Table of contents

Why does the Zel'dovich approximation (ZA) work well to describe gravitational collapse in the universe? This problem is examined by focusing on its dependence on the dimensionality of the collapse. The ZA is known to be exact for a one-dimensional collapse. We show that the ZA becomes progressively more accurate in the order of three-, two-, and one-dimensional collapses. Furthermore, using models for spheroidal collapse, we show that the ZA remains accurate in all collapses, which become progressively lower dimensional with the passage of time. That is, the ZA is accurate because the essence of the gravitational collapse is incorporated in the ZA.

We develop an algorithm to reconstruct the triaxial shapes of dark matter halos from the anisotropic spatial distributions of their substructures for the concordance background cosmology. First, we construct an analytic model for the anisotropic spatial distribution of dark halo substructures under the assumption that the tidal field with nonzero trace in the triaxial mass distribution of the host halos generates the substructure bulk motions toward the major principal axes of the hosts. Our analytic model implies that the degree of anisotropy depends sensitively on the triaxiality of the host halos as well as the correlation between the substructure locations and the tidal shear field. Second, we set the axis ratios of the triaxial host halos as free parameters in the analytic model, fit the model to the numerical results from high-resolution N-body simulation of the concordance cosmology, and reconstruct the two axis ratios of the host halos from the best-fit values of the free parameters. The comparison of the reconstructed axis ratios with the numerical results reveals good agreement. Finally, we conclude that our analytic model may provide a physical understanding of the anisotropic spatial distribution of dark halo substructures and a new way to reconstruct in principle the triaxial shapes of dark matter halos from the observables.

It has been known for some time that rotating bars in galaxies slow due to dynamical friction against the halo. However, recent attempts to use this process to place constraints on the dark matter density in galaxies and possibly also to drive dark matter out of the center have been challenged. This paper uses simplified numerical experiments to clarify several aspects of the friction mechanism. I explicitly demonstrate the Chandrasekhar scaling of the friction force with bar mass, halo density, and halo velocity dispersion. I present direct evidence that exchanges between the bar and halo orbits at major resonances are responsible for friction and study both individual orbits and the net changes at these resonances. I also show that friction alters the phase space density of particles in the vicinity of a major resonance, which is the reason the magnitude of the friction force depends on the prior evolution. I demonstrate that bar slowdown can be captured correctly in simulations having modest spatial resolution and a practicable numbers of particles. Subsequent papers in this series delineate the dark matter density that can be tolerated in halos of different density profiles.

We analyze the formation and evolution of stellar bars in galactic disks embedded in mildly triaxial cold dark matter (CDM) halos that have density distributions ranging from large flat cores to cuspy profiles. We have applied tailored numerical simulations of analytical and live halos that include the feedback from disk/bar system onto the halo in order to test and extend earlier work by El-Zant and Shlosman. The latter employed the method of Liapunov exponents to analyze the fate of bars in analytical asymmetric halos. We find the following: (1) The bar growth is very similar in all rigid axisymmetric and triaxial halos. (2) Bars in live models experience vertical buckling instability and the formation of a pseudobulge with a boxy/peanut shape, while bars in rigid halos do not buckle. (3) In live axisymmetric halos, the bar strength varies by a factor of ≲2, in growth or decay, during the secular evolution following the buckling. The bar pattern speed evolution (i.e., deceleration) anticorrelates with the halo core size. In such halos, the bar strength is larger for smaller disk-to-halo mass ratios (D/H) within disk radii, the bar size correlates with the halo core sizes, and the bar pattern speeds correlate with the halo central mass concentration. In contrast, bars embedded in live triaxial halos have a starkly different fate: they dissolve on a timescale of ~1.5-5 Gyr due to the onset of chaos over continuous zones, sometimes leaving behind a weak oval distortion. The onset of chaos is related to the halo triaxiality, the fast-rotating bar, and the halo cuspiness. Before the bar dissolves, the region outside it develops strong spiral structures, especially in the live triaxial halos. (4) More angular momentum is absorbed (fractionally) by the triaxial halos than in the axisymmetric models. The disk-halo angular momentum exchange is mediated by the lower resonances in the latter models. (5) Cuspy halos are more susceptible than flat-core halos to having their prolateness washed out by the action of the bar. The subsequent evolution is then similar to the case of cuspy axisymmetric halos. We analyze the above results on disk and bar evolution in terms of the stability of trajectories and development of chaos in the system. We set important constraints on the triaxiality of dark matter (DM) halos by comparing our predictions to recent observational results on the properties of bars out to intermediate redshifts z ~ 1.

We describe a practical measurement of the curvature of the universe which, unlike current constraints, relies purely on the properties of the Robertson-Walker metric rather than any assumed model for the dynamics and content of the universe. The observable quantity is the cross-correlation between foreground mass and gravitational shear of background galaxies, which depends on the angular diameter distances d_A(z_l), d_A(z_s), and d_A(z_s,z_l) on the degenerate triangle formed by observer, source, and lens. In a flat universe, d_A(z_l,z_s) = d_A(z_s) - d_A(z_l), but in curved universes an additional term ∝Ω_k appears and alters the lensing observables even if d_A(z) is fixed. We describe a method whereby weak-lensing data can be used to solve simultaneously for d_A and the curvature. This method is completely insensitive to the equation of state of the contents of the universe, or amendments to general relativity that alter the gravitational deflection of light or the growth of structure. The curvature estimate is also independent of biases in the photometric redshift scale. This measurement is shown to be subject to a degeneracy among d_A, Ω_k, and the galaxy bias factors that may be broken by using the same imaging data to measure the angular scale of baryon acoustic oscillations. Simplified estimates of the accuracy attainable by this method indicate that ambitious weak-lensing + baryon-oscillation surveys would measure Ω_k to an accuracy ≈0.04f(σ_{ln z}/0.04)^1/2, where σ_{ln z} is the photometric redshift error. The Fisher-matrix formalism developed here is also useful for predicting bounds on curvature and other characteristics of parametric dark energy models. We forecast some representative error levels and compare ours to other analyses of the weak-lensing cross-correlation method. We find both curvature and parametric constraints to be surprisingly insensitive to the systematic shear calibration errors.

If an extended source, such as a galaxy, is gravitationally lensed by a massive object in the foreground, the lensing distorts the observed image. It is straightforward to simulate what the observed image would be for a particular lens and source combination. In practice, one observes the lensed image on the sky, albeit blurred by atmospheric and telescopic effects and also contaminated with noise. The question that then arises is, given this incomplete data, what combinations of lens mass distribution and source surface brightness profile could plausibly have produced this image? This is a classic example of an inverse problem, and the method for solving it is given by the framework of Bayesian inference. In this paper we demonstrate the application of Bayesian inference to the problem of gravitational lens reconstruction and illustrate the use of Markov Chain Monte Carlo simulations, which can be used when the analytical calculations become too difficult. Previous methods for performing gravitational lens inversion are seen in a new light, as special cases of the general approach presented in this paper. Thus, we are able to answer, at least in principle, lingering questions about the uncertainties in the reconstructed source and lens parameters, taking into account all of the data and any prior information we may have.

The observed enhancement of the Fe Kα line in three gravitationally lensed QSOs (MG J0414+0534, QSO 2237+0305, and H1413+117) is interpreted in terms of microlensing, even when equivalent X-ray continuum amplification is not observed. In order to interpret these observations, first we studied the effects of microlensing on quasar spectra produced by a straight fold caustic crossing over a standard relativistic accretion disk. The disk emission was analyzed using the ray-tracing method, considering Schwarzschild and Kerr metrics. When the emission is separated into two regions (an inner disk corresponding to the Fe Kα line and an outer annulus corresponding to the continuum, or vice versa), we find microlensing events that enhance the Fe Kα line without noticeable amplification of the X-ray continuum, but only during a limited time interval. Continuum amplification is expected if a complete microlensing event is monitored. Second, we studied a more realistic case of amplification by a caustic magnification pattern. In this case we could satisfactorily explain the observations if the Fe Kα line is emitted from the innermost part of the accretion disk while the continuum is emitted from a larger region. We also studied the chromatic effects of microlensing, finding that the radial distribution of temperature in the accretion disk, combined with microlensing itself, can induce wavelength-dependent variability of ~30% for microlenses with very small masses. All these results show that X-ray monitoring of gravitational lenses is a method well suited for studying the innermost structure of active galactic nucleus accretion disks.

We explored the clustering properties of Lyman break galaxies at z = 4 and 5 with an angular two-point correlation function on the basis of the very deep and wide Subaru Deep Field data. We confirmed the previous result that the clustering strength of LBGs depends on the UV luminosity in the sense that brighter LBGs are more strongly clustered. In addition, we found an apparent dependence of the correlation function slope on UV luminosity for LBGs at both z = 4 and 5. More luminous LBGs have a steeper correlation function. The bias parameter was found to be a scale-dependent function for bright LBGs, whereas it appears to be almost scale-independent for faint LBGs. Luminous LBGs have a higher bias at smaller angular scales, which decreases as the scale increases. To compare these observational results, we constructed numerical mock LBG catalogs based on a semianalytic model of hierarchical clustering combined with high-resolution N-body simulation, carefully mimicking the observational selection effects. The luminosity functions and the overall correlation functions for LBGs at z = 4 and 5 predicted by this mock catalog were found to be almost consistent with the observation. The observed dependence of the clustering on UV luminosity was not reproduced by the model, unless subsamples of distinct halo mass were considered. That is, LBGs belonging to more massive dark halos had steeper and larger amplitude correlation functions. With this model, we found that LBG multiplicity in massive dark halos amplifies the clustering strength at small scales, which steepens the correlation function. The hierarchical clustering model could therefore be reconciled with the observed luminosity dependence of the correlation function if there is a tight correlation between UV luminosity and halo mass. Our finding that the slope of the correlation function depends on luminosity could be an indication that massive dark halos hosted multiple bright LBGs.

We present a comparative study of galaxies and intergalactic gas toward the z = 2.73 quasar HS 1700+6416, to explore the effects of galaxy formation feedback on the IGM. Our observations and ionization simulations indicate that the volume within 100-200 h physical kpc of high-redshift galaxies is populated by very small (ΔL ≲ 1 kpc), dense (ρ/ ~ 1000), and metal-rich (Z ≳ - Z_☉) absorption-line regions. These systems often contain shock-heated gas seen in O VI and may exhibit [Si/C] abundance enhancements suggestive of preferential enrichment by Type II supernovae. We argue that the absorber geometries resemble thin sheets or bubbles and that their unusual physical properties can be explained using a simple model of radiatively efficient shocks plowing through moderately overdense intergalactic filaments. The high metallicities suggest that these shocks are being expelled from, rather than falling into, star-forming galaxies. There is a drop-off in the intergalactic gas density at galaxy impact parameters of ≳300 physical kpc (≳1 comoving Mpc) that may represent boundaries of the gas structures where galaxies reside. The heavy-element enhancement near galaxies covers smaller distances: at galactocentric radii between 100 and 200 h kpc the observed abundances blend into the general metallicity field of the IGM. Our results suggest that either supernova-driven winds or dynamical stripping of interstellar gas alters the IGM near massive galaxies, even at R ≳ 100 kpc. However, only a few percent of the total mass in the Lyα forest is encompassed by this active feedback at z ~ 2.5. The effects could be more widespread if the more numerous metal-poor C IV systems at impact parameters ≳200 h kpc are the tepid remnants of very powerful late-time winds. However, based on present observations it is not clear that this scenario is to be favored over one involving preenrichment by smaller galaxies at z ≳ 6.

Jet formation is thought to be closely connected with the mass of the central supermassive black hole in active galactic nuclei. The radio luminosity commonly used in investigating this issue is merely an indirect measure of the energy transported through the jets from the central engine and is severely Doppler boosted in core-dominated radio quasars. In this work, we investigate the relationship between the jet power and the black hole mass, by estimating the jet power using extrapolated extended 151 MHz flux density from the VLA 5 GHz extended radio emission, for a sample of 146 radio-loud quasars compiled from the literature. After removing the effect of relativistic beaming in the radio and optical emission, we find a significant intrinsic correlation between the jet power and the black hole mass. It strongly implies that the jet power, like jet formation, is closely connected with the black hole mass. To eliminate the beaming effect in the conventional radio loudness, we define a new radio loudness as the ratio of the radio extended luminosity to the optical luminosity estimated from the broad-line luminosity. In a tentatively combined sample of radio-quiet with our radio-loud quasars, the apparent gap around the conventional radio loudness R = 10 is not prominent for the new-defined radio loudness. In this combined sample, we find a significant correlation between the black hole mass and new-defined radio loudness.

Before the official first-light images, the Chandra X-Ray Observatory obtained an X-ray image of the field to which its focal plane was first exposed. We describe this historic observation and report our study of the first Chandra field. Chandra's Advanced CCD Imaging Spectrometer (ACIS) detected 15 X-ray sources, the brightest being dubbed Leon X-1 to honor the Chandra telescope scientist Leon Van Speybroeck. Based on our analysis of the X-ray data and spectroscopy at the European Southern Observatory (ESO; La Silla, Chile), we find that Leon X-1 is a type-1 (unobscured) active galactic nucleus (AGN) at redshift z = 0.3207. Leon X-1 exhibits strong Fe II emission and a broad-line Balmer decrement that is unusually flat for an AGN. Within the context of the eigenvector-1 correlation space, these properties suggest that Leon X-1 may be a massive (≥10⁹ M_☉) black hole, accreting at a rate approaching its Eddington limit.

We discuss the contribution of kiloparsec-scale jets in FR I radio galaxies to the diffuse γ-ray background radiation. The analyzed γ-ray emission comes from inverse-Compton scattering of starlight photon fields by the ultrarelativistic electrons whose synchrotron radiation is detected from such sources at radio, optical, and X-ray energies. We find that these objects, under the minimum-power hypothesis (corresponding to a magnetic field of 300 μG in the brightest knots of these jets), can contribute about one percent to the extragalactic γ-ray background measured by EGRET. We point out that this result already indicates that the magnetic fields in kiloparsec-scale jets of low-power radio galaxies are not likely to be smaller than 10 μG on average, as otherwise the extragalactic γ-ray background would be overproduced.

With currently available XMM-Newton EPIC pn observations spanned over about 3 yr, we present a detailed spectral and temporal variability of the 0.2-10 keV X-ray emission from the X-ray-bright BL Lac object PKS 2155-304. The spectral variability is examined with a model-independent hardness ratio method. We find that the spectral evolution of the source follows the light curves well, indicating that the spectra harden when the fluxes increase. The plots of hardness ratios versus count rates show that the spectral changes are particularly significant during flares. The cross-correlation functions (CCFs) show that the light curves in the different energy bands are well correlated at different time lags. The CCF peaks (i.e., the maximum correlation coefficients) tend to become smaller with larger energy differences, and the variabilities in the different energy bands are more correlated for the flares than for the other cases. In most cases the higher energy band variations lead the lower energy band, but in two cases we observed the opposite behavior, that the lower energy variability possibly leads the higher energy variability. The time lags increase with the energy differences between the two cross-correlated light curves. The maximum lag is found to be up to about 1 hr, supporting the findings obtained with previous low Earth orbit X-ray missions. We discuss our results in the context of the particle acceleration, cooling, and light-crossing timescales.

The ROSAT All-Sky Survey-Green Bank (RGB) BL Lac object sample presented by Laurent-Muehleisen et al. includes not only interesting intermediate but also extreme HBL and LBL sources. These unique characteristics make it possible to unambiguously address the question of how HBLs and LBLs are related. In this paper, we study the relations between the radio-X-ray spectral index (α_rx) and each of the luminosities (L_r, L_o, L_x), as well as the relation between the X-ray spectral index (α_x) and the radio luminosity (L_r) for this sample. Our analysis results indicate that (1) plots of (α_rx, L_i) (i denotes r, o, or x) all exhibit the continuity of α_rx; (2) of 71 low-L_r RGB BL Lac objects (L_{5 GHz} < 10²⁵ W Hz^-1), 24% have α_rx steeper than 0.75, while 79% are distributed in the region 0.6 < α_rx < 0.9 and have such a broad scatter in α_rx that α_rx can range from 0.55 to 0.8 for any given value of L_r; (3) of 60 low-L_r RGB BL Lac objects (with α_x available), 20% have α_x flatter than 1, and no correlation is present for L_r versus α_x. In addition, no correlation is found between L_o and α_rx for the RGB BL Lac sample. These characteristics seem inconsistent with those derived from the earlier classic combined surveys. From our analysis, however, we also found that (1) there is a significant anticorrelation for L_x versus α_rx, which agrees with that reported by Mei et al., and (2) more importantly, α_rx is well correlated with L_r, which is consistent with the result expected in the blazar sequence but in contradiction with that derived from the DXRBS sample. All of these seemingly odd results show the importance of sampling enough parameter space in order to obtain an unbiased view or model of blazar properties.

It is well known that the simple criterion originally proposed by Polyachenko and Shukhman in their 1981 paper for the onset of the radial orbit instability, although generally a useful tool, faces significant exceptions for both mildly anisotropic systems (with some that can be proved to be unstable) and strongly anisotropic models (with some that can be shown to be stable). In this paper we address two issues: Are there processes of collisionless collapse that can lead to equilibria of the exceptional type? And what is the intrinsic structural property that is responsible for the sometimes-noted exceptional stability behavior? To clarify these issues, we have performed a series of simulations of collisionless collapse that start from homogeneous, highly symmetrized, cold initial conditions and, because of such special conditions, are characterized by very little mixing. For these runs, the end states can be associated with large values of the global pressure anisotropy parameter up to 2K_r/K_T ≈ 2.75. The highly anisotropic equilibrium states thus constructed show no significant traces of radial anisotropy in their central region, with a very sharp transition to a radially anisotropic envelope occurring well inside the half-mass radius (around 0.2 r_M). To check whether the existence of such an almost perfectly isotropic "nucleus" might be responsible for the apparent suppression of the radial orbit instability, we could not resort to equilibrium models with the above characteristics and with analytically available distribution function. Instead, we studied and confirmed the stability of configurations with those characteristics by initializing N-body approximate equilibria (with given density and pressure anisotropy profiles) with the help of the Jeans equations.

We present deep K_s < 21.5 (Vega) identifications, redshifts, and stellar masses for most of the sources composing the bulk of the 24 μm background in the GOODS/CDFS. Our identified sample consists of 747 Spitzer MIPS 24 μm objects and includes ~94% of all the 24 μm sources in the GOODS-South field that have fluxes S_ν(24 μm) > 83 μJy (the ~80% completeness limit of the Spitzer/GTO 24 μm catalog); 36% of our galaxies have spectroscopic redshifts (mostly at z < 1.5), and the remaining ones have photometric redshifts of very good quality, with a median of |dz| = |z_spec - z_phot|/(1 + z_spec) = 0.02. We find that MIPS 24 μm galaxies span the redshift range z ~ 0-4 and that a substantial fraction (28%) lie at high redshifts z ≳ 1.5. We determine the existence of a bump in the redshift distribution at z ~ 1.9, indicating the presence of a significant population of galaxies with PAH emission at these redshifts. The 24 μm galaxy population ranges from sources with intermediate luminosities (10¹⁰L_☉ < L_IR < 10¹¹L_☉) and low-to-intermediate assembled stellar masses (10⁹M_☉ ≲ M ≲ 10¹¹M_☉) at z ≲ 0.8, to massive (M ≳ 10¹¹M_☉) hyperluminous galaxies (L_IR > 10¹²L_☉) at redshifts z ~ 2-3. Massive star-forming galaxies at redshifts 2 ≲ z ≲ 3 are characterized by very high star formation rates (SFR > 500 M_☉ yr^-1), and some of them are able to construct a mass of ≈10¹⁰-10¹¹M_☉ in a single burst lifetime (~0.01-0.1 Gyr). At lower redshifts z ≲ 2, massive star-forming galaxies are also present but appear to be building their stars on long timescales, either quiescently or in multiple modest burstlike episodes. At redshifts z ~ 1-2, the ability of the burstlike mode to produce entire galaxies in a single event is limited to some lower (M ≲ 7 × 10¹⁰M_☉) mass systems, and it is basically negligible at z ≲ 1. Our results support a scenario in which star formation activity is differential with assembled stellar mass and redshift, and where the relative importance of the burstlike mode proceeds in a downsizing way from high to low redshifts.

We present new spectroscopic observations of 13 H II regions in the Local Group spiral galaxy M33. The regions observed range from 1 to 7 kpc in distance from the nucleus. Of the 13 H II regions observed, the [O III] λ4363 line was detected in six regions. Electron temperatures were thus able to be determined directly from the spectra using the [O III] λλ4959, 5007/λ4363 line ratio. Based on these temperature measurements, oxygen and neon abundances and their radial gradients were calculated. For neon, a gradient of -0.016 ± 0.017 dex kpc^-1 was computed, which agrees with the Ne/H gradient derived previously from ISO spectra. A gradient of -0.012 ± 0.011 dex kpc^-1 was computed for O/H, much shallower than was derived in previous studies. The newly calculated O/H and Ne/H gradients are in much better agreement with each other, as expected from predictions of stellar nucleosynthesis. We examine the correlation between the WC/WN ratio and metallicity, and find that the new M33 abundances do not impact the observed correlation significantly. We also identify two new He II-emitting H II regions in M33, the first to be discovered in a spiral galaxy other than the Milky Way. In both cases the nebular He II emission is not associated with Wolf-Rayet stars. Therefore, caution is warranted in interpreting the relationship between nebular He II emission and Wolf-Rayet stars when both are observed in the integrated spectrum of an H II region.

The observed spectra of the microlensed sources toward the Galactic bulge may be used as a tool for studying the kinematics and extinction effects in the Galactic bulge. In this paper, we first investigate the expected distribution of the microlensed sources as a function of depth within the Galactic bulge. Our analysis takes a magnitude-limited microlensing survey into account and includes the effects of extinction. We show that in the current magnitude-limited surveys, the probability that the source lies at the far side of the bulge is larger than the probability that the source lies at the near side. We then investigate the effects of extinction on the observed spectra of microlensed sources. Kurucz model spectra and the observed extinctions toward the Galactic bulge have been used to demonstrate that the microlensed sources should clearly show the effects of extinction, which, in turn, can be used as a statistical measure of the contribution of the disk lenses and bulge lenses at different depths. The spectra of the microlensed sources provide a unique probe to derive the radial velocities of a sample that lies preferentially at the far side of the Galactic bulge. The radial velocities, coupled with the microlensing timescales, can thus be useful in studying the three-dimensional kinematics of the Galactic bulge.

It is generally accepted that magnetic fields generated in the nonlinear development of the transverse Weibel instability provide effective collisionality in unmagnetized collisionless shocks. Recently, extensive two- and three-dimensional simulations improved our understanding of the growth and saturation of the instability in colliding plasma shells. However, the steady state structure of the shock wave transition layers remains poorly understood. We use basic physical considerations and order-of-magnitude arguments to study the steady state structure in relativistic unmagnetized collisionless shocks in pair plasmas. The shock contains an electrostatic layer resulting from the formation of stationary, magnetically focused current filaments. The filaments form where the cold upstream plasma and the counterstreaming thermal plasma interpenetrate. The filaments are not entirely neutral, and strong electrostatic fields are present. Most of the downstream particles cannot cross this layer into the upstream medium because they are trapped by the electrostatic field. We identify the critical location in the shock transition layer where the electromagnetic field ceases to be static. At this location, the degree of charge separation in the filaments reaches a maximum value and the current inside the filaments comes close to the Alfvén limit. We argue that the radius of the current filaments upstream of the critical location is about equal to the upstream plasma skin depth. Finally, we show that some downstream particles cross the electrostatic layer and run ahead of the shock into the preshock medium without causing instability. These particles may play an important role in diffusive particle acceleration.

We have analyzed the 9.7 and "18" μm interstellar silicate absorption features along the line of sight toward four heavily extincted galactic WC-type Wolf-Rayet (WR) stars. We construct two interstellar extinction curves from 1.25 to 25 μm using near-IR extinction measurements from the literature, along with the silicate profiles of WR 98a (representing the local ISM) and GCS 3 (representing the Galactic center). We have investigated the mineralogy of the interstellar silicates by comparing extinction profiles for amorphous silicates with olivine and pyroxene stoichiometry to the 9.7 and "18" μm absorption features in the WR 98a spectrum. In this analysis, we have considered solid and porous spheres and a continuous distribution of ellipsoids. While it is not possible to simultaneously provide a perfect match to both profiles, we find that the best match requires a mixture of these two types of compounds. We also consider iron oxides, aluminosilicates, and silicate carbide (SiC) as grain components. Iron oxides cannot be accommodated in the observed spectrum, while the amount of Si in SiC is limited to <4%. Finally, we discuss the cosmic elemental abundance constraints on the silicate mineralogy, grain shape, and porosity.

The Voyager 1 (V1), Voyager 2 (V2), and Pioneer 10 (P10) Lyα data sets are three of several diagnostic data sets available for the study of the very local interstellar medium (VLISM). Selected V1 data obtained on 1989 day 279 at heliocentric distance of 39.1 AU in the upstream direction relative to the incoming interstellar neutral hydrogen flow and V2 data obtained on 1990 day 143 at heliocentric distance of 32 AU, also in the upstream direction, have been used to estimate the local interstellar neutral hydrogen and proton densities and compared with P10 data obtained in 1981 at distances between 23.39 and 23.87 AU in the downstream direction, respectively. State-of-the-art plasma-neutral and radiative transfer models have been used in the interpretation of the data. It has been found that a VLISM heliospheric model with neutral hydrogen density of 0.18 cm^-3 and proton density of 0.06 cm^-3 best fits both the V1 data and the V2 data. The P10 data are best fitted by a VLISM model with neutral hydrogen density of 0.15 cm^-3 and proton density of 0.05 cm^-3. The failure to find a single best-fit stationary heliosphere plasma-neutral model suggests, among other possibilities, that a quantitative interpretation of the heliospheric Lyα glow would require the incorporation of magnetic field and time dependence in the heliospheric model.

Submillimeter lines of H₂O and NH₃ have been detected in the carbon star IRC +10216 (CW Leo) with the Odin submillimeter satellite. The detection of the J = 1₁₀ → 1₀₁ 557 GHz line of ortho-H₂O confirms the earlier detection in the same source with SWAS. The detection of the J_K = 1₀ → 0₀ 572 GHz line represents the first observation of the ground-state rotational transition of NH₃ in a stellar envelope. By fitting a molecular line transfer model to the observed lines, we derive an ortho-H₂O abundance of 2.4 × 10^-6, which is consistent with estimates from the SWAS observation. The derived ortho-NH₃ abundance of 1 × 10^-6 relative to H₂ is significantly higher than those derived from 24 GHz inversion transitions and is slightly higher than those from vibrational transitions in the infrared band. The high H₂O and NH₃ abundances in the carbon-rich star IRC +10216 underscore shortcomings in the conventional gas-phase LTE and non-LTE chemical models.

In this work, the gas infall and the formation of outflows around low- and high-mass protostars are investigated. A radial self-similar approach to model the transit of the molecular gas around the central object is employed. We include gravitational and radiative fields to produce heated pressure-driven outflows with magnetocentrifugal acceleration and collimation. Outflow solutions with negligible or vanishing magnetic fields are reported. They indicate that thermodynamics is a sufficient engine to generate an outflow. The magnetized solutions show dynamically significant differences in the axial region, precisely where the radial velocity and collimation are the largest. They compare quantitatively well with observations. The influence of the opacity on the transit solutions is also studied. It is found that, when dust is not the dominant coolant, such as in the primordial universe, mass infall rates have substantial larger values in the equatorial region. This suggests that stars forming in a dust-free environment should be able to accrete much more mass and become more massive than present-day protostars. It is also suggested that molecular outflows may be dominated by the global transit of material around the protostar during the very early stages of star formation, especially in the case of massive or dust-free star formation.

The class I protostar TMC-1 (IRAS 04381+2540) is oriented favorably for determining the properties of its circumstellar envelope and outflow cavity. Deep, high spatial resolution Hubble Space Telescope (HST) NICMOS images at 1.6 μm exhibit both a narrow jet and a wide-angle conical outflow cavity. Model images of the scattered-light distribution fit the data well, reproducing the intensity level, cavity width, and observed limb brightening. The best-fit geometry for TMC-1 has a 45° ± 5° source inclination and an 80° ± 5° deprojected wind opening angle (full width). The age, normally a poorly known quantity, is well constrained; the protostar age, i.e., time since the onset of cloud collapse, is 1 × 10⁵ yr to within a factor of 2. We offer a possible resolution to the well-known luminosity problem. By considering the efficiency of infall onto the protostar, we find that plausible parameters can give an efficiency, and hence accretion luminosity, as low as 10% of the value derived from the collapsing cloud core. The efficiency, together with a luminosity constraint, leads to a mass estimate that ranges from about 0.1 M_☉ for high efficiency to 0.2 M_☉ for low accretion efficiency onto the protostar. Similarly, the estimated mass accretion rate onto the protostar ranges over roughly (0.9-1.4) × 10^-6M_☉ yr^-1, which is smaller than the (1.6-3.5) × 10^-6M_☉ yr^-1 infall rate of the cloud. If low efficiency rates are prevalent for protostars, one important consequence is that it will take longer to assemble the central star than the time t = M_in/_in, a time that assumes all of the infalling material lands on the protostar.

The mid-infrared emission from a photodissociation region (PDR) viewed edge-on in the Orion Nebula is examined through 8.7-20.6 μm images and 8-13 μm spectra. The polycyclic aromatic hydrocarbon (PAH) emission is located between the edges of H II regions and layers of [C I] emission, agreeing with PDR theory. Using a simple model, the spatial variations in the emission from PAHs detected at 8.6, 11.2, and 12.7 μm are demonstrated to be directly proportional to the material column density and the intensity of the UV field. For a homogeneous, neutral cloud illuminated by a bright OB star, PDR theory predicts that the ultraviolet (UV) radiation is attenuated exponentially (e). The predicted UV attenuation is confirmed by observations of broad PAH emission features found at 8.6, 11.2, and 12.7 μm. The PAH emission is found in cool regions having greater optical depths relative to regions where mid-infrared emission from ionized gas is observed. Through modeling we determine a gas density of 9.7 × 10⁴ cm^-3. On large and small size scales, the relative strengths of the 8.6, 11.2, and 12.7 μm PAH features at the bar of the Orion Nebula indicate that there is not a simple transition from ionized to neutral PAHs across the PDR.

We investigate the effects of the addition of pre-main-sequence evolution to star cluster simulations. We allowed stars to follow pre-main-sequence tracks that begin at the deuterium-burning birth line and end at the zero-age main sequence. We compared our simulations to ones in which the stars began their lives at the zero-age main sequence, and also investigated the effects of particular choices for initial binary orbital parameters. We find that the inclusion of the pre-main-sequence phase results in a slightly higher core concentration, lower binary fraction, and fewer hard binary systems. In general, the global properties of star clusters remain almost unchanged, but the properties of the binary star population in the cluster can be dramatically modified by the correct treatment of the pre-main-sequence stage.

Observations of the H66α recombination line from the ionized gas in the cluster of newly formed massive stars, G10.6-0.4, show that most of the continuum emission derives from the dense gas in an ionized accretion flow that forms an ionized disk or torus around a group of stars in the center of the cluster. The inward motion observed in the accretion flow suggests that despite the equivalent luminosity and ionizing radiation of several O stars, neither radiation pressure nor thermal pressure has reversed the accretion flow. The observations indicate why the radiation pressure of the stars and the thermal pressure of the H II region are not effective in reversing the accretion flow. The observed rate of the accretion flow, 10^-3M_☉ yr^-1, is sufficient to form massive stars within the timescale imposed by their short main-sequence lifetimes. A simple model of disk accretion relates quenched H II regions, trapped hypercompact H II regions, and photoevaporating disks in an evolutionary sequence.

We report "infall asymmetry" in the HCO⁺(1-0) and (3-2) lines toward NGC 1333, extended over ~0.39 pc², a larger extent than has been reported before for any star-forming region. The infall asymmetry extends over a major portion of the star-forming complex, and is not limited to a single protostar, a single dense core, or a single spectral line. It seems likely that the infall asymmetry represents inward motions, and that these motions are physically associated with the complex. Both blue-asymmetric and red-asymmetric lines are seen, but in both the (3-2) and (1-0) lines of HCO⁺ the vast majority of the asymmetric lines are blue, indicating inward motions. The (3-2) line, tracing denser gas, has the spectra with the strongest asymmetry; these spectra are associated with the protostars IRAS 4A and 4B, which most likely indicates that a warm central source is affecting the line profiles. The (3-2) and (1-0) lines usually have the same sense of asymmetry in common positions, but their profiles differ significantly, and the (1-0) line appears to trace motions on much larger spatial scales than does the (3-2) line. Line profile models fit the spectra well, but do not strongly constrain their parameters. The mass accretion rate of the inward motions is of order 10^-4M_☉ yr^-1, similar to the ratio of stellar mass to cluster age.

We develop the theory of jitter radiation from GRB shocks containing small-scale magnetic fields and propagating at an angle with respect to the line of sight. We demonstrate that the spectra vary considerably: the low-energy photon index α ranges from 0 to -1 as the apparent viewing angle goes from 0 to π/2. Thus, we interpret the hard-to-soft evolution and the correlation of α with the photon flux observed in GRBs as a combined effect of temporal variation of the viewing angle and relativistic aberration of an individual thin, instantaneously illuminated shell. The model predicts that about a quarter of time-resolved spectra should have hard spectra, violating the synchrotron α = -2/3 line of death. The model also naturally explains why the peak of the distribution of α is at α ≈ -1. The presence of a low-energy break in the jitter spectrum at oblique angles also explains the appearance of a soft X-ray component in some GRBs and a relatively small number of them. We emphasize that our theory is based solely on the first principles and contains no ad hoc (phenomenological) assumptions.

The best-sampled afterglow light curves available are for GRB 030329. A distinguishing feature of this event is the obvious rebrightening at around 1.6 days after the burst. Proposed explanations for the rebrightening mainly include the two-component jet model and the refreshed-shock model, although a sudden density jump in the circumburst environment is also a potential choice. Here we reexamine the optical afterglow of GRB 030329 numerically in light of the three models. In the density-jump model, no obvious rebrightening can be produced at the jump moment. In addition, after the density jump, the predicted flux density decreases rapidly to a level that is significantly below observations. A simple density-jump model thus can be excluded. In the two-component jet model, although the observed late afterglow (after 1.6 days) can potentially be explained as emission from the wide component, the emergence of this emission actually is too slow, and it does not manifest as a rebrightening as previously expected. The energy-injection model seems to be the most preferred choice. By engaging a sequence of energy-injection events, it provides an acceptable fit to the rebrightening at ~1.6 days, as well as the whole observed light curve that extends to ~80 days. Further studies on these multiple energy-injection processes may provide a valuable insight into the nature of the central engines of gamma-ray bursts.

The Solar Mass Ejection Imager (SMEI) views nearly every point on the sky once every 102 minutes and can detect point sources as faint as R ~ 10 mag. Therefore, SMEI can detect or provide upper limits for the optical afterglow from gamma-ray bursts in the tens of minutes after the burst, when different shocked regions may emit optically. Here we provide upper limits for 58 bursts between 2003 February and 2005 April.

We present the results of a systematic analysis of the world sample of optical/near-infrared afterglow light curves observed in the pre-Swift era by the end of 2004. After selecting the best observed 16 afterglows with well-sampled light curves that can be described by a Beuermann equation, we explore the parameter space of the light-curve parameters and physical quantities related to them. In addition, we search for correlations between these parameters and the corresponding gamma-ray data, and we use our data set to look for a fine structure in the light curves.

The bright gamma-ray burst GRB 050525a has been detected with the Swift observatory, providing unique multiwavelength coverage from the very earliest phases of the burst. The X-ray and optical/UV afterglow decay light curves both exhibit a steeper slope ~0.15 days after the burst, indicative of a jet break. This jet break time combined with the total gamma-ray energy of the burst constrains the opening angle of the jet to be 32. We derive an empirical "time-lag" redshift from the BAT data of = 0.69 ± 0.02, in good agreement with the spectroscopic redshift of 0.61. Prior to the jet break, the X-ray data can be modeled by a simple power law with index α = -1.2. However, after 300 s the X-ray flux brightens by about 30% compared to the power-law fit. The optical/UV data have a more complex decay, with evidence of a rapidly falling reverse shock component that dominates in the first minute or so, giving way to a flatter forward shock component at later times. The multiwavelength X-ray/UV/optical spectrum of the afterglow shows evidence for migration of the electron cooling frequency through the optical range within 25,000 s. The measured temporal decay and spectral indexes in the X-ray and optical/UV regimes compare favorably with the standard fireball model for gamma-ray bursts assuming expansion into a constant-density interstellar medium.

Those massive stars that give rise to gamma-ray bursts (GRBs) during their deaths must be endowed with an unusually large amount of angular momentum in their inner regions, 1-2 orders of magnitude greater than the ones that make common pulsars. Yet the inclusion of mass loss and angular momentum transport by magnetic torques during the precollapse evolution is known to sap the core of the necessary rotation. Here we explore the evolution of very rapidly rotating massive stars, including stripped-down helium cores that might result from mergers or mass transfer in a binary, and single stars that rotate unusually rapidly on the main sequence. For the highest possible rotation rates (about 400 km s^-1), a novel sort of evolution is encountered in which single stars mix completely on the main sequence, never becoming red giants. Such stars, essentially massive "blue stragglers," produce helium-oxygen cores that rotate unusually rapidly. Such stars might comprise roughly 1% of all stars above 10 M_☉ and can, under certain circumstances, retain enough angular momentum to make GRBs. Because this possibility is very sensitive to mass loss, GRBs are much more probable in regions of low metallicity.

We introduce a low Mach number equation set for the large-scale numerical simulation of carbon-oxygen white dwarfs experiencing a thermonuclear deflagration. Since most of the interesting physics in a Type Ia supernova transpires at Mach numbers from 0.01 to 0.1, such an approach enables both a considerable increase in accuracy and a savings in computer time compared with frequently used compressible codes. Our equation set is derived from the fully compressible equations using low Mach number asymptotics, but without any restriction on the size of perturbations in density or temperature. Comparisons with simulations that use the fully compressible equations validate the low Mach number model in regimes where both are applicable. Comparisons to simulations based on the more traditional anelastic approximation also demonstrate the agreement of these models in the regime for which the anelastic approximation is valid. For low Mach number flows with potentially finite amplitude variations in density and temperature, the low Mach number model overcomes the limitations of each of the more traditional models and can serve as the basis for an accurate and efficient simulation tool.

We model the dynamical evolution of primordial black holes (BHs) in dense star clusters using a simplified treatment of stellar dynamics in which the BHs are assumed to remain concentrated in an inner core, completely decoupled from the background stars. Dynamical interactions involving BH binaries are computed exactly and are generated according to a Monte Carlo prescription. Recoil and ejections lead to complete evaporation of the BH core on a timescale ~10⁹ yr for typical globular cluster parameters. Orbital decay driven by gravitational radiation can make binaries merge, and, in some cases, successive mergers can lead to significant BH growth. Our highly simplified treatment of the cluster dynamics allows us to study a large number of models and to compute statistical distributions of outcomes, such as the probability of massive BH growth and retention in a cluster. We find that, in most models, there is a significant probability (~20%-80%) of BH growth with final masses ≳100 M_☉. In one case, a BH formed with mass ≈620 M_☉. However, if the typical merger recoil speed (due to asymmetric emission of gravitational radiation) significantly exceeds the cluster escape speed, no growth ever occurs. Independent of the recoil speed, we find that BH-BH mergers enhanced by dynamical interactions in cluster cores present an important source of gravitational waves for ground-based laser interferometers. Under optimistic conditions, the total rate of detections by Advanced LIGO could be as high as a few tens of events per year from inspiraling BHs from clusters.

We describe local shearing box simulations of turbulence driven by the magnetorotational instability (MRI) in a collisionless plasma. Collisionless effects may be important in radiatively inefficient accretion flows, such as near the black hole in the Galactic center. The MHD version of ZEUS is modified to evolve an anisotropic pressure tensor. A fluid closure approximation is used to calculate heat conduction along magnetic field lines. The anisotropic pressure tensor provides a qualitatively new mechanism for transporting angular momentum in accretion flows (in addition to the Maxwell and Reynolds stresses). We estimate limits on the pressure anisotropy due to pitch angle scattering by kinetic instabilities. Such instabilities provide an effective "collision" rate in a collisionless plasma and lead to more MHD-like dynamics. We find that the MRI leads to efficient growth of the magnetic field in a collisionless plasma, with saturation amplitudes comparable to those in MHD. In the saturated state, the anisotropic stress is comparable to the Maxwell stress, implying that the rate of angular momentum transport may be moderately enhanced in a collisionless plasma.

We have constructed numerically global solutions of advective accretion disks around black holes that describe a continuous transition between the effectively optically thick outer and optically thin inner disk regions. We have concentrated on models of accretion flows with large mass accretion rates, and we have employed a bridging formula for radiative losses at high and low effective optical depths. Contrary to the models neglecting advection, we have found that global solutions exist for the extended range of accretion rates. The presence of the effectively optically thin regions in the innermost parts of accretion disks with intermediate accretion rates (~10-50 Eddington units) results in a significant increase of the plasma temperature in those regions, and this increase can be discriminated in observations.

Using observations with the Rossi X-Ray Timing Explorer, we examine the behavior of 2-10 Hz quasi-periodic oscillations (QPOs) during spectrally hard dips in the X-ray light curve of GRS 1915+105 that are accompanied by infrared flares. Of the 12 light curves examined, 9 are β class and three are α class, following the scheme of Belloni et al. In most cases, the QPO frequency is most strongly correlated to the power-law flux, which partially contradicts some earlier claims that the strongest correlation is between QPO frequency and blackbody flux. Seven β-class curves are highly correlated to blackbody features. In several cases, the QPO evolution appears to decouple from the spectral evolution. We find that β-class light curves with strong correlations can be distinguished from those without by their "trigger spike" morphology. We also show that the origin and strength of the subsequent infrared flare may be causally linked to the variations in QPO frequency evolution and not solely tied to the onset of soft X-ray flaring behavior. We divide the 12 α- and β-class light curves into three groups based on the evolution of the QPO, the morphology of the trigger spike, and the infrared flare strength. An apparent crossover case leads us to conclude that these groups are not unique modes but represent part of a continuum of accretion behaviors. We believe the QPO behavior at the initiation of the hard dip can ultimately be used to determine the terminating X-ray behavior and the following infrared flaring behavior.

We present the results of detailed analysis of pointed X-ray observations by RXTE of the black hole X-ray binary (BHXRB) system V4641 Sgr (SAX J1819.3-2525) during its outburst of 2003 August. Soft X-ray (3-20 keV) flux variations by factors of 10 or more on timescales of minutes or shorter were seen. The rapid and strong variability of this source sets it apart from typical XRBs. In spite of large luminosity fluctuations, the spectral state of the source did not change significantly during the dwells, which suggests that the physical emission processes did not change much during the observations. The energy spectra during the dwells were dominated by a hard Comptonized power-law component, indicative of the canonical low/hard state observed in other BHXRBs. No soft thermal component was found in three out of the four RXTE pointings. However, spectral deconvolution of the observation with the largest average luminosity suggests an obscured, hot accretion disk. During one of the observations, we detected a short-term (~100 s) soft X-ray dropout, which is apparently due to variability in the observed column density. A strong Fe Kα fluorescent emission line near 6.5 keV was detected with large equivalent widths in the range 700-1000 eV. In the temporal domain, the Fourier power spectra were dominated by red noise below a few hertz. Poisson noise dominated at higher frequencies, and no high-frequency features were detected. The strong Comptonized spectra, broad iron emission line, absence of a disk component in the spectra, absence of any timing variability above a few hertz, and occasional large changes in the column density along the line of sight all support an enshrouded black hole with occasional outflow and a very dynamic environment.

The 2003 X-ray outburst of the candidate black hole binary, H1743-322, was investigated in frequent pointed observations (2-250 keV) with the Rossi X-Ray Timing Explorer (RXTE). One particular program of 130 observations is organized into 111 time intervals and searched for the presence of high-frequency quasi-periodic oscillations (HFQPOs) in the range 50-2000 Hz. Only a single observation (2003 June 13) yields a detection above 4 σ. The central frequency of 239 ± 4 Hz is consistent with the 240 Hz QPO reported for this source on 2003 May 28 (Homan and coworkers). We group the observations in several different ways and compute average power-density spectra (PDS) in a search for further evidence of HFQPOs. Significant results are found for two groups defined by the presence of low-frequency QPOs (0.1-20 Hz) and an absence of "band-limited" power continua. (1) The nine time intervals with the highest X-ray flux yield a QPO at 166 ± 5 Hz. (4.1 σ; 3-35 keV). (2) The group with lower X-ray flux (24 time intervals) produces a QPO at 242 ± 3 Hz (6.0 σ; 7-35 keV). The ratio of these two frequencies is 1.46 ± 0.05. This finding is consistent with results obtained for three other black hole systems that exhibit commensurate HFQPOs in a 3 : 2 ratio. Furthermore, the occurrence of H1743-322's slower HFQPO at times of higher X-ray luminosity closely resembles the behavior of XTE J1550-564 and GRO J1655-40. We discuss our results in terms of resonance models that invoke frequencies set by general relativity.

We show that the drift waves near the light cylinder can cause modulation of the emission with periods of the order of several seconds. These periods explain the intervals between successive pulses observed in "magnetars" and radio pulsars with long periods. The model under consideration makes it possible to calculate the real rotation periods of the host neutron stars. They are less than 1 s for the investigated objects. The magnetic fields at the surface of the neutron star are of the order of 10¹¹-10¹³ G and equal to the usual fields for known radio pulsars.

We report high time-resolution broadband spectroscopic observations of two radio bursts on the classical flare star AD Leonis. The observations were acquired by the 305 m telescope at Arecibo Observatory on 2003 June 13-14. Using the Wideband Arecibo Pulsar Processor, these observations sampled a total bandwidth of 400 MHz, distributed over a 500 MHz frequency range, 1120-1620 MHz, with a frequency resolution of 0.78 MHz and a time resolution of 10 ms. The radio burst observed on June 13 is characterized by the presence of multitudes of short-duration (Δt ~ 30 ms), high brightness temperature (T_b > 10¹⁴ K), highly circularly polarized, fast-drift radio sub-bursts, with median bandwidths Δν/ν ~ 5%. The inverse drift rates are small and have a symmetric distribution (both positive and negative frequency drifts), with a Gaussian FWHM inverse drift rate of 4.5 × 10^-4 s MHz^-1. The fast-drift sub-bursts occur at a mean rate of 13 s^-1 and show no evidence for periodic recurrence. The fast-drift radio events on AD Leo are highly reminiscent of solar decimetric spike bursts. We suggest that the emission is due to fundamental plasma radiation. A second highly circularly polarized radio burst, recorded June 14, has markedly different properties: a smoothly varying intensity profile characterized by a slow drift in frequency with time (-52 MHz s^-1). Under the assumption that the source is due to a disturbance propagating through the low corona, a source size of 0.1-1 R_⋆ is inferred, implying a brightness temperature range 6 × 10¹¹-6 × 10¹³ K; another example of a coherent radio burst.

We have determined accurate values of the product of the mass-loss rate and the ion fraction of P⁺⁴, q(P⁺⁴), for a sample of 40 Galactic O-type stars by fitting stellar wind profiles to observations of the P V resonance doublet obtained with FUSE, ORFEUS BEFS, and Copernicus. When P⁺⁴ is the dominant ion in the wind [i.e., 0.5 ≲ q(P⁺⁴) ≤ 1], q(P⁺⁴) approximates the mass-loss rate to within a factor of ≲2. Theory predicts that P⁺⁴ is the dominant ion in the winds of O7-O9.7 stars, although an empirical estimator suggests that the range O4-O7 may be more appropriate. However, we find that the mass-loss rates obtained from P V wind profiles are systematically smaller than those obtained from fits to Hα emission profiles or radio free-free emission by median factors of ~130 (if P⁺⁴ is dominant between O7 and O9.7) or ~20 (if P⁺⁴ is dominant between O4 and O7). These discordant measurements can be reconciled if the winds of O stars in the relevant temperature range are strongly clumped on small spatial scales. We use a simplified two-component model to investigate the volume filling factors of the denser regions. This clumping implies that mass-loss rates determined from "ρ²" diagnostics have been systematically overestimated by factors of 10 or more, at least for a subset of O stars. Reductions in the mass-loss rates of this size have important implications for the evolution of massive stars and quantitative estimates of the feedback that hot-star winds provide to their interstellar environments.

The outer atmosphere of the M supergiant Betelgeuse is puzzling. Published observations of different kinds have shed light on different aspects of the atmosphere, but no unified picture has emerged. They have shown, for example, evidence of a water envelope (MOLsphere) that in some studies is found to be optically thick in the mid-infrared. In this paper, we present high-resolution, mid-infrared spectra of Betelgeuse recorded with the TEXES spectrograph. The spectra clearly show absorption features of water vapor and OH. We show that a spectrum based on a spherical, hydrostatic model photosphere with T_eff = 3600 K, an effective temperature often assumed for Betelgeuse, fails to model the observed lines. Furthermore, we show that published MOLsphere scenarios are unable to explain our data. However, we are able to model the observed spectrum reasonably well by adopting a cooler outer photospheric structure corresponding to T_mod = 3250 K. The success of this model may indicate that the observed mid-infrared lines are formed in cool photospheric surface regions. Given the uncertainties of the temperature structure and the likely presence of inhomogeneities, we cannot rule out the possibility that our spectrum could be mostly photospheric, albeit nonclassical. Our data put new, strong constraints on atmospheric models of Betelgeuse, and we conclude that continued investigation requires consideration of nonclassical model photospheres, as well as possible effects of a MOLsphere. We show that the mid-infrared water vapor features have great diagnostic value for the environments of K and M (super)giant star atmospheres.

We present the results of a high-resolution imaging survey for brown dwarf binaries in the Pleiades open cluster. The observations were carried out with the Advanced Camera for Surveys (Pavlovsky and coworkers) on board the Hubble Space Telescope. Our sample consists of 15 bona fide brown dwarfs. We confirm two binaries and detect their orbital motion, but we did not resolve any new binary candidates in the separation range between 5.4 and 1700 AU and masses in the range 0.035-0.065 M_☉. Together with the results of our previous study (Martín and coworkers), we can derive a visual binary frequency of 13.3 % for separations greater than 7 AU, masses in the range 0.055-0.065 M_☉, and mass ratios in the range 0.45-0.9 < q < 1.0. The other observed properties of Pleiades brown dwarf binaries (distributions of separation and mass ratio) appear to be similar to their older counterparts in the field.

A revised near-infrared classification scheme for T dwarfs is presented, based on and superseding prior schemes developed by Burgasser and coworkers and Geballe and coworkers, and defined following the precepts of the MK process. Drawing from two large spectroscopic libraries of T dwarfs identified largely in the Sloan Digital Sky Survey and the Two Micron All Sky Survey, nine primary spectral standards and five alternate standards spanning spectral types T0-T8 are identified that match criteria of spectral character, brightness, absence of a resolved companion, and accessibility from both the Northern and Southern Hemispheres. The classification of T dwarfs is formally made by the direct comparison of near-infrared spectral data of equivalent resolution to the spectra of these standards. Alternately, we have redefined five key spectral indices measuring the strengths of the major H₂O and CH₄ bands in the 1-2.5 μm region that may be used as a proxy to direct spectral comparison. Two methods of determining T spectral type using these indices are outlined and yield equivalent results. These classifications are also equivalent to those from prior schemes, implying that no revision of existing spectral type trends is required. The one-dimensional scheme presented here provides a first step toward the observational characterization of the lowest luminosity brown dwarfs currently known. Future extensions to incorporate spectral variations arising from differences in photospheric dust content, gravity, and metallicity are briefly discussed. A compendium of all currently known T dwarfs with updated classifications is presented.

We report the detection of two short-period planets discovered at Keck Observatory. HD 149143 is a metal-rich G0 IV star with a planet of M sin i = 1.33M_J and an orbital radius of 0.053 AU. The best-fit Keplerian model has an orbital period, P = 4.072 days, semivelocity amplitude, K = 149.6 m s^-1, and eccentricity, e = 0.016 ± 0.01. The host star is chromospherically inactive and metal-rich, with [Fe/H] = 0.26. Based on the T_eff and stellar luminosity, we derive a stellar radius of 1.49 R_☉. Photometric observations of HD 149143 were carried out using the automated photometric telescopes at Fairborn Observatory. HD 149143 is photometrically constant over the radial velocity period to 0.0003 ± 0.0002 mag, supporting the existence of the planetary companion. No transits were detected down to a photometric limit of approximately 0.02%, eliminating transiting planets with a variety of compositions and constraining the orbital inclination to less than 83°. A short-period planet was also detected around HD 109749, a G3 IV star. HD 109749 is chromospherically inactive, with [Fe/H] = 0.25 and a stellar radius of 1.24. The radial velocities for HD 109749 are modeled by a Keplerian with P = 5.24 days and K = 28.7 m s^-1. The inferred planet mass is M sin i = 0.28M_J and the semimajor axis of this orbit is 0.0635 AU. Photometry of HD 109749 was obtained with the SMARTS consortium telescope, the PROMPT telescope, and by transitsearch.org observers in Adelaide and Pretoria. These observations did not detect a decrement in the brightness of the host star at the predicted ephemeris time, and they constrain the orbital inclination to less than 85° for gas giant planets with radii down to 0.7R_J.

We present empirical calibrations that provide estimates of stellar metallicity, effective temperature, and surface gravity as a function of Lick IDS indices. These calibrations have been derived from a training set of 261 stars for which (1) high-precision measurements of [Fe/H], T_eff, and log g have been made using spectral-synthesis analysis of HIRES spectra, and (2) Lick indices have also been measured. Estimation of atmospheric parameters with low-resolution spectroscopy rather than photometry has the advantage of producing a highly accurate metallicity calibration, and requires only one observation per star. Our calibrations have identified a number of bright (V < 9) metal-rich stars that are now being screened for hot-Jupiter-type planets. Using the Yonsei-Yale stellar models, we show that the calibrations provide distance estimates accurate to ~20% for nearby stars. We have also investigated the possibility of constructing a "planeticity" calibration to predict the presence of planets based on stellar abundance ratios but find no evidence that a convincing relation of this type can be established. High metallicity remains the best single indicator that a given star is likely to harbor extrasolar planets.

Depolarization of solar radio bursts requires reflection off boundary layers no thicker than about a wavelength (a few meters at most) between regions with large density ratios. The implied inhomogeneities suggest that the corona is much more highly and sharply structured than can be resolved from observations at other wavelengths. A simplified version of magnetoionic theory is used to derive a depolarization coefficient; the effect of the magnetic field is ignored in treating the dispersion, but taken into account in treating the (circular) polarization. Plots of the depolarization coefficient are used to infer conditions under which effective depolarization occurs, and it is concluded that favorable conditions require total internal reflection. For type I sources away from the central meridian, effective depolarization requires reflection off an overdense structure with a density ratio ξ ≈ 10. For type III bursts, a density ratio ξ ≈ 2 suffices, with at least two reflections off walls of ducts at ≈20° to the radial.

We report observations of reconnection inflows in extreme ultraviolet (EUV) Fe XII λ195 images with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO). Yokoyama and colleagues reported the first example observed on 1999 March 18. We survey the EIT data from 1996 to 2000 and find six new inflow events. We measure the inflow velocity v_inflow for each event and find that v_inflow is about 2.6-38 km s^-1. Furthermore, using the six EIT inflow events observed simultaneously with Yohkoh SXT (including the Yokoyama event), we calculate the reconnection rate as M_A ≡ v_inflow/v_A = 0.001-0.07. It is also found that the plasmoid ejection and/or coronal mass ejection (CME) are closely related to the inflow. The velocity of the CME exhibits a correlation with the inflow velocity.

In this paper we discuss the transport of toroidal magnetic field by a weak meridional flow at the base of the convection zone. We use the differential rotation and meridional flow model developed by Rempel and incorporate feedback of a purely toroidal magnetic field in two ways: directly through the Lorentz force (magnetic tension) and indirectly through quenching of the turbulent viscosity, which affects the parameterized turbulent angular momentum transport in the model. In the case of direct Lorentz force feedback, we find that a meridional flow with an amplitude of around 2 m s^-1 can transport a magnetic field with a strength of 20-30 kG. Quenching of turbulent viscosity leads to deflection of the meridional flow from the magnetized region and a significant reduction of the transport velocity if the magnetic field is above equipartition strength.

Previous mid-infrared, far-infrared, and new submillimeter data relating solely to ⁵²CrH in its X⁶Σ⁺ state have been reanalyzed by a least-squares fit using a Hund's case (b) Hamiltonian to determine the best obtainable set of parameters for the molecule. In particular, the fine structure and hyperfine constants have been improved. From these parameters, transition frequencies are determined that are more reliable than those published previously; the latter show systematic errors of up to 15 MHz. Such frequencies will facilitate the identification of CrH in the interstellar medium.

Erratum to (ApJ, 376, 278 [1991])

Gamma-ray burst (GRB) 050904 is the most distant X-ray source known, at z = 6.295, comparable to the farthest AGNs and galaxies. Its X-ray flux decays, but not as a power law; it is dominated by large variability from a few minutes to at least half a day. The spectra soften from a power law with photon index Γ = 1.2-1.9 and are well fit by an absorbed power law with possible evidence of large intrinsic absorption. There is no evidence for discrete features, in spite of the high signal-to-noise ratio. In the days after the burst, GRB 050904 was by far the brightest known X-ray source at z > 4. In the first minutes after the burst, the flux was >10^-9 ergs cm^-2 s^-1 in the 0.2-10 keV band, corresponding to an apparent luminosity >10⁵ times larger than the brightest AGNs at these distances. More photons were acquired in a few minutes with Swift XRT than XMM-Newton and Chandra obtained in ~300 ks of pointed observations of z > 5 AGNs. This observation is a clear demonstration of concept for efficient X-ray studies of the high-z IGM with large-area, high-resolution X-ray detectors and shows that early-phase GRBs are the only backlighting bright enough for X-ray absorption studies of the IGM at high redshift.

We demonstrate that gravitationally lensed quasars are easily recognized using image subtraction methods as time variable sources that are spatially extended. For Galactic latitudes |b| ≳ 20°, lensed quasars dominate the population of spatially extended variable sources, although there is some contamination from variable star pairs, variable star-quasar pairs, and binary quasars that can be easily controlled using other information in the survey such as the object light curves and colors. This method will allow planned large-scale synoptic surveys to find lensed quasars almost down to their detection limits without the need for extensive follow-up observations.

High-z Type Ia supernovae are expected to be gravitationally lensed by the foreground distribution of large-scale structure. The resulting magnification of supernovae is statistically measurable, and the angular correlation of the magnification pattern directly probes the integrated mass density along the line of sight. Measurements of the cosmic magnification of supernovae therefore complement measurements of galaxy shear in providing a direct measure of the clustering of the dark matter. As the surface density of supernovae is typically much smaller than that of sheared galaxies, the two-point correlation function of lensed Type Ia supernovae suffers from significantly increased shot noise. Nevertheless, we find that the magnification map of a large sample of supernovae provides an important cosmological tool. For example, a search over 20 deg² over 5 years leading to a sample of ~10,000 supernovae would measure the angular power spectrum of cosmic magnification with a cumulative signal-to-noise ratio of ~20. This detection can be further improved once the supernova distance measurements are cross-correlated with measurements of the foreground galaxy distribution. The magnification maps made using supernovae can be used for important cross-checks with traditional lensing shear statistics obtained in the same fields and can help to control systematics.

We present the discovery of a 70 kpc X-ray tail behind the small late-type galaxy ESO 137-001, in the nearby, hot (T = 6.5 keV) merging cluster A3627, from both Chandra and XMM-Newton observations. The tail has a length-to-width ratio of ~10. It is luminous (L_{0.5-2 keV} ~ 10⁴¹ ergs s^-1), with a temperature of ~0.7 keV and an X-ray M_gas of ~10⁹M_☉ (~10% of the galaxy's stellar mass). We interpret this tail as the stripped interstellar medium of ESO 137-001 mixed with the hot cluster medium, with this blue galaxy being converted into a gas-poor galaxy. Three X-ray point sources are detected in the axis of the tail, which may imply active star formation there. The straightness and narrowness of the tail also imply that the turbulence in the intracluster medium is not strong on scales of 20-70 kpc.

Seed black holes formed in the collapse of Population III stars have been invoked to explain the presence of supermassive black holes at high redshift. It has been suggested that a seed black hole can grow up to 10⁵-10⁶M_☉ through highly super-Eddington accretion for a period of ~10⁶-10⁷ yr at redshift z = 20-24. We studied the feedback of radiation pressure, Compton heating, and outflow during the seed black hole growth. It is found that its surrounding medium fueled to the seed hole is greatly heated by Compton heating. For a supercritical accretion onto a 10³M_☉ seed hole, a Compton sphere (with a temperature ~10⁶ K) will be formed on a timescale of 1.6 × 10³ yr so that the hole is only supplied by a rate of 10^-3 of the Eddington limit from the Compton sphere. The kinetic feedback of the strong outflow will heat the medium at a large distance from the black hole; this leads to a dramatic decrease of the outer Bondi accretion onto the black hole and avoids the accumulation of the matter. The highly supercritical accretion will be rapidly halted by the strong feedback. The seed black hole hardly grows up at the very early universe unless the strong feedback can be avoided.

We present Spitzer Space Telescope observations of the z = 2.38 Lyα emitter overdensity associated with galaxy cluster J2143-4423, the largest known structure (110 Mpc) above z = 2. We imaged 22 of the 37 known Lyα emitters within the filament-like structure, using the MIPS 24 μm band. We detected six of the Lyα emitters, including three of the four clouds of extended (>50 kpc) Lyα emission, also known as Lyα blobs. Conversion from a rest wavelength of 7 μm to total far-infrared luminosity using locally derived correlations suggests that all the detected sources are in the class of ultraluminous infrared galaxies (ULIRGs), with some reaching hyper-LIRG energies. Lyα blobs frequently show evidence of interaction, either in HST imaging or in the proximity of multiple MIPS sources within the Lyα cloud. This connection suggests that interaction or even mergers may be related to the production of Lyα blobs. A connection to mergers does not in itself help explain the origin of the Lyα blobs, as most of the suggested mechanisms for creating Lyα blobs (starbursts, active galactic nuclei, cooling flows) could also be associated with galaxy interactions.

The formation and structure of dark matter halos are investigated by means of constrained realizations of Gaussian fields using N-body simulations. Experiments in the formation of a 10¹²h^-1M_☉ halo are designed to study the dependence of the density profile on its merging history. We find that (1) the halo growth consists of several violent and quiescent phases, with the density well approximated by the Navarro-Frenk-White (NFW) profile at most times; (2) the NFW scale radius R_s stays constant during the quiet phases and grows abruptly during the violent ones, while the virial radius grows linearly during the quiet phases and grows abruptly during the violent phases; (3) the value of R_s reflects the violent merging history of the halo; (4) the central density stays unchanged during the quiet phases while dropping abruptly during the violent ones (its value does not reflect the formation time of the halo); and (5) the clear separation of the evolution of an individual halo into series of quiescent and violent phases explains the inability to fit its entire evolution by simple scaling relations, in agreement with previous studies.

Recent observations have discovered star formation activities in the extreme outer regions of disk galaxies. However, it remains unclear what physical mechanisms are responsible for triggering star formation in such low-density gaseous environments of galaxies. In order to understand the origin of these outer star-forming regions, we numerically investigate how the impact of dark matter subhalos orbiting a gas-rich disk galaxy embedded in a massive dark matter halo influences the dynamical evolution of the outer H I gas disk of the galaxy. We find that if the masses of the subhalos (M_sb) in a galaxy with an extended H I gas disk are as large as 10^-3M_h, where M_h is the total mass of the galaxy's dark halo, local fine structures can be formed in the extended H I disk. We also find that the gas densities of some apparently filamentary structures can exceed a threshold gas density for star formation and thus be likely to be converted into new stars in the outer part of the H I disk in some models with larger M_sb. These results thus imply that the impact of dark matter subhalos ("dark impact") can be important for better understanding the origin of recent star formation discovered in the extreme outer regions of disk galaxies. We also suggest that characteristic morphologies of local gaseous structures formed by the dark impact can indirectly prove the existence of dark matter subhalos in galaxies. We discuss the origin of giant H I holes observed in some gas-rich galaxies (e.g., NGC 6822) in the context of the dark impact.

A great deal of study has been conducted over the last 20 years on the origin of the magnetic activity in the Galactic center. One of the most popular hypotheses assumes a milligauss magnetic field with poloidal geometry, pervading the inner few hundred parsecs of the Galactic center region. However, there is growing observational evidence for the large-scale distribution of a much weaker field of B ≲ 10 μG in this region. Here we propose that the Galactic center magnetic field originates from turbulent activity, which is known to be extreme in the central hundred parsecs. In this picture, the spatial distribution of the magnetic field energy is highly intermittent, and the regions of strong field have filamentary structure. We propose that the observed nonthermal radio filaments appear in (or, possibly, may be identified with) such strongly magnetized regions. At the same time, the large-scale diffuse magnetic field is weak. Both results of our model can explain the magnetic field measurements of the Galactic center region. In addition, we discuss the role of ionized outflow from stellar clusters in producing the long magnetized filaments perpendicular to the Galactic plane.

We estimate the polarized thermal dust emission from MHD simulations of protostellar collapse and outflow formation in order to investigate the alignment of outflows with magnetic fields. The polarization maps indicate that the alignment of an outflow with the magnetic field depends on the field strength inside the cloud core; the direction of the outflow, projected on the plane of the sky, is aligned preferentially with the mean polarization vector for a cloud core with a magnetic field strength of 80 μG, while it does not tend to be aligned for 50 μG as long as the 1000 AU scale is considered. The direction of the magnetic field at the cloud center is probed by the direction of the outflow. In addition, the magnetic field at the cloud center can be revealed by the Atacama Large Millimeter Array (ALMA) even when the source is embedded deeply in the envelope. The Chandrasekhar-Fermi formula is examined using the polarization maps, indicating that the field strength predicted by the formula should be corrected by a factor of 0.24-0.44. The correction factor has a tendency to be lower for a cloud core with a weaker magnetic field.

Recent observational studies of ω Centauri by the Hubble Space Telescope have discovered a double main sequence in the color-magnitude diagrams (CMDs) of its stellar populations. These observations suggest that the stellar population with the blue main sequence (bMS) has a helium abundance much larger, by ΔY ~ 0.12, than that of the red main sequence (rMS). By using somewhat idealized models in which stars of the bMS are formed from gas ejected from stars of the rMS, we quantitatively investigate whether the helium overabundance of the bMS can result from self-enrichment from massive AGB stars, from mass loss of very massive young stars, or from Type II supernovae within ω Cen. We show that as long as the helium enrichment is due to ejecta from the rMS formed earlier than the bMS, none of these three enrichment scenarios can explain the observed properties of the bMS self-consistently for reasonable IMFs. The common, serious problem in all cases is that the observed number fraction of the bMS cannot be explained without assuming unusually top-heavy IMFs. This failure of the self-enrichment scenarios implies that most of the helium-enriched gas necessary for the formation of the bMS originated from other external sources. We thus suggest a new scenario, in which most of the second generation of stars (i.e., the bMS) in ω Cen could have formed from gas ejected from field stellar populations that surrounded ω Cen when it was the nucleus of an ancient dwarf galaxy.

We use a relativistic ray-tracing code coupled to a relativistic hydrodynamics code to analyze the X-ray emission from a pressure-supported oscillating torus around a black hole. Solving the relativistic radiative transfer equation along each photon path, we calculate the X-ray light curves, power spectra, and broadened emission lines, as seen by a distant observer from various inclinations. We show that a strong correlation exists between the intrinsic frequencies of the torus normal modes and the extrinsic frequencies seen in the observed light-curve power spectrum. This correlation demonstrates the feasibility of the oscillating-torus model to explain the multiple peaks seen in black hole high-frequency quasi-periodic oscillations. Observations of these features can provide important information about the torus as well as the black hole.

The recent discovery of high-frequency oscillations in giant flares from SGR 1806-20 and SGR 1900+14 may be the first direct detection of vibrations in a neutron star crust. If this interpretation is correct, it offers a novel means of testing the neutron star equation of state, crustal breaking strain, and magnetic field configuration. Using timing data from RHESSI, we have confirmed the detection of a 92.5 Hz quasi-periodic oscillation (QPO) in the tail of the SGR 1806-20 giant flare. We also find another, stronger QPO at higher energies, at 626.5 Hz. Both QPOs are visible only at particular (but different) rotational phases, implying an association with a specific area of the neutron star surface or magnetosphere. At lower frequencies we confirm the detection of an 18 Hz QPO, at the same rotational phase as the 92.5 Hz QPO, and report the additional presence of a broad 26 Hz QPO. We are, however, unable to make a robust confirmation of the presence of a 30 Hz QPO, despite higher count rates. We discuss our results in the light of neutron star vibration models.

In 2005 May-June, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed the Be/X-ray binary system A0535+26 during a giant outburst, during which time its hard X-ray intensity reached several crab. This bright source presented a unique opportunity to search for redshifted neutron-capture lines from the surface of the neutron star. Such lines, if discovered, would strongly constrain the neutron star equation of state, motivating this search. An upper limit on the narrow, unredshifted line has been set at 6.5 × 10^-4 photons cm^-2 s^-1, while width-dependent limits on a broadened, redshifted line are set in the range of (4.0-10.5) × 10^-4 photons cm^-2 s^-1. To our knowledge, these are the first measured upper limits on redshifted 2.2 MeV emission from an accreting neutron star.

We study the effects of an annular gap induced by an embedded protoplanet on disk scattered light images and the infrared spectral energy distribution (SED). We find that the outer edge of a gap is brighter in the scattered light images than a similar location in a gap-free disk. The stellar radiation that would have been scattered by material within the gap is instead scattered by the disk wall at the outer edge of the gap, producing a bright ring surrounding the dark gap in the images. Given sufficient resolution, such gaps can be detected by the presence of this bright ring in scattered light images. A gap in a disk also changes the shape of the SED. Radiation that would have been absorbed by material in the gap is instead reprocessed by the outer gap wall. This leads to a decrease in the SED at wavelengths corresponding to the temperature at the radius of the missing gap material, and to a corresponding flux increase at longer wavelengths corresponding to the temperature of the outer wall. We note, however, that the presence of an annular gap does not change the bolometric IR flux; it simply redistributes the radiation previously produced by material within the gap to longer wavelengths. Although it will be difficult on the basis of the SED alone to distinguish between the presence of a gap and other physical effects, the level of changes can be sufficiently large to be measurable with current instruments (e.g., Spitzer).

We observed the embedded, young 8-10 M_☉ star AFGL 490 at subarcsecond resolution with the Plateau de Bure Interferometer (PdBI) in the C¹⁷O (2-1) transition and found convincing evidence that AFGL 490 is surrounded by a rotating disk. Using two-dimensional modeling of the physical and chemical disk structure coupled to line radiative transfer, we constrain its basic parameters. We obtain a relatively high disk mass of 1 M_☉ and a radius of ~1500 AU. A plausible explanation for the apparent asymmetry of the disk morphology is given.

We present Keck Interferometer observations of TW Hya that spatially resolve its emission at 2 μm wavelength. Analyzing these data together with existing K-band veiling and near-infrared photometric measurements, we conclude that the inner disk consists of optically thin, submicron-sized dust extending from ~4 AU to within 0.06 AU of the central star. The inner disk edge may be magnetospherically truncated. Even if we account for the presence of gas in the inner disk, these small dust grains have survival times against radiation blowout that are orders of magnitude shorter than the age of the system, suggesting continual replenishment through collisions of larger bodies.

A new suite of three-dimensional radiative, gravitational hydrodynamical models is used to show that gas giant planets are unlikely to form by the disk-instability mechanism at distances of ~100-200 AU from young stars. A similar result seems to hold for the core accretion mechanism. These results appear to be consistent with the paucity of detections of gas giant planets on wide orbits by infrared imaging surveys, and they also imply that if the object orbiting GQ Lupus is a gas giant planet, it most likely did not form at a separation of ~100 AU. Instead, a wide planet around GQ Lup must have undergone a close encounter with a third body that tossed the planet outward to its present distance from its protostar. If it exists, the third body may be detectable by NASA's Space Interferometry Mission.