics. The results for the zonal flows agree well with previous results. The meridional flows of an order of 20 m s^-1 are found to remain poleward during the whole period of observations. In addition to the poleward meridional flows observed at the solar minimum, extra meridional circulation cells of flow converging toward the activity belts are found in both hemispheres, which may imply plasma downdrafts in the activity belts. These converging flow cells migrate toward the solar equator together with the activity belts as the solar cycle evolves. The vorticity distributions are largely linear with latitude, and the deviations from the vorticity caused by the mean differential rotation are presented. Patterns of large-scale flows are investigated for a large active region at different depths. Converging flows toward the center of the active region are found near the solar surface, and divergent flows in this large active region are found to be rooted much deeper than similar flows observed in individual sunspots. We conclude that the extremely rich and complicated dynamics of the upper convection zone reveal remarkable organization on the large scale, which can be correlated with the magnetic activity zones.

The Sun's large-scale magnetic field is believed to be generated deep within the Sun, at the base of the convection zone, in a predominantly stably stratified region of strong radial shear, known as the tachocline. Observations indicate that, owing to magnetic buoyancy, this field rises to the surface to form active regions with systematic properties. Whereas previous investigations of magnetic buoyancy instability have generally assumed a static basic state, here we investigate the role of an aligned shear in modifying this instability. We adopt two complementary approaches. Using the energy principle, we derive sufficient conditions for the stability of a magnetic field in a stratified, compressible shear flow. To investigate the nature of the unstable modes, we have performed a series of numerical calculations; these suggest that the shear has an "axisymmetrizing" and stabilizing effect on the magnetic buoyancy instability. We discuss the implications for the instability of the solar magnetic field.