Table of contents

We propose a method to study the epoch of reionization based on the possible observation of 2p-2s fine-structure lines from the neutral hydrogen outside the cosmological H II regions enveloping QSOs and other ionizing sources in the reionization era. We show that for parameters typical of luminous sources observed at z ≃ 6.3, the strength of this signal, which is proportional to the H I fraction, has a brightness temperature ≃20 μK for a fully neutral medium. The fine-structure line from this redshift is observable at ν ≃ 1 GHz, and we discuss prospects for the detection with several operational and future radio telescopes. We also compute the characteristics of this signal from the epoch of recombination; the peak brightness is expected to be ≃100 μK, and this signal appears in the frequency range 5-10 MHz. The signal from the recombination era is nearly impossible to detect owing to the extreme brightness of the Galactic emission at these frequencies.

We test the hypothesis that the temperature of the cosmic microwave background is consistent with a Gaussian random field defined on the celestial sphere, using the full sky debiased internal linear combination (DILC) map produced from the three-year WMAP data. We test the phases for spherical harmonic modes with ℓ ≤ 10 (which should be the cleanest) for uniformity, randomness, and correlation with phases of the foreground maps. The phases themselves are consistent with a uniform distribution, but the differences between phases (randomness) are not consistent with uniformity. For ℓ = 3 and ℓ = 6, the phases of the CMB maps cross-correlate with the foregrounds, suggesting the presence of residual contamination in the DILC map even on these large scales. We also use a one-dimensional Fourier representation to assemble a_ℓm into the ΔT_ℓ(φ) for each ℓ mode and test the positions of the resulting maxima and minima for consistency with uniformity randomness on the unit circle. The results show significant departures at the 0.5% level, with the one-dimensional peaks concentrated around φ = 180°. This strongly significant alignment with the Galactic meridian, together with the cross-correlation of DILC phases with the foreground maps, strongly suggests that even the lowest spherical harmonic modes in the map are significantly contaminated with foreground radiation.

We describe the methodology and compute the illumination of geometrically thin accretion disks around black holes of arbitrary spin parameter a exposed to the radiation of a pointlike isotropic source at arbitrary height above the disk on its symmetry axis. We then provide analytic fitting formulae for the illumination as a function of the source height h and the black hole angular momentum a. We find that for a source on the disk symmetry axis and with h/M > 3, the main effect of the parameter a is allowing the disk to extend to smaller radii (approaching r/M → 1 as a/M → 1) and thus allowing the illumination of regions of much higher rotational velocity and redshift. We also compute the illumination profiles for anisotropic emission associated with the motion of the source relative to the accretion disk and present the fractions of photons absorbed by the black hole, intercepted by the disk, or escaping to infinity for both isotropic and anisotropic emission for a/M = 0 and 0.99. As the anisotropy (of a source approaching the disk) increases, the illumination profile reduces (approximately) to a single power law, whose index q, because of absorption of the beamed photons by the black hole, saturates to a value no higher than q ≳ 3. Finally, we compute the fluorescent Fe line profiles associated with the specific illumination and compare them among various cases.

We derive linearized relativistic magnetohydrodynamic (RMHD) equations describing a uniform axially magnetized cylindrical relativistic jet spine embedded in a uniform axially magnetized relativistically moving sheath. The displacement current is retained in the equations, so that effects associated with Alfvén wave propagation near light speed can be studied. A dispersion relation for the normal modes is obtained. Analytical solutions for the normal modes in the low- and high-frequency limits are found, and a general stability condition is determined. A trans-Alfvénic and even a super-Alfvénic relativistic jet spine can be stable to velocity shear-driven Kelvin-Helmholtz modes. The resonance condition for maximum growth of the normal modes is obtained in the kinetically and magnetically dominated regimes. Numerical solution of the dispersion relation verifies the analytical solutions and is used to study the regime of high sound and Alfvén speeds.

By their star content, the galaxies split out into a red and a blue population; their color index peaked around u-r ≈ 2.5 or u-r ≈ 1, respectively, quantifies the ratio of the blue stars newly formed from cold galactic gas, to the redder ones left over by past generations. On the other hand, on accreting substantial gas amounts the central massive black holes energize active galactic nuclei (AGNs); here we investigate whether these show a similar, and possibly related, bimodal partition as for current accretion activity relative to the past. To this aim we use an updated semianalytic model; based on Monte Carlo simulations, this follows with a large statistics the galaxy assemblage, the star generations, and the black hole accretions in the cosmological framework over the redshift span from z = 10 to z = 0. We test our simulations for yielding in close detail the observed split of galaxies into a red, early and a blue, late population. We find that the black hole accretion activities likewise give rise to two source populations: early, bright quasars and later, dimmer AGNs. We predict for their Eddington parameter λ_E—the ratio of the current to the past black hole accretions—a bimodal distribution; the two branches sit now under λ_E ≈ 0.01 (mainly contributed by low-luminosity AGNs) and around λ_E ≈ 0.3-1. These not only mark out the two populations of AGNs, but also will turn out to correlate strongly with the red or blue color of their host galaxies.

We present a catalog of 36,120 QSO candidates from the Galaxy Evolution Explorer (GALEX) Release Two (GR2) UV catalog and the USNO-A2.0 optical catalog. The selection criteria are established using known quasars from the Sloan Digital Sky Survey (SDSS). The SDSS sample is then used to assign individual probabilities to our GALEX USNO candidates. The mean probability is ~50% and would rise to ~65% if better morphological information than that from USNO were available to eliminate galaxies. The sample is ~40% complete for i ≤ 19.1. Candidates are cross identified in 2MASS, FIRST, SDSS, and the XMM-Newton Slewing Survey (XMMSL1), whenever such counterparts exist. The present catalog covers the 8000 deg² of GR2 lying above = 25°, but can be extended to all 24,000 deg² that satisfy this criterion as new GALEX data become available.

We report the discovery of 40 bright quasars and active galactic nuclei (AGNs) at low Galactic latitude (b < ). The low Galactic latitude region has been considered a place to avoid when searching for extragalactic sources, because of the high Galactic extinction, as well as a large number of stars contaminating the sample selection. Bright quasars (R ≲ 17) suffer more from such difficulties because they look like bright stars, which are numerous at low b, yet their surface number density is very low. In order to find quasars in this region of the sky less explored for extragalactic sources, we have started a survey of low Galactic latitude bright quasars as a part of the Seoul National University Quasar Survey in Optical (SNUQSO). Quasar candidates have been selected from radio and near-infrared (NIR) data. Out of 88 targets, we identify 29 bright quasars/AGNs around the antigalactic center, and 11 bright quasars/AGNs in the outskirts of the Galactic center, from two observing runs in 2006 at the Bohyunsan Optical Astronomical Observatory (BOAO) in Korea. Our finding demonstrates that quasars/AGNs can be discovered effectively, even at low Galactic latitude, using multiwavelength data.

We present the first systematic investigation of the [Ne V] (14 μm/24 μm) and [S III] (18 μm/33 μm) infrared line flux ratios, traditionally used to estimate the density of the ionized gas, in a sample of 41 type 1 and type 2 active galactic nuclei (AGNs) observed with the Infrared Spectrograph on board Spitzer. The majority of galaxies with both [Ne V] lines detected have observed [Ne V] line flux ratios consistent with or below the theoretical low-density limit, based on calculations using currently available collision strengths and ignoring absorption and stimulated emission. We find that type 2 AGNs have lower line flux ratios than type 1 AGNs and that all of the galaxies with line flux ratios below the low-density limit are type 2 AGNs. We argue that differential infrared extinction to the [Ne V] emitting region due to dust in the obscuring torus is responsible for the ratios below the low-density limit and we suggest that the ratio may be a tracer of the inclination angle of the torus to our line of sight. Because the temperature of the gas, the amount of extinction, and the effect of absorption and stimulated emission on the line ratios are all unknown, we are not able to determine the electron densities associated with the [Ne V] line flux ratios for the objects in our sample. We also find that the [S III] emission from the galaxies in our sample is extended and originates primarily in star-forming regions. Since the emission from low-ionization species is extended, any analysis using line flux ratios from such species obtained from slits of different sizes is invalid for most nearby galaxies.

We present the results from simultaneous Chandra and RXTE observations of the X-ray bright broad-line radio galaxy (BLRG) 3C 382. The long (120 ks) exposure with Chandra HETG allows a detailed study of the soft X-ray continuum and of the narrow component of the Fe Kα line. The RXTE PCA data are used to put an upper limit on the broad-line component and constrain the hard X-ray continuum. A strong soft excess below 1 keV is observed in the time-averaged HETG spectrum, which can be parameterized with a steep power law or a thermal model. The flux variability at low energies indicates that the origin of the soft excess cannot be entirely ascribed to the circumnuclear diffuse emission, detected by Chandra on scales of 20''-30'' (22-33 kpc). A narrow (σ < 90 eV) Fe Kα line (with EW< 100 eV) is observed by the Chandra HEG. Similar values for the line parameters are measured by the RXTE PCA, suggesting that the contribution from a broad-line component is negligible. The fact that the exposure is split into two observations taken 3 days apart allows us to investigate the spectral and temporal evolution of the source on different timescales. Significant flux variability associated with spectral changes is observed on timescales of hours and days. The spectral variability is similar to that observed in radio-quiet AGN ruling out a jet-dominated origin of the X-rays.

Using Rossi X-Ray Timing Explorer Seyfert 1 and 1.2 data spanning 9 years, we study correlations between X-ray spectral features. The sample consists of 350 time-resolved spectra from 12 Seyfert 1 and 1.2 galaxies. Each spectrum is fitted to a model with an intrinsic power-law X-ray spectrum produced close to the central black hole that is reprocessed and absorbed by material around the black hole. To test the robustness of our results, we performed Monte Carlo simulations of the spectral sample. We find a complex relationship between the iron line equivalent width (EW) and the underlying power-law index (Γ). The data reveal a correlation between Γ and EW, which turns over at Γ ≲ 2, but finds a weak anticorrelation for steeper photon indices. We propose that this relationship is driven by dilution of a disk spectrum (which includes the narrow iron line) by a beamed jet component and hence could be used as a diagnostic of jet dominance. In addition, our sample shows a strong correlation between R and Γ, but we find that it is likely the result of modeling degeneracies. We also see the X-ray Baldwin effect (an anticorrelation between the 2-10 keV X-ray luminosity and EW) for the sample as a whole, but not for the individual galaxies and galaxy types.

The blazar PKS 0537-441 has been observed with all instruments of the Swift satellite between the end of 2004 and 2005 November. The BAT monitored it recurrently for a total of 2.7 Ms, and the XRT and UVOT pointed it on seven occasions for a total of 67 k. The automatic optical and near-infrared telescope REM has monitored simultaneously the source at all times. In 2005 January-February, PKS 0537-441 was detected at its brightest in optical and X-rays: more than a factor of 2 brighter in X-rays and about a factor of 60 brighter in the optical than observed in 2004 December. The 2005 July observation recorded a fainter X-ray state, albeit still brighter than the historical average. The simultaneous optical state is comparable to the one recorded in early 2005 January, before the outburst. In 2005 November, the source subsided both in X-rays and optical to a quiescent state. The optical and X-ray variations are well correlated, with no measurable time lag larger than about 1 month. On intraday timescales there is no obvious correlation between X-ray and optical variations, but the former tend to be more pronounced, opposite to what is observed on monthly timescales. The widely different amplitude of the long-term variability in optical and X-rays is very unusual. The spectral energy distributions are interpreted in terms of the synchrotron and inverse Compton mechanisms within a jet where the plasma radiates via internal shocks and the dissipation depends on the distance of the emitting region from the central engine.

The MareNostrum Universe is one of the largest cosmological smoothed particle hydrodynamics simulations done so far. It consists of 1024³ dark and 1024³ gas particles in a box of 500 h^-1 Mpc on a side. Here we study the shapes and spins of the dark matter and gas components of the 10,000 most massive objects extracted from the simulation as well as the gas fraction in those objects. We find that the shapes of objects tend to be prolate both in the dark matter and gas. There is a clear dependence of shape on halo mass, the more massive ones being less spherical than the less massive objects. The gas distribution is nevertheless much more spherical than the dark matter, although the triaxiality parameters of gas and dark matter differ only by a few percent, and it increases with cluster mass. The spin parameters of gas and dark matter can be well fitted by a lognormal distribution function. On average, the spin of gas is 1.4 times larger than the spin of dark matter. We find a similar behavior for the spins at higher redshifts, with a slight decrease of the spin ratios to 1.16 at z = 1. The cosmic normalized baryon fraction in the entire cluster sample ranges from Y_b = 0.94 at z = 1 to Y_b = 0.92 at z = 0. At both redshifts we find a slight, but statistically significant, decrease of Y_b with cluster mass.

We present the concentration (c)-virial mass (M) relation of 39 galaxy systems ranging in mass from individual early-type galaxies up to the most massive galaxy clusters, (0.06-20) × 10¹⁴M_☉. We selected for analysis the most relaxed systems possessing the highest quality data currently available in the Chandra and XMM-Newton public data archives. A power-law model fitted to the X-ray c-M relation requires at high significance (6.6 σ) that c decreases with increasing M, which is a general feature of CDM models. The median and scatter of the c-M relation produced by the flat, concordance ΛCDM model (Ω_m = 0.3, σ₈ = 0.9) agrees with the X-ray data, provided that the sample is comprised of the most relaxed, early-forming systems, which is consistent with our selection criteria. When allowing only σ₈ to vary in the concordance model, the c-M relation requires 0.76 < σ₈ < 1.07 (99% confidence), assuming a 10% upward bias in the concentrations for early-forming systems. The tilted, low-σ₈ model suggested by a new WMAP analysis is rejected at 99.99% confidence, but a model with the same tilt and normalization can be reconciled with the X-ray data by increasing the dark energy equation of state parameter to w ≈ -0.8. When imposing the additional constraint of the tight relation between σ₈ and Ω_m from studies of cluster abundances, the X-ray c-M relation excludes (>99% confidence) both open CDM models and flat CDM models with Ω_m ≈ 1. This result provides novel evidence for a flat, low-Ω_m universe with dark energy using observations only in the local (z ≪ 1) universe. Possible systematic errors in the X-ray mass measurements of a magnitude ≈10% suggested by CDM simulations do not change our conclusions.

In dilute astrophysical plasmas, thermal conduction is primarily along magnetic field lines, and therefore highly anisotropic. As a result, the usual convective stability criterion is modified from a condition on entropy to a condition on temperature. For small magnetic fields or small wavenumbers, instability occurs in any atmosphere where the temperature and pressure gradients point in the same direction. We refer to the resulting convective instability as the magnetothermal instability (MTI). We present fully three-dimensional simulations of the MTI and show that saturation results in an atmosphere with different vertical structure, dependent on the boundary conditions. When the temperature at the boundary of the unstable layer is allowed to vary, the temperature gradient relaxes until the unstable region is almost isothermal. When the temperature at the boundary of the unstable region is fixed, the magnetic field is reoriented to an almost vertical geometry as a result of buoyant motions. This case exhibits more vigorous turbulence. In both cases the resulting saturated heat flux is almost one-half of the value expected if the conduction were purely isotropic, ~ χΔT/L, where χ is the thermal conductivity, ΔT is the fixed temperature drop across the simulation domain, and L is the temperature gradient scale length. The action of the MTI results in dynamical processes that lead to significant transport perpendicular to the initial direction of the magnetic field. The resulting magnetoconvection in both cases amplifies the magnetic field until it is almost in equipartition with sustained subsonic turbulence. These results are relevant to understanding measurements of the temperature profiles of the intracluster medium of clusters of galaxies as well as the structure of radiatively inefficient accretion flows.

We study the ability of three-frequency, arcminute-resolution microwave measurements to detect galaxy clusters via their Sunyaev-Zel'dovich (SZ) distortion of the microwave background. For this purpose, we have constructed large-area simulations of the microwave sky, and we have made them publicly available to further investigations into optimal data reduction techniques for upcoming SZ cluster surveys. In these sky simulations, galaxy clusters are modeled using N-body simulated dark matter halos plus a gas prescription for the intracluster medium that allows the small-scale cluster physics such as star formation and feedback to be realistically incorporated. We also model the primary microwave background, radio and infrared point sources, galactic dust emission, and the SZ flux including kinetic and relativistic contributions. We make use of these simulations to study the scaling relation between integrated SZ flux and cluster mass and find that our clusters follow a power law with an index that is steeper than that for self-similar cluster models. Some evolution of the power-law index and normalization with redshift is also observed. These simulations are also used to study cluster detection for the Atacama Cosmology Telescope (ACT). Using a multifrequency Wiener filter to separate clusters from other microwave components, we find that ACT alone can recover a cluster sample that is ≈90% complete and ≈85% pure above 3 × 10¹⁴M_☉.

We provide a new framework for the joint analysis of cluster observations (JACO) using simultaneous fits to X-ray, Sunyaev-Zel'dovich (SZ), and weak-lensing data. Our method fits the mass models simultaneously to all data, provides explicit separation of the gaseous, dark, and stellar components, and—for the first time—allows joint constraints on all measurable physical parameters. JACO includes additional improvements to previous X-ray techniques, such as the treatment of the cluster termination shock and explicit inclusion of the BCG's stellar mass profile. An application of JACO to the rich galaxy cluster Abell 478 shows excellent agreement among the X-ray, lensing, and SZ data. We find that Abell 478 is consistent with a cuspy dark matter profile with inner slope n = 1. Accounting for the stellar mass profile of the BCG allows us to rule out inner dark matter slopes n > 1.1 at the 99% confidence level. At large radii, an r^-3 asymptotic slope is preferred over an r^-4 behavior. All single power-law dark matter models are ruled out at greater than the 99% confidence level. JACO shows that self-consistent modeling of multiwavelength data can provide powerful constraints on the shape of the dark profile.

We study the IR properties of galaxies in the cluster MS 1054-03 at z = 0.83 by combining MIPS 24 μm data with spectra of more than 400 galaxies and a very deep K-band-selected catalog. Nineteen IR cluster members are selected spectroscopically, and an additional 15 are selected by their photometric redshifts. We derive the IR luminosity function of the cluster and find strong evolution compared to the similar-mass Coma Cluster. The best-fitting Schechter function gives L = 11.49L_☉ with a fixed faint-end slope, about 1 order of magnitude larger than that in Coma. The rate of evolution of the IR luminosity from Coma to MS 1054-03 is consistent with that found in field galaxies, and it suggests that some internal mechanism, e.g., the consumption of the gas fuel, is responsible for the general decline of the cosmic SFR in different environments. The mass-normalized integrated SFR within 0.5R₂₀₀ in MS 1054-03 also shows evolution compared with other rich clusters at lower redshifts, but the trend is less conclusive if the mass selection effect is considered. A nonnegligible fraction (13% ± 3%) of cluster members are forming stars actively, and the overdensity of IR galaxies is about 20 compared to the field. It is unlikely that clusters only passively accrete star-forming galaxies from the surrounding fields and have their star formation quenched quickly afterward; instead, many cluster galaxies still have large amounts of gas, and their star formation may be enhanced by the interaction with the cluster.

We present a simple scenario where the formation of galactic bulges was regulated by the dark halo gravity and regulates the growth of the central supermassive black hole. Assuming the angular momentum is low, we suggest that bulges form in a runaway collapse due to the "gravothermal" instability once the central gas density or pressure exceeds a certain threshold. We emphasize that the threshold is nearly universal, set by the background NFW dark matter gravity g_DM ~ 1.2 × 10^-8 cm s^-2 in the central cusps of halos. Unlike known thresholds for gradual formation of galaxy disks, we show that the universal "halo-regulated" star formation threshold for spheroids matches the very high star formation rate and star formation efficiency seen in high-redshift observations of central starburst regions. The starburst feedback also builds up a pressure shortly after the collapse. This large pressure could act both outward to halt further infall of gas from larger scales and inward to counter the Compton-thick wind launched from the central black hole in an Eddington accretion. Assuming the feedback balances inward and outward forces, our scenario naturally gives rise to the black hole-bulge relationships observed in the local universe.

We present a new procedure to fit nonaxisymmetric flow patterns to two-dimensional velocity maps of spiral galaxies. We concentrate on flows caused by barlike or oval distortions to the total potential, which may arise either from a non-axially symmetric halo or a bar in the luminous disk. We apply our method to high-quality CO and Hα data for the nearby, low-mass spiral NGC 2976, previously obtained by Simon et al., and find that a barlike model fits the data at least as well as their model with large radial flows. We find supporting evidence for the existence of a bar in the baryonic disk. Our model suggests that the azimuthally averaged central attraction in the inner part of this galaxy is larger than estimated by these authors. It is likely that the disk is also more massive, which will limit the increase to the allowed dark halo density. Allowance for barlike distortions in other galaxies may either increase or decrease the estimated central attraction.

In integrated light, some color-color diagrams that use optical and near-infrared photometry show surprisingly orthogonal grids as age and metallicity are varied, and they are coming into common usage for estimating the average age and metallicity of spiral galaxies. In this paper we reconstruct these composite grids using simple stellar population models from several different groups convolved with some plausible functional forms of star formation histories at fixed metallicity. We find that the youngest populations present (t < 2 Gyr) dominate the light, and because of their presence the age-metallicity degeneracy can be partially broken with broadband colors, unlike older populations. The scatter among simple stellar population models by different authors is, however, large at ages t < 2 Gyr. The dominant uncertainties in stellar population models arise from convective core overshoot assumptions and the treatment of the thermally pulsing asymptotic giant branch phase and helium abundance may play a significant role at higher metallicities. Real spiral galaxies are unlikely to have smooth, exponential star formation histories, and burstiness will cause a partial reversion to the single-burst case, which has even larger model-to-model scatter. Finally, it is emphasized that the current composite stellar population models need some implementation of chemical enrichment histories for the proper analysis of the observational data.

We combine several HST investigations on the central structure of early-type galaxies to generate a large sample of surface photometry. The studies selected were those that used the "Nuker law" to characterize the inner light distributions of the galaxies. The sample comprises WFPC1 and WFPC2 V-band observations published earlier by our group, R-band WFPC2 photometry of Rest et al., NICMOS H-band photometry by Ravindranath et al. and Quillen et al., and the brightest cluster galaxy WFPC2 I-band photometry of Laine et al. The distribution of the logarithmic slopes of the central brightness profiles strongly affirms that the central structure of elliptical galaxies with M_V < -19 is bimodal, based on both parametric and nonparametric analysis. At the HST resolution limit, most galaxies are either power-law systems, which have steep cusps in surface brightness, or core systems, which have shallow cusps interior to a steeper envelope brightness distribution. A rapid transition between the two forms occurs over the luminosity range -22 < M_V < -20, with cores dominating at the highest luminosities and power laws at the lowest. There are a few "intermediate" systems that have both cusp slopes and total luminosities that fall within the core/power-law transition, but they are rare and do not fill in the overall bimodal distribution.

We present results from Planetary Nebula Spectrograph (PN.S) observations of the elliptical galaxy NGC 3379 and a description of the data reduction pipeline. We detected 214 planetary nebulae, of which 191 are ascribed to NGC 3379 and 23 to the companion galaxy NGC 3384. Comparison with data from the literature shows that the PN.S velocities have an internal error of ≲20 km s^-1 and a possible offset of similar magnitude. We present the results of kinematic modeling and show that the PN kinematics is consistent with absorption-line data in the region where they overlap. The resulting combined kinematic data set, running from the center of NGC 3379 out to more than 7 effective radii (R_eff), reveals a mean rotation velocity that is small compared to the random velocities and a dispersion profile that declines rapidly with radius. From a series of Jeans dynamical models we find the B-band mass-to-light ratio inside 5R_eff to be 8-12 in solar units, and the dark matter fraction inside this radius to be less than 40%. We compare these and other results of dynamical analysis with those of dark matter-dominated merger simulations, finding that significant discrepancies remain, reiterating the question of whether NGC 3379 has the kind of dark matter halo that the current ΛCDM paradigm requires.

We report the discovery of an X-ray source coincident with the nuclear star cluster at the dynamical center of the nearby late-type spiral galaxy NGC 2403. The X-ray luminosity of this source varies from below detection levels, ~10³⁵ erg s^-1 in the 0.5-8.0 keV band to 7 × 10³⁸ erg s^-1 on timescales between observations of <2 months. The X-ray spectrum is well fit by an accretion disk model consisting of multiple blackbody components and corresponding physically to a compact object mass of ≳5 M_☉. No pulsations nor aperiodic behavior is evident in its X-ray light curve on the short timescales of the individual observations. The X-ray properties of the source are more similar to those of the nuclear source X-8 in M33, believed to be a low-mass X-ray binary, then to those of the low-luminosity active galactic nucleus (AGN) in NGC 4395. The brightness of the nuclear star cluster, M_I ~ -11.8 mag, is typical of clusters in late-type spirals but its effective radius, r_e ~ 12 pc, is several times larger than average indicating a relatively relaxed cluster and a low probability of a central massive object. Estimating from its observed colors and brightness, the cluster has a mass of ≳10^6.5M_☉ and an age of ~1.4 Gyr.

We present Gemini optical spectroscopy of 23 young star clusters in NGC 3256. We find that the cluster ages range from few to ~150 Myr. All these clusters are relatively massive [(2-40) × 10⁵M_☉] and appear to be of roughly 1.5 Z_☉ metallicity. The majority of the clusters in our sample follow the same rotation curve as the gas and hence were presumably formed in the molecular-gas disk. However, a western subsample of five clusters has velocities that deviate significantly from the gas rotation curve. These clusters may either belong to the second spiral galaxy of the merger or may have formed in tidal-tail gas falling back into the system. We discuss our observations in light of other known cluster populations in merging galaxies, and suggest that NGC 3256 is similar to Arp 220, and hence may become an ultraluminous infrared galaxy as the merger progresses and the star formation rate increases. Some of the clusters that appeared as isolated in our ground-based images are clearly resolved into multiple subcomponents in the HST ACS images. The same effect has been observed in the Antennae galaxies, showing that clusters are often not formed in isolation, but instead tend to form in larger groups or cluster complexes.

We present UBVRI imaging polarimetry of NGC 2100 and its surrounding environment, which comprise a part of the LMC 2 supershell. The morphology of the observed position angle distribution provides a tracer of the projected magnetic field in this environment. Our polarization maps detail regions exhibiting similarly aligned polarization position angles, as well as more complex position angle patterns. We observe regions of coherent fields on spatial scales of 42 × 24 to 104 × 83 pc, and infer projected field strengths of ~14-30 μG. We propose that the superposition of global outflows from the LMC 2 environment, as well as outflows created within NGC 2100, produce the unique field geometry in the region.

We present the first far-ultraviolet (FUV) spectra of the four known Balmer-dominated supernova remnants (SNRs) in the Large Magellanic Cloud, acquired with the Far Ultraviolet Spectroscopic Explorer (FUSE). The remnants DEM L 71 (SNR 0505-67.9), SNR 0509-67.5, SNR 0519-69.0, and SNR 0548-70.4 are all in the nonradiative stages of evolution and exhibit expansion speeds ranging from ~500 to ~5000 km s^-1. We have detected broad emission lines of Lyβ, Lyγ, C III, and O VI in DEM L 71 (V_FWHM ~ 1000 km s^-1) and have detected broad Lyβ and O VI emission in SNR 0519-69.0 (V_FWHM ~ 3000 km s^-1). In addition, broad Lyβ emission (V_FWHM ~ 3700 km s^-1) has been observed in SNR 0509-67.5, the first detection of broad line emission from this SNR. No emission was detected in our FUSE spectrum of SNR 0548-70.4, allowing us to place only upper limits on the FUV line fluxes. The spectra of these SNRs are unaffected by postshock cooling and provide valuable probes of collisionless heating efficiency in high Mach number shocks. We have used the F(Lyβ)/F(1032) flux ratio and relative widths of the broad Lyβ and O VI lines to estimate the degree of electron-proton and proton-oxygen ion equilibration in DEM L 71, SNR 0509-67.5, and SNR 0519-69.0. Although our equilibration estimates are subject to considerable uncertainty due to the faintness of the FUV lines and contributions from bulk Doppler broadening, our results are consistent with a declining efficiency of electron-proton and proton-oxygen ion equilibration with increasing shock speed. From our shock velocity estimates, we obtain ages of 295-585 yr for SNR 0509-67.5 and 520-900 yr for SNR 0519-69.0, in good agreement with the ages obtained from SN light-echo studies.

We present the results of our investigation of the intermediate-age star cluster BS 90, located in the vicinity of the H II region N66 in the SMC, observed with HST ACS. The high-resolution data provide a unique opportunity for a very detailed photometric study performed on one of the rare intermediate-age rich SMC clusters. The complete set of observations is centered on the association NGC 346 and contains almost 100,000 stars down to V ≃ 28 mag. In this study we focus on the northern part of the region, which covers almost the whole stellar content of BS 90. We construct its stellar surface density profile and derive structural parameters. Isochrone fits on the CMD of the cluster results in an age of about 4.5 Gyr. The luminosity function is constructed and the present-day mass function of BS 90 has been obtained using the mass-luminosity relation, derived from the isochrone models. We found a slope between -1.30 and -0.95, comparable to or somewhat shallower than a typical Salpeter IMF. Examination of the radial dependence of the mass function shows a steeper slope at larger radial distances, indicating mass segregation in the cluster. The derived half-mass relaxation time of 0.95 Gyr suggests that the cluster is mass segregated due to its dynamical evolution. From the isochrone model fits we derive a metallicity for BS 90 of [Fe/H] = -0.72, which adds an important point to the age-metallicity relation of the SMC. We discuss our findings on this relation in comparison to other SMC clusters.

In response to the proposed high-helium content stars as an explanation for the double main sequence observed in ω Centauri, we investigated the consequences of such stars elsewhere on the color-magnitude diagram. We concentrated on the horizontal branch, where the effects of high helium are expected to show themselves more clearly. In the process we developed a procedure for comparing the mass loss suffered by differing stellar populations in a physically motivated manner. High-helium stars in the numbers proposed seem absent from the horizontal branch of ω Cen unless their mass-loss history is very different from that of the majority metal-poor stars. It is possible to generate a double main sequence with existing ω Cen stars via accretion of helium-rich pollution consistent with the latest AGB ejecta theoretical yields and such polluted stars are consistent with the observed HB morphology of ω Cen. Polluted models are consistent with observed merging of the main sequences as opposed to our models of helium-rich stars. Using the (B - R)/(B + V + R) statistic, we find that the high-helium bMS stars require an age difference compared to the rMS stars that is too great, whereas the pollution scenario stars have no such conflict for inferred ω Cen mass losses.

To date, all of the reported hypervelocity stars (HVSs), which are believed to be ejected from the Galactic center, are blue and therefore almost certainly young. Old-population HVSs could be much more numerous than the young ones that have been discovered, but still have escaped detection because they are hidden in a much denser background of Galactic halo stars. Discovery of these stars would shed light on star formation at the Galactic center, would constrain the mechanism by which they are ejected from it, and, if they prove numerous, would enable detailed studies of the structure of the dark halo. We analyze the problem of finding these stars and show that the search should be concentrated around the main-sequence turnoff (0.3 < g - i < 1.1) at relatively faint magnitudes (19.5 < g < 21.5). If the ratio of turnoff stars to B stars is the same for HVSs as it is in the local disk, such a search would yield about 1 old-population HVS per 45 deg². A telescope similar to the Sloan 2.5 m could search about 20 deg² per night, implying that such a population, should it exist, would show up in interesting numbers in short order.

The North Polar Spur (NPS) is the brightest filament of Loop I, a large circular feature in the radio continuum sky. In this paper, a model consisting of two synchrotron-emitting shells is presented that reproduces large-scale structures revealed by recent polarization surveys. The polarized emission of the NPS is reproduced by one of these shells. The other shell, which passes close to the Sun, gives rise to polarized emission toward the Galactic poles. It is proposed that X-ray emission seen toward the NPS is produced by the interaction of the two shells. Two OB associations coincide with the centers of the shells. A formation scenario of the Loop I region is suggested.

We combined sensitive near-infrared data obtained with ground-based imagers on the ESO NTT and VLT telescopes with space mid-infrared data acquired with the IRAC imager on the Spitzer Space Telescope to calculate the extinction law A_λ/A as a function of λ between 1.25 and 7.76 μm to an unprecedented depth in Barnard 59, a star-forming, dense core located in the Pipe Nebula. The ratios A_λ/A were calculated from the slopes of the distributions of sources in color-color diagrams λ-K_s versus H-K_s. The distributions in the color-color diagrams are fit well with single slopes to extinction levels of A ≈ 7 (A_V ≈ 59 mag). Consequently, there appears to be no significant variation of the extinction law with depth through the B59 line of sight. However, when slopes are translated into the relative extinction coefficients A_λ/A, we find an extinction law that departs from the simple extrapolation of the near-infrared power-law extinction curve, and agrees more closely with a dust extinction model for a cloud with a total to selective absorption R_V = 5.5 and a grain size distribution favoring larger grains than those in the diffuse interstellar medium. Thus, the difference we observe could possibly be due to the effect of grain growth in denser regions. Finally, the slopes in our diagrams are somewhat less steep than those from the study of Indebetouw et al. for clouds with lower column densities, and this indicates that the extinction law between 3 and 8 μm might vary slightly as a function of environment.

We examine the process that triggers molecular cloud formation around diffuse H II regions. We calculate the time evolution of the shell, as well as the H II region, in a two-phase neutral medium, solving the UV and FUV radiative transfer and the thermal and chemical processes with a time-dependent hydrodynamics code. In the cold neutral medium, the ambient gas is swept up in a cold (T ~ 30-40 K) and dense (n ~ 10³ cm^-3) shell around the H II region. In the shell, H₂ molecules are formed from the swept-up H I gas, but hardly any CO is formed. This is a result of the different efficiencies of self-shielding effects between H₂ and CO molecules. The physical and chemical properties of gas in the shell are just intermediate between those of the neutral medium and molecular clouds observed in CO emission. We suggest that the predicted cold "dark" H I/H₂ gas should be detectable as an H I self-absorption (HISA) feature. We have sought such features in recent observational data and found shell-like HISA features around the giant H II regions W4 and W5. These features shows good spatial correlation with dust emission, but poor correlation with CO emission. Our quantitative analysis shows that the HISA cloud can be as cold as a few tens of kelvins. In the warm neutral medium, on the other hand, an expanding diffuse H II region is much simpler, owing to a small pressure excess. The UV photons only ionize the neutral medium and produce a warm ionized medium.

We present mid-infrared (10.4, 11.7, and 18.3 μm) imaging intended to locate and characterize the suspected protostellar components within the Bok globule CB54. We detect and confirm the protostellar status for the near-infrared source CB54YC1-II. The mid-infrared luminosity for CB54YC1-II was found to be L_midir ≈ 8 L_☉, and we estimate a central source mass of M_* ≈ 0.8 M_☉ (for a mass accretion rate of = 10^-6M_☉ yr^-1). CB54 harbors another near-infrared source (CB54YC1-I), which was not detected by our observations. The nondetection is consistent with CB54YC1-I being a highly extinguished embedded young A or B star or a background G or F giant. An alternative explanation for CB54YC1-I is that the source is an embedded protostar viewed at an extremely high inclination angle, and the near-infrared detections are not of the central protostar, but of light scattered by the accretion disk into our line of sight. In addition, we have discovered three new mid-infrared sources, which are spatially coincident with the previously known dense core in CB54. The source temperatures (~100 K) and association of the mid-infrared sources with the dense core suggests that these mid-infrared objects may be embedded class 0 protostars.

GRB X-ray light curves display rapid declines followed by a gradual steepening or plateau phase in ≳30% of GRBs in the Swift sample. Treating the standard relativistic blast wave model in a uniform circumburst medium, it is shown that if GRBs accelerate ultra-high-energy cosmic rays through a Fermi mechanism, then the hadronic component can be rapidly depleted by means of photopion processes on timescales ~10²-10⁴ s after the GRB explosion. While discharging the hadronic energy in the form of ultra-high-energy cosmic-ray neutrals and escaping cosmic-ray ions, the blast wave goes through a strongly radiative phase, causing the steep declines observed with Swift. Following the discharge, the blast wave recovers its adiabatic behavior, forming the observed plateaus or slow declines. These effects are illustrated by calculations of model bolometric light curves. The results show that steep X-ray declines and plateau features occur when GRB sources take place in rather dense media, with n ≳ 10² cm^-3 out to ≳10¹⁷ cm.

Using the neutrino telescope AMANDA-II, we have conducted two analyses searching for neutrino-induced cascades from gamma-ray bursts. No evidence of astrophysical neutrinos was found, and limits are presented for several models. We also present neutrino effective areas which allow the calculation of limits for any neutrino production model. The first analysis looked for a statistical excess of events within a sliding window of 1 or 100 s (for short and long burst classes, respectively) during the years 2001-2003. The resulting upper limit on the diffuse flux normalization times E² for the Waxman-Bahcall model at 1 PeV is 1.6 × 10^-6 GeV cm^-2 s^-1 sr^-1 (a factor of 120 above the theoretical prediction). For this search 90% of the neutrinos would fall in the energy range 50 TeV to 7 PeV. The second analysis looked for neutrino-induced cascades in coincidence with 73 bursts detected by BATSE in the year 2000. The resulting upper limit on the diffuse flux normalization times E², also at 1 PeV, is 1.5 × 10^-6 GeV cm^-2 s^-1 sr^-1 (a factor of 110 above the theoretical prediction) for the same energy range. The neutrino-induced cascade channel is complementary to the up-going muon channel. We comment on its advantages for searches of neutrinos from GRBs and its future use with IceCube.

We present high-sensitivity Very Long Baseline Interferometry (VLBI) observations 806 days after the γ-ray burst of 2003 March 29 (GRB 030329). The angular diameter of the radio afterglow is measured to be 0.347 ± 0.09 mas, corresponding to 0.99 ± 0.26 pc at the redshift of GRB 030329 (z = 0.1685). The evolution of the image size favors a uniform external density over an R^-2 windlike density profile (at distances of R ≳ 10¹⁸ cm from the source), although the latter cannot be ruled out yet. The current apparent expansion velocity of the image size is only mildly relativistic, suggesting a nonrelativistic transition time of t_NR ~ 1 yr. A rebrightening, or at least a significant flattening in the flux decay, is expected within the next several years as the counterjet becomes visible (this has not yet been observed). An upper limit of <1.9c is set on the proper motion of the flux centroid.

We present here the first 2D rotating, multigroup, radiation magnetohydrodynamics (RMHD) simulations of supernova core collapse, bounce, and explosion. In the context of rapid rotation, we focus on the dynamical effects of magnetic stresses and the creation and propagation of MHD jets. We find that a quasi-steady state can be quickly established after bounce, during which a well-collimated MHD jet is maintained by continuous pumping of power from the differentially rotating core. If the initial spin period of the progenitor core is ≲2 s, the free energy reservoir in the secularly evolving proto-neutron star is adequate to power a supernova explosion and may be enough for a hypernova. The jets are well collimated by the infalling material and magnetic hoop stresses and maintain a small opening angle. We see evidence of sausage instabilities in the emerging jet stream. Neutrino heating is subdominant in the rapidly rotating models we explore but can contribute 10%-25% to the final explosion energy. Our simulations suggest that even in the case of modest or slow rotation, a supernova explosion might be followed by a secondary, weak MHD jet explosion, which, because of its weakness, may to date have gone unnoticed in supernova debris. Furthermore, we suggest that the generation of a nonrelativistic MHD precursor jet during the early proto-neutron star/supernova phase is implicit in both the collapsar and "millisecond magnetar" models of GRBs. The multidimensional, multigroup, rapidly rotating RMHD simulations we describe here are a start along the path toward more realistic simulations of the possible role of magnetic fields in some of nature's most dramatic events.

We present results on the X-ray and optical/UV emission from the Type II-P supernova (SN) 2006bp and the interaction of the SN shock with its environment, obtained with the X-Ray Telescope (XRT) and UV/Optical Telescope (UVOT) on board Swift. SN 2006bp is detected in X-rays at a 4.5 σ level of significance in the merged XRT data from days 1 to 12 after the explosion. If the 0.2-10 keV band X-ray luminosity of L_0.2-10 = (1.8 ± 0.4) × 10³⁹ ergs s^-1 is caused by interaction of the SN shock with circumstellar material (CSM) deposited by a stellar wind from the progenitor's companion star, a mass-loss rate of ≈ (1 × 10^-5M_☉ yr^-1)(v_w/10 km s^-1) is inferred. The mass-loss rate is consistent with the nondetection in the radio with the VLA on days 2, 9, and 11 after the explosion and is characteristic of a red supergiant progenitor with a mass of ≈12-15 M_☉ prior to the explosion. The Swift data further show a fading of the X-ray emission starting around day 12 after the explosion. In combination with a follow-up XMM-Newton observation obtained on day 21 after the explosion, an X-ray rate of decline of L_X ∝ t^-n with index n = 1.2 ± 0.6 is inferred. Since no other SN has been detected in X-rays prior to the optical peak, and since Type II-P SNe have an extended "plateau" phase in the optical, we discuss the scenario that the X-rays might be due to inverse Compton scattering of photospheric optical photons off relativistic electrons produced in circumstellar shocks. However, due to the high required value of the Lorentz factor (≈10-100), which is inconsistent with the ejecta velocity inferred from optical line widths, we conclude that inverse Compton scattering is an unlikely explanation for the observed X-ray emission.

We re-examine the evidence for alignment of the proper motion and fiducial polarization-angle directions in radio pulsars given by Johnston et al., and we find that the case is probably stronger than this paper asserts. Other alignments are examined using the published literature and recent Arecibo polarimetry, providing an enlarged case that pulsar rotation axis and supernovae "kick" directions are aligned or orthogonal at birth. Pulsars' "orthogonal" polarization modes complicate fixing the orientation of such "kicks," but we note that determining the absolute geometry of the polarization modes is a key objective for understanding the physical origins of pulsar emission.

On 2006 September 21, an intense (~10³⁹ erg s^-1) and short (20 ms) burst was detected by Swift BAT at a position consistent with that of the candidate anomalous X-ray pulsar (AXP) CXOU J164710.2-455216, discovered by Chandra in 2005. Swift follow-up observations began ~13 hr after the event and found the source at a 1-10 keV flux level of about 4.5 × 10^-11 erg cm^-2 s^-1, i.e., ~300 times brighter than measured 5 days earlier by XMM-Newton. We report the results obtained from Swift BAT observations of the burst and subsequent Swift XRT observations carried out during the first 4 months after the burst. These data are complemented with those from two XMM-Newton observations (carried out just before and after the BAT event) and four archival Chandra observations carried out between 2005 and 2007. We find a phase-coherent solution for the source pulsations after the burst. The evolution of the pulse phase comprises an exponential component decaying with timescale of 1.4 days, which we interpret as the recovery stage following a large glitch (Δν/ν ~ 6 × 10^-5). We also detect a quadratic component corresponding to a spin-down rate of ~ 9 × 10^-13 s s^-1, implying a magnetic field strength of 10¹⁴ G. During the first Swift XRT observation taken 0.6 days after the burst, the spectrum showed a kT ~ 0.65 keV blackbody (R_BB ~ 1.5 km) plus a Γ ~ 2.3 power law accounting for about 60% of the 1-10 keV observed flux. Analysis of Chandra archival data, taken during 2005 when the source was in quiescence, reveal that the modulation in quiescence is 100% pulsed at energies above ~4 keV and consistent with the (unusually small-sized) blackbody component being occulted by the neutron star as it rotates. These findings demonstrate that CXOU J164710.2-455216 is indeed an AXP; we compare them with the properties of three other AXPs which displayed similar behavior in the past.

Data from Chandra observations of 30 nearby galaxies were analyzed and 365 X-ray point sources were chosen whose spectra were not contaminated by excessive diffuse emission and not affected by photon pileup. The spectra of these sources were fitted using two spectral models (an absorbed power-law and a disk blackbody) to ascertain the dependence of estimated parameters on the spectral model used. It was found that the cumulative luminosity function depends on the choice of the spectral model, especially for luminosities >10⁴⁰ ergs s^-1. A large number (~80) of the sources have luminosities >10³⁹ ergs s^-1 (ultraluminous X-ray sources) with indistinguishable average spectral parameters (inner disk temperature ~1 keV and/or photon index Γ ~ 2) with those of the lower luminosity ones. We identify four sources whose minimum luminosity exceeds 10⁴⁰ ergs s^-1. Their spectra are in general better represented by the disk blackbody model than the power-law one. These extremely luminous X-ray sources (ELXs) can be grouped into two distinct spectral classes. Two of them have an inner disk temperature of <0.5 keV (supersoft), while the other two have temperatures ≳1.3 keV (hard). The estimated inner disk temperatures of the supersoft ELXs are compatible with the hypothesis that they harbor intermediate-size black holes, which are accreting at ~0.5 times their Eddington luminosity. The radiative mechanism for hard ELXs seems to be inverse Comptonization, which in contrast to standard black holes systems is probably saturated.

We report on X-ray observations of Nova Sagittarii 1998 (V4633 Sgr) performed with XMM-Newton at three different epochs, 934, 1083, and 1265 days after discovery. The nova was detected with the EPIC cameras at all three epochs, with emission spanning the whole energy range from 0.2 to 10 keV. The X-ray spectra do not change significantly at the different epochs and are well fitted for the first and third observations with a multitemperature optically thin thermal plasma, while lower statistics in the second observations lead to a poorer fit. The thermal plasma emission is most probably originated in the shock heated ejecta, with chemical composition similar to that of a CO nova. However, we cannot completely rule out reestablished accretion as the origin of the emission. We also obtain upper limits for the temperature and luminosity of a potential white dwarf atmospheric component and conclude that hydrogen burning had already turned off by the time of our observations.

Understanding and quantifying the contribution of known classes of transient and variable sources is an important lesson to be learned from the manifold of precursor programs of the near-future large synoptic sky survey programs such as SkyMapper, Pan-STARRS, and LSST. With this goal in mind, we undertook photometric and spectroscopic follow-up observations of four recently reported unidentified transients. For two sources, WFI J132813.7-214237 and WFI J161953.3+031909, we show that unfortunate coincidences led to their previous designation as transients. While the former is now interpreted as the spatial coincidence of a solar system object with a faint background star, the latter is merely a cataclysmic variable unfortunately caught in and out of eclipse. The third candidate, ROTSE3 J160213.1-021311.7, is identified as an SU UMa type dwarf nova with quiescent brightness of R ~ 22.7 and an outburst amplitude of ~5 mag. The fourth event, SDSS-SN 15207, similarly shows evidence for a dwarf nova origin. Our main conclusion is that cataclysmic variables in their various avatars will contribute moderately to the population of transient objects.

We present multiwavelength optical and IR photometry of 170 previously known low-mass stars and brown dwarfs of the 5 Myr Collinder 69 cluster (λ Orionis). The new photometry supports cluster membership for most of them, with less than 15% of the previous candidates identified as probable nonmembers. The near-IR photometry allows us to identify stars with IR excesses, and we find that the Class II population is very large, around 25% for stars (in the spectral range M0-M6.5) and 40% for brown dwarfs, down to 0.04 M_☉, despite the fact that the Hα equivalent width is low for a significant fraction of them. In addition, there are a number of substellar objects, classified as Class III, that have optically thin disks. The Class II members are distributed in an inhomogeneous way, lying preferentially in a filament running toward the southeast. The IR excesses for the Collinder 69 members range from pure Class II (flat or nearly flat spectra longward of 1 μm), to transition disks with no near-IR excess but excesses beginning within the IRAC wavelength range, to two stars with excess only detected at 24 μm. Collinder 69 thus appears to be at an age where it provides a natural laboratory for the study of primordial disks and their dissipation.

High-resolution infrared spectroscopy in the 2.3-4.6 μm region is reported for the peculiar A supergiant, single-lined spectroscopic binary HR 4049. Lines from the CO fundamental and first overtone, OH fundamental, and several H₂O vibration-rotation transitions have been observed in the near-infrared spectrum. The spectrum of HR 4049 appears principally in emission through the 3 and 4.6 μm region and in absorption in the 2 μm region. The 4.6 μm spectrum shows a rich "forest" of emission lines. All the spectral lines observed in the 2.3-4.6 μm spectrum are shown to be circumbinary in origin. The presence of OH and H₂O lines confirm the oxygen-rich nature of the circumbinary gas, which is in contrast to the previously detected carbon-rich material. The emission and absorption line profiles show that the circumbinary gas is located in a thin, rotating layer near the dust disk. The properties of the dust and gas circumbinary disk and the spectroscopic orbit yield masses for the individual stars, M_{A I} ~ 0.58 M_☉ and M_secondary ~ 0.34 M_☉. Gas in the disk also has an outward flow with a velocity of ≳1 km s^-1. The severe depletion of refractory elements but near-solar abundances of volatile elements observed in HR 4049 results from abundance winnowing. The separation of the volatiles from the grains in the disk and the subsequent accretion by the star are discussed. Contrary to prior reports, the HR 4049 carbon and oxygen isotopic abundances are typical AGB values, ¹²C/¹³C = 6 and ¹⁶O/¹⁷O > 200.

We present follow-up observations of pulsating subdwarf B (sdB) stars as part of our efforts to resolve the pulsation spectra for use in asteroseismological analyses. This paper reports on multisite campaigns of the pulsating sdB stars PG 1618+563B and PG 0048+091. Data were obtained from observatories placed around the globe for coverage from all longitudes. For PG 1618+563B, our five-site campaign uncovered a dichotomy of pulsation states. Early during the campaign the amplitudes and phases (and perhaps frequencies) were quite variable, while data obtained late in the campaign were able to fully resolve five stable pulsation frequencies. For PG 0048+091, our five-site campaign uncovered a plethora of frequencies with short pulsation lifetimes. We find them to have observed properties consistent with stochastically excited oscillations, an unexpected result for subdwarf B stars. We discuss our findings and their impact on subdwarf B asteroseismology.

We present Very Large Array observations at 7 mm wavelength that resolve the dust emission structure in the disk around the young star TW Hydrae at the scale of the ~4 AU (0.16'') radius inner hole inferred from spectral energy distribution modeling. These high-resolution data directly confirm the presence of an inner hole in the dust disk and reveal a high-brightness ring that we associate with the directly illuminated inner edge of the disk. The clearing of the inner disk plausibly results from the dynamical effects of a giant planet in formation. In an appendix, we develop an analytical framework for the interpretation of visibility curves from power-law disk models with inner holes.

Kinetic and magnetic energy spectra in the ecliptic plane near 1 AU are found to exhibit different power-law behaviors in the inertial range, with the magnetic spectrum often having a power-law exponent near 5/3 and the kinetic energy spectrum often having a power-law exponent near 3/2 (the inertial range extends from approximately 5 × 10^-4 to 10^-1 Hz). The total energy, kinetic plus magnetic, has a power-law exponent that lies between 3/2 and 5/3, with a value near 1.6. The Alfvén ratio, the ratio of kinetic to magnetic energy, is found to be a slowly increasing function of frequency in the inertial range, increasing from roughly 0.5 to 0.9 in the frequency range from 10^-3 to 10^-1 Hz. These conclusions are based on the analysis of four distinct time intervals of solar wind magnetic field and plasma data obtained by the Wind spacecraft near the end of solar cycle 22 and at different times throughout solar cycle 23. Three 54 day intervals and one 81 day interval are used to compute power spectra in the range from 10^-5 to 1.7 × 10^-1 Hz. Power-law exponents are estimated from linear least-squares fits to the logarithm of the power spectral density versus the logarithm of the frequency over the frequency interval from 10^-3 to 10^-2 Hz. To prevent errors due to spectral aliasing, the last decade of the spectrum is omitted from the calculation of the power-law exponents. The results show that a measurable difference exists between the power-law exponents of velocity and magnetic field fluctuations and that this difference persists throughout the solar cycle.

The possibility of velocity shear-induced linear transformations of different magnetohydrodynamic waves in the solar wind is studied both analytically and numerically. A quantitative analysis of the wave transformation processes for all possible plasma-β regimes is performed. By applying the obtained criteria for effective wave coupling to the solar wind parameters, we show that velocity shear-induced linear transformations of Alfvén waves into magnetoacoustic waves could effectively take place for the relatively low frequency Alfvén waves in the energy-containing interval. The obtained results are in a good qualitative agreement with the observed features of density perturbations in the solar wind.

We examine the nature of large-scale, coronal, propagating wave fronts ("EIT waves") and find they are incongruous with solutions using fast-mode MHD plane-wave theory. Specifically, we consider the following properties: nondispersive single pulse manifestations, observed velocities below the local Alfvén speed, and different pulses which travel at any number of constant velocities, rather than at the "predicted" fast-mode speed. We discuss the possibility of a soliton-like explanation for these phenomena, and show how it is consistent with the above-mentioned aspects.

A method for quantifying the asymmetry attributes of coronal mass ejections (CMEs) is described. The technique developed is applied to a set of full-halo CMEs observed from 1997 to the end of 2000 by the Large Angle Spectrometric Coronagraph (LASCO) instrument on board the Solar and Heliospheric Observatory (SOHO) spacecraft. A cross section of events is examined, with the progression of asymmetry characteristics as the ejecta proceeds outward illustrated for a symmetrical full-halo CME, an asymmetrical full-halo CME, and multiple CMEs. A control sample for a day with minimal solar activity is presented for comparison. A qualitative evaluation of the results with visual inspection of the coronagraph data is discussed. Relationships between the asymmetry parameters derived from the data are examined, and good correlations are found among the asymmetry magnitudes, the average sector pixel intensities, and the frontal CME speeds. This computerized method can also be designed to automatically detect and measure the asymmetry and relative brightness of any CME. We compare the results with storms at Earth to demonstrate that the calculated asymmetry magnitude by itself indicates the geoeffectiveness of a CME, with any associated solar surface activity helping to determine whether the CME is aimed Earthward or not. Thus, this test suggests that our technique is useful as a forecast tool for space weather applications. With modifications, the algorithm is applicable to analyses of the data returned from the Solar Mass Ejection Imager (SMEI) aboard the Coriolis spacecraft and the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) coronagraph and heliospheric imagers aboard the two Solar Terrestrial Relations Observatory (STEREO) spacecraft.

The three seismic sources, S1, S2, and S3, detected from MDI Dopplergrams using the time-distance (TD) diagram technique are presented with the locations, areas, and vertical and horizontal velocities of the visible wave displacements. Within the data cube of 120 Mm, the horizontal velocities and the wave propagation times vary slightly from source to source. The momenta and start times measured from the TD diagrams in sources S1-S3 are compared with those delivered to the photosphere by different kinds of high-energy particles with the parameters deduced from hard X-ray and γ-ray emission, as well as by the hydrodynamic shocks caused by these particles. The energetic protons (power laws combined with quasi-thermal ones, or jets) are shown to deliver momentum high enough and to form the hydrodynamic shocks deep enough in a flaring atmosphere to allow them to be delivered to the photosphere through much shorter distances and times. Then the seismic waves observed in the sources S2 and S3 can be explained by the momenta produced by hydrodynamic shocks, which are caused by mixed proton beams and jets occurring nearly simultaneously with the third burst of hard X-ray and γ-ray emission in the loops with footpoints in the locations of these sources. The seismic wave in source S1, delayed by 4 and 2 minutes from the first and second hard X-ray bursts, respectively, is likely to be associated with a hydrodynamic shock occurring in this loop from precipitation of a very powerful and hard electron beam with higher energy cutoff mixed with quasi-thermal protons generated by either of these two bursts.

A complete model of helium-like line and continuum emission has been incorporated into the plasma simulation code Cloudy. All elements between He and Zn are treated, any number of levels can be considered, and radiative and collisional processes are included. This includes photoionization from all levels, line transfer, including continuum pumping and destruction by background opacities, scattering, and collisional processes. The model is calculated self-consistently along with the ionization and thermal structure of the surrounding nebula. The result is a complete line and continuum spectrum of the plasma. Here we focus on the ions of the He I sequence and reconsider the standard helium-like X-ray diagnostics. We first consider semianalytical predictions and compare these with previous work in the low-density, optically thin limit. We then perform numerical calculations of helium-like X-ray emission (such as is observed in some regions of Seyfert galaxies) and predict line ratios as a function of ionizing flux, hydrogen density, and column density. In particular, we demonstrate that, in photoionized plasmas, the R ratio, a density indicator in a collisional plasma, depends on the ionization fraction and is strongly affected by optical depth for large column densities. We also introduce the notion that the R ratio is a measure of the incident continuum at UV wavelengths. The G ratio, which is temperature sensitive in a collisional plasma, is also discussed and shown to be strongly affected by continuum pumping and optical depth as well. These distinguish a photoionized plasma from the more commonly studied collisional case.

In recent years, increasing evidence has emerged for a thermal component in the γ- and X-ray spectrum of the prompt emission phase in gamma-ray bursts. The temperature and flux of the thermal component show a characteristic break in the temporal behavior after a few seconds. We show here that measurements of the temperature and flux of the thermal component at early times (before the break) allow the determination of the values of two of the least restricted fireball model parameters: the size at the base of the flow and the outflow bulk Lorentz factor. Relying on the thermal emission component only, this measurement is insensitive to the inherent uncertainties of previous estimates of the bulk motion Lorentz factor. We give specific examples of the use of this method: for GRB 970828 at redshift z = 0.9578, we show that the physical size at the base of the flow is r₀ = (2.9 ± 1.8) × 10⁸Y cm and the Lorentz factor of the flow is Γ = (305 ± 28)Y, and for GRB 990510 at z = 1.619, r₀ = (1.7 ± 1.7) × 10⁸Y cm and Γ = (384 ± 71)Y, where Y = 1Y₀ is the ratio between the total fireball energy and the energy emitted in γ-rays.

The collapse of a massive star is believed to be the most probable progenitor of a long gamma-ray burst (GRB). Such a star is expected to have its environment modified by the stellar wind. The effect of such a circumstellar wind medium is expected to be seen in the evolution of a GRB afterglow, but so far this has not been conclusively found. We claim that a signature of the transition from wind to constant density medium of the circumburst medium is visible in the afterglow of GRB 050319. Along with the optical observations of the afterglow of GRB 050319, we present a model for the multiband afterglow of GRB 050319. We show that the break seen in the optical light curve at ~0.02 days could be explained as being due to the transition from wind to constant density medium of the circumburst medium, in which case, to our knowledge, this could be the first ever detection of such a transition at any given frequency band. Detection of such a transition could also serve as confirmation of the massive star collapse scenario for GRB progenitors, independent of supernova signatures.

We present the first measurement of the AGN fraction in high-redshift clusters of galaxies (z ~ 0.6) with spectroscopy of one cluster and archival data for three additional clusters. We identify eight active galactic nuclei (AGNs) in these clusters from Chandra data that are sensitive to AGNs with hard X-ray (2-10 keV) luminosity L_{X, H} > 10⁴³ ergs s^-1 in host galaxies more luminous than a rest-frame M_R < -20 mag. This stands in sharp contrast to the one AGN with L_{X, H} > 10⁴³ ergs s^-1 that we discovered in our earlier study of eight low-redshift clusters with z = 0.06-0.31 ( ~ 0.2). Three of the four high-redshift cluster data sets are sensitive to L_{X, H} > 10⁴² ergs s^-1, and we identify seven AGNs above this luminosity limit, compared to two AGNs in eight low-redshift clusters. Based on membership estimates for each cluster, we determine that the AGN fractions at z ~ 0.6 are f_A(L_X > 10⁴²; M_R < -20) = 0.028 and f_A(L_X > 10⁴³; M_R < -20) = 0.020. These values are approximately a factor of 20 greater than the AGN fractions in lower redshift ( ~ 0.2) clusters of galaxies. The cluster increase also represents substantially faster evolution than observed in the field over this redshift range (a factor of 1.3 and 3.3 for L_X > 10⁴² and L_X > 10⁴³, respectively). The cluster AGN fraction increases more rapidly with redshift than the field, and the increase in cluster AGNs indicates the presence of an AGN Butcher-Oemler effect.

We examine recent evidence from the luminosity-redshift relation of Type Ia Supernovae (SNe Ia) for the ~3 σ detection of a "Hubble bubble"—a departure of the local value of the Hubble constant from its globally averaged value. By comparing the MLCS2k2 fits used in that study to the results from other light-curve fitters applied to the same data, we demonstrate that this is related to the interpretation of SN color excesses (after correction for a light-curve shape-color relation) and the presence of a color gradient across the local sample. If the slope of the linear relation (β) between SN color excess and luminosity is fit empirically, then the bubble disappears. If, on the other hand, the color excess arises purely from Milky Way-like dust, then SN data clearly favor a Hubble bubble. We find that SN data give β ≃ 2, instead of the β ≃ 4 one would expect from purely Milky Way-like dust. This suggests that either SN intrinsic colors are more complicated than can be described with a single light-curve shape parameter, or that dust around SN is unusual. Disentangling these possibilities is both a challenge and an opportunity for large-survey SN Ia cosmology.

We describe a compact cluster or group of massive red galaxies at z = 1.5 discovered in one of the Gemini Deep Deep Survey (GDDS) fields. Deep H-band imaging from the Hubble Space Telescope (HST) reveals a high density of red galaxies associated with a galaxy with a spectroscopic redshift of 1.51. These galaxies have spectral energy distributions (SEDs) that peak between 3.6 and 4.5 μm, and fits to 12-band photometry reveal 12 or more galaxies with spectral shapes consistent with z = 1.5. Most are within ~170 comoving kpc of the GDDS galaxy, and the enclosed stellar mass is >6 × 10¹¹M_☉. The colors of the most massive galaxies are close to those expected from passive evolution of simple stellar populations (SSPs) formed at much higher redshifts. We suggest that several of these galaxies will merge to form a single, very massive galaxy by the present day. This system may represent an example of a short-lived dense group or cluster core typical of the progenitors of massive clusters in the present day and suggests that the red sequence was in place in overdense regions at early times.

Based on its strong CO⁺ emission, it is argued that the M82 starburst galaxy is exposed to a combination of FUV and X-ray radiation. The latter is likely to be the result of the starburst superwind, which leads to diffuse thermal emission, and a compact hard source (but not an AGN). Although a photon-dominated region (FUV) component is clearly present in the nucleus of M82, and capable of forming CO⁺, only X-ray irradiated gas of density 10³-10⁵ cm^-3 can reproduce the large, ~(1-4) × 10¹³ cm^-2, columns of CO⁺ that are observed toward the prototypical starburst M82. The total X-ray luminosity produced by M82 is weak, ~10⁴¹ ergs s^-1, but this is sufficient to drive the formation of CO⁺.

We report the discovery of a high CO (J = 3-2)/CO (J = 1-0) ratio gas with an arclike distribution ("high-ratio gas arc") surrounding the central star cluster of the supergiant H II region NGC 604 in the nearby spiral galaxy M33, based on multi-J CO observations of a 5' × 5' region of NGC 604 conducted using the ASTE 10 m and NRO 45 m telescopes. The discovered "high-ratio gas arc" extends to the southeast-northwest direction with a size of ~200 pc. The western part of the high-ratio gas arc closely coincides with the shells of the H II regions traced by Hα and radio continuum peaks. The CO (J = 3-2)/CO (J = 1-0) ratio, R_3-2/1-0, ranges between 0.3 and 1.2 in the observed region, and the R_3-2/1-0 values of the high-ratio gas arc are around or higher than unity, indicating very warm (T_kin ≥ 60 K) and dense (n ≥ 10³-10⁴ cm^-3) conditions of the high-ratio gas arc. We suggest that the dense gas formation and second-generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova of the first-generation stars of NGC 604, i.e., the central star cluster of NGC 604.

Previous work has related the Galactic bar to structure in the local stellar velocity distribution. Here we show that the bar also influences the spatial gradients of the velocity vector via the Oort constants. By numerical integration of test particles we simulate measurements of the Oort C-value in a gravitational potential including the Galactic bar. We account for the observed trend that C is increasingly negative for stars with higher velocity dispersion. By comparing measurements of C with our simulations we improve on previous models of the bar, estimating that the bar pattern speed is Ω_b/Ω₀ = 1.87 ± 0.04, where Ω₀ is the local circular frequency, and the bar angle lies within 20° ⩽ ϕ₀ ⩽ 45°. We find that the Galactic bar affects measurements of the Oort constants A and B less than ~2 km s^-1 kpc^-1 for the hot stars.

We analyze all X-ray timing data on 1E 1207.4-5209 in supernova remnant PKS 1209-51/52 gathered from 1993-2005 and find a highly stable rotation with P = 424.130749(4) ms and = (6.6 ± 9.0) × 10^-17 (1 σ errors). This refutes previous claims of large timing irregularities in these data. In the dipole spin-down formalism, the 2 σ upper limit on implies a spin-down luminosity < 1.3 × 10³² ergs s^-1, surface magnetic field strength B_p < 3.3 × 10¹¹ G, and characteristic age τ_c ≡ P/2 > 27 Myr. This τ_c exceeds the remnant age by 3 orders of magnitude, requiring that the pulsar was born spinning at its present period. The X-ray luminosity of 1E 1207.4-5209, L_bol ≈ 2 × 10³³(d/2 kpc)² ergs s^-1, exceeds its , implying that L_bol derives from residual cooling and perhaps partly from accretion of supernova debris. The upper limit on B_p is small enough to favor the electron-cyclotron model for at least one of the prominent absorption lines in its soft X-ray spectrum. This is the second demonstrable case of a pulsar born spinning slowly and with a weak B-field, after PSR J1852+0040 in Kesteven 79. We suggest that these properties define the class of central compact objects.

We present a 50 ks observation of the gamma-ray binary LS I +61 303 carried out with the ACIS-I array aboard the Chandra X-Ray Observatory. This is the highest resolution X-ray observation of the source conducted so far. Possible evidence of an extended structure at a distance between 5'' and 12'' toward the north of LS I +61 303 has been found at a significance level of 3.2 σ. The asymmetry of the extended emission excludes an interpretation in the context of a dust-scattered halo, suggesting an intrinsic nature. On the other hand, while the obtained source flux of F_{0.3-10 keV} = 7.1 × 10^-12 ergs cm^-2 s^-1 and hydrogen column density N_H = (0.70 ± 0.06) × 10²² cm^-2 are compatible with previous results, the photon index Γ = 1.25 ± 0.09 is the hardest ever found. In light of these new results, we briefly discuss the physics behind the X-ray emission, the location of the emitter, and the possible origin of the extended emission ~0.1 pc away from LS I +61 303.

The negative molecular ion C₈H^- has been detected in the Galactic molecular source TMC-1. Four rotational transitions have been observed in the centimeter-wave band with the NRAO 100 m Green Bank Telescope (GBT) at precisely the frequencies calculated from the recent laboratory spectroscopy of this large carbon chain anion. C₈H^- is about 5% as abundant as C₈H, or somewhat more than C₆H^- relative to C₆H (1.6%). Improved values of the column densities of C₆H^- and C₆H, and an upper limit for the abundance of the smaller carbon chain C₄H^- of 0.014% with respect to C₄H, have also been determined.

We report the detection of new transitions of octatetraynyl (C₈H) toward the circumstellar envelope IRC +10 216 using data taken with the 100 m Green Bank Telescope (GBT). In addition, we report five features from the Ku, K, and Q bands that have been identified as transitions of the octatetraynyl anion (C₈H^-). From a rotational temperature diagram and an assumed source size of 30'', we find a total C₈H column density of 8(3) × 10¹² cm^-2 and a rotational temperature of ~13 K. From the five detected transitions of C₈H^-, we find a total C₈H^- column density of ~2.1 × 10¹² cm^-2 consistent with a rotational temperature of ~34 K for a total C₈H/C₈H^- column density ratio of ~3.8. This observed C₈H/C₈H^- column density ratio is similar to the theoretical prediction of 3.6, while the observed column densities were lower than that predicted by a factor of ~30. This prompted us to reinvestigate the initial conditions of the circumstellar envelope (CSE) model. The new model results are presented, and they more closely match the C₈H and C₈H^- abundances observed with the GBT. Finally, we use the new CSE model results to predict the abundance of decapentaynyl (C₁₀H), and we compare them with the measured upper limit found from the GBT observations.

Theoretical studies predict that Trojans are likely a frequent by-product of planet formation and evolution. We examine the sensitivity of transit timing observations for detecting Trojan companions to transiting extrasolar planets. We demonstrate that this method offers the potential to detect terrestrial-mass Trojans using existing ground-based observatories. We compare the transit timing variation (TTV) method with other techniques for detecting extrasolar Trojans and outline the future prospects for this method.

The first challenge in the formation of both terrestrial planets and the cores of gas giants is the retention of grains in protoplanetary disks. In most regions of these disks, gas attains sub-Keplerian speeds as a consequence of a negative pressure gradient. Hydrodynamic drag leads to orbital decay and depletion of the solid material in the disk, with characteristic timescales as short as only a few hundred years for meter-sized objects at 1 AU. In this Letter, we suggest a particle retention mechanism that promotes the accumulation of grains and the formation of planetesimals near the water sublimation front or "snow line." This model is based on the assumption that, in the regions most interesting for planet formation, the viscous evolution of the disk is due to turbulence driven by the magnetorotational instability (MRI) in the surface layers of the disk. The depth to which MRI effectively generates turbulence is a strong function of grain size and abundance. A sharp increase in the grain-to-gas density ratio across the snow line reduces the column depth of the active layer. As the disk evolves toward a quasi-steady state, this change in the active layer creates a local maximum in radial distribution of the gas surface density and pressure, causing the gas to rotate at super-Keplerian speed and halting the inward migration of grains. This scenario presents a robust process for grain retention that may aid in the formation of proto-gas giant cores preferentially near the snow line.

Correlated isotopic anomalies in osmium (¹⁸⁶Os, ¹⁸⁸Os, ¹⁹⁰Os, measured with respect to ¹⁸⁹Os) extracted from primitive carbonaceous chondrites tightly constrain the σ_n(¹⁹⁰Os)/σ_n(¹⁸⁸Os) ratio to be 0.859 ± 0.042 (±5%). A recent measurement of the Maxwellian-averaged neutron-capture cross sections (MACSs) for ^{186, 187, 188}Os lowered the σ_n(¹⁸⁸Os) by 27% but did not measure σ_n(¹⁹⁰Os). From the σ_n(¹⁹⁰Os)/σ_n(¹⁸⁸Os) ratio, we infer σ_n(¹⁹⁰Os) = 249 ± 18 mbarns for internal consistency with the new MACSs for the other Os isotopes. This approach is applicable to other isotopic anomalies in r-process/s-process ratios derived from meteorites for nuclei that do not have branching points between them. Branching at ¹⁸⁵W and ¹⁸⁶Re makes the ¹⁸⁶Os/¹⁸⁸Os ratio a neutron dosimeter for the s-process that, with the new cross sections, yields an average neutron density, n_n ~ 3 × 10⁸ cm^-3. This low neutron density is consistent with previous results from Sr, Zr, Mo, and Ba isotopes that indicated a minor contribution from the ²²Ne(α, n)²⁵Mg neutron source relative to the ¹³C(α, n)¹⁶O neutron source.