Table of contents

Statistical observations of the epoch of reionization using the 21 cm line of neutral hydrogen have the potential to revolutionize our understanding of structure formation and the first luminous objects. However, these observations are complicated by a host of strong foreground sources. Several foreground-removal techniques have been proposed in the literature, and it has been assumed that these would be used in combination to reveal the epoch of reionization (EOR) signal. By studying the characteristic subtraction errors of the proposed foreground-removal techniques, we identify an additional subtraction stage that can further reduce the EOR foreground contamination, and study the interactions between the foreground-removal algorithms. This enables us to outline a comprehensive foreground-removal strategy that incorporates all previously proposed subtraction techniques. Using this foreground-removal framework and the characteristic subtraction errors, we discuss the complementarity of different foreground-removal techniques and the implications for array design and the analysis of EOR data.

We calculate the intergalactic photon density as a function of both energy and redshift for 0 < z < 6 for photon energies from.003 eV to the Lyman limit cutoff at 13.6 eV in a ΛCDM universe with Ω_Λ = 0.7 and Ω_m = 0.3. The basic features of our backward-evolution model for galaxies were developed in earlier papers by Malkan & Stecker. With a few improvements, we find that this evolutionary model gives predictions of new deep number counts from Spitzer, as well as a calculation of the spectral energy distribution of the diffuse infrared background, which are in good agreement with the data. We then use our calculated intergalactic photon densities to extend previous work on the absorption of high-energy γ-rays in intergalactic space owing to interactions with low-energy photons and the 2.7 K cosmic microwave background radiation. We calculate the optical depth of the universe, τ, for γ-rays having energies from 4 GeV to 100 TeV emitted by sources at redshifts from 0 to 5. We also give an analytic fit with numerical coefficients for approximating τ(E_γ, z). As an example of the application of our results, we calculate the absorbed spectrum of the blazar PKS 2155-304 at z = 0.117 and compare it with the spectrum observed by the HESS air Cerenkov γ-ray telescope array.

We present a new approach to foreground removal for CMB maps. Rather than relying on prior knowledge about the foreground components, we first extract the necessary information about them directly from the microwave sky maps by taking differences of temperature maps at different frequencies. These difference maps, which we refer to as internal templates, consist only of linear combinations of Galactic foregrounds and noise, with no CMB component. We obtain the foreground-cleaned maps by fitting these internal templates to, and subsequently subtracting the appropriately scaled contributions of them from, the CMB-dominated channels. The fitting operation is performed in wavelet space, making the analysis feasible at high resolution with only a minor loss of precision. Applying this procedure to the WMAP data, we obtain a power spectrum that matches the spectrum obtained by the WMAP team at the signal-dominated scales. The fact that we obtain basically identical results without using any external templates has considerable relevance for future observations of the CMB polarization, where very little is known about the Galactic foregrounds. Finally, we have revisited previous claims about a north-south power asymmetry on large angular scales and confirm that these remain unchanged with this completely different approach to foreground separation. This also holds when fitting the foreground contribution independently to the northern and southern hemisphere, indicating that the asymmetry is unlikely to have its origin in different foreground properties of the hemispheres. This conclusion is further strengthened by the lack of any observed frequency dependence.

The strongest evidence for dark energy at present comes from geometric techniques such as the supernova distance-redshift relation. By combining the measured expansion history with the Friedmann equation, one determines the energy density and its time evolution and hence the equation of state of dark energy. Because these methods rely on the Friedmann equation, which has not been independently tested, it is desirable to find alternative methods that work for both general relativity and other theories of gravity. Assuming that sufficiently large patches of a perturbed Robertson-Walker spacetime evolve like separate Robertson-Walker universes, that shear stress is unimportant on large scales, and that energy and momentum are locally conserved, we derive several relations between long-wavelength metric and matter perturbations. These relations include generalizations of the initial-value constraints of general relativity. For a class of theories including general relativity we reduce the long-wavelength metric, density, and velocity potential perturbations to quadratures including curvature perturbations, entropy perturbations, and the effects of nonzero background curvature. When combined with the expansion history measured geometrically, the long-wavelength solution provides a test that could distinguish modified gravity from other explanations of dark energy.

We study the growth of galactic disks in live triaxial dark matter (DM) halos. The halos have been assembled using the constrained realizations method and evolved from the linear regime using cosmological simulations. The "seed" disks have been inserted at redshift z = 3 and increased in mass tenfold over various time periods, ~1-3 Gyr, with the halo responding quasi-adiabatically to this process. We follow the dynamical and secular evolution of the disk-halo system and analyze changes in the most important parameters, such as three-dimensional DM shapes, stellar (disk) and DM (halo) radial density profiles, and stellar bar development. We find that a growing disk is responsible for washing out the halo prolateness (in the disk plane) and for diluting its flatness over a period of time comparable to the disk growth. Moreover, we find that a disk that contributes more to the overall rotation curve in the system is also more efficient in axisymmetrizing the halo, without accelerating the halo figure rotation. The observational corollary is that the maximal disks probably reside in nearly axisymmetric halos, while disks whose rotation is dominated by the halo at all radii are expected to reside in more prolate halos. The halo shape is sensitive to the final disk mass, but is independent of how the seed disk is introduced into the system, abruptly or quasi-adiabatically. It is weakly sensitive to the timescale of the disk growth. We also expect that the massive disks are subject to a bar instability, while in light disks this instability is damped by the halo triaxiality. Implications of these results for the cosmological evolution of disks embedded in asymmetric halos are discussed and so are the corollaries for the observed fraction of stellar bars. Finally, the halo responds to the stellar bar by developing a gravitational wake—a "ghost" bar of its own, which is almost in phase with that in the disk.

We study whether hierarchical galaxy formation in a concordance ΛCDM universe can produce enough massive and red galaxies compared to the observations. We implement a semianalytical model in which the central black holes gain their mass during major mergers of galaxies and the energy feedback from active galaxy nuclei (AGNs) suppresses the gas cooling in their host halos. The energy feedback from AGNs acts effectively only in massive galaxies when supermassive black holes have been formed in the central bulges. Compared with previous models without black hole formation, our model predicts more massive and luminous galaxies at high redshift, agreeing with the observations of K20 up to z ~ 3. Also, the predicted stellar mass density from massive galaxies agrees with the observations of GDDS. Because of the energy feedback from AGNs, the formation of new stars is stopped in massive galaxies with the termination of gas cooling and these galaxies soon become red with color R - K > 5 (Vega magnitude), comparable to the extremely red objects (EROs) observed at redshift z ~ 1-2. Still, the predicted number density of very EROs is lower than observed at z ~ 2, and it may be related to inadequate descriptions of dust extinction, star formation history and AGN feedback in those luminous galaxies.

In the context of recent observational results that show massive ellipticals were in place at high redshifts, we reassess the status of monolithic collapse in a ΛCDM universe. Using a sample of over 2000 galaxies from the Sloan Digital Sky Survey, by comparing the dynamical mass and stellar mass (estimated from colors) we find that ellipticals have "cores" that are baryon-dominated within their half-light radius. These galaxies correspond to 3 σ peaks in the spherical collapse model if the total mass in the halo is assumed to be 20 times the dynamical mass within the half-light radius. This value yields stellar mass-to-total mass ratios of 8%, compared to a cosmological baryon fraction of 18% derived from the first 3 years of WMAP observations alone. We further develop a method for reconstructing the concentration halo parameter c of the progenitors of these galaxies by utilizing adiabatic contraction. Although the analysis is done within the framework of monolithic collapse, the resulting distribution of c is lognormal with a peak value of c ~ 3-10 and a distribution width similar to the results of N-body simulations. We also derive scaling relations between stellar and dynamical mass and the velocity dispersion, and find that these are sufficient to recover the tilt of the fundamental plane.

We use three-dimensional hydrodynamic simulations to investigate the effects of a transient photoionizing UV flux on the collapse and cooling of pregalactic clouds. These clouds have masses in the range 10⁵-10⁷M_☉, form at high redshifts (z ≳ 18), and are assumed to lie within the short-lived cosmological H II regions around the first generation of stars. In addition, we study the combined effects of this transient UV flux and a persistent Lyman-Werner (LW) background (at photon energies below 13.6 eV) from distant sources. In the absence of a LW background, we find that a critical specific intensity of J_UV ~ 0.1 × 10^-21 ergs s^-1 cm^-2 Hz^-1 sr^-1 demarcates a transition from net negative to positive feedback for the halo population. A weaker UV flux stimulates subsequent star formation inside the fossil H II regions, by enhancing the H₂ molecule abundance. A stronger UV flux significantly delays star formation by reducing the gas density, and increasing the cooling time, at the centers of collapsing halos. At a fixed J_UV, the sign of the feedback also depends strongly on the density of the gas at the time of UV illumination. Regardless of whether the feedback is positive or negative, we find that once the UV flux is turned off, its impact starts to diminish after ~30% of the Hubble time. In the more realistic case when a LW background is present, with J_LW ≳ 0.01 × 10^-21 ergs s^-1 cm^-2 Hz^-1 sr^-1, strong suppression persists down to the lowest redshift (z = 18) in our simulations. Finally, we find evidence that heating and photoevaporation by the transient UV flux render the ~10⁶M_☉ halos inside fossil H II regions more vulnerable to subsequent H₂ photodissociation by a LW background.

We critically analyze the measurement of galaxy cluster gas masses, which is central to cosmological studies that rely on the galaxy cluster gas mass fraction. Using synthetic observations of numerically simulated clusters viewed through their X-ray emission and thermal Sunyaev-Zeldovich effect (SZE), we reduce the observations to obtain measurements of the cluster gas mass. We quantify the possible sources of uncertainty and systematic bias associated with the common simplifying assumptions used in reducing real cluster observations, including isothermality and hydrostatic equilibrium. We find that intrinsic variations in clusters limit the precision of observational gas mass estimation to ~10% to 1 σ confidence, excluding instrumental effects. Gas mass estimates show surprisingly little trending in the scatter as a function of cluster redshift. For the full cluster sample, methods that use SZE profiles out to roughly the virial radius are the simplest, most accurate, and unbiased way to estimate cluster mass. X-ray methods are systematically more precise mass estimators than are SZE methods if merger and cool-core systems are removed, but slightly overestimate (5%-10%) the cluster gas mass on average. We find that cool-core clusters in our samples are particularly poor candidates for observational mass estimation, even when excluding emission from the core region. The effects of cooling in the cluster gas alter the radial profile of the X-ray and SZE surface brightness even outside the cool-core region. Finally, we find that methods using a universal temperature profile estimate cluster masses to higher precision than those assuming isothermality.

We show that Type Ia supernovae (SNe Ia) are formed within both very young and old stellar populations, with observed rates that depend on the stellar mass and mean star formation rates (SFRs) of their host galaxies. Models in which the SN Ia rate depends solely on host galaxy stellar mass are ruled out with >99% confidence. Our analysis is based on 100 spectroscopically confirmed SNe Ia, plus 24 photometrically classified events, all from the Supernova Legacy Survey (SNLS) and distributed over 0.2 < z < 0.75. We estimate stellar masses and SFRs for the SN Ia host galaxies by fitting their broadband spectral energy distributions with the galaxy spectral synthesis code PÉGASE.2. We show that the SN Ia rate per unit mass is proportional to the specific SFR of the parent galaxies—more vigorously star-forming galaxies host more SNe Ia per unit stellar mass, broadly equivalent to the trend of increasing SN Ia rate in later type galaxies seen in the local universe. Following earlier suggestions for a simple "two-component" model approximating the SN Ia rate, we find bivariate linear dependencies of the SN Ia rate on both the stellar masses and the mean SFRs of the host systems. We find that the SN Ia rate can be well represented as the sum of 5.3 ± 1.1 × 10^-14 SNe yr^-1M_☉^-1 and 3.9 ± 0.7 × 10^-4 SNe yr^-1 (M_☉ yr^-1)^-1 of star formation. We also demonstrate a dependence of distant SN Ia light-curve shapes on star formation in the host galaxy, similar to trends observed locally. Passive galaxies, with no star formation, preferentially host faster declining/dimmer SNe Ia, while brighter events are found in systems with ongoing star formation.

Future measurements of the nature of dark energy using Type Ia supernovae will require a precise characterization of systematic sources of error. Evolutionary effects remain the most uncertain contributor to the overall systematic error budget. Present plans to probe evolution with cosmology-independent explosion parameters could yield an absence of evidence for evolution without definitive evidence of its absence. Here we show that observations of Type Ia supernovae in the redshift interval 1.5 < z < 3.0, where dark-energy-dependent effects are relatively negligible, should provide direct evidence to discern evolutionary effects. As examples of our approach to constraining evolution, we examine the impact of changing progenitor metallicity and age on the degree of potential luminosity evolution. We show that the observations we propose can be carried out by existing space telescopes, or ones that are already under development.

After being destroyed by a binary supermassive black hole, a stellar density cusp can regrow at the center of a galaxy via energy exchange between stars moving in the gravitational field of the single, coalesced hole. We illustrate this process via high-accuracy N-body simulations. Regeneration requires roughly one relaxation time, and the new cusp extends to a distance of roughly one-fifth the black hole's influence radius, with density ρ ~ r^-7/4; the mass in the cusp is of the order of 10% of the mass of the black hole. The growth of the cusp is preceded by a stage in which the stellar velocity dispersion evolves toward isotropy and away from the tangentially anisotropic state induced by the binary. We show that density profiles similar to those observed at the center of the Milky Way and M32 can regenerate themselves in several Gyr following the infall of a second black hole; the presence of density cusps at the centers of these galaxies can therefore not be used to infer that no merger has occurred. We argue that ρ ~ r^-7/4 density cusps are ubiquitous in stellar spheroids fainter than M_V ≈ -18.5 that contain supermassive black holes, but the cusps have not been detected outside of the Local Group, since their angular sizes are less than ~01. We show that the presence of a cusp implies a lower limit of ~10^-4 yr^-1 on the rate of stellar tidal disruptions and discuss the consequences of the cusps for gravitational lensing and the distribution of dark matter on subparsec scales.

The jet in 3C 273 is a high-power quasar jet with radio, optical, and X-ray emission, whose size and brightness allow a detailed study of the emission processes acting in it. We present deep Chandra observations of this jet and analyze the spectral properties of the jet emission from radio through X-rays. We find that the X-ray spectra are significantly softer than the radio spectra in all regions of the bright part of the jet, except for the first bright "knot A," ruling out a model in which the X-ray emission from the entire jet arises from beamed inverse Compton scattering of cosmic microwave background photons in a single-zone jet flow. Within two-zone jet models, we find that a synchrotron origin for the jet's X-rays requires fewer additional assumptions than an inverse Compton model, especially if velocity shear leads to efficient particle acceleration in jet flows.

We have performed infrared imaging of the jet of the quasar 3C 273 at wavelengths of 3.6 and 5.8 μm with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. When combined with the radio, optical, and X-ray measurements, the IRAC photometry of the X-ray-bright jet knots clearly shows that the optical emission is dominated by the high-energy emission component of the jet, not by the radio synchrotron component, as had been assumed to date. The high-energy component, represented by a power law from the optical through X-ray, may be due to a second synchrotron component or to inverse Compton scattering of ambient photons. In the former case, we argue that the acceleration of protons exceeding energies of E_p ~ 10¹⁶ eV or possibly even to E_p ~ 10¹⁹ eV would be taking place in the jet knots of 3C 273, assuming that the acceleration time is proportional to the particle gyroradius. In contrast, the inverse Compton model, into which highly relativistic Doppler beaming has to be incorporated, requires very low energy electrons of E_e ~ 1 MeV in the jet knots. The present polarization data in the radio and optical would favor the former interpretation in the case of the 3C 273 jet. Sensitive and detailed measurements of optical polarization are important in order to establish the radiation mechanism responsible for the high-energy emission. The present study offers new clues as to the controversial origin of the X-ray emission seen in many quasar jets.

We derive the equations for the propagation of relativistic ionization fronts (I-fronts) in both static and moving gases, including the cosmologically expanding intergalactic medium (IGM). For the supersonic R-type phase that occurs right after a source turns on, relativistic corrections can be significant up until the light crossing time of the equilibrium Strömgren sphere. When q, the ratio of this light-crossing time to the recombination time, exceeds unity, the time for the expanding I-front to reach the Strömgren radius is delayed by a factor of q. For a static medium, we obtain exact analytical solutions and apply them to the illustrative problems of an O star in a molecular cloud and a starburst in a high-redshift cosmological halo. Relativistic corrections can be important both at early times when the H II regions are small and at later times if a density gradient causes the I-front to accelerate. An analytical solution is also derived for a steady source in a cosmologically expanding IGM. Here relativistic corrections are significant for short-lived, highly luminous sources such as QSOs at the end of reionization (z ≃ 6) but negligible for weaker or higher redshift sources. Finally, we numerically solve the equations for relativistic, cosmological I-fronts in the presence of evolving small-scale structure, for a large-galaxy starburst and a luminous QSO. For such strong and short-lived sources at z ~ 7, relativistic corrections are quite significant, and small-scale structure can decrease the size of the H II region by up to an additional ~25%. However, most of the IGM was ionized by smaller, higher redshift sources. Thus, the effect of relativistic corrections on global reionization is small and can usually be neglected.

Using N-body simulations, we have modeled the production and evolution of diffuse, low surface brightness intracluster light (ICL) in three simulated galaxy clusters. Using an observational definition of ICL to be luminosity at a surface brightness μ_V > 26.5 mag arcsec^-2, we have found that the fraction of cluster luminosity contained in ICL generally increases as clusters evolve, although there are large deviations from this trend over short timescales, including sustained periods of decreasing ICL luminosity. Most ICL luminosity increases come in short, discrete events that are highly correlated with group accretion events within the cluster. In evolved clusters we find that ≈10%-15% of the clusters' luminosity is at ICL surface brightness. The morphological structure of the ICL changes with time, evolving from a complex of filaments and small-scale, relatively high surface brightness features early in a cluster's history to a more diffuse and amorphous cluster-scale ICL envelope at later times. Finally, we also see a correlation between the evolution of ICL at different surface brightnesses, including a time delay between the evolution of faint and extremely faint surface brightness features that is traced to the differing dynamical timescales in the group and cluster environment.

We use a 54.4 ks Chandra observation to study nuclear outflow activity in NGC 4552 (M89), an elliptical galaxy in the Virgo Cluster. Chandra images in the 0.5-2 keV band show two ringlike features ~1.7 kpc in diameter in the core of NGC 4552, as reported previously by Filho et al. We use spherically symmetric point explosion shock models to argue that the shape of the surface brightness profile across the rims of the rings and the temperature of hot gas in the rings are consistent with a Mach 1.7 shock carrying mean mechanical power L_shock ~ 3 × 10⁴¹ ergs s^-1 produced by a ~1.4 × 10⁵⁵ ergs nuclear outburst ~1-2 Myr ago. We find the gas temperature in the central ~100 pc of the galaxy to be 1.0 ± 0.2 keV, hotter than elsewhere in the galaxy, suggesting that we may be directly observing the reheating of the galaxy ISM by the outburst.

We investigate the relationship between soft X-ray luminosity and mass for low-redshift clusters of galaxies by comparing observed number counts and scaling laws to halo-based expectations of ΛCDM cosmologies. We model the conditional likelihood of halo luminosity as a lognormal distribution of fixed width, centered on a scaling relation, L ∝ M^pρ(z), and consider two values for s, appropriate for self-similar evolution or no evolution. Convolving with the halo mass function, we compute expected counts in redshift and flux that, after appropriate survey effects are included, we compare to REFLEX survey data. Counts alone provide only an upper limit on the scatter in mass at fixed luminosity, σ_{ln M} < 0.4. We argue that the observed, intrinsic variance in the temperature-luminosity relation is directly indicative of mass-luminosity variance and derive σ_{ln M} = 0.43 ± 0.06 from HIFLUGCS data. When added to the likelihood analysis, we derive values p = 1.59 ± 0.05, ln L_15,0 = 1.34 ± 0.09, and σ_{ln M} = 0.37 ± 0.05 for self-similar redshift evolution in a concordance (Ω_m = 0.3, Ω_Λ = 0.7, σ₈ = 0.9) universe. The present-epoch intercept is sensitive to power spectrum normalization, L_15,0 ∝ σ, and the slope is weakly sensitive to the matter density, p ∝ Ω. We find a substantially (factor 2) dimmer intercept and slightly steeper slope than the values published using hydrostatic mass estimates of the HIFLUGCS sample and show that a Malmquist bias of the X-ray flux-limited sample accounts for this effect. In light of new WMAP constraints, we discuss the interplay between parameters and sources of systematic error and offer a compromise model with Ω_m = 0.24, σ₈ = 0.85, and somewhat lower scatter σ_{ln M} = 0.25, in which hydrostatic mass estimates remain accurate to ~15%. We stress the need for independent calibration of the L-M relation via weak gravitational lensing.

We use a cosmological numerical simulation to study the tidal features produced by a minor merger with an elliptical galaxy. We find that the simulated tidal features are quantitatively similar to the red tidal features, i.e., dry tidal features, recently found in deep images of elliptical galaxies at intermediate redshifts. The minor merger in our simulation does not trigger star formation due to active galactic nuclei heating. Therefore, both the tidal features and the host galaxy are red, i.e., a dry minor merger. The stellar mass of the infalling satellite galaxy is about 10¹⁰M_☉, and the tidal debris reach the surface brightness of μ_R ~ 27 mag arcsec^-2. Thus, we conclude that tidal debris from minor mergers can explain the observed dry tidal features in elliptical galaxies at intermediate redshifts, although other mechanisms (such as major dry mergers) may also be important.

A binary supermassive black hole leaves an imprint on a galactic nucleus in the form of a "mass deficit," a decrease in the mass of the nucleus due to ejection of stars by the binary. The magnitude of the mass deficit is in principle related to the galaxy's merger history, but the relation has never been quantified. Here high-accuracy N-body simulations are used to calibrate this relation. Mass deficits are shown to be M_def ≈ 0.5M₁₂, with M₁₂ the total mass of the binary; the coefficient in this relation is found to depend only weakly on M₂/M₁ or on the galaxy's preexisting nuclear density profile. Hence, after mergers, M_def ≈ 0.5M_•, with M_• the final (current) black hole mass. When compared with observed mass deficits, this result implies 1 ≲ ≲ 3, in accord with hierarchical structure formation models. Implications for binary stalling radii, the origin of hypervelocity stars, and the distribution of dark matter at the centers of galaxies are discussed.

The recent star formation (SF) in the early-type spiral galaxy M81 is characterized using imaging observations from the far-ultraviolet to the far-infrared. We compare these data with models of the stellar, gas, and dust emission for subgalactic regions. Our results suggest the existence of a diffuse dust emission not directly linked to the recent star formation. We find a radial decrease of the dust temperature and dust mass density, and in the attenuation of the stellar light. The IR emission in M81 can be modeled with three components: (1) cold dust with a temperature = 18 ± 2 K, concentrated near the H II regions but also presenting a diffuse distribution; (2) warm dust with = 53 ± 7 K, directly linked with the H II regions; and (3) aromatic molecules, with diffuse morphology peaking around the H II regions. We derive several relationships to obtain total IR luminosities from IR monochromatic fluxes, and we compare five different star formation rate (SFR) estimators for H II regions in M81 and M51: the UV, Hα, and three estimators based on Spitzer data. We find that the Hα luminosity absorbed by dust correlates tightly with the 24 μm emission. The correlation with the total IR luminosity is not as good. Important variations from galaxy to galaxy are found when estimating the total SFR with the 24 μm or the total IR emission alone. The most reliable estimations of the total SFRs are obtained by combining the Hα emission (or the UV) and an IR luminosity (especially the 24 μm emission), which probe the unobscured and obscured SF, respectively. For the entire M81 galaxy, about 50% of the total SF is obscured by dust. The percentage of obscured SF ranges from 60% in the inner regions of the galaxy to 30% in the outer zones.

As part of the Araucaria Project, we present the first spectral catalog of supergiant stars in the Local Group dwarf irregular galaxy WLM. In assigning a spectral classification to these stars we accounted for the low metal content of WLM relative to the galactic standards used in the MK process, by using classification criteria developed for B and A supergiants contained in the Small Magellanic Cloud. Our spectral catalog shows that our higher signal-to-noise ratio (S/N) spectroscopic sample of 19 objects contains at least six early B (B0-B5) supergiants and six late B and early A (B8-A2) stars of luminosity class between Ia and II, as well as an O7 V star and an O9.7 Ia star. The spectra of several of these stars are of sufficiently high quality for a determination of the stellar parameters and abundances. We have also acquired a second set of lower S/N spectra for mostly BA stars; however, their quality does not allow a further analysis. We have carried out a quantitative analysis for three early B supergiants. The mean oxygen abundance we derive is 12 + log(O/H) = 7.83 ± 0.12. This value agrees very well with the measurement that is obtained from H II regions. We therefore find no additional evidence for the discrepancy between stellar and nebular oxygen abundances measured for a single A-type supergiant by Venn et al. The analysis of B- and A-type supergiants yields compatible results for nitrogen, silicon, and magnesium. We show that the photometric variability of the blue supergiants included in our spectroscopic sample is negligible for the use of these stars as distance indicators.

The near-IR and X-ray flares in Sagittarius A* are believed to be produced by relativistic electrons via synchrotron and synchrotron self-Comptonization, respectively. These electrons are likely energized by turbulent plasma waves through second-order Fermi acceleration that, in combination with the radiative cooling processes, produces a relativistic Maxwellian distribution in the steady state. This model has four principal parameters, namely the magnetic field B, the electron density n and temperature γ_cm_ec², and the size of the flare region R. In the context of stochastic acceleration, the quantities Rn^1/2B and γ_cRn should remain nearly constant in time. Therefore, simultaneous spectroscopic observations in the NIR and X-ray bands can readily test the model, which, if proven to be valid, may be used to determine the evolution of the plasma properties during an eruptive event with spectroscopic observations in either band or simultaneous flux density measurements in both bands. The formulae can be applied to other isolated or confined systems, where electrons are accelerated to relativistic energies by plasma wave turbulence and produce most of the emission via synchrotron processes.

We have surveyed the blue straggler star population of the Galactic globular cluster M5 using high-resolution images of the core along with wide-field ground-based images reaching to more than 19 core radii. To gauge M5's relative efficiency of producing stragglers, we compared our sample to five studies of other globular clusters. Using a "bright" sample selected in the same way as that of Ferraro and coworkers, we found a bimodal radial distribution similar to those found in three other luminous clusters. When the radial distributions for different clusters are scaled using the core radius, there is good cluster-to-cluster agreement in the size of the core straggler sample and the center of the "zone of avoidance." However, M5 has the smallest fraction of stragglers in the zone of avoidance of any of the clusters measured to date, and its zone of avoidance appears to be wider (in r/r_c) than that of M3, which has a very similar surface brightness profile. Both of these facts indicate that M5's straggler population has dynamically evolved to a larger extent than M3's. Using an ultraviolet sample from Hubble Space Telescope selected in the same way as that of Ferraro and coworkers, we find that the frequency of blue stragglers in M5 is lower than in all but two of the clusters examined. We also identified seven bright blue stragglers that were previously misidentified as HB stars by Sandquist & Bolte. These bright stragglers are most likely the result of stellar collisions involving binary stars.

We present a deep observation with the Chandra X-Ray Observatory of the neutron star bow shock G189.22+2.90 in the supernova remnant (SNR) IC 443. Our data confirm the cometary morphology and central point source seen previously, but also reveal considerable new structure. Specifically, we find that the X-ray nebula consists of two distinct components: a "tongue" of bright emission close to the neutron star, enveloped by a larger, fainter "tail." We interpret the tongue and tail as delineating the termination shock and the postshock flow, respectively, as previously identified also in the pulsar bow shock G359.23-0.82 ("the Mouse"). However, for G189.22+2.90 the tongue is much less elongated than for the Mouse, while the tail is much broader. These differences are consistent with the low Mach number, M ≲ 2, expected for a neutron star moving through the hot gas in a SNR's interior, supporting the case for a physical association between G189.22+2.90 and IC 443. We resolve the standoff distance between the star and the head of the bow shock, which allows us to estimate a space velocity for the neutron star of ~230 km s^-1, independent of distance. We detect thermal emission from the neutron star surface at a temperature of 102 ± 22 eV, which is consistent with the age of SNR IC 443 for standard neutron star cooling models. We also identify two compact knots of hard emission located 1''-2'' north and south of the neutron star.

We present models of turbulent mixing at the boundaries between hot (T ~ 10⁶-10⁷ K) and warm material (T ~ 10⁴ K) in the interstellar medium, using a three-dimensional magnetohydrodynamic code, with radiative cooling. The source of turbulence in our simulations is a Kelvin-Helmholtz instability, produced by shear between the two media. We find that because the growth rate of the large-scale modes in the instability is rather slow, it takes a significant amount of time (~1 Myr) for turbulence to produce effective mixing. We also find that the total column densities of the highly ionized species (C IV, N V, and O VI) per interface (assuming ionization equilibrium) are similar to previous steady state nonequilibrium ionization models but grow slowly from log N ~ 10¹¹ to a few ×10¹² cm^-2 as the interface evolves. However, the column density ratios can differ significantly from previous estimates, with an order of magnitude variation in N(C )/N(O ) as the mixing develops.

Molecular cloud complexes (MCCs) are highly structured and "turbulent." Observational evidence suggests that MCCs are dynamically dominated systems, rather than quasi-equilibrium entities. The observed structure is more likely a consequence of the formation process than something that is imprinted after the formation of the MCC. Converging flows provide a natural mechanism to generate MCC structure. We present a detailed numerical analysis of this scenario. Our study addresses the evolution of an MCC from its birth in colliding atomic hydrogen flows up until the point when H₂ may begin to form. A combination of dynamical and thermal instabilities breaks up coherent flows efficiently, seeding the small-scale nonlinear density perturbations necessary for local gravitational collapse and thus allowing (close to) instantaneous star formation. Many observed properties of MCCs come as a natural consequence of this formation scenario. Since converging flows are omnipresent in the ISM, we discuss the general applicability of this mechanism, from local star formation regions to galaxy mergers.

We present high-resolution spectroscopy of the neon K-shell and iron L-shell interstellar absorption edges in nine X-ray binaries using the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra X-Ray Observatory. We found that the iron absorption is well fit by an experimental determination of the cross section for metallic iron, although with a slight wavelength shift of ≈20 mÅ. The neon edge region is best fit by a model that includes the neutral neon edge and three Gaussian absorption lines. We identify these lines as due to the 1s-2p transitions from Ne II, Ne III, and Ne IX. As we found in our oxygen edge study, the theoretical predictions for neutral and low-ionization lines all require shifts of ≈20 mÅ to match our data. Combined with our earlier oxygen edge study, we find that a best-fit O/Ne ratio of 5.4 ± 1.6, consistent with standard interstellar abundances. Our best-fit Fe/Ne ratio of 0.20 ± 0.03 is significantly lower than the interstellar value. We attribute this difference to iron depletion into dust grains in the interstellar medium. We make the first measurement of the neon ionization fraction in the ISM. We find Ne /Ne ≈ 0.3 and Ne /Ne ≈ 0.07. These values are larger than is expected given the measured ionization of interstellar helium. For Ne IX, our results confirm the detection of the hot ionized interstellar medium of the Galaxy.

We have obtained maps of the large-scale outflow associated with the UC H II region G5.89-0.39 in CO and ¹³CO (J = 3-2), SiO (J = 8-7, J = 5-4), SO₂ (J = 13_2,12-13_1,13),¹³ and H¹³CO⁺(J = 4-3). From these maps we have been able to determine the mass (3.3 M_☉), momentum (96 M_☉ km s^-1), energy (3.5 × 10⁴⁶ ergs), mechanical luminosity (141 L_☉), and mass-loss rate (~ 1 × 10^-3M_☉ yr^-1) in the large-scale outflow. The observationally derived parameters were used to guide three-dimensional magnetohydrodynamic models of the jet-entrained outflow. Through the combination of observations and simulations, we suggest that the large-scale outflow may be inclined by approximately 45° to the line of sight, and that the jet entraining the observed molecular outflow may have been active for as little as 1000 years, half the kinematic age of the outflow.

We present results from 1078 high-resolution spectra of 990 stars in the young open cluster NGC 2264, obtained with the Hectochelle multiobject echelle spectrograph on the 6.5 m MMT. We confirm 471 stars as members on the basis of their radial velocity and/or Hα emission. The radial velocity distribution of cluster members is non-Gaussian, with a dispersion of σ ≈ 3.5 km s^-1. We find a substantial north-south velocity gradient and spatially coherent structure in the radial velocity distribution, similar to that seen in the molecular gas in the region. Our results suggest that there are at least three distinguishable subclusters in NGC 2264, correlated with similar structure seen in ¹³CO emission, which is likely to be a remnant of initial structure in this very young cluster. We propose that this substructure is the result of gravitational amplification of initial inhomogeneities during overall collapse to a filamentary distribution of gas and stars, as found in simulations by Burkert & Hartmann (2004).

We present 5-35 μm spectra, taken with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope, of five FU Orionis objects: FU Ori, V1515 Cyg, V1057 Cyg, BBW 76, and V346 Nor. All but V346 Nor reveal amorphous silicate grains in emission at 10 and 20 μm, and show water-vapor absorption bands at 5.8 and 6.8 μm and SiO or possibly methane absorption at 8 μm. These absorption features closely match these bands in model stellar photospheres—signs of the gaseous photospheres of the inner regions of these objects' accretion disks. The continuum emission at 5-8 μm is also consistent with such disks, and, for FU Orionis and BBW 76, longer wavelength emission may be fit by a model that includes moderate disk flaring. V1057 Cyg and V1515 Cyg have much more emission at longer wavelengths than the others, perhaps evidence of a substantial remnant of their natal, infalling envelopes.

We present a theoretical study of double compact objects as potential short/hard gamma-ray burst (GRB) progenitors. An updated population synthesis code, StarTrack, is used to calculate properties of double neutron stars and black hole-neutron star binaries. We obtain their formation rates, estimate merger times, and finally predict their most likely merger locations and afterglow properties for different types of host galaxies. Our results serve for a direct comparison with the recent HETE-2 and Swift observations of several short bursts, for which afterglows and host galaxies were detected. We also discuss the possible constraints these observations put on the evolutionary models of double compact object formation. We emphasize that our double compact object models can successfully reproduce at the same time short GRBs within both young, star-forming galaxies (e.g., GRB 050709 and GRB 051221A), as well as within old, elliptical hosts (e.g., GRB 050724 and probably GRB 050509B).

The long burst GRB 050717 was observed simultaneously by the Burst Alert Telescope (BAT) on Swift and the Konus instrument on Wind. Significant hard to soft spectral evolution was seen. Early gamma-ray and X-ray emission was detected by both BAT and the X-Ray Telescope (XRT) on Swift. The XRT continued to observe the burst for 7.1 days and detect it for 1.4 days. The X-ray light curve showed a classic decay pattern; the afterglow was too faint for a jet break to be detected. No optical, infrared, or ultraviolet counterpart was discovered despite deep searches within 14 hr of the burst. Two particular features of the prompt emission make GRB 050717 a very unusual burst. First, the peak of the νF(ν) spectrum was observed to be 2401 keV for the main peak, which is the highest value of E_peak ever observed. Second, the spectral lag for GRB 050717 was determined to be 2.5 ± 2.6 ms, consistent with zero and unusually short for a long burst. This lag measurement suggests that this burst has a high intrinsic luminosity and hence is at high redshift (z > 2.7). Despite these unusual features, GRB 050717 exhibits the classic prompt and afterglow behavior of a gamma-ray burst.

The 2 m robotic Liverpool Telescope (LT) reacted promptly to the high-redshift (z = 4.048) gamma-ray burst GRB 060206. The afterglow was identified automatically, and the multicolor r'i'z' imaging program was triggered without human intervention. Combining our data with those obtained from later follow-ups provides a well-sampled optical light curve from 5 minutes to more than 2days after the gamma event. The light curve is highly structured, with at least three bumps evident in the first 75 minutes, including a major rebrightening (Δr' ≈ -1.6 at t ≈ 3000 s), interpreted as late energy injection. At early time (t ≈ 440 s), we find evidence for fast (Δt_rest < 4 s ≪ t) variability, indicating ongoing internal-engine activity. We emphasize that a low-redshift GRB (z < 1) with similar intrinsic properties would have been interpreted completely differently, due to undersampling of the light curve in the rest frame at early times; the light-curve behavior of GRB 060206 should therefore not be considered peculiar. Finally, although the observed late-time steepening of the optical light curve resembles a jet break if taken in isolation, the lack of a corresponding change in the X-ray slope rules out a jet-break interpretation. Traditionally, GRB jet breaks have been inferred from optical data in the absence of simultaneous X-ray data. We therefore suggest that current estimates of the jet-opening angle distribution might be biased by events like GRB 060206. Consequently, the GRB explosion energy distribution and event rates may have to be revised.

We present the discovery of XRF 050215B and its afterglow. The burst was detected by the Swift BAT during the check-out phase, and observations with the X-Ray Telescope began approximately 30 minutes after the burst. These observations found a faint, slowly fading X-ray afterglow near the center of the error box as reported by the BAT. Infrared data obtained at UKIRT after 10 hr also revealed a very faint K-band afterglow. The afterglow appears unusual since it is very faint, especially in the infrared, with K > 20 only 9 hr postburst. The X-ray and infrared light curves exhibit a slow, monotonic decay with α ~ 0.8 and no evidence for a steepening associated with the jet break to 10 days postburst. We discuss possible explanations for the faintness and slow decay in the context of present models for the production of X-ray flashes.

We present a comprehensive investigation of the two broad absorption features observed in the X-ray spectrum of the neutron star 1E1207.4-5209 based on a recent analysis of the 260 ks XMM-Newton data by Mori et al. Expanding on earlier work by Hailey & Mori, we have examined all previously proposed atmospheric models for 1E1207.4-5209. Using our atomic code, which rapidly solves Schrödinger's equation for arbitrary ions in strong magnetic field, we have systematically constructed atmospheric models by calculating polarization-dependent LTE opacities and addressed all the physics relevant to strongly magnetized plasmas. We have been able to rule out virtually all atmospheric models because they either do not sustain an ionization balance consistent with the claimed atmosphere composition or because they predict line strengths and line widths that are inconsistent with the data. Only oxygen or neon atmospheres at B ~ 10¹² G provide self-consistent atmospheric solutions of the appropriate ionization balance that also have line widths, strengths, and energies consistent with the observations. The observed features are likely composed of several bound-bound transition lines from highly ionized oxygen/neon, and they are broadened primarily by motional Stark effects and magnetic field variation over the line-emitting region. Further considerations of plausible mechanisms for the formation of a mid-Z atmosphere likely rule out neon atmospheres and have important implications for the fallback mechanism in supernova ejecta. Future high-resolution spectroscopy missions such as Constellation-X will be able to resolve predicted substructure in the absorption features and will measure magnetic field strength and gravitational redshift independently to better than 10% accuracy.

We present high time-resolution multiwavelength observations of X-ray bursts in the low-mass X-ray binary UY Vol. Strong reprocessed signals are present in the ultraviolet and optical, lagged and smeared with respect to the X-rays. The addition of far-ultraviolet coverage for one burst allows much tighter constraints on the temperature and geometry of the reprocessing region than previously possible. A blackbody reprocessing model for this burst suggests a rise in temperatures during the burst from 18,000 to 35,000 K and an emitting area comparable to that expected for the disk and/or irradiated companion star. The lags are consistent with those expected. The single-zone blackbody model cannot reproduce the ratio of optical to ultraviolet flux during the burst, however. The discrepancy seems too large to explain with deviations from a local blackbody spectrum and more likely indicates that a range of reprocessing temperatures are required. Comparable results are derived from other bursts, and in particular the lag and smearing both appear shorter when the companion star is on the near side of the disk as predicted. The burst observed by HST also yielded a spectrum of the reprocessed light. It is dominated by continuum, with a spectral shape consistent with the temperatures derived from lightcurve modeling. Taken as a whole, our observations confirm the standard paradigm of prompt reprocessing distributed across the disk and companion star, with the response dominated by a thermalized continuum rather than by emission lines.

We present optical and ultraviolet spectra, light curves, and Doppler tomograms of the low-mass X-ray binary EXO 0748-676. Using an extensive set of 15 emission-line tomograms, we show that, along with the usual emission from the stream and "hot spot," there is extended nonaxisymmetric emission from the disk rim. Some of the emission and Hα and Hβ absorption features lend weight to the hypothesis that part of the stream overflows the disk rim and forms a two phase medium. The data are consistent with a 1.35 M_☉ neutron star with a main-sequence companion and hence a mass ratio q ≈ 0.34.

Thermochemical equilibrium and kinetic calculations are used to model sulfur and phosphorus chemistry in giant planets, brown dwarfs, and extrasolar giant planets (EGPs). The chemical behavior of individual S- and P-bearing gases and condensates is determined as a function of pressure, temperature, and metallicity. The results are independent of particular model atmospheres, and in principle, the equilibrium composition along the pressure-temperature profile of any object can be determined. Hydrogen sulfide (H₂S) is the dominant S-bearing gas throughout substellar atmospheres and approximately represents the atmospheric sulfur inventory. Silicon sulfide (SiS) is a potential tracer of weather in substellar atmospheres. Disequilibrium abundances of phosphine (PH₃) approximately representative of the total atmospheric phosphorus inventory are expected to be mixed upward into the observable atmospheres of giant planets and T dwarfs. In hotter objects, several P-bearing gases (e.g., P₂, PH₃, PH ₂, PH, and HCP) become increasingly important at high temperatures.

Moons of giant planets may represent an alternative to the classical picture of habitable worlds. They may exist within the circumstellar habitable zone of a parent star, and through tidal energy dissipation they may also offer alternative habitable zones, where stellar insolation plays a secondary, or complementary, role. We investigate the potential extent of stable satellite orbits around a set of 74 known extrasolar giant planets located beyond 0.6 AU from their parent stars—where moons should be long-lived with respect to removal by stellar tides. For this sample, the typical stable satellite orbital radii span a band some ~0.02 AU in width, compared to the ~0.12-0.15 AU bands for the Jovian and Saturnian systems. Approximately 60% of these giant planets can sustain satellites or moons in bands up to ~0.04 AU in width. For comparison, the Galilean satellites extend to ~0.013 AU. We discuss how the actual number and characteristics of satellites will depend strongly on the formation pathways. We investigate the stellar insolation that moons would experience for these exoplanet systems and the implications for sublimation loss of volatiles. We find that between 15% and 27% of all known exoplanets may be capable of harboring small, icy moons. In addition, some 22%-28% of all known exoplanets could harbor moons within a "sublimation zone," with insolation temperatures between 273 and 170 K. A simplified energy-balance model is applied to the situation of temperate moons, maintained by a combination of stellar insolation and tidal heat flow. We demonstrate that large moons (>0.1 M_⊕), at orbital radii commensurate with those of the Galilean satellites, could maintain temperate, or habitable, surface conditions during episodes of tidal heat dissipation of the order 1-100 times that currently seen on Io.

We report the findings of a comprehensive study of disk accretion and related phenomena in four of the nearest young stellar associations spanning 6-30 million years in age, an epoch that may coincide with the late stages of planet formation. We have obtained ~650 multiepoch high-resolution optical spectra of 100 low-mass stars that are likely members of the η Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), β Pictoris (~12 Myr), and Tucanae-Horologium (~30 Myr) groups. Our data were collected over 12 nights between 2004 December and 2005 July on the Magellan Clay 6.5 m telescope. Based on Hα line profiles, along with a variety of other emission lines, we find clear evidence of ongoing accretion in 3 out of 11 η Cha stars and 2 out of 32 TW Hydrae members. None of the 57 β Pic or Tuc-Hor members shows measurable signs of accretion. Together, these results imply significant evolution of the disk-accretion process within the first several Myr of a low-mass star's life. While a few disks can continue to accrete for up to ~10 Myr, our findings suggest that disks accreting for beyond that timescale are rather rare. This result provides an indirect constraint on the timescale for gas dissipation in inner disks and, in turn, on gas-giant planet formation. All accretors in our sample are slow rotators, whereas nonaccretors cover a large range in rotational velocities. This may hint at rotational braking by disks at ages up to ~8 Myr. Our multiepoch spectra confirm that emission-line variability is common even in somewhat older T Tauri stars, among which accretors tend to show particularly strong variations. Thus, our results indicate that accretion and wind activity undergo significant and sustained variations throughout the lifetime of accretion disks.

We discuss the possibility of the erosion of dusty bodies in protoplanetary disks by a subsonic laminar gas flow. Our analysis is based on wind tunnel experiments on centimeter-size dust targets in an air gas flow of 63 m s^-1 at static gas pressures between 0.1 and 4.5 mbar. We compare the results to numerical calculations of gas flow through porous bodies and the resulting drag force on dust aggregates at the surface. Our studies imply that a dusty body can be efficiently eroded if the dynamic gas pressure of the surface flow exceeds gravity and/or cohesion. In protoplanetary disks, we find that objects on circular orbits might be relatively safe against erosion in a laminar gas flow even in a dense disk. However, if a body is stirred up to an eccentric orbit, its motion relative to the gas increases. Such objects can be significantly eroded if they consist of dust. As an extreme, a 100 m body with the rather low eccentricity of an Earth orbit might be eroded in a single orbit. This effect leads to a bias for planetesimals in low-eccentricity orbits, as objects with large eccentricities are destroyed more easily. Erosion of bodies in high-eccentricity orbits, and reaccretion of the dust aggregates by low-eccentricity planetesimals, might provide a special growth mode of planetesimals and protoplanets.

A planet transits an 11th magnitude, G1 V star in the constellation Corona Borealis. We designate the planet XO-1b and the star XO-1, also known as GSC 02041-01657. XO-1 lacks a trigonometric distance; we estimate it to be 200 ± 20 pc. Of the 10 stars currently known to host extrasolar transiting planets, the star XO-1 is the most similar to the Sun in its physical characteristics: its radius is 1.0 ± 0.08 R_☉, its mass is 1.0 ± 0.03 M_☉, its V sin i < 3 km s^-1, and its metallicity [Fe/H] is 0.015 ± 0.04. The orbital period of the planet XO-1b is 3.941534 ± 0.000027 days, one of the longer ones known. The planetary mass is 0.90 ± 0.07M_J, which is marginally larger than that of other transiting planets with periods between 3 and 4 days. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. If the stellar radius is 1.0 ± 0.08 R_☉, then the planetary radius is 1.30 ± 0.11R_J and the inclination of the orbit is 877 ± 12. We have demonstrated a productive international collaboration between professional and amateur astronomers that was important to distinguishing this planet from many other similar candidates.

We report on the analysis of a thermal flare observed by a newly developed balloon-borne hard X-ray spectrometer. This instrument uses CdTe detectors and can observe the 20-120 keV hard X-ray range, with 3.0 keV energy resolution at 60 keV. During the 2002 May 24 flight, it successfully observed a class M1.1 flare. This flare observation shows no detectable flux above 35 keV, and its spectrum is consistent with a superhot thermal source with the temperature varying from 44 to 20 MK. Partial observation of the flare by the RHESSI satellite is consistent with this result. The Nobeyama Radio Polarimeters (NORP) observation of this flare shows no detectable polarization. The NORP light curves show impulsive features at 3.75 GHz that can be explained as thermal gyrosynchrotron emission, and this flux is consistent with observed X-ray spectra if a magnetic field of 275 G is assumed. Slower varying features seen in the NORP data are consistent with the lower temperature ("hot") thermal source of 10-15 MK seen in soft X-rays. We conclude that this flare shows no observable signature of nonthermal electrons, and all observed features are consistent with a purely thermal event. This serves as a strong indication that a nonthermal electron beam is not always the dominant energy source of plasma heating in solar flares.

We have identified the sources of six impulsive ³He-rich solar energetic particle events using imaging radio, optical, and energetic ion and electron data, together with calculated coronal fields obtained from extrapolating photospheric magnetograms using a potential field source surface (PFSS) model. These events were all studied in 2006 by Wang et al., who identified the particle sources as typically small, flaring active regions lying next to a coronal hole containing Earth-directed open field lines, located between W33° and W65°. By introducing radio imaging data we were able in one case to conclusively identify which of two simultaneous EUV jets was associated with the particle source. In addition, type III radio burst and energetic electron data introduced in this study constrain the injection times much more accurately than possible with low-energy ion data used in Wang et al. These new observations confirm the source identifications of Wang et al. and remove many of the remaining uncertainties. All of these events were associated with narrow, fast coronal mass ejections (CMEs), which are unusual for ³He-rich solar energetic particle (SEP) events. Although the CMEs generally were ejected in directions well off the ecliptic plane, the PFSS calculations show the presence of magnetic field lines that made it possible for the energetic particle to quickly reach Earth. Some of these impulsive events were observed during periods in which ³He was observed continuously over several days.

We present a method to correct MDI high-resolution Dopplergrams in active regions for systematic observational errors due to the spectral line shape changes. We use a measurement campaign with the Advanced Stokes Polarimeter to investigate the influence of line shape changes in active regions on MDI Dopplergram calibration. An estimate of the width of the Ni I λ6768 absorption line used by MDI to measure velocity provides a linear correction to the standard calibration. This correction can be largely explained by the Zeeman broadening of the absorption line.

Throughout the past decade, detailed helioseismic analyses of observations of solar surface oscillations have led to advances in our knowledge of the structure and dynamics of the solar interior. Such analyses involve the decomposition of time series of the observed surface oscillation pattern into its constituent wave modes, followed by inversion procedures that yield inferences of properties of the solar interior. While this inverse problem has been a major focus in recent years, the corresponding forward problem has received much less attention. We aim to rectify this situation by taking the first steps toward validating and determining the efficacy of the helioseismic measurement procedure. The goal of this effort is to design a means to perform differential studies of various effects such as flows and thermal perturbations on helioseismic observables such as resonant frequencies, travel-time shifts, etc. Here we describe our first efforts to simulate wave propagation within a spherical shell, which extends from 0.2 to about 1.0004 R_☉ (where R_☉ is the radius of the Sun) and which possesses a solar-like stratification. We consider a model containing no flows that will serve as a reference model for later studies. We discuss the computational procedure, some difficulties encountered in a simulation of this kind, and the means to overcome them. We also present techniques used to validate the simulation.

One of the biggest challenges associated with a nulling-interferometer-based approach to detecting extrasolar Earth-like planets comes from the extremely stringent requirements of path length, polarization, and amplitude matching in the interferometer. To the extent that the light from multiple apertures is not matched in these properties, light will leak through the nuller and confuse the search for a planetary signal. Here we explore the possibility of using the coherence properties of the starlight to separate contributions from the planet and nuller leakage. We find that straightforward modifications to the optical layout of a nulling interferometer will allow one to measure and correct for the leakage to a high degree of precision. This nulling calibration relaxes the field matching requirements substantially and should consequently simplify the instrument design.

Ions embedded in icy grain mantles are thought to account for various observed infrared spectroscopic features, particularly in certain young stellar objects. The dissociation of formic acid (HCOOH) in astrophysical ices to form the formate ion (HCOO^-) was modeled with density functional theory cluster calculations. Like isocyanic acid (HOCN), HCOOH was found to spontaneously deprotonate when sufficient water is present to stabilize charge transfer complexes. Both ammonia and water can serve as proton acceptors, yielding ammonium (NH) and hydronium (H₃O⁺) counterions. Computed frequencies of weak infrared features produced by stretching and bending modes in both HCOO^- and HCOOH were compared with experimental and astronomical data. Our results confirm laboratory assignments that a band at 1381 cm^-1 can be attributed to the CH bend in either HCOO^- or HCOOH, but a band at 1349 cm^-1 corresponds to CO stretching in HCOO^-. Another feature at 1710 cm^-1 (5.85 μm) can possibly be assigned to a CO stretching mode in HCOOH, as suggested by experiment, but the agreement is less satisfactory. In addition, we examine and analyze spectroscopic features associated with NH, both as a counterion to HCOO^- or OCN^- and in isolation, in order to compare with experimental and astronomical data in the 7 μm region.

Since the discovery of the first short-population γ-ray burst (GRB) afterglows in 2005, the handful of observed events have been found to be embedded in nearby (z < 1), bright underlying galaxies. We present multiwavelength observations of GRB 060121, the first short burst observed to clearly outshine its host galaxy (by a factor >10²). A photometric redshift for this event places the progenitor at a most probable redshift of z = 4.6, with a less probable scenario of z = 1.7. In either case, GRB 060121 could be the farthermost short-population GRB detected to date and implies an isotropic-equivalent energy release in gamma rays comparable to that seen in long-population bursts. We discuss the implications of the released energy on the nature of the progenitor. These results suggest that GRB 060121 may belong to a family of energetic short-population events, lying at z > 1 and whose optical afterglows would outshine their host galaxies, unlike the first short GRBs observed in 2005. The possibility of GRB 060121 being an intermediate-duration burst is also discussed.

Excited Si⁺ and Fe⁺ species are routinely observed in the host environment of gamma-ray burst (GRB) afterglows but are not commonly seen in other extragalactic locations. Their presence signals unusual properties in the gaseous environment of these GRB hosts that arise either as a result of the intense ionizing radiation of the afterglow or through collision excitation in a dense cloud. In particular, the photon pumping scenario has explicit expectations for temporal variation in the strength of the excited lines, owing to the decline in the ionizing flux of the GRB afterglow. We analyze afterglow spectra of GRB 020813 obtained in two epochs ≈16 hr apart and examine transitions from the first excited state of Fe⁺ at J = 7/2 in these two sets of data. We report a significant decline by at least a factor of 5 in the equivalent width of the Fe II λ2396 transition, the strongest from the J = 7/2 state. We perform a Monte Carlo analysis and determine that this temporal variation is present at more than 3 σ level of significance. This observation represents the first detection in the temporal variation of the excited Fe⁺ states in the GRB host interstellar medium, a direct influence of the burst itself on its environment. We further estimate that the Fe⁺ gas resides 50-100 pc from the afterglow, based on the afterglow light curve and the presence and absence of the excited Fe II λ2396 in the two-epoch observations.

We report on a survey for strong (rest equivalent width W_r ≥ 1 Å), intervening Mg II systems along the sight lines to long-duration gamma-ray bursts (GRBs). The GRB spectra that comprise the survey have a heterogeneous mix of resolution and wavelength coverage, but we implement a strict, uniform set of search criteria to derive a well-defined statistical sample. We identify 14 strong Mg II absorbers along 14 GRB sight lines (nearly every sight line exhibits at least one absorber) with spectra covering a total path length Δz = 15.5 at a mean redshift = 1.1. In contrast, the predicted incidence of such absorber systems along the same path length to quasar sight lines is only 3.8. The roughly 4 times higher incidence along GRB sight lines is inconsistent with a statistical fluctuation at greater than 99.9% c.l. Several effects could explain the result: (1) dust within the Mg II absorbers obscures faint quasars giving a lower observed incidence along quasar sight lines, (2) the gas is intrinsic to the GRB event, and (3) the GRBs are gravitationally lensed by these absorbers. We present strong arguments against the first two effects and also consider lensing to be an unlikely explanation. The results suggest that at least one of our fundamental assumptions underpinning extragalactic absorption line research is flawed.

We present abundance measurements for two super-Lyman limit systems (SLLSs; quasar absorption-line systems with 10¹⁹ cm^-2 < N < 10^20.3 cm^-2) selected from a set of metal-strong absorbers in the SDSS quasar database. After applying estimate corrections for photoionization effects, we derive gas-phase metallicities of [M/H] = +0.7 ± 0.2 dex for the SLLS at z = 1.7749 toward SDSS 0927+5621 and [M/H] = +0.05 ± 0.1 dex for the SLLS at z = 1.7678 toward SDSS 0953+5230. The former exhibits one of the highest gas metallicities of any astrophysical environment, and its total metal surface density exceeds that of nearly every known damped Lyα system. The properties of these absorbers—high metallicity and large velocity width (Δv > 300 km s^-1)—resemble those of gas observed in absorption in the spectra of bright, star-forming galaxies at high redshift. We discuss the metal mass density of the SLLSs based on these observations and our SLLS survey, and we argue that a conservative estimate to the total metal budget at z = 2 is greater than 15% of the total, suggesting that metal-rich LLSs may represent the dominant metal reservoir in the young universe.

Unified schemes of active galactic nuclei (AGNs) require an obscuring dusty torus around the central engine. The compact sizes (only a few parsecs) determined in recent high-resolution observations require that the obscuring matter be clumpy and located inside the region where the black hole gravity dominates over the galactic bulge. This location is in line with the scenario depicting the torus as the region of the clumpy wind coming off the accretion disk in which the clouds are dusty and optically thick. We study here the outflow scenario within the framework of hydromagnetic disk winds, incorporating the cloud properties determined from detailed modeling of the IR emission from clumpy tori. We find that torus clouds were likely detected in recent water maser observations of NGC 3079. In the wind scenario, the AGN main dynamic channel for release of accreted mass seems to be switching at low luminosities from torus outflow to radio jets. The torus disappears when the bolometric luminosity decreases below ~10⁴² ergs s^-1 because the accretion onto the central black hole can no longer sustain the required cloud outflow rate. This disappearance seems to have been observed in both LINERs and radio galaxies. With further luminosity decrease, suppression of cloud outflow spreads radially inward from the disk's dusty, molecular region into its atomic, ionized zone, resulting in the disappearance of the broad emission-line region at lower luminosities yet to be determined.

The high-frequency-peaked BL Lacertae object Markarian 180 (Mrk 180) was observed to have an optical outburst in 2006 March, triggering a Target of Opportunity observation with the MAGIC telescope. The source was observed for 12.4 hr, and very high energy γ-ray emission was detected with a significance of 5.5 σ. An integral flux above 200 GeV of (2.3 ± 0.7) × 10^-11 cm^-2 s^-1 was measured, corresponding to 11% of the Crab Nebula flux. A rather soft spectrum with a photon index of -3.3 ± 0.7 has been determined. No significant flux variation was found.

We present new weak-lensing observations of 1E 0657-558 (z = 0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray-emitting plasma are spatially segregated. By using both wide-field ground-based images and HST/ACS images of the cluster cores, we create gravitational lensing maps showing that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8 σ significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law and thus proves that the majority of the matter in the system is unseen.

We present a complete analysis, which includes morphology, kinematics, stellar populations, and N-body simulations, of CGCG 480-022, the most distant (cz = 14,317 km s^-1) isolated galaxy studied so far in such detail. The results all support the hypothesis that this galaxy has suffered a major merger event with a companion of ~0.1 times its mass. Morphology reveals the presence of a circumnuclear ring and possibly further ring debris. The radial velocity curve looks symmetrical, while the velocity dispersion increases with radius, reaching values that do not correspond to a virialized system. Moreover, this galaxy deviates significantly from the fundamental plane and the Faber-Jackson relation. The stellar population analysis show that the ring is younger and more metal-rich, which suggest that it has undergone a fairly recent burst of star formation. Both morphological and dynamical results are in broad agreement with our N-body simulations.

We present a study of the early (days to weeks) X-ray and UV properties of eight Type Ia supernovae (SNe Ia) that have been extensively observed with the X-Ray Telescope (XRT) and UV/Optical Telescope (UVOT) on board Swift, ranging from 5 to 132 days after the outburst. SN 2005ke is tentatively detected (at a 3-3.6 σ level of significance) in X-rays based on deep monitoring with the XRT ranging from 8 to 120 days after the outburst. The inferred X-ray luminosity [L_0.3-2 = (2 ± 1) × 10³⁸ ergs s^-1; 0.3-2 keV band] is likely caused by interaction of the SN shock with circumstellar material (CSM) deposited by a stellar wind from the progenitor's companion star with a mass-loss rate of ≈ 3 × 10^-6M_☉ yr^-1 (v_w/10 km s^-1). Evidence of CSM interaction in X-rays is independently confirmed by an excess of UV emission, as observed with the UVOT on board Swift, starting around 35 days after the explosion. The nondetection of SN 2005ke with Chandra 105 days after the outburst implies a rate of decline steeper than L_X ∝ t^-0.75, consistent with the decline expected from the interaction of the SN shock with a spherically symmetric CSM (t^-1). None of the other seven SNe Ia is detected in X-rays or shows a UV excess, which allows us to put tight constraints on the mass-loss rates of the progenitor systems.

We investigate the effect of the hot CNO cycle breakout reaction ¹⁵O(α, γ)¹⁹Ne on the occurrence of type I X-ray bursts on accreting neutron stars. For f_rp ≲ 0.1, where f_rp is a dimensionless factor by which we multiply the ¹⁵O(α, γ)¹⁹Ne reaction rate of Caughlan & Fowler, our model predicts that bursts should occur only for accretion rates below a critical value ≈ 0.3_Edd. This agrees with observations. For larger values of f_rp, including the standard choice f_rp = 1, the model switches to a new regime in which bursts occur all the way up to ≈ _Edd. Since the latter regime disagrees with observations, we suggest that the true ¹⁵O(α, γ)¹⁹Ne reaction rate is lower than usually assumed.

We present new high-resolution observations of Sagittarius A* at wavelengths of 17.4-23.8 cm with the Very Large Array in the A configuration with the Pie Town Very Long Baseline Array (VLBA) antenna. We use the measured sizes to calibrate the interstellar scattering law and find that the major axis size of the scattering law is smaller by ~6% than previous estimates. Using the new scattering law, we are able to determine the intrinsic size of Sgr A* at wavelengths from 0.35 to 6 cm using existing results from the VLBA. The new law increases the intrinsic size by ~20% at 0.7 cm and <5% at 0.35 cm. The intrinsic size is 13R_S at 0.35 cm and is proportional to λ^γ, where γ is in the range 1.3-1.7.

We quantitatively examine the significance of star formation triggered in the swept-up shell around an expanding H II region. If the swept-up molecular gas is sufficiently massive, new OB stars massive enough to repeat the triggering process will form in the shell. We determine the lower limit (M_thr) for the mass of the star that sweeps up the molecular gas, where at least one new star with mass M_* > M_thr forms after shell fragmentation. To calculate the threshold stellar mass M_thr, we examine how massive molecular shells can form around various central stars, by performing detailed numerical radiation hydrodynamics calculations. The mass of the photodissociated gas is generally larger than the mass of the photoionized gas. However, the swept-up molecular mass exceeds the photodissociated mass with a higher mass star of M_* ≳ 20 M_☉. The accumulated molecular mass generally increases with the stellar mass, and amounts to 10⁴-10⁵M_☉ for M_* ≳ 20 M_☉ with an ambient density of n ~ 10² cm^-3. The threshold stellar mass is M_thr ~ 18 M_☉ with a star formation efficiency of ~ 0.1 and n ~ 10² cm^-3. We examine the generality of this mode of runaway triggering for different sets of parameters and find that M_thr ~ 15-20 M_☉ in various situations. If the ambient density is too high or the star formation efficiency is too low, the triggering is not runaway, but a single event.

We report the discovery of excess 4.5 and 8 μm emission from three quiescent black hole low-mass X-ray binaries, A0620-00, GS 2023+338, and XTE J1118+480, and the lack of similar excess emission from Cen X-4. The mid-infrared emission from GS 2023+338 probably originates in the accretion disk. However, the excess emission from A0620-00 and XTE J1118+480 is brighter and peaks at longer wavelengths, and thus most likely originates from circumbinary dust that is heated by the light of the secondary star. For these two sources, we find that the inner edges of the dust distributions lie near 1.7 times the binary separations, which are the minimum radii at which circumbinary disks would be stable against tidal disruption. The excesses are weak at 24 μm, which implies that the dust does not extend beyond about 3 times the binary separations. The total masses of circumbinary material are between 10²² and 10²⁴ g. The material could be the remains of fallback disks produced in supernovae, or material from the companions injected into circumbinary orbits during mass transfer.

The binary X-ray pulsar A0535+262 was observed with the Suzaku X-ray observatory on 2005 September 14 for a net exposure of 22 ks. The source was in the declining phase of a minor outburst, exhibiting 3-50 keV luminosity of ~3.7 × 10³⁵ ergs s^-1 at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mcrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-Ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as that measured when the source is almost 2 orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X0331+53.

Neutral fluorine (F I) lines are identified in the optical spectra of cool extreme helium (EHe) stars. These are the first F I lines identified in a star's spectrum, and they provide the first measurement of fluorine abundances in EHe stars. The results show that fluorine is overabundant in EHe stars. The overabundance of fluorine provides us with evidence for the synthesis of fluorine in these stars, which is discussed in light of asymptotic giant branch evolution and the expectation from accretion of an He white dwarf by a C-O white dwarf.

The profile of the silicate 10 μm IR band contains important information about the evolutional stage of dust in circumstellar environments and the possible ongoing process of planetesimal formation. In order to extract this information, the observed band profiles are compared with calculated or laboratory-measured absorption cross sections of amorphous and crystalline grains with different sizes and compositions. We present in this study the first laboratory measurements of the 10 μm band profiles of nonembedded, i.e., free-flying, particles of amorphous and crystalline Mg₂SiO₄ (with two different particle shapes), amorphous and crystalline MgSiO₃, and crystalline olivine. We compare the spectra with those measured on embedded grains and discuss the potential of the new experimental method for comparison with observed spectra, as well as for future studies of agglomeration and surface manipulation of the grains.

We show that inclined magnetic field lines at the boundaries of large-scale convective cells (supergranules) provide "portals" through which low-frequency (<5 mHz) magnetoacoustic waves can propagate into the solar chromosphere. The energy flux carried by these waves at a height of 400 km above the solar surface is found to be a factor of 4 greater than that carried by the high-frequency (>5 mHz) acoustic waves, which are believed to provide the dominant source of wave heating of the chromosphere. This result opens up the possibility that low-frequency magnetoacoustic waves provide a significant source of energy for balancing the radiative losses of the ambient solar chromosphere.

We present the first results from three-dimensional spherical shell simulations of magnetic dynamo action realized by turbulent convection penetrating downward into a tachocline of rotational shear. This permits us to assess several dynamical elements believed to be crucial to the operation of the solar global dynamo, variously involving differential rotation resulting from convection, magnetic pumping, and amplification of fields by stretching within the tachocline. The simulations reveal that strong axisymmetric toroidal magnetic fields (about 3000 G in strength) are realized within the lower stable layer, unlike in the convection zone where fluctuating fields are predominant. The toroidal fields in the stable layer possess a striking persistent antisymmetric parity, with fields in the northern hemisphere largely of opposite polarity to those in the southern hemisphere. The associated mean poloidal magnetic fields there have a clear dipolar geometry, but we have not yet observed any distinctive reversals or latitudinal propagation. The presence of these deep magnetic fields appears to stabilize the sense of mean fields produced by vigorous dynamo action in the bulk of the convection zone.