Table of contents

We show that the baryonic oscillations expected in the galaxy power spectrum may be used as a "standard cosmological ruler" to facilitate accurate measurement of the cosmological equation of state. Our approach involves a straightforward measurement of the oscillation "wavelength" in Fourier space, which is fixed by fundamental linear physics in the early universe and hence is highly model-independent. We quantify the ability of future large-scale galaxy redshift surveys with mean redshifts z ~ 1 and ~3 to delineate the baryonic peaks in the power spectrum, and we derive corresponding constraints on the parameter w describing the equation of state of the dark energy. For example, a survey of 4 times the Sloan volume at z ~ 1 can produce a measurement with accuracy Δw ≈ 0.1. We suggest that this method of measuring the dark energy powerfully complements other probes, such as Type Ia supernovae, and suffers from a different (and arguably less serious) set of systematic uncertainties.

We present a complete sample of 29 gamma-ray bursts (GRBs) for which it has been possible to determine temporal breaks (or limits) from their afterglow light curves. We interpret these breaks within the framework of the uniform conical jet model, incorporating realistic estimates of the ambient density and propagating error estimates on the measured quantities. In agreement with our previous analysis of a smaller sample, the derived jet opening angles of those 16 bursts with redshifts result in a narrow clustering of geometrically corrected gamma-ray energies about _γ = 1.33 × 10⁵¹ ergs; the burst-to-burst variance about this value is 0.35 dex, a factor of 2.2. Despite this rather small scatter, we demonstrate in a series of GRB Hubble diagrams that the current sample cannot place meaningful constraints upon the fundamental parameters of the universe. Indeed, for GRBs to ever be useful in cosmographic measurements, we argue the necessity of two directions. First, GRB Hubble diagrams should be based upon fundamental physical quantities such as energy, rather than empirically derived and physically ill-understood distance indicators (such as those based upon prompt burst time-profiles and spectra). Second, a more homogeneous set should be constructed by culling subclasses from the larger sample. These subclasses, although now first recognizable by deviant energies, ultimately must be identifiable by properties other than those directly related to energy. We identify a new subclass of GRBs ("f-GRBs") that appear both underluminous by factors of at least 10 and exhibit a rapid fading (f_ν ∝ t^-2) at early times (t ≲ 0.5 day). About 10%-20% of observed long-duration bursts appear to be f-GRBs.

The redshift distribution of flat-spectrum radio sources with 5 GHz flux densities S₅ ≳ 5 mJy is a key component in using current radio lens surveys to probe the cosmological model. We have constructed the first flat-spectrum radio sample in the flux density range 3-20 mJy. Our new sample has 33 sources; we have determined the redshifts of 14 of these (42% complete). The low mean redshift, ⟨z⟩ ≃ 0.75, of our faintest sample needs to be confirmed by further observations to improve the sample completeness. We also increased the redshift completeness of several surveys of brighter flat-spectrum sources. While the mean redshift ⟨z⟩ ≃ 1.1 of flat-spectrum samples fainter than 1 Jy is nearly constant, the fraction of the sources identifiable as quasars steadily drops from ~80% to ~10% as the flux density of the sources decreases.

In order to search for metals in the Lyα forest at redshifts z_abs > 4, we have obtained spectra of high signal-to-noise ratio and moderately high resolution of three QSOs at z_em > 5.4 discovered by the Sloan Digital Sky Survey. These data allow us to probe to metal enrichment of the intergalactic medium at early times with higher sensitivity than previous studies. We find 16 C IV absorption systems with column densities log N(C ) = 12.50-13.98 over a total redshift path ΔX = 3.29. In the redshift interval z = 4.5-5.0, where our statistics are most reliable, we deduce a comoving mass density of C³⁺ ions Ω = (4.3 ± 2.5) × 10^-8 (90% confidence limits) for absorption systems with log N(C ) ≥ 13.0 (for an Einstein-de Sitter cosmology with h = 0.65). This value of Ω is entirely consistent with those measured at z < 4; we confirm the earlier finding by Songaila that neither the column density distribution of C IV absorbers nor its integral show significant redshift evolution over a period of time that stretches from ~1.25 to ~4.5 Gyr after the big bang. This somewhat surprising conclusion may be an indication that the intergalactic medium was enriched in metals at z ≫ 5, perhaps by the sources responsible for its reionization. Alternatively, the C IV systems we see may be associated with outflows from massive star-forming galaxies at later times, while the truly intergalactic metals may reside in regions of the Lyα forest of lower density than those probed up to now.

We present Chandra ACIS-S observations of the enigmatic Seyfert 2 galaxy NGC 4698. This object, together with several other bona fide Seyfert 2 galaxies, shows no absorption in the low spatial resolution ASCA data, in contrast to the standard unification models. Our Chandra observations of NGC 4698 probe directly the nucleus, allowing us to check whether nearby sources contaminate the ASCA spectrum. Indeed, the Chandra observations show that the ASCA spectrum is dominated by two nearby AGNs. The X-ray flux of NGC 4698 is dominated by a nuclear source with luminosity L_{0.3-8 keV} ~ 10³⁹ ergs s^-1, coincident with the radio nucleus. Its spectrum is well represented by a power law, Γ ≈ 2.2, obscured by a small column density of 5 × 10²⁰ cm^-2, suggesting that NGC 4698 is an atypical Seyfert galaxy. On the basis of its low luminosity, we then interpret NGC 4698 as a Seyfert galaxy that lacks a broad-line region.

We have developed a numerical model for the temporal evolution of particle and photon spectra resulting from nonthermal processes at the shock fronts formed in merging clusters of galaxies. Fermi acceleration is approximated by injecting power-law distributions of particles during a merger event, subject to constraints on maximum particle energies. We consider synchrotron, bremsstrahlung, Compton, and Coulomb processes for the electrons, nuclear, photomeson, and Coulomb processes for the protons, and knock-on electron production during the merging process. The broadband radio through γ-ray emission radiated by nonthermal protons and primary and secondary electrons is calculated both during and after the merger event. Using ROSAT observations to establish typical parameters for the matter density profile of clusters of galaxies, we find that typical merger shocks are weak and accelerate particles with relatively soft spectra. We consider the prospects for detecting nonthermal radio and γ-ray emission from clusters of galaxies and implications for the origin of ultra-high-energy cosmic rays and the diffuse γ-ray background. Our results suggest that only a few of the isotropically distributed unidentified EGRET sources are due to shocks formed in cluster mergers and that only a minor contribution to the diffuse extragalactic γ-ray background can originate from cluster merger shocks. Cluster merger shocks can accelerate protons to ≲10¹⁹ eV for the standard parameters considered here. We predict that GLAST will detect several cluster mergers, and depending on the mean magnetic fields in the intracluster medium, the Low Frequency Array could detect anywhere from several to several hundred.

We investigate theoretical models for the radio halo and hard X-ray (HXR) excess in the Coma galaxy cluster. Time-independent and time-dependent reacceleration models for relativistic electrons have been carried out to study the formation of the radio halo and HXR excess. In these models, the relativistic electrons are injected by merger shocks and reaccelerated by ensuing violent turbulence. The effects of different Mach numbers of the merger shocks on the radio and HXR excess emission are also investigated. We adopt 6 μG as the central magnetic field and reproduce the observed radio spectra via the synchrotron emission. We also obtain a central "plateau" in the distribution of the radio spectral-indices, which have been observed in the radio emission distribution. Our models can also produce the observed HXR excess emission via the inverse Compton scattering of the cosmic microwave background photons. We find that only the merger shocks with Mach numbers around 1.6-2 can produce results in agreement with both the radio and HXR emission in the Coma Cluster.

Using new XMM and Chandra observations, we present an analysis of the temperature structure of the hot gas within a radius of 100 kpc of the bright nearby galaxy group NGC 5044. A spectral deprojection analysis of data extracted from circular annuli reveals that a two-temperature model (2T) of the hot gas is favored over single-phase or cooling flow ( = 4.5 ± 0.2 M_☉ yr^-1) models within the central ~30 kpc. Alternatively, the data can be fitted equally well if the temperature within each spherical shell varies continuously from ~T_h to T_c ~ T_h/2, but no lower. The high spatial resolution of the Chandra data allows us to determine that the temperature excursion T_h → T_c required in each shell exceeds the temperature range between the boundaries of the same shell in the best-fitting single-phase model. This is strong evidence for a multiphase gas having a limited temperature range. We do not find any evidence that azimuthal temperature variations within each annulus on the sky can account for the range in temperatures within each shell. We provide a detailed investigation of the systematic errors on the derived spectral models considering the effects of calibration, plasma codes, bandwidth, variable N_H, and background rate. We find that the RGS gratings and the EPIC and ACIS CCDs give fully consistent results when the same models are fitted over the same energy ranges for each instrument. The cooler component of the 2T model has a temperature (T_c ~ 0.7 keV) similar to the kinetic temperature of the stars. The hot phase has a temperature (T_h ~ 1.4 keV) characteristic of the virial temperature of the ~10¹³M_☉ halo expected in the NGC 5044 group. However, in view of the morphological disturbances and X-ray holes visible in the Chandra image within R ≈ 10 kpc, bubbles of gas heated to ~T_h in this region may be formed by intermittent AGN feedback. Some additional heating at larger radii may be associated with the evolution of the cold front near R ~ 50 kpc, as suggested by the sharp edge in the EPIC images.

We present a study of the [C II] 157.74 μm fine-structure line in a sample of 15 ultraluminous infrared (IR) galaxies (IR luminosity L_IR ⩾ 10¹²L_☉; ULIRGs) using the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). We confirm the observed order of magnitude deficit (compared to normal and starburst galaxies) in the strength of the [C II] line relative to the far-infrared (FIR) dust continuum emission found in our initial report, but here with a sample that is twice as large. This result suggests that the deficit is a general phenomenon affecting 4 out of 5 ULIRGs. We present an analysis using observations of generally acknowledged photodissociation region (PDR) tracers ([C II], [O I] 63 and 145 μm, and FIR continuum emission), which suggests that a high ultraviolet flux G₀ incident on a moderate density n PDR could explain the deficit. However, comparisons with other ULIRG observations, including CO (1-0), [C I] (1-0), and 6.2 μm polycyclic aromatic hydrocarbon (PAH) emission, suggest that high G₀/n PDRs alone cannot produce a self-consistent solution that is compatible with all of the observations. We propose that non-PDR contributions to the FIR continuum can explain the apparent [C II] deficiency. Here, unusually high G₀ and/or n physical conditions in ULIRGs as compared to those in normal and starburst galaxies are not required to explain the [C II] deficit. Dust-bounded photoionization regions, which generate much of the FIR emission but do not contribute significant [C II] emission, offer one possible physical origin for this additional non-PDR component. Such environments may also contribute to the observed suppression of FIR fine-structure emission from ionized gas and PAHs, as well as the warmer FIR colors found in ULIRGs. The implications for observations at higher redshifts are also revisited.

Deep Chandra observations (53 ks, ACIS-S3) of NGC 3077, a starburst dwarf galaxy in the M81 triplet, resolve the X-ray emission from several supershells. The emission is brightest in the cavities defined by expanding shells detected previously in Hα emission. Thermal emission models fitted to the data imply temperatures ranging from ~1.3 to 4.9 × 10⁶ K and indicate that the strongest absorption is coincident with the densest clouds traced by CO emission. The fitted emission measures give pressures of P/k ≈ (10⁵-10⁶)ξ^-0.5f K cm^-3 (where ξ is the metallicity of the hot gas in solar units and f_v is the volume filling factor). Despite these high pressures, the radial density profile of the hot gas is not as steep as that expected in a freely expanding wind (e.g., as seen in the neighboring starburst galaxy M82), implying that the hot gas is still confined by the Hα shells. The chaotic dynamical state of NGC 3077 undermines reliable estimates of the escape velocity. The more relevant quantity for the ultimate fate of the outflow is probably the gas density in the rich intragroup medium. Based on the H I distribution of NGC 3077 and a connected tidal tail we argue that the wind has the potential to leave the gravitational well of NGC 3077 to the north but not to the south. The total 0.3-6.0 keV X-ray luminosity is ~(2-5) × 10³⁹ ergs s^-1 (depending on the selected thermal plasma model). Most (~85%) of the X-ray luminosity in NGC 3077 comes from the hot interstellar gas; the remainder comes from six X-ray point sources. In spite of previous claims to the contrary, we do not find X-ray emission originating from the prominent tidal tail near NGC 3077.

We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). We detect 46 X-ray sources, of which 12 lie within 1 core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources, for which we determine the X-ray luminosity function down to a limit of about 6 × 10³⁰ ergs s^-1. We measure the radial distribution of the X-ray sources and fit it to a King profile finding a core radius of r_c,X ≈ 11''. We measure for the first time the unconfused phase-averaged X-ray spectrum of the 3.05 ms pulsar B1821-24 and find that it is best described by a power law with photon index Γ ≃ 1.2. We find marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum, which could be interpreted as cyclotron emission from a corona above the pulsar's polar cap if the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5-8.0 keV band is ≈3.5 × 10^-13 ergs s^-1 cm^-2. We present spectral analyses of the five brightest unidentified sources. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T = 90 eV and the radius R = 14.5 km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts.

We analyze deep near-IR adaptive optics imaging (taken with NAOS/CONICA on the Very Large Telescope at the European Southern Observatory, Chile), as well as new proper-motion data of the nuclear star cluster of the Milky Way. The surface density distribution of faint (H ≤ 20, K_s ≤ 19) stars peaks within 02 of the black hole candidate Sgr A*. The radial density distribution of this stellar "cusp" follows a power law of exponent α ~ 1.3-1.4. The K-band luminosity function of the overall nuclear stellar cluster (within 9'' of Sgr A*) resembles that of the large-scale Galactic bulge but shows an excess of stars at K_s≤ 14. It fits population synthesis models of an old, metal-rich stellar population with a contribution from young, early, and late-type stars at the bright end. In contrast, the cusp within ≤15 of Sgr A* appears to have a featureless luminosity function, suggesting that old, low-mass, horizontal-branch/red-clump stars are lacking. Likewise, there appear to be fewer late-type giants. The innermost cusp also contains a group of moderately bright, early-type stars that are tightly bound to the black hole. We interpret these results as evidence that the stellar properties change significantly from the outer cluster (≥a few arcseconds) to the dense innermost region around the black hole. We find that most of the massive early-type stars at distances of 1''-10'' from Sgr A* are located in two rotating and geometrically thin disks. These disks are inclined at large angles and counterrotate with respect to each other. Their stellar content is essentially the same, indicating that they formed at the same time. We conclude that of the possible formation scenarios for these massive stars the most probable one is that 5-8 million years ago two clouds fell into the center, collided, were shock compressed, and then formed two rotating (accretion) disks orbiting the central black hole. For the OB stars in the central arcsecond, on the other hand, a stellar merger model is the most appealing explanation. These stars may thus be "super-blue stragglers," formed and "rejuvenated" through mergers of lower mass stars in the very dense (≥10⁸M_☉ pc^-3) environment of the cusp. The "collider model" also accounts for the lack of giants within the central few arcseconds. The star closest to Sgr A* in 2002, S2, exhibits a 3.8 μm excess. We propose that the mid-IR emission comes either from the accretion flow around the black hole itself or from dust in the accretion flow that is heated by the ultraviolet emission of S2.

We present new high-resolution H I images of the Galactic chimney GSH 277+00+36. The chimney is at a distance of ~6.5 kpc, is more than 600 pc in diameter, and extends at least 1 kpc above and below the Galactic midplane. Using the Australia Telescope Compact Array and the Parkes Radiotelescope as part of the Southern Galactic Plane Survey, we have imaged the H I associated with this chimney, with a spatial resolution of ~6 pc. These are among the highest spatial resolution images of an H I chimney. We find very narrow well-defined shell walls, a remarkably empty interior, and complex small-scale structures. The shell walls show a very steep reduction in emission at the interior edge and a more gradual decline toward the exterior. We suggest that this structure is characteristic of compression and may be used to distinguish stellar by-product shells from shell-like structures resulting from random turbulent motions. The shell and chimney walls also exhibit a great deal of small-scale structure, which we discuss in the context of hydrodynamic instabilities. We find that these structures are primarily cold gas with narrow line widths in the range 1.5-2.5 km s^-1.

We use recent results on interstellar gas toward nearby stars and interstellar by-products within the solar system to select among the equilibrium radiative transfer models of the nearest interstellar material presented in Slavin & Frisch. For the assumption that O/H ~ 400 parts per million, models 2 and 8 are found to yield good fits to available data on interstellar material inside and outside of the heliosphere, with the exception of the Ne abundance in the pickup ion and anomalous cosmic-ray populations. For these models, the interstellar medium (ISM) at the entry point to the heliosphere has n(H⁰) = 0.202-0.208 cm^-3, n(He⁰) = 0.0137-0.0152 cm^-3, and ionizations χ(H) = 0.29-0.30, χ(He) = 0.47-0.51. These best models suggest that the chemical composition of the nearby ISM is ~60%-70% subsolar if S is undepleted. Both H⁰ and H⁺ need to be included when evaluating abundances of ions found in warm diffuse clouds. Models 2 and 8 yield an H filtration factor of ~0.46. Gas-to-dust mass ratios for the ISM toward CMa are R_gd = 178-183 for solar abundances of Holweger or R_gd = 611-657 for an interstellar abundance standard 70% solar. Direct observations of dust grains in the solar system by Ulysses and Galileo yield R_gd ≃ 115 for models 2 and 8, supporting earlier results (Frisch and coworkers). If the local ISM abundances are subsolar, then gas and dust are decoupled over small spatial scales. The inferred variation in R_gd over parsec length scales is consistent with the fact that the ISM near the Sun is part of a dynamically active cluster of cloudlets flowing away from the Sco-Cen association. Observations toward stars within ~500 pc show that R_gd correlates with the percentage of the dust mass that is carried by iron, suggesting that an Fe-rich grain core (by mass) remains after grain destruction. Evidently large dust grains (>10^-13 g) and small dust grains (<10^-13 g) are not well mixed over parsec length spatial scales in the ISM. It also appears that very small C-dominated dust grains have been destroyed in the ISM within several parsecs of the Sun, since C appears to be essentially undepleted. However, if gas-dust coupling breaks down over the cloud lifetime, the missing mass arguments applied here to determine R_gd and dust grain mineralogy are not appropriate.

We present results of mapping observations of the DNC, HN¹³C, and H¹³CO⁺ lines (J = 1-0) toward four nearby dark cloud cores, TMC-1, L1512, L1544, and L63, along with observations of the DNC and HN¹³C lines (J = 2-1) toward selected positions. By use of statistical equilibrium calculations based on the large velocity gradient (LVG) model, the H₂ densities are derived to be × 10⁵ cm^-3, and the [DNC]/[HN¹³C] ratios are derived to be 1.25-5.44, with a typical uncertainty of a factor of 2. The observed [DNC]/[HNC] ratios range from 0.02 to 0.09, assuming a [¹²C]/[¹³C] ratio of 60. Distributions of DNC and HN¹³C are generally similar to each other, whereas the distribution of H¹³CO⁺ is more extended than those of DNC and HN¹³C, indicating that they reside in a more inward part of the cores than HCO⁺. The [DNC]/[HN¹³C] ratio is rather constant within each core, although small systematic gradients are observed in TMC-1 and L63. In particular, no such systematic gradient is found in L1512 and L1544, where a significant effect of depletion of molecules is reported toward the central part of the cores. This suggests that the [DNC]/[HNC] ratio would not be very sensitive to the depletion factor, unlike the [DCO⁺]/[HCO⁺] ratio. On the other hand, the core-to-core variation of the [DNC]/[HNC] ratio, which ranges over an order of magnitude, is more remarkable than the variation within each core. These results are interpreted qualitatively by a combination of three competing time-dependent processes: gas-phase deuterium fractionation, depletion of molecules onto grain surfaces, and dynamical evolution of a core.

We report B-LYP/6-31G* and B3-LYP/6-31G* density functional theory calculations on a set of polycyclic hydrocarbons, ranging in size from C₁₉H₂₂ to C₃₆H₃₂, combining aromatic (unsaturated) and aliphatic (saturated, sp³-hybridized carbon) ring systems. These locally aromatic polycyclic hydrocarbons (LAPHs), generally exhibiting large deviations from planarity, may be considered as intermediate structures between polycyclic aromatic hydrocarbons (PAHs) and nanodiamonds. Calculated infrared vibrational frequencies are found to be similar to those observed experimentally in spectra of hydrogenated amorphous carbon (HAC) and other carbonaceous solids. In the C-H stretching region (~3.1-3.6 μm) these species are characterized by strong absorption/emission within both the aliphatic and aromatic C-H bands. They also show spectral features associated with tertiary C-H. Similar features are evident in calculated spectra of the corresponding ions, which we have characterized in some cases. Ionization results in the particular enhancement of a spectral feature typically seen at ~6.4 μm, in the aromatic C-C stretching region. In keeping with previous experimental and theoretical studies on the spectra of neutral and cationic PAHs, we find that the influence of ionization on the relative intensities of C-C and C-H stretching features is much greater than the influence of molecular structure. We suggest that LAPHs may be significant contributors to emission in Type B unidentified infrared emission sources.

We have imaged the planetary nebula (PN) NGC 246 in the near-ultraviolet wavelengths [Ne V] 342.6 nm, the Bowen fluorescence line of O III at 344.4 nm, and a nearby line-free region centered on 338.6 nm, as well as Hα, [O III] 500.7 nm, and [S II] 673.0 and 671.5 nm. Imaging in the 344.4 nm line is necessary to deconvolve contamination of the [Ne V] images by O III 342.9 nm. The emission from the shell and inner parts of the nebula is detected in [Ne V]. The radial profiles of the [Ne V] brightness decrease with radius from the exciting star, indicating that the bulk of the emission from this ion is due to the hard UV stellar radiation field, with a (probably) marginal contribution from collisional ionization in a shock between the PN shell and the interstellar medium (ISM). In contrast, the radial profiles of the emission in Hα, [O III] 500.7 nm, and [S II] are flatter and peak at the location of the shell. The emission of [S II] probably traces the interaction of the PN with the ambient ISM. We also present two-dimensional numerical simulations for this PN-ISM interaction. The simulations consider the stellar motion with respect to the ambient ISM, with a velocity of 85 km s^-1, and include the time evolution of the wind parameters and UV radiation field from the progenitor star.

We present results of numerical simulations carried out with a two-dimensional radiation hydrodynamics code in order to study the impact of massive stars on their surrounding interstellar medium. This first paper deals with the evolution of the circumstellar gas around an isolated 60 M_☉ star. The interaction of the photoionized H II region with the stellar wind bubble forms a variety of interesting structures like shells, clouds, fingers, and spokes. These results demonstrate that complex structures found in H II regions are not necessarily relics from the time before the gas became ionized but may result from dynamical processes during the course of the H II region evolution. We have also analyzed the transfer and deposit of the stellar wind and radiation energy into the circumstellar medium until the star explodes as a supernova. Although the total mechanical wind energy supplied by the star is negligible compared to the accumulated energy of the Lyman continuum photons, the kinetic energy imparted to the circumstellar gas over the star's lifetime is 4 times higher than for a comparable windless simulation. Furthermore, the thermal energy of warm photoionized gas is lower by some 55%. Our results document the necessity to consider both ionizing radiation and stellar winds for an appropriate description of the interaction of OB stars with their circumstellar environment.

In an attempt to probe the magnetic field morphology near the massive young star Orion IRc2, we mapped the linear polarization of its J = 2-1 SiO masers, in both the v = 0 and v = 1 vibrational levels, with 05 resolution. The intense v = 1 masers are confined to a narrow zone 40 AU from the star. Their polarization position angles vary significantly on timescales of years. For the v = 1 masers the stimulated emission rate R is likely to exceed the Zeeman splitting gΩ caused by any plausible magnetic field; in this case, the maser polarization need not correlate with the field direction. The much weaker v = 0 masers in the ground vibrational level lie 100-700 AU from IRc2, in what appears to be a flared disk. Their fractional polarizations are as high as 50%. The polarization position angles vary little across the line profile or the emission region and appear to be stable in time. The position angle (P.A. = 80^°) we measure for the J = 2-1 masers differs by 70° from that measured for the J = 1-0 SiO transition, possibly because of Faraday rotation in the foreground, Orion A, H II region. A rotation measure 3.3 × 10⁴ rad m^-2 is required to bring the J = 2-1 and J = 1-0 position angles into concordance. The intrinsic polarization position angle for both transitions is then 57°, parallel to the plane of the putative disk. The magnetic field probably threads the disk poloidally. There is little evidence for a pinched or twisted field near the star.

The idea that gamma-ray bursts might be a phenomenon associated with neutron star kicks was first proposed by Dar & Plaga. Here we study this mechanism in more detail and point out that the neutron star should be a high-speed one (with proper motion larger than ~1000 km s^-1). It is shown that the model agrees well with observations in many aspects, such as the energetics, the event rate, the collimation, the bimodal distribution of durations, the narrowly clustered intrinsic energy, and the association of gamma-ray bursts with supernovae and star-forming regions. We also discuss the implications of this model on the neutron star kick mechanism and suggest that the high kick speed was probably acquired as the result of the electromagnetic rocket effect of a millisecond magnetar with an off-centered magnetic dipole.

We examine the stability of self-similar solutions for an accelerating relativistic blast wave that is generated by a point explosion in an external medium with a steep radial density profile of a power-law index greater than 4.134. These accelerating solutions apply, for example, to the breakout of a gamma-ray burst outflow from the boundary of a massive star, as assumed in the popular collapsar model. We show that short wavelength perturbations may grow, but only by a modest factor ≲10.

A large number of neutron stars (NSs), ~10⁹, populate the Galaxy, but only a tiny fraction of them is observable during the short radio pulsar lifetime. The majority of these isolated NSs, too cold to be detectable by their own thermal emission, should be visible in X-rays as a result of accretion from the interstellar medium. The ROSAT All-Sky Survey has, however, shown that such accreting isolated NSs are very elusive: only a few tentative candidates have been identified, contrary to theoretical predictions that up to several thousand should be seen. We suggest that the fundamental reason for this discrepancy lies in the use of the standard Bondi formula to estimate the accretion rates. We compute the expected source counts using updated estimates of the pulsar velocity distribution, realistic hydrogen atmosphere spectra, and a modified expression for the Bondi accretion rate, as suggested by recent MHD simulations and supported by direct observations in the case of accretion around supermassive black holes in nearby galaxies and in our own. We find that, whereas the inclusion of atmospheric spectra partly compensates for the reduction in the counts due to the higher mean velocities of the new distribution, the modified Bondi formula dramatically suppresses the source counts. The new predictions are consistent with a null detection at the ROSAT sensitivity.

We have used the Arecibo Radio Telescope to search for millisecond pulsars in two intermediate Galactic latitude regions (7° < |b| < 20°) accessible to this telescope. For these latitudes the useful millisecond pulsar search volume achieved by Arecibo's 430 MHz beam is predicted to be maximal. Searching a total of 130 deg², we have discovered nine new pulsars and detected four previously known objects. We compare the results of this survey with those of other 430 MHz surveys carried out at Arecibo and an intermediate-latitude survey made at Parkes that included part of our search area; the latter independently found two of the nine pulsars we have discovered. At least six of our discoveries are isolated pulsars with ages between 5 and 300 Myr; one of these, PSR J1819+1305, exhibits very marked and periodic nulling. We have also found a recycled pulsar, PSR J2016+1948. With a rotational period of 65 ms, this is a member of a binary system with a 635 day orbital period. We discuss some of the properties of this system in detail and indicate its potential to provide a test of the strong equivalence principle. This pulsar and PSR J0407+16, a similar system now being timed at Arecibo, are by far the best systems known for such a test.

After almost 2.5 yr of actively accreting, the neutron star X-ray transient and eclipsing binary MXB 1659-29 returned to quiescence in 2001 September. We report on a Chandra observation of this source taken a little over a month after this transition. The source was detected at an unabsorbed 0.5-10 keV flux of only (2.7-3.6) × 10^-13 ergs cm^-2 s^-1, which implies a 0.5-10 keV X-ray luminosity of approximately (3.2-4.3) × 10³³ (d/10 kpc)² ergs s^-1, with d the distance to the source in kiloparsecs. Its spectrum had a thermal shape and could be well fitted by either a blackbody with a temperature kT of ~0.3 keV or a neutron star atmosphere model with a kT of ~0.1 keV. The luminosity and spectral shape of MXB 1659-29 are very similar to those observed of the other neutron star X-ray transients when they are in their quiescent state. The source was variable during our observation, exhibiting a complete eclipse of the inner part of the system by the companion star. Dipping behavior was observed before the eclipse, likely due to obscuration by an extended feature in the outer part of a residual accretion disk. We discuss our observation in the context of the cooling neutron star model proposed to explain the quiescent properties of neutron star X-ray transients.

We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt ≤ (5.5 ± 1.9) × 10^-15 s s^-1, after correcting for proper motion. Our results are consistent with previous values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is ∣ P/ ∣ ≥ 1.2 Gyr, comparable to the theoretical cooling timescale for the star. Our current stability limit for the period 213.13 s is only slightly less than the present measurement for another DAV, G117-B15A, for the period 215.2 s, establishing this mode in ZZ Ceti as the second most stable optical clock known, comparable to atomic clocks and more stable than most pulsars. Constraining the cooling rate of ZZ Ceti aids theoretical evolutionary models and white dwarf cosmochronology. The drift rate of this clock is small enough that we can set interesting limits on reflex motion due to planetary companions.

We have studied the photometric variability of very young brown dwarfs and very low mass stars (masses well below 0.2 M_☉) in the Cha I star-forming region. We have determined photometric periods in the Gunn i and R bands for the three M6.5-M7 type brown dwarf candidates Cha Hα 2, Cha Hα 3, and Cha Hα 6 of 2.2-3.4 days. These are the longest photometric periods found for any brown dwarf so far. If interpreted as rotationally induced, they correspond to moderately fast rotational velocities, which is fully consistent with their v sin i values and their relatively large radii. We have also determined periods for the two M5-M5.5 type very low mass stars B34 and CHXR 78C. In addition to the Gunn i- and R-band data, we have analyzed JHK_S monitoring data of the targets, which have been taken a few weeks earlier and confirm the periods found in the optical data. Upper limits for the errors in the period determination are between 2 and 9 hr. The observed periodic variations of the brown dwarf candidates as well as of the T Tauri stars are interpreted as modulation of the flux at the rotation period by magnetically driven surface features, on the basis of a consistency with v sin i values as well as R-i color variations typical for spots. Furthermore, the temperatures even for the brown dwarfs in the sample are relatively high (>2800 K) because the objects are very young. Therefore, the atmospheric gas should be sufficiently ionized for the formation of spots on one hand, and the temperatures are too high for significant dust condensation and hence variabilities due to clouds on the other hand. A comparison with rotational properties of older brown dwarfs shows that most of the acceleration of brown dwarfs takes place within the first 30 Myr or less. If magnetic braking plays a role, this suggests that the disk dissipation for brown dwarfs occurs between a few and 36 Myr.

We present results from a Chandra ACIS observation of the M8 dwarf star VB 10, a star near the hydrogen-burning mass limit. Until now, VB 10 has only been detected to flare at X-ray wavelengths. We can now report that nonflare, "quiescent," X-ray emission has been detected with a luminosity, L_X = (2.4 ± 0.05) × 10²⁵ ergs s^-1 and log(L_X/L_bol) = -4.9. This is consistent with the previous ROSAT nondetections of quiescent emission from VB 10. We discuss the implications of this discovery for the nature of coronae in ultracool dwarfs.

We model the infrared (IR) emission from the double-ring disk of HD 141569A, using a porous dust model that was previously shown to be successful in reproducing the spectral energy distributions (SEDs) of the disks around β Pictoris and HR 4796A. The dust consists of either unaltered or highly processed interstellar materials and vacuum with a volume fraction of ~90%. Together with a population of polycyclic aromatic hydrocarbon (PAH) molecules, of which a large fraction are charged, the porous dust model made of coagulated but otherwise unaltered protostellar interstellar grains provides an excellent fit to the entire SED from the mid-IR to millimeter wavelengths, including the PAH mid-IR emission features. The nondetection of the 21 μm crystalline silicate feature predicted from the porous dust model composed of highly processed grains (mainly crystalline silicate dust) imposes an upper limit of ~10% on the mass fraction of crystalline silicates. Spectroscopic and broadband photometric predictions are made for SIRTF observations, which will provide further tests of the applicability of the porous dust model to circumstellar disks.

We show that under certain circumstances the differences between the absorption mean and Planck mean opacities can lead to multiple solutions for an LTE atmospheric structure. Since the absorption and Planck mean opacities are not expected to differ significantly in the usual case of radiative equilibrium, nonirradiated atmospheres, the most interesting situations in which the effect may play a role are strongly irradiated stars and planets, and also possibly structures in which there is a significant deposition of mechanical energy, such as stellar chromospheres and accretion disks. We have presented an illustrative example of a strongly irradiated giant planet in which the bifurcation effect is predicted to occur for a certain range of distances from the star.

A set of radial velocity measurements of HD 160691 has been recently published by H. Jones and coauthors. It reveals a linear trend that indicates the presence of a second planet in this system. The preliminary double-Keplerian orbital fit to the observations, announced by the discovery team, describes a highly unstable, self-disrupting configuration. Because the observational window of the HD 160691 system is narrow, the orbital parameters of the hypothetical second companion are unconstrained. In this paper we try to find out whether a second giant planet can exist out to the distance of Jupiter and search for the dynamical constraints on its orbital parameters. Our analysis employs a combination of fitting algorithms and simultaneous examination of the dynamical stability of the obtained orbital fits. It reveals that if the semimajor axis of the second planet is smaller than ≃5.2 AU, the observations are consistent with quasi-periodic, regular motions of the system confined to the islands of various low-order mean motion resonances, e.g., 3:1, 7:2, 4:1, 5:1, or to their vicinity. In such cases the second planet has smaller eccentricity, ≃0.2-0.5, than estimated in previous works. We show that the currently available Doppler data rather preclude the 2:1 mean motion resonance expected by some authors to be present in the HD 160691 system. We also demonstrate that the MEGNO penalty method (MEGNO is an acronym for the mean exponential growth factor of nearby orbits), developed in this paper, which is a combination of the genetic minimization algorithm and the MEGNO stability analysis, can be efficiently used for predicting stable planetary configurations when only a limited number of observations is given or the data do not provide tight constraints on the orbital elements.

We compute the magnetic helicity injected by transient photospheric horizontal flows in six solar active regions associated with halo coronal mass ejections (CMEs) that produced major geomagnetic storms and magnetic clouds (MCs) at 1 AU. The velocities are computed using the local correlation tracking (LCT) method. Our computations cover time intervals of 110-150 hr, and in four active regions the accumulated helicities due to transient flows are factors of 8-12 larger than the accumulated helicities due to differential rotation. As was first pointed out by Démoulin and Berger, we suggest that the helicity computed with the LCT method yields not only the helicity injected from shearing motions but also the helicity coming from flux emergence. We compare the computed helicities injected into the corona with the helicities carried away by the CMEs using the MC helicity computations as proxies to the CME helicities. If we assume that the length of the MC flux tubes is l = 2 AU, then the total helicities injected into the corona are a factor of 2.9-4 lower than the total CME helicities. If we use the values of l determined by the condition for the initiation of the kink instability in the coronal flux rope or l = 0.5 AU then the total CME helicities and the total helicities injected into the corona are broadly consistent. Our study, at least partially, clears up some of the discrepancies in the helicity budget of active regions because the discrepancies appearing in our paper are much smaller than the ones reported in previous studies. However, they point out the uncertainties in the MC/CME helicity calculations and also the limitations of the LCT method, which underestimates the computed helicities.

The density of interplanetary dust increases sunward to reach its maximum in the F corona, where its scattered white-light emission dominates that of the electron K corona above about 3 R_☉. The dust will interact with both the particles and fields of antisunward propagating coronal mass ejections (CMEs). To understand the effects of the CME/dust interactions we consider the dominant forces, with and without CMEs, acting on the dust in the 3-5 R_☉ region. Dust grain orbits are then computed to compare the drift rates from 5 to 3 R_☉ for periods of minimum and maximum solar activity, where a simple CME model is adopted to distinguish between the two periods. The ion-drag force, even in the quiet solar wind, reduces the drift time by a significant factor from its value estimated with the Poynting-Robertson drag force alone. The ion-drag effects of CMEs result in even shorter drift times of the large (≳3 μm) dust grains, hence faster depletion rates and lower dust-grain densities, at solar maxima. If dominated by thermal emission, the near-infrared brightness will thus display solar cycle variations close to the dust plane of symmetry. While trapping the smallest of the grains, the CME magnetic fields also scatter the grains of intermediate size (0.1-3 μm) in latitude. If light scattering by small grains close to the Sun dominates the optical brightness, the scattering by the CME magnetic fields will result in a solar cycle variation of the optical brightness distribution not exceeding 10% at high latitudes, with a higher isotropy reached at solar maxima. A good degree of latitudinal isotropy is already reached at low solar activity since the magnetic fields of the quiet solar wind so close to the Sun are able to scatter the small (≲3 μm) grains up to the polar regions in only a few days or less, producing strong perturbations of their trajectories in less than half their orbital periods. Finally, we consider possible observable consequences of individual CME/dust interactions. We show that the dust grains very likely have no observable effect on the dynamics of CMEs. The effect of an individual CME on the dust grains, however, might serve as a forecasting tool for the directions and amplitudes of the magnetic fields within the CME.

This paper follows up on our recent paper on the role of prominence mass in the storage of magnetic energy for driving a coronal mass ejection (CME). The previous paper erroneously rejected a set of sheet-prominence solutions, the recovery of which allows for a simple theoretical estimate of the mass of a quiescent prominence. For coronal fields of 5-10 G, these hydromagnetic solutions suggest that a prominence mass of (1-26) × 10¹⁶ g is needed to hold detached magnetic fields of intensity comparable to the coronal fields in an unbounded atmosphere such that the global magnetic field is energetically able to spontaneously open up and still have enough energy to account for the kinetic and gravitational potential energies carried away in a CME. This simple result is discussed in relation to observed prominence magnetic field intensities, densities, and masses, pointing to the relevance of such observations to the question of magnetic energy storage in the solar corona.

In the eruptive process of the Kopp-Pneuman type, the closed magnetic field is stretched by the eruption so much that it is usually believed to be "open" to infinity. Formation of the current sheet in such a configuration makes it possible for the energy in the coronal magnetic field to quickly convert into thermal and kinetic energies and cause significant observational consequences, such as growing postflare/CME loop system in the corona, separating bright flare ribbons in the chromosphere, and fast ejections of the plasma and the magnetic flux. An eruption on 2002 January 8 provides us a good opportunity to look into these observational signatures of and place constraints on the theories of eruptions. The event started with the expansion of a magnetic arcade over an active region, developed into a coronal mass ejection (CME), and left some thin streamer-like structures with successively growing loop systems beneath them. The plasma outflow and the highly ionized states of the plasma inside these streamer-like structures, as well as the growing loops beneath them, lead us to conclude that these structures are associated with a magnetic reconnection site, namely, the current sheet, of this eruptive process. We combine the data from the Ultraviolet Coronagraph Spectrometer, Large Angle and Spectrometric Coronagraph Experiment, EUV Imaging Telescope, and Coronal Diagnostic Spectrometer on board the Solar and Heliospheric Observatory, as well as from the Mauna Loa Solar Observatory Mark IV K-coronameter, to investigate the morphological and dynamical properties of this event, as well as the physical properties of the current sheet. The velocity and acceleration of the CME reached up to 1800 km s^-1 and 1 km s^-2, respectively. The acceleration is found to occur mainly at the lower corona (<2.76 R_☉). The post-CME loop systems showed behaviors of both postflare loops (upward motion with decreasing speed) and soft X-ray giant arches (upward motion with constant speed, or acceleration) according to the definition of Švestka. In the current sheet, the presence of highly ionized ions, such as Fe⁺¹⁷ and Ca⁺¹³, suggests temperature as high as (3-4) × 10⁶ K, and the plasma outflows have speeds ranging from 300 to 650 km s^-1. Absolute elemental abundances in the current sheet show a strong first ionization potential effect and have values similar to those found in the active region streamers. The magnetic field strength in the vicinity of the current sheet is found to be of the order of 1 G.

We have used the Zurich Imaging Stokes Polarimeter (ZIMPOL I) with the McMath-Pierce 1.5 m main telescope on Kitt Peak to obtain linear polarization measurements of the off-limb chromosphere with a sensitivity better than 1 × 10^-5. We found that the off-disk observations require a combination of good seeing (to show the emission lines) and a clean heliostat (to avoid contamination by scattered light from the Sun's disk). When these conditions were met, we obtained the following principal results:

A recently released data set of Super-Kamiokande data comprises 184 bins of about 10 days each, from 1996 May to 2001 July. The power spectrum of the acquisition times has a huge peak at frequency ν = 35.98 (period 10.15 days), where frequencies are measured in cycles per year, leading to severe aliasing of the power spectrum that complicates the analysis. The strongest peak in the range 0-100 in a power spectrum formed by a likelihood procedure is at ν = 26.57 (period 13.75 days) with S = 11.26. For the range 0-40, the second-strongest peak is at 9.42 (period 38.82 days) with S = 7.3. Since 26.57 + 9.42 = 35.99, we infer that the weaker peak at 9.42 is an alias of the stronger at 26.57. When we subtract the oscillation at 26.57 from the data, the resulting CLEAN spectrum has no peak with power in excess of 6.3 in the range 0-100, and none with power in excess of 5.0 in the range 0-40. We note that 26.57 falls in the band 26.36-27.66, formed from twice the range of synodic rotation frequencies of an equatorial section of the Sun. We find from the shuffle test that the probability of obtaining a peak of S = 11.26 or more by chance in this band is 0.1%. This new result therefore supports previous evidence, found in Homestake and GALLEX-GNO data, for rotational modulation of the solar neutrino flux. The frequency 25.57 yr^-1 points to a source of modulation at or near the tachocline.

Radiative transfer effects due to overlapping X-ray lines in a high-temperature, optically thick, highly ionized medium are investigated. One particular example, in which the O VIII Lyα doublet (2 ²P_1/2,3/2-1 ²S_1/2) coincides in frequency with the N VII Lyζ lines (7 ²P_1/2,3/2-1 ²S_1/2), is studied in detail to illustrate the effects on the properties of the emergent line spectrum. We solve the radiative transfer equation to study the energy transport of resonance-line radiation in a static, infinite, plane-parallel geometry, which is used to compute the destruction/escape probabilities for each of the lines for various total optical thicknesses of the medium, as well as destruction probabilities by sources of underlying photoelectric opacity. It is found that a large fraction of the O VIII Lyα line radiation can be destroyed by N VII, which can result in a reversal of the O VIII Lyα/N VII Lyα line intensity ratio similar to what may be seen under nonsolar abundances. Photoelectric absorption by ionized carbon and nitrogen can also subsequently increase the emission-line intensities of these ions. We show that line ratios, which are directly proportional to the abundance ratios in optically thin plasmas, are not good indicators of the true CNO abundances. Conversely, global spectral modeling that assumes optically thin conditions may yield incorrect abundance estimates when compared with observations, especially if the optical depth is large. Other potentially important overlapping lines and continua in the X-ray band are also identified, and their possible relevance to recent high-resolution spectroscopic observations with Chandra and XMM-Newton are briefly discussed.

The cosmic microwave background (CMB) anisotropy data from the COBE Far Infrared Absolute Spectrophotometer (FIRAS) is reanalyzed in light of the Wilkinson Microwave Anisotropy Probe (WMAP) observations. The frequency spectrum of the FIRAS signal that has the spatial distribution seen by WMAP is shown to be consistent with CMB temperature fluctuations well into the Wien region of the spectrum. The consistency of these data, from very different instruments with very different observing strategies, provides compelling support for the interpretation that the signal seen by WMAP is temperature anisotropy of cosmological origin. The data also limit rms fluctuations in the Compton y parameter, observable via the Sunyaev-Zeldovich effect, to Δy < 3 × 10^-6 (95% confidence level) on ~5° angular scales.

We consider the possibility that massive primordial black holes are the dominant form of dark matter. Black hole formation adds a Poisson noise to the matter density fluctuations. We use Lyα forest observations to constrain this Poisson term, which constrains the black hole masses to be less than approximately a few times 10⁴M_☉. We also find that structures with less than ~10³ black holes evaporate by now.

The intergalactic gas in groups and clusters of galaxies bears the indelible stamp of galaxy formation. We present a comparison between observations and simple theoretical models indicating that radiative cooling governs the entropy scale that sets the core radius of the intracluster medium. Entropy measured at the radius 0.1r₂₀₀ scales as T^2/3, in accord with cooling threshold models for the regulation of core entropy. Cooling of baryons to form galaxies is likely to lead to feedback, and the signature of feedback may appear farther out in the cluster. Entropy measured at the radius r₅₀₀ in all but the most massive clusters exceeds the amount that can be generated by hierarchical accretion. However, feedback that smoothes the density distribution of accreting baryons, perhaps via galactic winds, can boost entropy production at the accretion shock by a factor of ~2-4. An initial comparison of entropy at r₅₀₀ to smooth accretion models shows that smooth accretion is a plausible explanation for this excess entropy and suggests that baryon accretion onto groups was smoother than baryon accretion onto clusters.

Using a simple off-axis jet model of gamma-ray bursts (GRBs), we can reproduce the observed unusual properties of the prompt emission of GRB 980425, such as the extremely low isotropic equivalent γ-ray energy, the low peak energy, the high fluence ratio, and the long spectral lag when the jet with the standard energy of ~10⁵¹ ergs and the opening half-angle of 10° ≲ Δθ ≲ 30° is seen from the off-axis viewing angle θ_v ~ Δθ + 10γ^-1, where γ is a Lorentz factor of the jet. For our adopted fiducial parameters, if the jet that caused GRB 980425 is viewed from the on-axis direction, the intrinsic peak energy E_p(1 + z) is ~2.0-4.0 MeV, which corresponds to those of GRB 990123 and GRB 021004. We also discuss the connection of GRB 980425 in our model with the X-ray flash, and the origin of a class of GRBs with small E_γ.

Linear polarization at the level of ~1%-3% has by now been measured in several gamma-ray burst afterglows. Whereas the degree of polarization, P, was found to vary in some sources, the position angle, θ_p, was roughly constant in all cases. Until now, the polarization has been commonly attributed to synchrotron radiation from a jet with a tangled magnetic field that is viewed somewhat off-axis. However, this model predicts either a peak in P or a 90° change in θ_p around the "jet break" time in the light curve, for which there has so far been no observational confirmation. We propose an alternative interpretation, wherein the polarization is attributed, at least in part, to a large-scale, ordered magnetic field in the ambient medium. The ordered component may dominate the polarization even if the total emissivity is dominated by a tangled field generated by postshock turbulence. In this picture, θ_p is roughly constant because of the uniformity of the field, whereas P varies as a result of changes in the ratio of the ordered-to-random mean-squared field amplitudes. We point out that variable afterglow light curves should be accompanied by a variable polarization. The radiation from the original ejecta, which includes the prompt γ-ray emission and the emission from the reverse shock (the "optical flash" and "radio flare"), could potentially exhibit a high degree of polarization (up to ~60%) induced by an ordered transverse magnetic field advected from the central source.

The relative proximity of the recent gamma-ray burst GRB 030329 resulted in a large gamma-ray fluence and in the brightest-ever afterglow (AG), hours after the burst, in the radio, optical, and X-ray bands, permitting precise AG measurements, sensitive tests of models, and an excellent occasion to investigate the association of gamma-ray bursts (GRBs) with supernovae (SNe). The Cannonball model provides a good, simple, and universal description of all AGs of GRBs of known redshift, so that it is straightforward to use it to predict what the expected SN signatures are. In the case of GRB 030329, 10 days after the burst, the AG should begin to reveal the light curve, spectrum, and polarization of an underlying SN—akin to SN 1998bw—which will peak in the optical band around day 15.

The recent detection of Hα emission in the Type Ia supernova SN 2002ic could be taken to mean that the elusive progenitor systems of Type Ia supernovae have finally been identified. At first glance, the observation appears to support a single-degenerate scenario, in which the white dwarf accretes from a normal companion. In this Letter we show that the opposite may be true, and the observations may support the merger of two white dwarfs as the cause for Type Ia supernovae.

We present near-infrared J- and K-band spectra of the z = 6.4 quasar SDSS J114816.64+525150.3 obtained with the NIRSPEC spectrograph at the Keck II telescope, covering the rest-frame spectral regions surrounding the C IV λ1549 and Mg II λ2800 emission lines. The iron emission blend at rest wavelength 2900-3000 Å is clearly detected, and its strength appears nearly indistinguishable from that of typical quasars at lower redshifts. The Fe II/Mg II ratio is also similar to values found for lower redshift quasars, demonstrating that there is no strong evolution in Fe/α broad-line emission ratios even out to z = 6.4. In the context of current models for iron enrichment from Type Ia supernovae (SNe Ia), this implies that the SN Ia progenitor stars formed at z ≳ 10. We apply the scaling relations of Vestergaard and of McLure & Jarvis to estimate the black hole mass from the widths of the C IV and Mg II emission lines and the ultraviolet continuum luminosity. The derived mass is in the range (2-6) × 10⁹M_☉, with an additional uncertainty of a factor of 3 due to the intrinsic scatter in the scaling relations. This result is in agreement with the previous mass estimate of 3 × 10⁹M_☉ by Willott, McLure, & Jarvis and supports their conclusion that the quasar is radiating close to its Eddington luminosity.

In a previous paper, we presented the global solution of a new accretion flow model, namely, luminous hot accretion flows (LHAFs). In this Letter, we first show the corresponding thermal equilibrium curve of LHAFs in the mass accretion rate versus surface density diagram. Then we examine its thermal stability against local perturbations. We find that LHAFs are thermally unstable when thermal conduction is neglected. However, when the accretion rate is not very large, the timescale of the growth of perturbations is longer than the accretion timescale; therefore, the instability has no dynamical effect on the accretion flow. When the accretion rate is large, the perturbations can grow very fast at a certain radius. As a result, some cold clumps may form and the accretion flow will become multiphase.

Existing radio images of a few X-shaped radio galaxies reveal Z-symmetric morphologies in their weaker secondary lobes that cannot be naturally explained by either the galactic merger or radio-lobe backflow scenarios, the two dominant models for these X-shaped radio sources. We show that the merger picture can explain these morphologies provided one takes into account that, prior to the coalescence of their supermassive black holes, the smaller galaxy releases significant amounts of gas into the interstellar medium of the dominant active galaxy. This rotating gas, whose angular momentum axis will typically not be aligned with the original jets, is likely to provide sufficient ram pressure at a distance ~10 kpc from the nucleus to bend the extant jets emerging from the central engine, thus producing a Z-symmetry in the pair of radio lobes. Once the two black holes have coalesced some 10⁷ yr later, a rapid reorientation of the jets along a direction close to that of the orbital angular momentum of the swallowed galaxy relative to the primary galaxy would create the younger primary lobes of the X-shaped radio galaxy. This picture naturally explains why such sources typically have powers close to the FR I/II break. We suggest that purely Z-symmetric radio sources are often en route to coalescence and the concomitant emission of substantial gravitational radiation, while X-shaped ones have already merged and radiated.

We study the cluster of H I and O VI absorption systems and the claimed detection of O VIII absorption from the intergalactic medium at z ≈ 0.0567, associated with a group of galaxies toward the BL Lac object PKS 2155-304. As measured by spectrographs on the Hubble Space Telescope, Far Ultraviolet Spectroscopic Explorer, and Chandra, this system appears to contain gas at a variety of temperatures. We analyze this multiphase gas in a clumpy-infall model. From the absence of C IV and Si III absorption in the Lyα clouds, we infer metallicities less than 2.5%-10% of solar values. The only metals are detected in two O VI absorption components, offset by ±400 km s^-1 from the group barycenter (cz ≈ 16,600 km s^-1). The O VI components may signify "nearside" and "backside" infall into the group potential well, which coincides with the claimed O VIII absorption. If the claimed O VIII detection is real, our analysis suggests that clusters of strong Lyα and O VI absorbers, associated with groups of galaxies, may be the "signposts" of shock-heated metal-enriched baryons. Through combined UV and X-ray spectra of H I and O VI, O VII, and O VIII, one may be able to clarify the heating mechanism of this multiphase gas.

We present new high-resolution radio and X-ray observations of the supernova remnant (SNR) B0453-685 in the Large Magellanic Cloud, carried out with the Australia Telescope Compact Array and the Chandra X-Ray Observatory, respectively. Embedded in the SNR shell is a compact central nebula producing both flat-spectrum polarized radio emission and nonthermal X-rays; we identify this source as a pulsar wind nebula (PWN) powered by an unseen central neutron star. We present a new approach by which the properties of a SNR and PWN can be used to infer upper limits on the initial spin period and surface magnetic field of the unseen pulsar, and we conclude that this star was an initial rapid rotator with current properties similar to those of the Vela pulsar. As is the case for other similarly aged sources, there is currently an interaction taking place between the PWN and the SNR's reverse shock.

The recently discovered, ringlike structure just outside the Galactic disk in Monoceros is detected and traced among 2MASS M giant stars. We have developed a method to recover the signature of this structure from the distance probability density function of stars along a given line of sight. Application of this method reveals the presence of a large group of M giant stars at a Galactocentric distance of 18 ± 2 kpc, over +36° < b < +12° and 100° < l < 270°. Evidence that the stream extends to high negative latitudes is also found. That the structure contains M giants shows that it contains populations of at least an order-of-magnitude higher abundance than the [Fe/H] = -1.6 mean metallicity previously reported for this system. The structural characteristics of the stellar stream as traced by M giants do not support the interpretation of this structure as a homogeneously dense ring that surrounds the Galaxy but possibly as a merging dwarf galaxy with tidal arms, like the Sagittarius dwarf galaxy.

To determine the nature of the recently discovered, ringlike stellar structure at the Galactic anticenter, we have collected spectra of a set of presumed constituent M giants selected from the Two Micron All Sky Survey Point Source Catalog. Radial velocities have been obtained for stars spanning ~100°, exhibiting a trend in velocity with Galactic longitude and an estimated dispersion of σ_v = 20 ± 4 km s^-1. A mean metallicity [Fe/H] = -0.4 ± 0.3 measured for these stars combines with previous evidence from the literature to suggest a population with a significant metallicity spread. In addition, a curious alignment of at least four globular clusters of lower mean metallicity is noted to be spatially and kinematically consistent with this stellar distribution. We interpret the M giant sample position and velocity variation with Galactic longitude as suggestive of a satellite galaxy currently undergoing tidal disruption in a noncircular, prograde orbit about the Milky Way.

We show that the "puzzling" chemical composition observed in the extremely metal-poor star HE 0107-5240 may be naturally explained by the concurrent pollution of at least two supernovae. In the simplest possible model, a supernova of quite low mass (~15 M_☉) underwent a "normal" explosion and ejected ~0.06 M_☉ of ⁵⁶Ni while a second one was massive enough (~35 M_☉) to experience a strong fallback that locked in a compact remnant all the carbon-oxygen core. In a more general scenario, the pristine gas clouds were polluted by one or more supernovae of relatively low mass (less than ~25 M_☉). The successive explosion of a quite massive star experiencing an extended fallback would have largely raised the abundances of the light elements in its close neighborhood.

V4743 Sagittarii (Nova Sgr 2002 No. 3) was discovered on 2002 September 20. We obtained a 5 ks ACIS-S spectrum in 2002 November and found that the nova was faint in X-rays. We then obtained a 25 ks Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation on 2003 March 19. By this time, it had evolved into the supersoft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large-amplitude oscillations with a period of 1325 s (22 minutes) followed by a decline in the total count rate after ~13 ks of observations. The count rate dropped from ~40 counts s^-1 to practically zero within ~6 ks and stayed low for the rest of the observation (~6 ks). The spectral hardness ratio changed from maxima to minima in correlation with the oscillations and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1 and 2 MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400 km s^-1. The spectrum obtained after the decline in count rate showed emission lines of C VI, N VI, and N VII, suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton Target of Opportunity observation, obtained on 2002 April 4 and a later LETGS observation from 2003 July 18 also showed oscillations, but with smaller amplitudes.

IRAS 16547-4247 is the most luminous (6.2 × 10⁴L_☉) embedded young stellar object known to harbor a thermal radio jet. We report the discovery using the Very Large Telescope Infrared Spectrometer and Array Camera of a chain of H₂ 2.12 μm emission knots that trace a collimated flow extending over 1.5 pc. The alignment of the H₂ flow and the central location of the radio jet imply that these phenomena are intimately linked. We have also detected an isolated unresolved 12 μm infrared source toward the radio jet using TIMMI2. Our findings affirm that IRAS 16547-4247 is excited by a single O-type star that is driving a collimated jet. We argue that the accretion mechanism that produces jets in low-mass star formation also operates in the higher mass regime.