Table of contents

The nonlinear evolution of a system consisting of collisional baryons and collisionless dark matter is generally characterized by strong shocks and discontinuities in the baryon fluid. The baryons slow down significantly at postshock areas of gravitational strong shocks, which can occur in high overdense as well as low overdense regions. On the other hand, the shocks do not affect the collapse of the dark matter. Consequently, the baryon fraction would be nonuniform on large scales. We studied these phenomena with simulation samples produced by the weighted essentially nonoscillatory (WENO) hybrid cosmological hydrodynamic/N-body code, which is effective at capturing shocks and complex structures with high precision. We find that the baryon fraction in high mass density regions is lower on average than the cosmic baryon fraction, and many baryons accumulate in the regions with moderate mass density to form a high baryon fraction phase (HBFP). In dense regions with ρ/ > 100, which are the possible hosts for galaxy clusters, the baryon fraction can be lower than the cosmic baryon fraction by about 10%-20% at z ≃ 0. We also find that at z < 2, almost all the HBFP gas locates in the regions with mass density 0.5 < ρ/ < 5 and temperature T > 10⁵ K, and conversely, almost all the gas in the areas of 0.5 < ρ/ < 5 and with temperature T > 10⁵ K has high baryon fraction. Our simulation samples show that about 3% of the cosmic baryon budget was hidden in the HBFP at redshift z = 3, while this percentage increases to about 14% at the present day. The gas in the HBFP cannot be detected either by Lyα forests of QSO absorption spectra or by soft X-ray background. That is, the HBFP would be missed in the baryon budget given by current observations.

We study the soft X-ray emission (0.1-1 keV) from the warm-hot intergalactic medium (WHIM) in a hydrodynamic simulation of a cold dark matter universe. Our main goal is to investigate how such emission can be explored with a combination of imaging and spectroscopy and to motivate future X-ray missions. We first present high-resolution images of the X-ray emission in several energy bands in which emission from different ion species dominates. We pick three different areas to study the high-resolution spectra of X-rays from the WHIM: (1) a galaxy group, (2) a filament, and (3) an underluminous region. By taking into account the background X-ray emission from AGNs and foreground emission from the Galaxy, we compute composite X-ray spectra of the selected regions. We briefly investigate angular clustering of the soft X-ray emission, finding a strong signal. Most interestingly, the combination of high spectral resolution and angular information allows us to map the emission from the WHIM in three dimensions. We cross-correlate the positions of galaxies in the simulation with this redshift map of emission and detect the presence of six different ion species (Ne IX, Fe XVII, O VII, O VIII, N VII, and C VI) in the large-scale structure traced by the galaxies. Finally, we show how such emission can be detected and studied with future X-ray satellites, with particular attention to a proposed mission, the Missing Baryon Explorer (MBE). We present simulated observations of the WHIM gas with MBE.

At redshifts beyond z ≳ 6, as the mean fraction of neutral hydrogen ⟨x_{H i}⟩ in the intergalactic medium (IGM) increases, the line flux of Lyα emitters can be significantly attenuated, which can result in a decrease in the observed number of emitters at a given Lyα flux. However, cosmological H II regions surrounding the Lyα emitting galaxies alleviate these effects. We use simple models of the Lyα line attenuation that incorporate the presence of H II regions to predict the overall effect of the Lyα absorption on the Lyα luminosity function (LF). We find, in agreement with other recent studies, that a fully neutral IGM is inconsistent with the Lyα LF remaining unchanged from z = 5.7 to z = 6.5, as suggested by recent observations. However, the presence of local H II regions prohibits placing a tight constraint on the mean neutral fraction. We find ⟨x⟩ ≲ 0.25; the presence of strong winds and/or the clustering of ionizing sources would further weaken this constraint. We conclude that the evolution of the Lyα LF is consistent with reionization occurring near this redshift, as suggested by other observations. Finally, we suggest that a measurement of observed Lyα line width as a function of the Lyα luminosity, in a future, larger sample of Lyα emitters, may serve as a robust diagnostic of the neutral fraction in the IGM.

In cosmological structure formation models, massive nonlinear objects in the process of formation, such as galaxy clusters, are surrounded by large-scale shocks at or around the expected virial radius. Direct observational evidence for such virial shocks is currently lacking, but we show here that their presence can be inferred from future, high-resolution, high-sensitivity observations of the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters. We study the detectability of virial shocks in mock SZ maps, using simple models of cluster structure (gas density and temperature distributions) and noise (background and foreground galaxy clusters projected along the line of sight, as well as the cosmic microwave background anisotropies). We find that at an angular resolution of 2'' and sensitivity of 10 μK, expected to be reached at ~100 GHz frequencies in a ~20 hr integration (for a cluster at z ~ 2) with the forthcoming ALMA instrument, virial shocks associated with massive (M ~ 10¹⁵M_☉) clusters will stand out from the noise and can be detected at high significance. More generally, our results imply that the projected SZ surface brightness profile in future, high-resolution experiments will provide sensitive constraints on the density profile of cluster gas.

We study the formation of globular clusters in a Milky Way-size galaxy using a high-resolution cosmological simulation. The clusters in our model form in the strongly baryon-dominated cores of supergiant molecular clouds in the gaseous disks of high-redshift galaxies. The properties of clusters are estimated using a physically motivated subgrid model of the isothermal cloud collapse. The first clusters in the simulation form at z ≈ 12, while we conjecture that the best conditions for globular cluster formation appear to be at z ~ 3-5. Most clusters form in the progenitor galaxies of the virial mass M_h > 10⁹M_☉, and the total mass of the cluster population is strongly correlated with the mass of its host galaxy: M_GC = 3 × 10⁶M_☉(M_h/10¹¹M_☉)^1.1. This corresponds to a fraction ~2 × 10^-4 of the galactic baryons being in the form of globular clusters. In addition, the mass of the globular cluster population and the maximum cluster mass in a given region strongly correlate with the local average star formation rate. We find that the mass, size, and metallicity distributions of the globular cluster population identified in the simulation are remarkably similar to the corresponding distributions of the Milky Way globular clusters. We find no clear mass-metallicity or age-metallicity correlations for the old clusters. The zero-age mass function of globular clusters can be approximated by a power law dN/dM ∝ M^-α with α ≈ 2, in agreement with the mass function of young stellar clusters in starbursting galaxies. We discuss in detail the origin and universality of the globular cluster mass function. Our results indicate that globular clusters with properties similar to those of observed clusters can form naturally within dense gaseous disks at z ≳ 3 in the concordance ΛCDM cosmology.

We introduce a framework for simultaneously investigating the structure and luminosity evolution of early-type gravitational lens galaxies. The method is based on the fundamental plane, which we interpret using the aperture mass-radius relations derived from lensed image geometries. We apply this method to our previous sample of 22 lens galaxies with measured redshifts and excellent photometry. Modeling the population with a single mass profile and evolutionary history, we find that early-type galaxies are nearly isothermal (logarithmic density slope n = 2.06 ± 0.17, 68% C.L.) and that their stars evolve at a rate of d log(M/L)_B/dz = -0.50 ± 0.19 (68% C.L.) in the rest-frame B band. For a Salpeter initial mass function and a concordance cosmology, this implies a mean star formation redshift of ⟨z_f⟩ > 1.5 at 95% confidence. While this model can neatly describe the mean properties of early-type galaxies, it is clear that the scatter of the lens sample is too large to be explained by observational uncertainties alone. We therefore consider statistical models in which the galaxy population is described by a distribution of star formation redshifts. We find that stars must form over a significant range of redshifts (Δz_f > 1.7, 68% C.L.), which can extend as low as z_f ~ 1 for some acceptable models. However, the typical galaxy will still have an old stellar population (⟨z_f⟩ > 1.5). The lens sample therefore favors early star formation in field ellipticals, even if we make no a priori assumption regarding the shape of the mass distribution in lenses and include the range of possible deviations from homology in the uncertainties. Our evolution results call into question several recent claims that early-type galaxies in low-density environments have much younger stars than those in rich clusters.

Although the highest redshift QSOs (z > 6.1) are embedded in a significantly neutral background universe (mass-averaged neutral hydrogen fraction >1%) as suggested by the Gunn-Peterson absorption troughs in their spectra, the intergalactic medium in their vicinity is highly ionized. The highly ionized region is generally idealized as spherical and called the Strömgren sphere. In this paper, by combining the expected evolution of the Strömgren sphere with the rule that the speed of light is finite, we illustrate the apparent shape of the ionization fronts around the highest redshift QSOs and its evolution, which depends on the age, luminosity evolution, and environment of the QSO (e.g., the hydrogen reionization history). The apparent shape may systematically deviate from a spherical shape, unless the QSO age is significantly long compared to the hydrogen recombination process within the ionization front and the QSO luminosity evolution is significantly slow. Effects of anisotropy of QSO emission are also discussed. The apparent shape of the Strömgren sphere may be directly mapped by transmitted spectra of background sources behind or inside the ionized regions or by surveys of the hyperfine transition (21 cm) line emission of neutral hydrogen.

Space-borne gravitational-wave interferometers such as LISA will detect the gravitational wave (GW) signal from the inspiral, plunge, and ringdown phases of massive black hole binary mergers at cosmological distances. From the inspiral waves, we will be able to measure the masses of the binaries' members; from the ringdown waves, we will be able to measure the mass of the final merged remnant. A subset of detected events allow the identification of both the inspiral and the ringdown waveforms in a given source and thus allow a measurement of the total mass-energy lost to GWs over the coalescence, M_GW. We define "golden" binary mergers to be those with measurement errors likely to be small enough for a physically useful determination of M_GW. A detailed sensitivity study, combined with simple black hole population models, suggests that a few golden binary mergers may be detected during a 3 yr LISA mission lifetime. Any such mass deficit measurement would constitute a robust and valuable observational test of strong-field relativistic gravity. An extension of this concept to include spin measurements may allow a direct empirical test of the black hole area theorem.

We present optical spectra and emission-line ratios of 12 narrow-line Seyfert 1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A common feature in the EUV continuum of active galactic nuclei is the big blue bump (BBB), generally associated with thermal accretion disk emission. While Galactic absorption prevents direct access to the EUV range, it can be mapped by measuring the strength of a variety of forbidden optical emission lines that respond to different EUV continuum regions. We find that narrow emission line ratios involving [O II] λ3727, Hβ, [O III] λ5007, [O I] λ6300, Hα, [N II] λ6583, and [S II] λλ6716, 6731 indicate no significant difference between NLS1 galaxies and broad-line Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy distributions of their ionizing EUV-soft X-ray continua are similar. The relative strength of important forbidden high-ionization lines like [Ne V] λ3426 compared to He II λ4686 and the relative strength of [Fe X] λ6374 appear to show the same range as in BLS1 galaxies. However, a trend of weaker F([O I] λ6300)/F(Hα) emission-line ratios is indicated for NLS1 galaxies compared to BLS1 galaxies. To recover the broad emission line profiles, we used Gaussian components. This approach indicates that the broad Hβ profile can be well described with a broad component (FWHM ≃ 3275 ± 800 km s^-1) and an intermediate broad component (FWHM ≃ 1200 ± 300 km s^-1). The width of the broad component is in the typical range of normal BLS1 galaxies. The emission-line flux that is associated with the broad component in these NLS1 galaxies amounts to at least 60% of the total flux. Thus, it dominates the total line flux, similar to BLS1 galaxies.

We measure the morphology-density relation (MDR) and morphology-radius relation (MRR) for galaxies in seven z ~ 1 clusters that have been observed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. Simulations and independent comparisons of our visually derived morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies down to z₈₅₀ = 24, corresponding to L/L* = 0.21 and 0.30 at z = 0.83 and 1.24, respectively. We adopt density and radius estimation methods that match those used at lower redshift in order to study the evolution of the MDR and MRR. We detect a change in the MDR between 0.8 < z < 1.2 and that observed at z ~ 0, consistent with recent work; specifically, the growth in the bulge-dominated galaxy fraction, f_E+S0, with increasing density proceeds less rapidly at z ~ 1 than it does at z ~ 0. At z ~ 1 and Σ ≥ 500 galaxies Mpc^-2, we find ⟨f_E+S0⟩ = 0.72 ± 0.10. At z ~ 0, an E+S0 population fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is confined to densities Σ ≳ 40 galaxies Mpc^-2 and appears to be primarily due to a deficit of S0 galaxies and an excess of Sp+Irr galaxies relative to the local galaxy population. The f_E-density relation exhibits no significant evolution between z = 1 and 0. We find mild evidence to suggest that the MDR is dependent on the bolometric X-ray luminosity of the intracluster medium. Implications for the evolution of the disk galaxy population in dense regions are discussed in the context of these observations.

We present results of a new deep 1.1 mm survey using Bolocam, a millimeter-wavelength bolometer array camera designed for mapping large fields at fast scan rates, without chopping. A map, galaxy candidate list, and derived number counts are presented. This survey encompasses 324 arcmin² to an rms noise level (filtered for point sources) of σ_{1.1 mm} ≃ 1.4 mJy beam^-1 and includes the entire regions surveyed by the published 8 mJy 850 μm JCMT SCUBA and 1.2 mm IRAM MAMBO surveys. We reduced the data using a custom software pipeline to remove correlated sky and instrument noise via a principal component analysis. Extensive simulations and jackknife tests were performed to confirm the robustness of our source candidates and estimate the effects of false detections, bias, and completeness. In total, 17 source candidates were detected at a significance ≥3.0 σ, with six expected false detections. Nine candidates are new detections, while eight candidates have coincident SCUBA 850 μm and/or MAMBO 1.2 mm detections. From our observed number counts, we estimate the underlying differential number count distribution of submillimeter galaxies and find it to be in general agreement with previous surveys. Modeling the spectral energy distributions of these submillimeter galaxies after observations of dusty nearby galaxies suggests extreme luminosities of L = (1.0-1.6) × 10¹³L_☉ and, if powered by star formation, star formation rates of 500-800 M_☉ yr^-1.

This paper presents sensitive new observations of the fine-structure line ³P₂ → ³P₁ (J = 2-1) of the neutral atomic carbon C I (ν_rest ~ 809 GHz) in the strongly lensed ultraluminous infrared galaxy (ULIRG) IRAS F10214+4724 at z = 2.3 obtained with the millimeter/submillimeter James Clerk Maxwell telescope (JCMT). These do not confirm the presence of emission from this line at the flux levels or angular extent previously reported in the literature. The new 2 σ upper limits are S ≲ 7 Jy km s^-1 (central position), and ⟨S⟩ ≲ 8.5 Jy km s^-1 (average over the two Δα = 0'', Δδ = ±10'' positions). C I emission assumed to be fully concomitant with the bulk of H₂ and confined entirely within the strongly lensed object yields an upper limit of M(H₂) ≲ 1.5 × 10¹⁰M_☉, compatible with the reported CO-derived H₂ gas mass, within the uncertainties of the two methods. A comparison is made with the recent detection of the ³P₁ → ³P₀ (J = 1-0) line in this galaxy by Weiss et al., and the large discrepancy with the previous C I measurements is briefly discussed.

The UV spectrum of the bright quasar PHL 1811 at z_em = 0.192 reveals a foreground gas system at z = 0.080923 with log N(H ) = 17.98 ± 0.05. We have determined the abundances of various atomic species in this system from a spectrum covering the wavelength range 1160-1730 Å recorded at 7 km s^-1 resolution by the E140M grating of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), supplemented by coverage at shorter wavelengths by the Far Ultraviolet Spectroscopic Explorer (FUSE). The abundances of C II, Si II, S II, and Fe II compared to that of O I indicate that a considerable fraction of the gas is in locations where the hydrogen is ionized. An oxygen abundance [O/H] = -0.19 ± 0.08 in the H I-bearing gas indicates that the chemical enrichment of the gas is unusually high for an extragalactic QSO absorption system. However, this same material has an unusually low abundance of nitrogen, [N/O] < -0.59, indicating that there may not have been enough time during this enrichment for secondary nitrogen to arise from low- and intermediate-mass stars. From the convergence of high Lyman series lines we can determine the velocity width of H I, and after correcting for turbulent broadening shown by the O I absorption feature, we derive a temperature T = 7070 K. We determine a lower bound for the electron density n(e) > 10^-3 cm^-3 by modeling the ionization by the intergalactic radiation field and an upper bound n(e) < 0.07 cm^-3 from the absence of much C II in an excited fine-structure level. The thermal pressure in the range 4 cm^-3 K < p/k < 140 cm^-3 K could be confined by a warm-hot intergalactic medium (WHIM) structure with ρ/ ~ 20 that might accompany a wall of galaxies at the same redshift, seen in data from the Sloan Digital Sky Survey. An r-band image of the field surrounding PHL 1811 recorded by the ACS instrument on HST shows that two galaxies at the same redshift as the gas are S0 galaxies, separated by only 34 and 87 h kpc from the line of sight. One or both of these galaxies may be the source of the material in the Lyman limit system, which may have been expelled from them in a fast wind, by tidal stripping, or by ram pressure stripping. Subtraction of the ACS point-spread function from the image of the QSO reveals the presence of a face-on spiral galaxy under the glare of the quasar; while it is possible that this galaxy may be responsible for the Lyman limit absorption, the exact alignment of the QSO with the center of the galaxy suggests that the spiral is the quasar host.

We investigate the effects of dust extinction on integrated absorption-line indices that are widely used to derive constraints on the ages and metallicities of composite stellar systems. Typically, absorption-line studies have been performed on globular clusters or elliptical galaxies, which are mostly dust-free systems. However, many recent studies of integrated stellar populations have focused on spiral galaxies that may contain significant amounts of dust. It is almost universally assumed that the effects of dust extinction on absorption-line measurements are entirely negligible given the narrow baseline of the spectral features, but no rigorous study has yet been performed to verify this conjecture. In this analysis, we explore the sensitivity of the standard set of Lick absorption-line indices, the higher order Balmer line indices, the 4000 Å break, the near-IR calcium triplet indices, and the Rose indices to dust absorption according to population synthesis models that incorporate a multicomponent model for the line and continuum attenuation due to dust. The latter takes into account the finite lifetime of stellar birth clouds. While dust does not greatly affect the line-index measurements for single stellar populations, its effect can be significant for the 4000 Å break or when there is a significant amount of current star formation.

The E3 giant elliptical galaxy NGC 5018 was observed with the Chandra X-Ray Observatory's Advanced CCD Imaging Spectrometer for 30 ks on 2001 April 14. Results of the analysis of these X-ray data as well as of complementary optical, infrared, and radio data are reported. Seven X-ray point sources, including the nucleus, were detected. If they are intrinsic to NGC 5018, then all six nonnuclear sources have luminosities exceeding 10³⁹ ergs s^-1 in the 0.5-8.0 keV energy band, placing them in the class of ultraluminous X-ray sources. Comparison of X-ray source positions to archival Hubble Space Telescope Wide Field Planetary Camera 2 (HST WFPC2) images reveals that four of the six nonnuclear sources are spatially coincident with bright, M_V ≲ -8.6 mag objects. These four objects have optical magnitudes and (V - I) colors consistent with globular clusters in NGC 5018, but they also have X-ray-to-optical flux ratios consistent with background active galactic nuclei (AGNs). Strong, unpolarized radio emission has been detected from one of the optically bright counterparts. Another optically bright counterpart was observed to vary by ~1 mag in optical observations taken on 1997 July 28 and 1999 February 4. Extended X-ray emission is detected within an ~15'' radius of the galaxy center at a luminosity of ~10⁴⁰ ergs s^-1 in the X-ray band. Its thermal X-ray spectrum (kT ~ 0.4 keV) and its spatial coincidence with strong Hα emission are consistent with a hot gas origin. The nucleus itself may be a weak X-ray source, L_X ≲ 3.5 × 10³⁹ ergs s^-1, which displays a radio spectrum typical of AGNs.

We investigate the properties of giant molecular clouds (GMCs) in the molecule-rich galaxy M64 (NGC 4826). In M64, the mean surface density of molecular gas is 2N(H₂) ~ 10²² cm^-2 over a 2 kpc region, equal to the surface densities of individual GMCs in the Milky Way. We observed the J = 1 → 0 transitions of CO, ¹³CO, and HCN. The line ratio W_CO/W_13CO for 200 pc < R_gal < 800 pc is comparable to that found in the Milky Way and increases significantly outside this region, in part because of a large contribution to the CO emission from diffuse gas, which composes 25% of the molecular mass in the galaxy. We developed a modified CLUMPFIND algorithm to decompose the ¹³CO emission into 25 resolved clouds. The clouds have a luminosity-line width relationship L ∝ ΔV^2.2±0.4, substantially different from the Milky Way trend, L ∝ ΔV⁵. Similarly, the clouds have a line width-size relationship of ΔV ∝ R, compared to ΔV ∝ R in the Milky Way. Estimates of the kinetic and binding energies of the clouds suggest that the clouds are self-gravitating and significantly overpressured with respect to the remainder of the ISM in M64. The ¹³CO-to-H₂ conversion factor is comparable to what is seen in the Galaxy. The M64 clouds have a mean surface density at least 2.5 times larger than observed in Local Group GMCs, and the surface density is not independent of mass as it is in the Local Group: Σ_H2 ∝ M^0.7±0.2. The clouds are correlated with the recombination emission from the galaxy, implying that they are star forming; the rate is comparable to that in other galaxies despite the increased densities of the clouds. The gas-to-dust ratio is similar to the Galactic value, but the low extinction in the visual band requires that the molecular gas be clumpy on small scales. We note that the internal pressures of clouds in several galaxies scale with the external pressure exerted on the clouds by the ambient ISM: P_int ∝ P. We attribute the differences between M64 molecular clouds and those in the Local Group to the high ambient pressures and large molecular gas content found in M64.

We present V- and I-equivalent Hubble Space Telescope (HST) WFPC2 photometry of two areas in the Large Magellanic Cloud: the southern part of the stellar association LH 52, located on the western edge of the supershell LMC 4, and a field between two associations, which is located on the southwestern edge of the shell and accounts for the general background field of the galaxy. The HST WFPC2 observations reach magnitudes as faint as V = 25 mag, much deeper than have been observed earlier in stellar associations in the LMC. We determine the mass function (MF) for main-sequence stars in the areas. Its slope in both areas is steeper for stars with masses M ≲ 2 M_☉ (-4 ≲ Γ ≲ -6) than for stars of M ≳ 2 M_☉ (-1 ≲ Γ ≲ -2). Thus, as far as the field of the LMC is concerned, the MF does not have a uniform slope throughout its observed mass range. The MF of the general field of the LMC was found previously to be steeper than the MF of a stellar association for massive stars with M ≳ 5 M_☉. We conclude that this seems to also be the case toward lower masses down to M ~ 1 M_☉. Our data allow us to construct the field-subtracted, incompleteness-corrected, main-sequence MF of the southwestern part of the young stellar association LH 52, which accounts for the initial mass function (IMF) of the system. Its mean slope is found to be comparable to, but more shallow than, a typical Salpeter IMF (Γ ≃ -1.12 ± 0.24) for masses down to ~1 M_☉. We found indications that the IMF of the association probably is "top heavy," owing to the large number of intermediate-mass stars in the field of the system, while the general LMC field is found to be responsible for the low-mass population, with M ≲ 2 M_☉, observed in both fields. This finding suggests that the local conditions seem to favor the formation of higher mass stars in associations, and not in the background field. No evidence for flattening of the IMF toward the low-mass regime or for a lower mass cutoff in the IMF was detected in our data.

We present three different optical and near-infrared (NIR) data sets for evolved stars in the Galactic globular cluster ω Centauri. The comparison between observations and homogeneous sets of stellar isochrones and zero-age horizontal branches provides two reasonable fits. Both of them suggest that the so-called anomalous branch has a metal-intermediate chemical composition (-1.1 ≤ [Fe/H] ≤ -0.8) and is located ~500 pc beyond the bulk of ω Cen stars. These findings are mainly supported by the shape of the subgiant branch in four different color-magnitude diagrams (CMDs). The most plausible fit requires a higher reddening, E(B - V) = 0.155 versus 0.12, and suggests that the anomalous branch is coeval with the bulk of ω Cen stellar populations to within empirical and theoretical uncertainties. This result is supported by the identification of a sample of faint horizontal-branch (HB) stars that might be connected with the anomalous branch. Circumstantial empirical evidence seems to suggest that the stars in this branch form a clump of stars located beyond the cluster.

We present an 11'' resolution map of the central 2 pc of the Galaxy in the CO J = 7 → 6 rotational transition. The CO emission shows rotation about Sgr A* but also evidence for noncircular turbulent motion and a clumpy morphology. We combine our data set with available CO measurements to model the physical conditions in the disk. We find that the molecular gas in the region is both warm and dense, with T ~ 200-300 K and n ~ (5-7) × 10⁴ cm^-3. The mass of warm molecular gas we measure in the central 2 pc is at least 2000 M_☉, about 20 times the UV-excited atomic gas mass, ruling out a UV heating scenario for the molecular material. We compare the available spectral tracers with theoretical models and conclude that molecular gas is heated with magnetohydrodynamic shocks with v ~ 10-20 km s^-1 and B ~ 0.3-0.5 mG. Using the conditions derived with the CO analysis, we include the other important coolants, neutral oxygen and molecular hydrogen, to estimate the total cooling budget of the molecular material. We derive a mass-to-luminosity ratio of ~2-3 M_☉L, which is consistent with the total power dissipated via turbulent decay in 0.1 pc cells with v_rms ~ 15 km s^-1. These size and velocity scales are comparable to the observed clumping scale and the velocity dispersion. At this rate, the material near Sgr A* is dissipating its orbital energy on an orbital timescale and cannot last for more than a few orbits. Our conclusions support a scenario in which the features near Sgr A* such as the circumnuclear disk and northern arm are generated by infalling clouds with low specific angular momentum.

The rare isotope ⁶Li is made only by cosmic rays, predominantly in αα → ⁶Li fusion reactions with interstellar medium (ISM) helium. Consequently, this nuclide provides a unique diagnostic of the history of cosmic rays in our Galaxy. The same hadronic cosmic-ray interactions also produce high-energy γ-rays (mostly via pp → π⁰ → γγ). Thus, hadronic γ-rays and ⁶Li are intimately linked. Specifically, ⁶Li directly encodes the local cosmic-ray fluence over cosmic time, while extragalactic hadronic γ-rays encode an average cosmic-ray fluence over lines of sight out to the horizon. We examine this link and show how ⁶Li and γ-rays can be used together to place important model-independent limits on the cosmic-ray history of our Galaxy and the universe. We first constrain γ-ray production from ordinary Galactic cosmic rays, using the local ⁶Li abundance. We find that the solar ⁶Li abundance demands an accompanying extragalactic pionic γ-ray intensity that exceeds that of the entire observed extragalactic γ-ray background (EGRB) by a factor of 2-6. Possible explanations for this discrepancy are discussed. We then constrain Li production using recent determinations of the EGRB. We note that cosmic rays created during cosmic structure formation would lead to pre-Galactic Li production, which would act as a "contaminant" to the primordial ⁷Li content of metal-poor halo stars; the EGRB can place an upper limit on this contamination if we attribute the entire EGRB pionic contribution to structure-forming cosmic rays. Unfortunately, the uncertainties in the determination of the EGRB are so large that the present γ-ray data cannot guarantee that the pre-Galactic Li contribution is small compared to primordial ⁷Li; thus, an improved determination of the EGRB will shed important new light on this issue. Our limits and their more model-dependent extensions will improve significantly with additional observations of ⁶Li in halo stars and with improved measurements of the EGRB spectrum by GLAST.

We present a coarsely sampled longitude-velocity (l-V) map of the region l = 300°-354°, b = 0° in the 492 GHz fine-structure transition of neutral atomic carbon (C⁰³P₁-³P₀; [C I]), observed with the Portable 18 cm Submillimeter-wave Telescope (POST18). The l-V distribution of the [C I] emission resembles closely that of the CO J = 1-0 emission, showing a widespread distribution of atomic carbon on the Galactic scale. The ratio of the antenna temperatures, R, concentrates on the narrow range from 0.05 to 0.3. A large velocity gradient (LVG) analysis shows that the [C I] emission from the Galactic disk is dominated by a population of neutral gas with high C⁰/CO abundance ratios and moderate column densities, which can be categorized as diffuse translucent clouds. The ratio of bulk emissivity, J/J_CO, shows a systematic trend, suggesting the bulk C⁰/CO abundance ratio increasing with the Galactic radius. A mechanism related to kiloparsec-scale structure of the Galaxy may control the bulk C⁰/CO abundance ratio in the Galactic disk. Two groups of high-ratio (R > 0.3) areas reside in the l-V loci several degrees inside of tangential points of the Galactic spiral arms. These could be gas condensations just accumulated in the potential well of spiral arms and be in the early stages of molecular cloud formation.

We present near-infrared (JHKL) photometry of 103 southern stars located behind translucent interstellar clouds at high Galactic latitude. Our data are combined with visual photometry and spectral type information from the literature in a detailed analysis of the wavelength dependence of interstellar extinction by dust in these high-latitude clouds. We investigate the shape of the near-infrared extinction curve and evaluate the total line-of-sight extinction (A_V) and ratio of total-to-selective extinction (R_V) in each line of sight. Sources of uncertainty in R_V arising from photometric errors and spectral classification errors are carefully assessed and quantified as functions of the line-of-sight reddening. We detect appreciable differences in our results compared with the average extinction curve for dust in the diffuse interstellar medium (ISM) close to the Galactic plane. Assuming a power-law form in the near-infrared, we find the mean for high-latitude clouds to be well described by ⟨A_λ⟩ ∝ λ^-2.3, somewhat steeper than that for the diffuse ISM (⟨A_λ⟩ ∝ λ^-1.8). Our sample includes a substantial number of lines of sight with low R_V values (47% with R_V < 2.8) relative to the diffuse ISM average of 3.05 ± 0.15. We conclude that many high-latitude clouds have enhanced abundances of relatively small grains.

The Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) was designed to study diffuse emission from hot gas in the local interstellar cavity in the wavelength range 90-265 Å. Between launch in 2003 January and early 2004, the instrument was operated in narrow-slit mode, achieving a peak spectral resolution of about 1.4 Å FWHM. Observations were carried out preferentially at high Galactic latitudes; weighted by observing time, the mean absolute value of the Galactic latitude for all narrow-slit observations combined is about 45°. The total integration time is about 13.2 Ms (74% day, 26% night). In the context of a standard collisional ionization equilibrium plasma model, the CHIPS data set tight constraints on the emission measure at temperatures between 10^5.55 and 10^6.4 K. At 10^6.0 K, the 95% upper limit on the emission measure is about 0.0004 cm^-6 pc for solar-abundance plasma with a foreground neutral hydrogen column of 2 × 10¹⁸ cm^-2. This constraint, derived primarily from limits on the extreme ultraviolet emission lines of highly ionized iron, is well below the range for the local hot bubble estimated previously from soft X-ray studies. If the pattern of elemental depletion in the hot gas follows that observed in much denser interstellar clouds, the gas-phase abundance of iron, relative to other heavy elements that contribute more to the soft X-ray emission, might be much lower than solar. However, to support the emission measures inferred previously from X-ray data would require depletions much higher than the moderate values reported previously for hot gas. Excluding the He II Lyman lines, which are known to be primarily terrestrial in origin, the brightest feature we find in the integrated spectrum is an Fe IX line at 171.1 Å. The sky-averaged flux of the feature is about 6 photons cm^-2 s^-1 sr^-1, a flux that exceeds the 1 σ shot noise significantly but is comparable to the systematic uncertainty. We find bright 171.1 Å emission (flux greater than 10 photons cm^-2 s^-1 sr^-1 and S/N >2) in about 10% of the observing time. However, these bright observations overwhelmingly select for daytime (96% of 1.3 Ms). Thus, a local rather than interstellar origin for much of the 171.1 Å emission seems likely.

We analyze the dynamical expansion of the H II region and outer photodissociation region (PDR) around a massive star by solving the UV and FUV radiation transfer and the thermal and chemical processes in a time-dependent hydrodynamics code. We focus on the physical structure of the shell swept up by the shock front (SF) preceding the ionization front (IF). After the IF reaches the initial Strömgren radius, the SF emerges in front of the IF and a geometrically thin shell bounded by the IF and SF is formed. The gas density inside the shell is about 10¹-10² times as high as the ambient gas density. Initially, the dissociation fronts (DFs) expands faster than the IF, and the PDR is formed outside the H II region. Thereafter, the IF and SF gradually overtake the preceding DFs, and eventually the DFs are taken into the shell. The chemical composition within the shell is initially atomic, but hydrogen and carbon monoxide molecules are gradually formed. This is partly because the IF and SF overtake the DFs and the SF enters the molecular region, and partly because the reformation timescales of the molecules become shorter than the dynamical timescale. The gas shell becomes dominated by the molecular gas by the time of gravitational fragmentation, which agrees with some recent observations. A simple estimation of the star formation rate in the shell shows that these processes contribute significantly to the star formation rate in our Galaxy.

We reexamine accretion onto a protobinary based on two-dimensional numerical simulations with high spatial resolution. We focus our attention on the ratio of the primary and secondary accretion rates. Fifty-eight models are made for studying the dependence of the accretion rates on the specific angular momentum of infalling gas j_inf, the mass ratio of the binary q, and the sound speed c_s. When j_inf is small, the binary accretes the gas mainly through two channels (type I): one through the Lagrange point L2 and the other through L3. When j_inf is large, the binary accretes the gas only through the L2 point (type II). The primary accretes more than the secondary in both the cases, although the L2 point is closer to the secondary. After flowing through the L2 point, the gas flows halfway around the secondary and through the L1 point to the primary. Only a small amount of gas flows back to the secondary, and the rest forms a circumstellar ring around the primary. The boundary between types I and II depends on q. When j_inf is very large, the accretion begins after several rotations (type III). The beginning of the accretion is later when j_inf is larger and c_s is smaller. Our result that the primary accretion rate is higher for a large j_inf is qualitatively different from results of earlier simulations. The difference is mainly due to limited spatial resolution and large numerical viscosity in the numerical simulations thus far.

We investigate the dynamical decay of nonhierarchical accreting triple systems and its implications on the ejection model as a brown dwarf formation scenario. A modified chain-regularization scheme is used to integrate the equations of motion that also allows for mass changes over time as well as for momentum transfer from the accreted gas mass onto the bodies. We integrate an ensemble of triple systems within a certain volume with different accretion rates, assuming several prescriptions of how momentum is transferred onto the bodies. We follow their evolution until the systems have decayed. We analyze the end states and decay times of these systems and determine the fraction of brown dwarfs formed and their escape speeds, as well as the semimajor axis distribution of the formed brown dwarf binaries. We find that the formation probability of brown dwarfs depends strongly on the assumed momentum transfer, which is related to the motion of the gas. Because of ongoing accretion and consequent shrinkage of the systems, the median escape velocity is increased by a factor of 2, and the binary separations are decreased by a factor of 5 compared with nonaccreting systems. Furthermore, the obtained semimajor axis distribution drops off sharply to either side of the median, which is also supported by observations. We conclude that accretion and momentum transfer of accreted gas during the dynamical decay of triple systems is able to produce the observed distribution of close binary brown dwarfs, making the ejection model a viable option as brown dwarf formation scenario.

We determine inner disk sizes and temperatures for four solar-type (1-2 M_☉) classical T Tauri stars, AS 207A, V2508 Oph, AS 205A, and PX Vul, using 2.2 μm observations from the Keck Interferometer. Nearly contemporaneous near-IR adaptive optics imaging photometry, optical photometry, and high-dispersion optical spectroscopy are used to distinguish contributions from the inner disks and central stars in the interferometric observations. In addition, the spectroscopic and photometric data provide estimates of stellar properties, mass accretion rates, and disk corotation radii. We model our interferometric and photometric data in the context of geometrically flat accretion disk models with inner holes, and flared disks with puffed-up inner walls. Models incorporating puffed-up inner disk walls generally provide better fits to the data, similar to previous results for higher mass Herbig Ae stars. Our measured inner disk sizes are larger than disk truncation radii predicted by magnetospheric accretion models, with larger discrepancies for sources with higher mass accretion rates. We suggest that our measured sizes correspond to dust sublimation radii, and that optically thin gaseous material may extend farther inward to the magnetospheric truncation radii. Finally, our inner disk measurements constrain the location of terrestrial planet formation as well as potential mechanisms for halting giant planet migration.

Using Monte Carlo simulations, we demonstrate photopion production from Fermi-accelerated protons and the resulting neutrino production in gamma-ray bursts. Unless internal shocks occur at quite large distances from the center, ultra-high-energy protons are depleted by photopion production and synchrotron radiation. Internal shocks at fiducial distance cause neutrino bursts, which accompany gamma-ray bursts originating from electromagnetic cascades.

We present uniform, detailed spectral analyses of γ-ray burst (GRB) X-ray afterglows observed with ASCA, BeppoSAX, Chandra, and XMM-Newton and critically evaluate the statistical significances of X-ray emission and absorption features in these spectra. The sample consists of 21 X-ray afterglow observations up to and including that of GRB 040106 with spectra of sufficient statistical quality to allow meaningful line searches, chosen here somewhat arbitrarily to be detections with more than 100 total (source plus background) counts. This sample includes all nine X-ray afterglows with published claims of line detections. Moderate resolution spectra are available for 16 of the 21 sources and for the remaining five the Chandra transmission grating spectrometers obtained high-resolution data. All of the data are available from the public archive. We test a simple hypothesis in which the observed spectra are produced by a power-law continuum model modified by photoelectric absorption by neutral material both in our Galaxy and possibly also local to the burst. As a sample, these afterglow spectra are consistent with this relatively simple model. However, since the χ² statistic is not sensitive to weak and/or localized fluctuations, we have performed Monte Carlo simulations to search for discrete features and to estimate their significances. Our analysis shows that there are four afterglows (GRB 011211, GRB 030227, GRB 021004, and GRB 040106) with line-like features that are significant at the 3 σ level. We cautiously note that, in two cases, the features are associated with an unusual background feature; in the other two, the fractional magnitudes of the lines are small, and comparable to the expected level of systematic uncertainty in the spectral response. In addition, none of the statistically significant features are seen in more than one detector or spectral order where available. We conclude that, to date, no credible X-ray line feature has been detected in a GRB afterglow. Finally, in a majority of cases, we find no evidence for significant absorbing columns local to the GRB host galaxy, implying that there is little evidence from X-ray observations that GRB preferentially explode in high-density environments.

In order to infer the effects of rotation on the revival of a stalled shock in supernova explosions, we investigate steady accretion flows with a standing shock. We first obtain a series of solutions for equations describing nonrotating, spherically symmetric flows and confirm the results of preceding papers, that for a given mass accretion rate, there is a critical luminosity of irradiating neutrinos above which there exists no steady solution. Below the critical value, we find two branches of solutions; one is stable and the other is unstable against radial perturbations. With a simple argument based on the Riemann problem, we can identify the critical luminosity as that at which the stalled shock revives. We also obtain a condition satisfied by the flow velocity for the critical luminosity, which can easily be applied to the rotational case. If a collapsing star rotates, the accretion flow is nonspherical as a consequence of centrifugal forces. Flows are accelerated near the rotation axis, whereas they are decelerated near the equatorial plane. As a result, the critical luminosity is lowered; that is, rotation assists the revival of a stalled shock. According to our calculations, the critical luminosity is ~25% lower for a mass accretion rate of 1 M_☉ s^-1 and a rotational frequency of 0.1 Hz at a radius of 1000 km than that for a spherically symmetric flow with the same mass accretion rate. We find that the condition on the flow velocity at the critical luminosity is first satisfied at the rotation axis. This suggests that shock revival is triggered on the rotation axis and a jetlike explosion ensues.

The photometric and spectroscopic properties of 26 well-observed Type Ia Supernovae (SNe Ia) were analyzed with the aim of exploring SN Ia diversity. The sample includes (Branch) normal SNe, as well as extreme events such as SN 1991T and SN 1991bg, while the truly peculiar SNe Ia, SN 2000cx and SN 2002cx, are not included in our sample. A statistical treatment reveals the existence of three different groups. The first group (FAINT) consists of faint SNe Ia similar to SN 1991bg, with low expansion velocities and rapid evolution of Si II velocity. A second group consists of normal SNe Ia, also with high temporal velocity gradient (HVG), but with brighter mean absolute magnitude ⟨M_B⟩ = -19.3 and higher expansion velocities than the FAINT SNe. The third group includes both normal and SN 1991T-like SNe Ia: these SNe populate a narrow strip in the Si II velocity evolution plot, with a low-velocity gradient (LVG), but have absolute magnitudes similar to HVGs. While the FAINT and HVG SNe Ia together seem to define a relation between (Si II) and Δm₁₅(B), the LVG SNe either do not conform to that relation or define a new, looser one. The (Si II) premaximum evolution of HVGs is strikingly different from that of LVGs. We discuss the impact of this evidence on the understanding of SN Ia diversity, in terms of explosion mechanisms, degree of ejecta mixing, and ejecta-circumstellar material interaction.

We investigate the dependence of observational properties of black hole X-ray binaries on the inclination angle i of their orbits. We find the following: (1) Transient black hole binaries show no trend in their quiescent X-ray luminosities as a function of i, suggesting that the radiation is not significantly beamed. This is consistent with emission from an accretion disk. If the X-rays are from a jet, then the Lorentz factor γ of the jet is <1.24 at the 90% confidence level. (2) The X-ray binary 4U 1543-47 with i ~ 21° has a surprisingly strong fluorescent iron line in the high soft state. Quantifying an earlier argument by Park et al., we conclude that if the continuum X-ray emission in this source is from a jet, then γ < 1.04. (3) None of the known binaries has cos i < 0.25 or i > 75°. This fact, plus the lack of eclipses among the 20 black hole binaries in our sample, strongly suggests at the 99.5% confidence level that systems with large inclination angles are hidden from view. The obscuration could be the result of disk flaring, as suggested by Milgrom for neutron star X-ray binaries. (4) Transient black hole binaries with i ~ 70°-75° have significantly more complex X-ray light curves than systems with i ≲ 65°. This may be the result of variable obscuration and/or variable height above the disk of the radiating gas.

We compile optical and UV spectra of a sample of "typical" short-period black hole X-ray transients in outburst. We also survey determinations of interstellar extinction and distance to deredden spectra and compare absolute fluxes. Hence, we perform a comparative study of the broadband spectral energy distributions (SEDs). We find that, given such a homogeneous sample of typical sources, the optical SEDs form a relatively uniform set, all exhibiting quasi-power-law spectra with F_ν ∝ ν^α, where 0.5 ≲ α ≲ 1.5 (steeper than the canonical ν^1/3 disk spectrum). All become flatter in the UV, although there is more diversity here. The SEDs studied can be broadly divided into two optical-UV spectral states. The UV-hard spectra, e.g., A0620-00 and GS 1124-684, continue to rise in the far-UV. The UV-soft spectra, e.g., GRO J0422+32, drop off. XTE J1859+226 evolved from UV-soft to UV-hard as it decayed, indicating that this effect is a real difference, not a dereddening artifact. All the spectra can be fitted by a generalized blackbody disk model with two forms of heating, resulting in the two states. The UV-soft state is consistent with a disk illuminated by a central point source, with irradiative heating dominating over viscous. The UV-hard state is well described by a viscously heated disk, although this requires very high mass flow rates in the case of GS 1124-684. Alternatively, a UV-hard spectrum can be produced if the disk is illuminated by a vertically extended X-ray source such as a central scattering corona or jet. Since scattering is assumed by some numerical simulations, it is worth emphasizing that when illumination comes from (nonlocal) scattering high above the disk, we generically expect a steeper radial dependence of X-ray heating (F ∝ R^-3) than is usually assumed; it is this steep dependence that leads to the UV-hard spectrum.

The amplitude of the gravitational radiation from an accreting neutron star undergoing polar magnetic burial is calculated. During accretion, the magnetic field of a neutron star is compressed into a narrow belt at the magnetic equator by material spreading equatorward from the polar cap. In turn, the compressed field confines the accreted material in a polar mountain, which is generally misaligned with the rotation axis, producing gravitational waves. The equilibrium hydromagnetic structure of the polar mountain, and its associated mass quadrupole moment, are computed as functions of the accreted mass M_a by solving a Grad-Shafranov boundary value problem. The orientation- and polarization-averaged gravitational wave strain at Earth is found to be h_c = 6 × 10^-24(M_a/M_c)(1 + M_ab²/8M_c)^-1(f/0.6 kHz)²(d/1 kpc)^-1, where f is the wave frequency, d is the distance to the source, b is the ratio of the hemispheric to polar magnetic flux, and the cutoff mass M_c ~ 10^-5M_☉ is a function of the natal magnetic field, temperature, and electrical conductivity of the crust. This value of h_c exceeds previous estimates that failed to treat equatorward spreading and flux freezing self-consistently. It is concluded that an accreting millisecond pulsar emits a persistent, sinusoidal gravitational wave signal at levels detectable, in principle, by long-baseline interferometers after phase-coherent integration, provided that the polar mountain is hydromagnetically stable. Magnetic burial also reduces the magnetic dipole moment μ monotonically as μ ∝ (1 + 3M_a/4M_c)^-1, implying a novel, observationally testable scaling h_c(μ). The implications for the rotational evolution of (accreting) X-ray and (isolated) radio millisecond pulsars are explored.

XMM-Newton EPIC observations of PSR B0656+14, PSR B1055-52, and Geminga have substantially increased the collection of statistics available for these three isolated neutron stars, so apparently similar to deserve the nickname of the Three Musketeers, given to them by Becker & Trümper. Here we take advantage of the EPIC statistics to perform phase-resolved spectroscopy for all three objects. The phase-averaged spectrum of the Three Musketeers is best described by a three-component model. This includes two blackbody components—a cooler one, possibly originating from the bulk of the star surface, and a hotter one, coming from a smaller portion of the star surface (a "hot spot")—plus a power law. The relative contributions of the three components are seen to vary as a function of phase, as the stars' rotation brings into view different emitting regions. The hot spots, which have very different apparent dimensions (in spite of the similarity of the three neutron stars polar cap radii) are responsible for the bulk of the phase variation. The amplitude of the observed phase modulation is also markedly different for the three sources. Another striking aspect of our phase-resolved phenomenology is the apparent lack of any common phase alignment between the observed modulation patterns for the two blackbody components. They are seen to vary in phase in the case of PSR B1055-52 but in antiphase in the case of PSR B0656+14. These findings do not support standard and simplistic models of neutron star magnetic field configuration and surface temperature distribution.

The emission from low-mass X-ray binaries (LMXBs) arises from the accretion of mass onto a neutron star or black hole. A knowledge of the amount of mass being accreted as well as changes in this value are therefore essential inputs into models of these systems. Despite the need for this information, we currently lack an easily applied method that allows the accretion rate to be measured. X-ray color-color plots and UV observations can be used for this purpose, but these methods require access to oversubscribed satellites. Even if time is granted on these facilities, there is no guarantee that the source will be in a desired state when the observations take place. In this paper we show that an estimate of the ratio of the mass accretion rate to the Eddington rate can be obtained for Sco X-1 by using the Johnson B magnitude. Based on correlated X-ray and ground-based observations, we find that for Sco X-1, /_E = -(0.123 ± 0.007)B + 2.543 ± 0.085. This relation is valid when the system is on its normal and lower flaring branches. Based on theoretical models, we suggest that similar relations should also exist for other LMXBs.

We report the discovery of spectroscopic variations in GD 323, the prototypical DAB white dwarf. Simultaneous optical spectroscopic observations over five consecutive nights of GD 323 and PG 1234+482, a nonvariable comparison DA white dwarf of similar brightness, are used to reveal quasi-periodic variations in both the hydrogen and helium absorption lines over a timescale of hours. The amplitude of the variation of the equivalent width of Hβ is ~30%. Moreover, the strength of the hydrogen lines is shown to vary in opposite phase from that of He I λ4471. These results suggest that the model currently thought to be the most viable to account for the simultaneous presence of hydrogen and helium lines in GD 323, namely, a static stratified atmosphere, may need to be reexamined. Instead, a model with an inhomogeneous surface composition, resulting perhaps from the dilution of a thin hydrogen atmosphere with the underlying helium convection zone, may be a better representation of GD 323. The observed variation timescale of ~3.5 hr is consistent with the slow rotation rate of white dwarf stars.

Big bang nucleosynthesis (BBN) and the cosmic baryon density from cosmic microwave background anisotropies together predict a primordial ⁷Li abundance a factor of 2-3 higher than that observed in galactic halo dwarf stars. A recent analysis of ⁷Li observations in halo stars, using significantly higher surface temperature for these stars, found a higher Li plateau abundance. These results go a long way toward resolving the discrepancy with BBN. Here we examine the implications of the higher surface temperatures on the abundances of Be and B that are thought to have been produced in galactic cosmic-ray nucleosynthesis by spallation of CNO together with Li (produced in α + α collisions). While the Be abundance is not overly sensitive to the surface temperature, the derived B abundances and more importantly the derived oxygen abundances are very temperature-dependent. If the new temperature scale is correct, the implied increased abundances of these elements pose a serious challenge to models of galactic cosmic-ray nucleosynthesis and galactic chemical evolution.

In this paper we present unfiltered and multiband (i.e., UBVRI) polarimetric observations of the short-period Wolf-Rayet binary CQ Cep. Using the basic assumptions of an optically thin, corotating envelope and pointlike sources (i.e., BME78 assumptions), we determined the orbital parameters of the system (i.e., i = 99° ± 1° and Ω = 76° ± 2° at the 2 σ level) with an accuracy many times better than any previous work. Residual non-BME78 variability around phase 0.0 was present in our data, which we associate with the polarimetric eclipse of the dense central parts of the Wolf-Rayet (W-R) wind by the orbiting O star. We attribute the observed phase lag of -0.15 between our residuals and those expected for a standard polarimetric eclipse to a wind-wind interaction (WWI) region distorted by Coriolis forces using the model presented by Marchenko et al. This model was also able to explain the strong wavelength dependence of the polarimetric amplitudes in our multiband observations. Our analysis also reveals important epoch-dependent departures of the matter distribution from spherical symmetry that were not related to the orbital plane and therefore cannot be the result of tidal interaction. We conclude that binarity is not playing an important role in driving the wind of the W-R star in CQ Cep and contributing to the observed nonspherical matter distribution. On the other hand, this asymmetry could be explained by a rotationally induced disk misaligned with the orbital plane.

We report the results of a 20 year campaign to monitor the infrared (IR) spectral energy distribution (SED) of the "born again" star FG Sge, which is embedded in an old planetary nebula. The IR data show that the central star apparently reestablished an essentially steady state post-asymptotic giant branch (AGB) type wind in late 1992. Since then, the visual light curve has shown irregular large amplitude variations on timescales of 200-400 days, while the IR SED has maintained a constant luminosity. The IR SED has been dominated by thermal emission from carbon dust at a temperature of ≃1000 K for nearly 10 years. The thermal emission is primarily from grains forming in the 1000 K condensation zone at the base of the outflow. The wind appears to be optically thick in the visual, and the central engine may be undergoing episodes of pulsational activity that cause the visual optical depth of the wind to vary. The central engine appears to be cooling steadily at constant luminosity. We estimate the mass-loss rate of the FG Sge wind to be between 2.3 × 10^-5 and 1.2 × 10^-4 M_☉ yr^-1 as would be expected for a star rejoining the tip of the AGB.

We present a 0.6-4.1 μm spectroscopic sequence of M, L, and T dwarfs. The spectra have R ≡ λ/Δλ ≈ 2000 from 0.9 to 2.4 μm and R = 2500-200 from 2.9 to 4.1 μm. These new data nearly double the number of L and T dwarfs that have reported L-band spectra. The near-infrared spectra are combined with previously published red-optical spectra to extend the wavelength coverage to ~0.6 μm. Prominent atomic and molecular absorption features are identified including neutral lines of Al, Fe, Mg, Ca, Ti, Na, and K and 19 new weak CH₄ absorption features in the H-band spectra of mid- to late-type T dwarfs. In addition, we detect for the first time the 0-0 band of the A⁴Π-X⁴Σ^- transition of VO at ~1.06 μm in the spectra of L dwarfs and the P- and R-branches of the ν₃ band of CH₄ in the spectrum of a T dwarf. The equivalent widths of the refractory atomic features all decrease with increasing spectral type and are absent by a spectral type of ~L0, except for the 1.189 μm Fe I line, which persists to at least ~L3. We compute the bolometric luminosities of the dwarfs in our sample with measured parallaxes and find good agreement with previously published results that use L'-band photometry to account for the flux emitted from 2.5 to 3.6 μm. Finally, 2MASS J2224381-0158521 (L4.5) has an anomalously red spectrum and the strongest Δν = +2 CO bands in our sample. This may be indicative of unusually thick condensate clouds and/or low surface gravity.

We present the results of a search for substellar companions to members of the star-forming cluster IC 348. Using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we have obtained deep, high-resolution images of the cluster through the F791W and F850LP filters. These data encompass 150 known members of IC 348, including 14 primaries that are likely to be substellar (M₁ = 0.015-0.08 M_☉). The detection limits for companions to low-mass stars and brown dwarfs in the PC images are Δm₇₉₁ = 0, 2.5, and 5.5 at separations of 005, 01, and 03, respectively, which correspond to M₂/M₁ = 1, 0.3, and 0.1 at 15, 30, and 90 AU. Meanwhile, for heavily saturated solar-mass primaries in the WFC images, the limits are Δm₇₉₁ = 0 and 6 (M₂/M₁ = 1 and 0.04) at 02 and 04. The sky limiting magnitude of m₇₉₁ ~ 26 at large separations from a primary corresponds to a mass of ~0.006 M_☉ according to the evolutionary models of Chabrier and Baraffe. Point sources appearing near known and candidate cluster members are classified as either field stars or likely cluster members through their positions on the color-magnitude diagram constructed from the WFPC2 photometry. For the two faintest candidate companions appearing in these data, we have obtained 0.8-2.5 μm spectra with SpeX at the IRTF. Through a comparison to spectra of optically classified dwarfs, giants, and pre-main-sequence objects, we classify these two sources as cluster members with spectral types near M6, corresponding to masses of ~0.1 M_☉ with the models of Chabrier and Baraffe. Thus, no probable substellar companions are detected in this survey. After considering all potential binaries within our WFPC2 images, we find that the frequencies of stellar and substellar companions within 04-5'' (120-1600 AU) from low-mass stars (M₁ = 0.08-0.5) in IC 348 agree within the uncertainties with measurements in the field. The factor of ~3-10 deficiency in brown dwarfs relative to stars among companions at wide separations in IC 348 and across the much larger range of separations probed for field stars in previous work is equal within the uncertainties to the deficiency in brown dwarfs in measurements of mass functions of isolated objects. In other words, when defined relative to stars, the brown dwarf "desert" among companions is also present among isolated objects, which is expected if stellar and substellar companions form in the same manner as their free-floating counterparts. Meanwhile, among the 14 substellar primaries in our survey of IC 348, no companions are detected. This absence of wide binary brown dwarfs is statistically consistent with the frequency of wide binary stars in IC 348.

We present the results of high-resolution, three-dimensional hydrodynamic simulations of the dynamics and formation of coherent, long-lived vortices in stably stratified protoplanetary disks. Tall, columnar vortices that extend vertically through many scale heights in the disk are unstable to small perturbations; such vortices cannot maintain vertical alignment over more than a few scale heights and are ripped apart by the Keplerian shear. Short, finite-height vortices that extend only 1 scale height above and below the midplane are also unstable, but for a different reason: we have isolated an antisymmetric (with respect to the midplane) eigenmode that grows with an e-folding time of only a few orbital periods; the nonlinear evolution of this instability leads to the destruction of the vortex. Serendipitously, we observe the formation of three-dimensional vortices that are centered not in the midplane, but at 1-3 scale heights above and below. Breaking internal gravity waves create vorticity; anticyclonic regions of vorticity roll up and coalesce into new vortices, whereas cyclonic regions shear into thin azimuthal bands. Unlike the midplane-centered vortices that were placed ad hoc in the disk and turned out to be linearly unstable, the off-midplane vortices form naturally out of perturbations in the disk and are stable and robust for many hundreds of orbits.

We report the detection of three new low-mass planets from the Anglo-Australian Planet Search. The three parent stars of these planets are chromospherically quiet main-sequence G dwarfs with metallicities ranging from roughly solar (HD 117618 and HD 208487) to metal enriched (HD 102117). The orbital periods range from 20.8 to 130 days, the minimum masses from roughly 0.5M_Sat to 0.5M_Jup, and the eccentricities from 0.08 to 0.37, with the planet in the smallest orbit (HD 102117) having the smallest eccentricity. With semiamplitudes of 10.6-19 m s^-1, these planets induce Doppler amplitudes similar to those of Jupiter analogs, albeit with shorter periods. Many of the most interesting future Doppler planets will be detected at these semiamplitude levels, placing a premium on measurement precision. The detection of such amplitudes in data extending back 6 yr gives confidence in the Anglo-Australian Planet Search's ability to detect Jupiter analogs as our time baseline extends to 12 yr. We discuss the criticality of such detections for the design of the next generation of extremely large telescopes and also highlight prospects for suitable observing strategies to push to below 1 m s^-1 precisions for bright stars in a search for sub-Neptunian planets.

We have examined radio data from the WAVES experiment on the Wind spacecraft in conjunction with ground-based data in order to investigate the relationship between the shocks responsible for metric type II radio bursts and the shocks in front of coronal mass ejections (CMEs). The bow shocks of fast, large CMEs are strong interplanetary (IP) shocks, and the associated radio emissions often consist of single broad bands starting below ~4 MHz; such emissions were previously called IP type II events. In contrast, metric type II bursts are usually narrowbanded and display two harmonically related bands. In addition to displaying complete dynamic spectra for a number of events, we also analyze the 135 WAVES 1-14 MHz slow-drift time periods in 2001-2003. We find that most of the periods contain multiple phenomena, which we divide into three groups: metric type II extensions, IP type II events, and "blobs and bands." About half of the WAVES listings include probable extensions of metric type II radio bursts, but in more than half of these events, there were also other slow-drift features. In the 3 yr study period, there were 31 IP type II events; these were associated with the very fastest CMEs. The most common form of activity in the WAVES events, blobs and bands in the frequency range between 1 and 8 MHz, fall below an envelope consistent with the early signatures of an IP type II event. However, most of this activity lasts only a few tens of minutes, whereas IP type II events last for many hours. In this study we find many examples in the radio data of two shock-like phenomena with different characteristics that occur simultaneously in the metric and decametric/hectometric bands, and no clear example of a metric type II burst that extends continuously down in frequency to become an IP type II event. The simplest interpretation is that metric type II bursts, unlike IP type II events, are not caused by shocks driven in front of CMEs.

We present observations of rapid penumbral decay associated with a major flare in solar NOAA Active Region 9026 on 2000 June 6. Within 1.5 hr, an X2.3 flare accompanied by an 11° long filament eruption and a full-halo coronal mass ejection (CME) originated near the neutral line of a large δ-spot region, which was associated with significant changes in white-light structure and magnetic field topology: an increase of moving magnetic features (MMFs), flux emergence and cancellation, and, in particular, the rapid disappearance of two penumbral segments located in opposite-polarity regions on the north and south sides of the δ-spot. The rapid penumbral decay is believed to be the result of magnetic field topology change that was caused by rapid magnetic reconnection during the flare, rather than part of overall long-term evolution. We present a possible explanation of this event, using a "magnetic breakout" model for solar flares, considering its complex multipolar δ-configuration and associated filament eruption and CME, i.e., previously closed magnetic field lines opened up and reconnected at a null point above the neutral line of this δ-spot. The magnetic breakout caused an energy release from a highly sheared magnetic field in the umbrae and a transition of the magnetic arcades from low lying to high lying, which led to an increase of the inclination angle of the magnetic field lines in the peripheral penumbrae; i.e., the magnetic field turned from more inclined to more vertical and toward the inner umbrae. Once the magnetic field in the penumbrae was vertical enough, the Evershed flow ceased, the manifestation of which in white-light structure is the disappearance of peripheral penumbrae. We also discuss other possible flare models for this event and compare them in several observational features. The present observations provide further evidence that highly energetic events have a distinct associated photospheric magnetic field signature and support the findings of recent analyses of photospheric line-of-sight magnetograms from the Big Bear Solar Observatory (BBSO) and the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) that show rapid and permanent changes of photospheric magnetic fields associated with flares.

Improving the accuracy and resolution of helioseismic inversions calls for more accurate modeling of the observational p-mode power spectra from which the solar oscillation frequencies are traditionally measured. We present a new technique of calculating the response function (leakage matrix) for Doppler velocity measurements that is based largely on an analytical description of the relevant instrumental and physical effects. The computational efficiency of the new approach allows us to implement the response function in an adaptive manner: i.e., the compensation for instrumental or optical distortions of unknown magnitude can be performed as a part of the spectral fitting procedure.

The high-frequency spectrum above the acoustic cutoff frequency (ν_ac) of the solar atmosphere has been observed in velocity measurements at low and high angular degrees and reveals significant evidence for the presence of pseudomodes characterized by their equally spaced signal. In this paper we study this region of the spectra using for the first time disk-integrated intensity measurements from the VIRGO/SPM photometers on board the SOHO probe. These pseudomodes have high visibility, and whereas the frequency separation between the pseudomodes is the same, some differences are found among the three wavelengths. We have studied the evolution of the pseudomodes with the solar cycle; nevertheless, no significant variations are found. Finally, the intensity pseudomodes of VIRGO/SPM are compared with those of velocity as seen by the GOLF instrument during the same periods and with the same analytical techniques.

Thermochemical equilibrium and kinetic calculations for the trace gases CO, PH₃, and SiH₄ give three independent constraints on the water and total oxygen abundances of Saturn's deep atmosphere. A lower limit to the water abundance of H₂O/H₂ ≥ (1.7) × 10^-3 is given by CO chemistry, whereas an upper limit of H₂O/H₂ ≤ (5.5) × 10^-3 is given by PH₃ chemistry. A combination of the CO and PH₃ constraints indicates a water enrichment on Saturn of 1.9-6.1 times the solar system abundance (H₂O/H₂ = 8.96 × 10^-4). The total oxygen abundance must be at least 1.7 times the solar system abundance (O/H₂ = 1.16 × 10^-3) in order for SiH₄ to remain below the detection limit of SiH₄/H₂ < 2 × 10^-10. A combination of the CO, PH₃, and SiH₄ constraints suggests that the total oxygen abundance on Saturn is 3.2-6.4 times the solar system abundance. Our results indicate that oxygen on Saturn is less enriched than other heavy elements (such as C and P) relative to the solar system composition.

We investigate empirical scaling relations between the thermal Sunyaev-Zeldovich effect (SZE) and cluster mass in simulated clusters of galaxies. The simulated clusters have been compiled from four different samples that differ only in their assumed baryonic physics. We show that the strength of the thermal SZE integrated over a significant fraction of the virialized region of the clusters is relatively insensitive to the detailed heating and cooling processes in the cores of clusters, by demonstrating that the derived scaling relations are nearly identical among the four cluster samples considered. For our synthetic images, the central Comptonization parameter shows significant boosting during transient merging events, but the integrated SZE appears to be relatively insensitive to these events. Most importantly, the integrated SZE closely tracks the underlying cluster mass. Observations through the thermal SZE allow a strikingly accurate mass estimation from relatively simple measurements that do not require either parametric modeling or geometric deprojection and thus avoid assumptions regarding the physics of the intracluster medium or the symmetry of the cluster. This result offers significant promise for precision cosmology using clusters of galaxies.

We analyze the effect of dissipation on the orbital evolution of supermassive black holes (SMBHs) using high-resolution self-consistent gasdynamical simulations of binary equal- and unequal-mass mergers of disk galaxies. The galaxy models are consistent with the ΛCDM paradigm of structure formation, and the simulations include the effects of radiative cooling and star formation. We find that equal-mass mergers always lead to the formation of a close SMBH pair at the center of the remnant, with separations limited solely by the adopted force resolution of ~100 pc. Instead, the final SMBH separation in unequal-mass mergers depends sensitively on how the central structure of the merging galaxies is modified by dissipation. In the absence of dissipation, the satellite galaxy can be entirely disrupted before the merger is completed, leaving its SMBH wandering at a distance too far from the center of the remnant for the formation of a close pair. In contrast, gas cooling facilitates the pairing process by increasing the resilience of the companion galaxy to tidal disruption. Moreover, we demonstrate that merging disk galaxies constructed to obey the M_BH-σ relation move relative to it depending on whether they undergo a dissipational or collisionless merger, regardless of the mass ratio of the merging systems. Collisionless simulations reveal that remnants tend to move away from the mean relation, highlighting the role of gas-poor mergers as a possible source of scatter. In dissipational mergers, the interplay between strong gas inflows associated with the formation of massive nuclear disks and the consumption of gas by star formation provides the necessary fuel to the SMBHs and allows the merger remnants to satisfy the relation

Elliptical galaxies larger than 10 pixels in the Hubble Ultra Deep Field (UDF) were surveyed for internal structure; 30 out of 100 in a sample of 884 morphologically classified galaxies exhibit large blue clumps near their centers. Unsharp-masked images of the clearest cases are presented. The distributions of the clumps on color-color and color-magnitude diagrams are about the same as the distributions of isolated objects in the UDF with the same size, suggesting a possible accretion origin. In the few cases for which redshifts have been published, the clump masses and star formation ages were determined from stellar evolution models, as were the galaxy masses. The clump mass scales with galaxy mass, probably because of selection effects, and ranges from 10⁶ to 10⁸M_☉ for galaxies with masses from 10⁹ to 10¹¹M_☉. The clump star formation age ranges between 10⁷ and 2 × 10⁸ yr. With partial evaporation and core contraction in the intervening years, some of these clumps could resemble globular clusters today. Stars that evaporate will contribute to the field population in the elliptical galaxies.

We study the star formation rate (SFR) as a function of environment for UV-selected Lyman break galaxies (LBGs) at redshift 3. From deep [μ_{I, AB}(sky) ≃ 27.6] UBVI MOSAIC images, covering a total of 0.90 deg², we select 334 LBGs in slices 100 h^-1 Mpc (comoving) deep spanning the redshift range 2.9 < z < 3.4 based on Bayesian photometric redshifts that include the I magnitude as a prior. The slice width (100 h^-1 Mpc) corresponds to the photometric redshift accuracy (Δ_z ~ 0.15). We used mock catalogs from the GIF2 cosmological simulations to show that this redshift resolution is sufficient to statistically differentiate the high-density regions from the low-density regions using Σ₅, the projected density to the fifth nearest neighbor. These mock catalogs have a redshift depth of 110 h^-1 Mpc, similar to our slice width. The large area of the MOSAIC images, ~40 × 40 Mpc (comoving) per field, allows us to measure the SFR from the dust-corrected UV continuum as a function of Σ₅. In contrast to low-redshift galaxies, we find that the SFR (or UV luminosity) of LBGs at z = 3 shows no detectable dependence on environment over 2 orders of magnitude in densities. To test the significance of our result, we use Monte Carlo simulations (from the mock catalogs) and the same projected density estimators that we applied to our data. We find that we can reject the steep z = 0 SFR-density relation at the 5 σ level. We conclude that the SFR-density relation at z = 3 must be at least 3.6 times flatter than it is locally; i.e., the SFR of LBGs is significantly less dependent on environment than the SFR of local star-forming galaxies. We find that the rest-frame UV colors are also independent of environment.

We study the spatial distribution of a 95% complete sample of 508 X-ray point sources (XPSs) detected in the 0.5-2.0 keV band in Chandra ACIS-I observations of 51 massive galaxy clusters found in the MAssive Cluster Survey (MACS). Covering the redshift range z = 0.3-0.7, our cluster sample is statistically complete and comprises all MACS clusters with X-ray luminosities in excess of 4.5 × 10⁴⁴ ergs s^-1 (0.1-2.4 keV, h₀ = 0.7, ΛCDM). Also studied are 20 control fields that do not contain clusters. We find the XPS surface density, computed in the cluster rest frame, to exhibit a pronounced excess within 3.5 Mpc of the cluster centers. The excess, believed to be caused by active galactic nuclei (AGNs) in the cluster, is significant at the 8.0 σ confidence level compared to the XPS density observed at the field edges. No significant central excess is found in the control fields. To investigate the physical origin of the AGN excess, we study the radial AGN density profile for a subset of 24 virialized clusters. We find a pronounced central spike (r < 0.5 Mpc), followed by a depletion region at about 1.5 Mpc, and a broad secondary excess centered at approximately the virial radius of the host clusters (≈2.5 Mpc). We present evidence that the central AGN excess reflects increased nuclear activity triggered by close encounters between infalling galaxies and the giant cD-type elliptical galaxy occupying the very cluster center. By contrast, the secondary excess at the cluster-field interface is likely due to black holes being fueled by galaxy mergers. In-depth spectroscopic and photometric follow-up observations of the optical counterparts of the XPSs in a subset of our sample are being conducted to confirm this picture.

We report the discovery of a massive, X-ray-luminous cluster of galaxies at z = 1.393, the most distant X-ray-selected cluster found to date. XMMU J2235.3-2557 was serendipitously detected as an extended X-ray source in an archival XMM-Newton observation of NGC 7314. VLT FORS2 R- and z-band snapshot imaging reveals an overdensity of red galaxies in both angular and color spaces. The galaxy enhancement is coincident in the sky with the X-ray emission; the cluster red sequence at R-z ≃ 2.1 identifies it as a high-redshift candidate. Subsequent FORS2 multiobject spectroscopy unambiguously confirms the presence of a massive cluster based on 12 concordant redshifts in the interval 1.38 < z < 1.40. The preliminary cluster velocity dispersion is 762 ± 265 km s^-1. VLT ISAAC K_s- and J-band images underscore the rich distribution of red galaxies associated with the cluster. Based on a 45 ks XMM-Newton observation, we find that the cluster has an aperture-corrected unabsorbed X-ray flux of f_X = (3.6 ± 0.3) × 10^-14 ergs cm^-2 s^-1, a rest-frame X-ray luminosity of L_X = (3.0 ± 0.2) × 10⁴⁴h ergs s^-1 (0.5-2.0 keV), and a temperature of kT = 6.0 keV. Though XMMU J2235.3-2557 is likely the first confirmed z > 1 cluster found with XMM-Newton, the relative ease and efficiency of discovery demonstrates that it should be possible to build large samples of z > 1 clusters through the joint use of X-ray and large ground-based telescopes.

Plasma outflows from gamma-ray bursts, supernovae, and relativistic jets, in general, interact with the surrounding medium through collisionless shocks. The microphysics of such shocks are still poorly understood, which, potentially, can introduce uncertainties in the interpretation of observations. It is now well established that the Weibel two-stream instability is capable of generating strong electromagnetic fields in the transition region between the jet and the ambient plasma. However, the parameter space of collisionless shocks is vast and still remains unexplored. In this Letter, we focus on how an ambient magnetic field affects the evolution of the electron Weibel instability and the associated shock. Using a particle-in-cell code, we have performed three-dimensional numerical experiments on such shocks. We compare simulations in which a jet is injected into an unmagnetized plasma with simulations in which the jet is injected into a plasma with an ambient magnetic field both parallel and perpendicular to the jet flow. We find that there exists a threshold of the magnetic field strength below which the Weibel two-stream instability dominates, and we note that the interstellar medium magnetic field strength lies well below this value. In the case of a strong magnetic field parallel to the jet, the Weibel instability is quenched. In the strong perpendicular case, ambient and jet electrons are strongly accelerated because of the charge separation between deflected jet electrons and less deflected jet ions. Also, the electromagnetic topologies become highly nonlinear and complex with the appearance of antiparallel field configurations.

We present multiple Chandra and XMM-Newton observations of the type 1.8 Seyfert galaxy NGC 1365, which shows the most dramatic X-ray spectral changes observed so far in an active galactic nucleus: the source switched from reflection-dominated to transmission-dominated and back in just 6 weeks. During this time the soft thermal component, arising from an ~1 kpc region around the center, remained constant. The reflection component is constant at all timescales, and its high flux relative to the primary component implies the presence of thick gas covering a large fraction of the solid angle. The presence of this gas, and the fast variability timescale, suggest that the Compton-thick to Compton-thin change is due to variation in the line-of-sight absorber rather than to extreme intrinsic emission variability. We discuss a structure of the circumnuclear absorber/reflector that can explain the observed X-ray spectral and temporal properties.

We use the observed number and column density distributions of intergalactic O VI absorbers to constrain the distribution of metals in the low-redshift intergalactic medium (IGM). In this simple model, the metals in the O VI absorbers are assumed to be produced in and propagated from low-redshift galaxies drawn from a real sample, in this case the Sloan Digital Sky Survey. This model can explain the observed d/dz of metals borne by O VI absorbers if these metals are dispersed out to ~200 kpc by galaxies down to L ~ L. Massive galaxies (L ~ L) by themselves cannot provide the metals unless they can enrich volumes out to ≳0.5-1 Mpc. This model suggests that the turnover in d/dNdz below log N(O ) ≃ 13.5 (W_λ = 50 mÅ) is likely a real effect resulting from the apparently limited volumes over which galaxies can disperse metals, rather than a consequence of survey incompleteness. If so, it would indicate that metals are not as widespread throughout the IGM, as they are assumed to be in cosmological simulations of the warm-hot IGM. Alternatively, our model estimates the fraction of O VI absorbers directly caused by galaxies, such as the known populations of highly ionized high-velocity clouds surrounding the Milky Way, rather than the hot IGM.

A new generation of sensitive X-ray measurements are indicating that the existence of X-ray attenuation column densities, N_H > 10²⁴ cm^-2, is quite common among broad absorption line quasars (BALQSOs). This is significant to the geometry of the broad absorption line (BAL) outflow. In particular, such an X-ray shield also shields equatorial accretion disk winds from the UV, thereby preventing high-velocity equatorial outflows from being launched. By contrast, bipolar winds initiated by continuum radiation pressure from the funnel of a slim accretion disk flare outward (like a trumpet) and offer vastly different absorbing columns to the X-ray and UV emission that are emitted from distinct regions of the disk, ~6M and ~10M-40M, respectively (where M is the radius of the black hole). Recent numerical work indicates that it is also possible to launch bipolar outflows from the inner regions of a thin disk. The recent discovery with VLBI that the Galactic analog of a BALQSO, the X-ray binary Circinus X-1 (with high-velocity P Cygni X-ray absorption lines), is viewed virtually along the radio jet axis (and therefore along the spin axis of the black hole and the normal to the accretion disk) has rekindled interest in the bipolar models of BALQSOs. We explore this possibility by studying the nearest BALQSO, Mrk 231. High-resolution two-dimensional optical spectroscopy and VLBI mappings of the radio jet axis indicate that the BAL outflow is parallel to the parsec-scale radio jet.

Using imaging from the Hubble Space Telescope, we derive surface brightness profiles for ultracompact dwarfs in the Fornax Cluster and for the nuclei of dwarf elliptical galaxies in the Virgo Cluster. Ultracompact dwarfs are more extended and have higher surface brightnesses than typical dwarf nuclei, while the luminosities, colors, and sizes of the nuclei are closer to those of Galactic globular clusters. This calls into question the production of ultracompact dwarfs via "threshing," whereby the lower surface brightness envelope of a dwarf elliptical galaxy is removed by tidal processes, leaving behind a bare nucleus. Threshing may still be a viable model if the relatively bright Fornax ultracompact dwarfs considered here are descended from dwarf elliptical galaxies whose nuclei are at the upper end of their luminosity and size distributions.

We report the detection of radio emission coincident with the ultraluminous X-ray source (ULX) in Holmberg II. The radio emission is diffuse and resolved, covering an area ~60 × 40 pc in extent and well matched to the recently discovered He II nebula surrounding the X-ray source. Comparison of the radio and optical properties of this extended radio emission argue against its association with either an H II region or a supernova remnant. This is additional evidence that this ULX is not powered by a stellar mass object whose emission is relativistically beamed toward the observer, and thus is either a super-Eddington source or an intermediate-mass black hole as suggested by optical observations. Implications of this result to future and existing radio studies of ULXs are discussed.

The Laser Interferometer Space Antenna (LISA) is expected to detect N ~ 22 × 10^±1 close white dwarf binaries in the Large Magellanic Cloud (LMC) through their gravitational radiation with signal-to-noise ratios greater than ~10 in observational durations of 3 yr or more. In addition to chirp mass, location on the sky, and other binary parameters, the distance to each binary is an independent parameter that can be extracted from an analysis of gravitational waves from these binaries. Using a sample of binaries, one can establish the mean distance to the LMC as well as the variance of this distance. Assuming no confusion noise at frequencies above 2 mHz, for data collected over 10 yr, LISA might determine the LMC distance to ~4.5(N/22)^1/2(1 yr/T_obs)^1/2% and the line-of-sight extent of the LMC to ~15(N/22)^1/4%, relative to its distance, at the 1 σ confidence. For a 3 yr observational window, these estimates are degraded by a factor of ~2.5. In general, the estimates based on LISA are competitive with some of the proposed direct geometric techniques of measuring the LMC distance in the future with missions such as the Space Interferometry Mission and the Global Astrometric Interferometer for Astrophysics.

We report on the discovery of a large number of RR Lyrae variable stars in the moderately metal-rich Galactic globular cluster M62 (NGC 6266), which places it among the top three most RR Lyrae-rich globular clusters known. Likely members of the cluster in our studied field, from our preliminary number counts, include ≈130 fundamental-mode (RRab) pulsators, with ⟨P_ab⟩ = 0.548 days, and ≈75 first-overtone (RRc) pulsators, with ⟨P_c⟩ = 0.300 days. The average periods and the position of the RRab variables with well-defined light curves in the Bailey diagram both suggest that the cluster is of Oosterhoff type I. However, the morphology of the cluster's horizontal branch (HB) is strikingly similar to that of the Oosterhoff type II globular cluster M15 (NGC 7078), with a dominant blue HB component and a very extended blue tail. Since M15 and M62 differ in metallicity by about 1 dex, we conclude that metallicity, at a fixed HB type, is a key parameter determining the Oosterhoff status of a globular cluster and the position of its variables in the Bailey diagram.

We present the results of a Chandra 30 ks observation of the low-mass X-ray binary and atoll source 4U 1705-44. Here we concentrate on the study of discrete features in the energy spectrum at energies below ~3 keV, as well as on the iron Kα line, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. Below 3 keV, three narrow emission lines are found at 1.47, 2.0, and 2.6 keV. The 1.47 and 2.6 keV lines are probably identified with Lyα emission from Mg XII and S XVI, respectively. The identification of the feature at ~2.0 keV is uncertain because of the presence of an instrumental feature at the same energy. The iron Kα line at ~6.5 keV is found to be intrinsically broad (FWHM ~ 1.2 keV); its width can be explained by reflection from a cold accretion disk extending down to ~15 km from the neutron star center or by Compton broadening in the external parts of a hot (~2 keV) Comptonizing corona. We finally report here precise X-ray coordinates of the source.

We present K'-band (2.12 μm) imaging observations of the SGR 1806-20 field taken during its very active phase in mid-2004; these observations reveal brightening of sources within the Chandra X-ray error circle when compared with earlier images obtained in 2002. One source brightened by more than a factor of 2, and so we consider this to be the probable infrared counterpart to SGR 1806-20. The other two sources are located in close proximity to the probable counterpart and show marginal brightening, which may suggest that the high-energy photons emitted from the soft gamma-ray repeater (SGR) during its active phase have induced dust sublimation or brightening of the unresolved background around the SGR.

Recent observations of blue stragglers by O. De Marco et al. have revealed continuum deficits on the blue side of the Balmer discontinuity, leading these authors to infer the presence of disks around the stars. This intriguing possibility may throw light on aspects of the mechanisms responsible for at least some of these objects; current theories of blue straggler formation invoke stellar collisions or interacting binaries, both of which appear capable of forming a circumstellar disk. However, by synthesizing photospheric spectra for models of rotating blue stragglers, we demonstrate that the Balmer jump enhancements can be wholly attributed to the influence of oblateness and gravity darkening on the formation of the continuum. Therefore, we are led to conclude that the observations of De Marco et al. can be ascribed a more prosaic explanation, that of rapid stellar rotation arising from the merger/interaction formation process.

We present new theoretical period-radius-metallicity relations for RR Lyrae stars. Current predictions are based on a large set of nonlinear, convective models that cover a broad range of chemical abundances and input parameters. We also provide new and homogeneous estimates of angular diameters for a sample of field RR Lyrae stars using a recent calibration of the Barnes-Evans surface brightness relation. Predicted and empirical radii are, within the errors, in reasonable agreement, but in the short-period range the latter present a larger scatter. As a working hypothesis we suggest that this discrepancy might be due to the occurrence either of nonlinear features such as bumps or a steep rising branch. A new distance determination for RR Lyr itself is in very good agreement with the Hubble Space Telescope trigonometric parallax and with the pulsation parallax.

We report the Chandra ACIS-S detection of a bright soft X-ray transient in the Mira AB interacting symbiotic-like binary. We have resolved the system for the first time in X-rays. Using Chandra and Hubble Space Telescope images, we determine that the unprecedented outburst is likely associated with the cool asymptotic giant branch (AGB) star, Mira A, the prototype of the Mira class of variables. X-rays have never before been detected from an AGB star, and the recent activity signals that the system is undergoing dramatic changes. The total X-ray luminosity of the system is several times higher than the luminosity estimated using previous XMM-Newton and ROSAT observations. The outburst may be caused by a giant flare in Mira A associated with a mass ejection or a jet and may have long-term consequences on the system.

We report the discovery of a uniquely large excess of ¹⁸O in the hydrogen-deficient carbon (HdC) star HD 137613 based on a spectrum of the first-overtone bands of CO at 2.3-2.4 μm in which three strong absorption bands of ¹²C¹⁸O are clearly present. Bands of ¹²C¹⁶O also are present, but no bands of ¹³C¹⁶O or ¹²C¹⁷O are seen. We estimate an isotopic ratio ¹⁶O/¹⁸O ≲ 1. The solar value of this ratio is ~500. Neither He-core burning nor He-shell flash burning can produce the isotopic ratios of oxygen and carbon observed in HD 137613. However, a remarkable similarity exists between the observed abundances and those found in the outer layers of the broad He shell of early-asymptotic giant branch (AGB) stars, soon after the end of He-core burning. It is not known how the outer envelope down to the He shell could be lost, but some mechanism of enhanced mass loss must be involved. HD 137613 may be a post-early-AGB star with the outer layers of the former He-burning shell as its photosphere. The unusual elemental abundances of the HdC stars resemble those of the R Coronae Borealis (RCB) stars, but HdC stars do not produce clouds of dust that produce declines in brightness. None of the other RCB or HdC stars observed show significant ¹⁸O.

Twenty-four days of highly precise Microvariability and Oscillations of Stars (MOST) satellite photometry obtained in mid-2004 of the rapidly rotating O9.5 V star ζ Oph have yielded at least a dozen significant oscillation frequencies between 1 and 10 cycles day^-1, clearly indicating its relationship to β Cephei variables. Eight periods were found in He I λ4922 and Hβ line profile variations (LPV) of which six coincide with those from the MOST photometry. This unique photometric and spectroscopic detection of radial and nonradial pulsations leads to a plausible model in which high l-modes are excited when their frequencies in the corotating frame are similar to those of low-order radial modes. We propose that the dominant photometric 4.6 hr period (f₁) corresponds to a radial first overtone excited by the κ-mechanism associated with the Fe opacity bump. No unambiguous rotational period can be identified in either the light curve or the LPV.

We propose that UV radiation can stimulate the formation of planetesimals in externally illuminated protoplanetary disks. We present a numerical model of disk evolution including vertical sedimentation and photoevaporation by an external O or B star. As solid material grows and settles toward the disk midplane, the outer layers of the disk become dust-depleted. When such a disk is exposed to UV radiation, heating drives photoevaporative mass loss from its surface, generating a dust-depleted outflow. The dust-to-gas surface density ratio in the disk interior grows until dust in the disk midplane becomes gravitationally unstable. Thus, UV radiation fields may induce the rapid formation of planetesimals in disks where sedimentation has occurred.

The neutron capture cross section of ⁶²Ni has been measured in the energy range from 5.5 to 90 keV by employing a prompt discrete γ-ray detection method with the use of an anti-Compton NaI(Tl) spectrometer. The Maxwellian-averaged capture cross section at 30 keV is derived as being 37.0 ± 3.2 mbarn, 3 times larger than the value currently used for the nucleosynthetic yield estimations in massive stars. The result could provide crucial information for the long-standing problem of the largest overproduction of ⁶²Ni. The intense discrete γ-ray feeding from a neutron capture state to the ground state of ⁶³Ni was for the first time observed at stellar neutron energy. This observation and the neutron energy dependence of the total neutron capture cross section of ⁶²Ni indicate clearly that the dominant capture process of ⁶²Ni is a nonresonant direct s-wave capture process and not a p-wave capture process as previously predicted.