Table of contents

The gravitational magnification and demagnification of Type Ia supernovae (SNe) modify their positions on the Hubble diagram, shifting the distance estimates from the underlying luminosity-distance relation. This can introduce a systematic uncertainty in the dark energy equation of state (EOS) estimated from SNe, although this systematic is expected to average away for sufficiently large data sets. Using mock SN samples over the redshift range 0 < z ⩽ 1.7, we quantify the lensing bias. We find that the bias on the dark energy EOS is less than half a percent for large data sets (≳2000 SNe). However, if highly magnified events (SNe deviating by more than 2.5 σ) are systematically removed from the analysis, the bias increases to ~0.8%. Given that the EOS parameters measured from such a sample have a 1 σ uncertainty of 10%, the systematic bias related to lensing in SN data out to z ∼ 1.7 can be safely ignored in future cosmological measurements.

By comparing a collisionless cosmological N-body simulation (DM) to a smoothed particle hydrodynamics simulation (SPH) with the same initial conditions, we investigate the correspondence between dark matter subhalos produced by collisionless dynamics and galaxies produced by dissipative gas dynamics in a dark matter background. When galaxies in the SPH simulation fall into larger groups and become satellites, they retain local dark matter concentrations (SPH subhalos) whose mass is typically 5 times the galaxy baryonic mass (compared to the simulation's universal ratio Ω_dm/Ω_b ≈ 7.5). The more massive subhalos of the SPH simulation generally have corresponding subhalos of similar mass and spatial position in the DM simulation; at lower masses, there is still fairly good correspondence, but some DM subhalos are in different spatial positions and some have suffered tidal stripping or disruption. The halo occupation statistics of DM subhalos—the mean number of subhalos, pairs, and triples as a function of host halo mass—are very similar to those of SPH subhalos and SPH galaxies. The gravity of the dissipative baryon component amplifies the density contrast of subhalos in the SPH simulation, making them more resistant to tidal disruption. Relative to SPH galaxies and SPH subhalos, the DM subhalo population is depleted in the densest regions of the most massive halos. The good agreement of halo occupation statistics between the DM subhalo and SPH galaxy populations leads to good agreement of their two-point correlation functions and higher order moments on large scales. The depletion of DM subhalos in dense regions depresses their clustering at R < 1 h⁻¹ Mpc. In these simulations, the "conversation" between dark matter and baryons is mostly one-way, with dark matter dynamics telling galaxies where to form and how to cluster, but the "back talk" of the baryons influences small-scale clustering by enhancing the survival of substructure in the densest environments.

We present results from an archival study of 70 medium-redshift QSOs observed with HST's WFPC2. The QSOs have magnitudes M_V ⩽ − 23 (total nuclear plus host light) and redshifts 0.06 ⩽ z ⩽ 0.46. The aim of the present study is to investigate the connections between the nuclear and host properties of QSOs, using high-resolution images and removing the central point source to reveal the host structure. We confirm that more luminous QSO nuclei are found in more luminous host galaxies. Using central black hole masses from the literature, we find that nuclear luminosity also generally increases with black hole mass, but it is not tightly correlated. Nuclear luminosities range from 2.3% to 200% of the Eddington limit. Those in elliptical hosts cover the range fairly evenly, while those in spirals are clustered near the Eddington limit. Using a principal components analysis, we find a kind of fundamental plane relating the nuclear luminosity to the size and effective surface magnitude of the bulge. Using optical nuclear luminosity, this relationship explains 96.1% of the variance in the overall sample, while another version of the relationship uses X-ray nuclear luminosity and explains 95.2% of the variance. The form of this QSO fundamental plane shows similarities to the well-studied fundamental plane of elliptical galaxies, and we examine the possible relationship between them, as well as the difficulties involved in establishing this connection.

The detection of large quantities of dust in z ∼ 6 quasars by infrared and radio surveys presents puzzles for the formation and evolution of dust in these early systems. Previously, Li et al. showed that luminous quasars at z≳ 6 can form through hierarchical mergers of gas-rich galaxies, and that these systems are expected to evolve from starburst through quasar phases. Here, we calculate the dust properties of simulated quasars and their progenitors using a three-dimensional Monte Carlo radiative transfer code, ART² (All-wavelength Radiative Transfer with Adaptive Refinement Tree). ART² incorporates a radiative equilibrium algorithm which treats dust emission self-consistently, an adaptive grid method which can efficiently cover a large dynamic range in both spatial and density scales, a multiphase model of the interstellar medium which accounts for the observed scaling relations of molecular clouds, and a supernova-origin model for dust which can explain the existence of dust in cosmologically young objects. By applying ART² to the hydrodynamic simulations of Li et al., we reproduce the observed spectral energy distribution (SED) and inferred dust properties of SDSS J1148+5251, the most distant Sloan quasar. We find that the dust and infrared emission are closely associated with the formation and evolution of the quasar host. The system evolves from a cold to a warm ultraluminous infrared galaxy (ULIRG) owing to heating and feedback from stars and the active galactic nucleus (AGN). Furthermore, the AGN activity has significant implications for the interpretation of observation of the hosts. Our results suggest that vigorous star formation in merging progenitors is necessary to reproduce the observed dust properties of z ∼ 6 quasars, supporting a merger-driven origin for luminous quasars at high redshifts and the starburst-to-quasar evolutionary hypothesis.

We present Very Long Baseline Array (VLBA) observations of the TeV blazars H1426+428, 1ES 1959+650, and PKS 2155–304 obtained during the years 2001-2004. We observed H1426+428 at four epochs at 8 GHz and found that its parsec-scale structure consisted of a ~17 mJy core and a single ~3 mJy jet component with an apparent speed of 2.09c ± 0.53c. The blazar 1ES 1959+650 was observed at three epochs at frequencies of 15 and 22 GHz. Spectral index information from these dual-frequency observations was used to definitively identify the core of the parsec-scale structure. PKS 2155–304 was observed at a single epoch at 15 GHz with dual-circular polarization, and we present the first VLBI polarimetry image of this source. For 1ES 1959+650 and PKS 2155–304, the current observations are combined with the VLBA observations from our earlier paper to yield improved apparent speed measurements for these sources with greatly reduced measurement errors. The new apparent speed measured for component C2 in 1ES 1959+650 is 0.00c ± 0.04c (stationary), and the new apparent speed measured for component C1 in PKS 2155–304 is 0.93c ± 0.31c. We combine the new apparent speed measurements from this paper with the apparent speeds measured in TeV blazar jets from our earlier papers to form a current set of apparent speed measurements in TeV high-frequency peaked BL Lac objects (HBLs). The mean peak apparent pattern speed in the jets of the TeV HBLs is about 1c. We conclude the paper with a detailed discussion of the interpretation of the collected VLBA data on TeV blazars in the context of current theoretical models for the parsec-scale structure of TeV blazar jets.

We present the first X-ray monitoring observations of the X-ray-bright FR I radio galaxy NGC 6251 observed with RXTE for 1 yr. The primary goal of this study is to shed light on the origin of the X-rays by investigating the spectral variability with model-independent methods coupled with time-resolved and flux-selected spectroscopy. The main results can be summarized as follows: (1) Throughout the monitoring campaign, NGC 6251 was in relatively high-flux state with an average 2-10 keV absorbed flux of the order of 4.5 × 10⁻¹² erg cm⁻² s⁻¹ and a corresponding intrinsic luminosity of 6 × 10⁴² erg s⁻¹. (2) The flux persistently changed with fluctuations of the order of ~2 on timescales of 20-30 days. (3) When the hardness ratio is plotted against the average count rate, there is evidence for a spectral hardening as the source brightens; this finding is confirmed by a flux-selected spectral analysis. (4) The fractional variability appears to be more pronounced in the hard energy band (5-12 keV) than in the soft one (2.5-5 keV). (5) Two month averaged and flux-limited energy spectra are adequately fitted by a power law. A Fe Kα line is never statistically required, although the presence of a strong iron line cannot be ruled out, due to the high upper limits on the line equivalent width. The inconsistency of the spectral variability behavior of NGC 6251 with the typical trend observed in Seyfert galaxies and the similarity with blazars lend support to a jet-dominated scenario during the RXTE monitoring campaign. However, a possible contribution from a disk-corona system cannot be ruled out.

We present a Very Long Baseline Interferometry image of the water maser emission in the nuclear region of NGC 3393. The maser emission has a linear distribution oriented at a position angle of ~–34°, perpendicular to both the kiloparsec-scale radio jet and the axis of the narrow-line region. The position-velocity diagram displays a red-blue asymmetry about the systemic velocity and the estimated dynamical center, and is thus consistent with rotation. Assuming Keplerian rotation in an edge-on disk, we obtain an enclosed mass of (3.1 ± 0.2) × 10⁷M_☉ within 0.36 ± 0.02 pc (1.48 ± 0.06 mas), which corresponds to a mean mass density of ~10^8.2M_☉ pc⁻³. We also report the measurement with the Green Bank Telescope of a velocity drift, a manifestation of centripetal acceleration within the disk, of 5 ± 1 km s⁻¹ yr⁻¹ in the ~3880 km s⁻¹ maser feature, which is most likely located along the line of sight to the dynamical center of the system. From the acceleration of this feature, we estimate a disk radius of 0.17 ± 0.02 pc, which is smaller than the inner disk radius (0.36 ± 0.02 pc) of emission that occurs along the midline (i.e., the line of nodes). The emission along the line of sight to the dynamical center evidently occurs much closer to the center than the emission from the disk midline, contrary to the situation in the archetypal maser systems NGC 4258 and NGC 1068. The outer radius of the disk as traced by the masers along the midline is about 1.5 pc.

Using the Green Bank Telescope, we conducted a "snapshot" survey for water maser emission toward the nuclei of 611 galaxies and detected eight new sources. The sample consisted of nearby (v < 5000 km s⁻¹) and luminous (M_B < − 19.5) galaxies, some with known nuclear activity but most not previously known to host AGNs. Our detections include both megamasers associated with AGNs and relatively low luminosity masers probably associated with star formation. The detection in UGC 3789 is particularly intriguing because the spectrum shows both systemic and high-velocity lines indicative of emission from an AGN accretion disk seen edge-on. Based on 6 months of monitoring, we detected accelerations among the systemic features ranging from 2 to 8 km s⁻¹ yr⁻¹, the larger values belonging to the most redshifted systemic components. High-velocity maser lines in UGC 3789 show no detectable drift over the same period. Although UGC 3789 was not known to be an AGN prior to this survey, the presence of a disk maser is strong evidence for nuclear activity, and an optical spectrum obtained later has confirmed it. With follow-up observations, it may be possible to measure a geometric distance to UGC 3789.

We applied the maximum likelihood (ML) method, as an image reconstruction algorithm, to the BAT X-Ray Survey (BXS). This method was specifically designed to preserve the full statistical information in the data and to avoid mosaicking of many exposures with different pointing directions, thus reducing systematic errors when co-adding images. We reconstructed, in the 14-170 keV energy band, the image of a 90 × 90 deg² sky region, centered on (R .A ., decl .) = (105°, − 25°) , which BAT surveyed with an exposure time of ~1 Ms (in 2005 November). The best sensitivity in our image is ~0.85 mcrab or 2.0 × 10⁻¹¹ ergs cm⁻². We detect 49 hard X-ray sources above the 4.5 σ level; of these, only 12 were previously known as hard X-ray sources (>15 keV). Swift XRT observations allowed us to firmly identify the counterparts for 15 objects, while 2 objects have Einstein IPC counterparts (Harris et al. 1990); in addition to those, we found a likely counterpart for 13 objects by correlating our sample with the ROSAT All-Sky Survey Bright Source Catalog (Voges et al. 1999). Seven objects remain unidentified. Analysis of the noise properties of our image shows that ~75% of the area is surveyed to a flux limit of ~1 mcrab. This study shows that the coupling of the ML method to the most sensitive, all-sky surveying, hard X-ray instrument, BAT, is able to probe for the first time the hard X-ray sky to the millicrab flux level. The successful application of this method to BAT demonstrates that it could also be applied with advantage to similar instruments such as INTEGRAL IBIS.

We study galaxies that host both nuclear star clusters and AGNs, implying the presence of a massive black hole. We select a sample of 176 galaxies with previously detected nuclear star clusters that range from ellipticals to late-type spirals. We search for AGNs in this sample using optical spectroscopy and archival radio and X-ray data. We find galaxies of all Hubble types and with a wide range of masses (10⁹-10¹¹M_☉) hosting both AGNs and nuclear star clusters. From the optical spectra, we classify 10% of the galaxies as AGN and an additional 15% as composite, indicating a mix of AGN and star formation spectra. The fraction of nucleated galaxies with AGNs increases strongly as a function of galaxy and nuclear star cluster mass. For galaxies with both a nuclear star cluster and a black hole, we find that the masses of these two objects are quite similar. However, nondetections of black holes in Local Group nuclear star clusters show that not all clusters host black holes of similar masses. We discuss the implications of our results for the formation of nuclear star clusters and massive black holes.

MOND predicts that a mass, M, contained within its transition radius, r_t ≡ (MG/a₀)^1/2, may exhibit a feature at about that radius in the form of a shell, or projected ring, in the deduced distribution of its phantom dark matter. This is despite the absence of any underlying feature in the true ("baryon") source distribution itself. The phenomenon is similar to the appearance of an event horizon and other unusual physics "in the middle of nothing" near the transition radius of general relativity, which is MG/c². We consider the possibility that this pure MOND phenomenon is in the basis of the recent finding of such a ring in the galaxy cluster Cl 0024+17 by Jee and coworkers. We find that the parameters of the observed ring can be naturally explained in this way; this feature may therefore turn out to be direct evidence for MOND. We study this phenomenon in simple, axisymmetric configurations aligned with the line of sight: spherical masses, a dumbbell of spherical masses, and an elongated, thin structure. The properties of the apparent ring, including its radius, surface density, and contrast, depend on the form of the MOND interpolating function and on the exact three-dimensional distribution of the sources (the thin-lens approximation is quite invalid in MOND). We also comment on the possible appearance of orphan features, marking the Newtonian-to-MOND transition, in high surface brightness galaxies. In particular, we find that previously unexplained structure in the rotation curves of some galaxies may be evidence for such features.

Traditional photometric redshift methods use only color information about the objects in question to estimate their redshifts. This paper introduces a new method utilizing colors, luminosity, surface brightness, and radial light profile to measure the redshifts of galaxies in the Sloan Digital Sky Survey (SDSS). We take a statistical approach: distributions of galaxies from the SDSS large-scale structure (LSS; spectroscopic) sample are constructed at a range of redshifts, and target galaxies are compared to these distributions. An adaptive mesh is implemented to increase the percentage of the parameter space populated by the LSS galaxies. We test the method on a subset of galaxies from the LSS sample, yielding rms Δz of 0.025 for red galaxies and 0.030 for blue galaxies (all with z < 0.25). Possible future improvements to this promising technique are described, as is our ongoing work to extend the method to galaxies at higher redshift.

Using a hydrodynamic adaptive mesh refinement code, we simulate the growth and evolution of a galaxy, which could potentially host a supermassive black hole, within a cosmological volume. Reaching a dynamical range in excess of 10 million, the simulation follows the evolution of the gas structure from supergalactic scales all the way down to the outer edge of the accretion disk. Here we focus on global instabilities in the self-gravitating, cold, turbulence-supported, molecular gas disk at the center of the model galaxy, which provide a natural mechanism for angular momentum transport down to subparsec scales. The gas density profile follows a power law ∝ r^−8/3, consistent with an analytic description of turbulence in a quasi-stationary circumnuclear disk. We analyze the properties of the disk which contribute to the instabilities and investigate the significance of instability for the galaxy's evolution and the growth of a supermassive black hole at the center.

We measure F814W surface brightness fluctuations (SBFs) for a sample of distant shell galaxies with radial velocities ranging from 4000 to 8000 km s⁻¹. The distance at galaxies is then evaluated by using the SBF method. For this purpose, theoretical SBF magnitudes for the HST ACS filters are computed for single-burst stellar populations covering a wide range of ages (t = 1.5–14 Gyr) and metallicities (Z = 0.008–0.04). Using these stellar population models, we provide the first _F814W versus (F 475W − F 814W)₀ calibration and we extend the previous M₁ versus (B − I)₀ color relation to colors (B − I)₀ ⩽ 2.0 mag. Coupling our SBF measurements with the theoretical calibration we derive distances with a statistical uncertainty of ~8%, and systematic error of ~6%. The procedure developed to analyze data ensures that the indetermination due to possible unmasked residual shells is well below ~12%. The results suggest that optical SBFs can be measured at d ⩾ 100 Mpc with HST ACS imaging. SBF-based distances coupled with recession velocities corrected for peculiar motion, allow us obtain H₀ = 76 ± 6 (statistical) ±5 (systematic) km s⁻¹ Mpc⁻¹.

The Antennae galaxies are the closest example of an ongoing major galaxy merger and, as such, represent a unique laboratory for furthering the understanding of the formation of exotic objects (e.g., tidal dwarf galaxies, ultraluminous X-ray sources, super stellar clusters). In a previous paper HST WFPC2 observations were used to demonstrate that the Antennae system might be at a distance considerably less than that conventionally assumed in the literature. Here we report new, much deeper HST ACS imaging that resolves the composite stellar populations and, most importantly, reveals a well-defined red giant branch. The tip of this red giant branch (TRGB) is unambiguously detected at I₀^TRGB = 26.65 ± 0.09 mag. Adopting the most recent calibration of the luminosity of the TRGB then yields a distance modulus for the Antennae of (m − M)₀ = 30.62 ± 0.17 corresponding to a distance of 13.3 ± 1.0 Mpc. This is consistent with our earlier result, once the different calibrations for the standard candle are considered. We briefly discuss the implications of this now well-determined shorter distance.

We present several different statistical methods to determine the transverse velocity vector of M31. The underlying assumptions are that the M31 satellites on average follow the motion of M31 through space and that the galaxies in the outer parts of the Local Group on average follow the motion of the Local Group barycenter through space. We apply the methods to the line-of-sight velocities of 17 M31 satellites, to the proper motions of the two satellites M33 and IC 10, and to the line-of-sight velocities of five galaxies near the Local Group turnaround radius, respectively. This yields four independent but mutually consistent determinations of the heliocentric M31 transverse velocities in the west and north directions, with weighted averages ⟨v_W⟩ = − 78 ± 41 km s⁻¹ and ⟨v_N⟩ = − 38 ± 34 km s⁻¹. The uncertainties correspond to proper motions of ~10 μas yr⁻¹, which will be difficult to verify observationally within the next decade. The galactocentric tangential velocity of M31 is 42 km s⁻¹, with 1 σ confidence interval V_tan ⩽ 56 km s⁻¹. The implied M31-Milky Way orbit is bound if the total Local Group mass M exceeds 1.72_−0.25^+0.26 × 10¹²M_☉. If the orbit is indeed bound, then the timing argument combined with the known age of the universe implies that M = 5.58_−0.72^+0.85 × 10¹²M_☉. This is on the high end of the allowed mass range suggested by cosmologically motivated models for the individual structure and dynamics of M31 and the Milky Way, respectively. It is therefore possible that the timing mass is an overestimate of the true mass, especially if one takes into account recent results from the Millenium Simulation that show that there is also a theoretical uncertainty of 41% (Gaussian dispersion) in timing mass estimates. The M31 transverse velocity implies that M33 is in a tightly bound orbit around M31. This may have led to some tidal deformation of M33. It will be worthwhile to search for observational evidence of this.

We used Spitzer infrared observations to find the young stars of two H II regions in the Large Magellanic Cloud, N63 and N180. The young stellar object (YSO) candidates were identified in each nebula by means of color-color, color-magnitude diagrams, and the shapes of their spectral energy distributions (SEDs). The most luminous YSOs are found near the ionization fronts within strong 8 μm emission clumps. Most YSOs, less luminous, are seen in projection inside the H II regions. HST images show several Class I stars that have emerged along the borders of the H II regions; other YSOs are embedded in cometary clouds. The most luminous YSO of N63 is connected to a string of pointlike sources. Its SED can be modeled by a central source of stellar mass M_⋆ between 7 and 11 M_☉, with a circumstellar disk of outer radius R_d of ~55 AU, and an envelope of moderate accretion rate, of ~2 × 10⁻⁵M_☉ yr⁻¹. N180 is experiencing a phase of star formation more intense than N63, attested by the properties of its most luminous YSO: M_⋆ of 25 M_☉, R_d of ~200 AU, and of ~1.5 × 10⁻³M_☉ yr⁻¹. The modes of triggered star formation in N63 and N180 appear similar to those seen in Galactic H II regions.

We present a new analysis of the diffuse gas in the Magellanic Bridge (R.A. ≳ 3^h) based on HST STIS E140M and FUSE spectra of two early-type stars lying within the Bridge and a QSO behind it. We derive the column densities of the H I (from Lyα), N I, O I, Ar I, Si II, S II, and Fe II of the gas in the Bridge. Using the atomic species, we determine the first gas-phase metallicity of the Magellanic Bridge, [Z/H] = − 1.02 ± 0.07 toward one sight line and -1.7 < [Z/H] < − 0.9 toward the other, a factor of 2 or more smaller than the present-day SMC metallicity. Using the metallicity and N(H I), we show that the Bridge gas along our three lines of sight is ~70%-90% ionized, despite high H I columns, log N(H I) ≃ 19.6 − 20.1. Possible sources for the ongoing ionization are certainly the hot stars within the Bridge, hot gas (revealed by O VI absorption), and leaking photons from the SMC and LMC. From the analysis of C II*, we deduce that the overall density of the Bridge must be low (<0.03-0.1 cm⁻³). We argue that our findings combined with other recent observational results should motivate new models of the evolution of the SMC-LMC-Galaxy system.

We present numerical simulations of the growth and saturation of the Kelvin-Helmholtz instability in a compressible fluid layer with and without a weak magnetic field. In the absence of a magnetic field, the instability generates a single eddy that flattens the velocity profile, stabilizing it against further perturbations. Adding a weak magnetic field—weak in the sense that it has almost no effect on the linear instability—leads to a complex flow morphology driven by MHD forces and to enhanced broadening of the layer due to Maxwell stresses. We corroborate earlier studies, which showed that magnetic fields destroy the large-scale eddy structure through periodic cycles of windup and resistive decay, but we show that the rate of decay decreases with decreasing plasma resistivity η, at least within the range of η accessible to our simulations. Magnetization increases the efficiency of momentum transport, and the transport increases with decreasing η.

We present a systematic study of the HNCO, C¹⁸O,¹³CS, and C³⁴S emission toward 13 selected molecular clouds in the Galactic center region. The molecular emission in these positions is used as a template of the different physical and chemical processes claimed to be dominant in the circumnuclear molecular gas of galaxies. The relative abundance of HNCO shows a variation of more than a factor of 20 among the observed sources. The HNCO/¹³CS abundance ratio is highly contrasted (up to a factor of 30) between the shielded molecular clouds mostly affected by shocks, where HNCO is released to gas phase from grain mantles, and those pervaded by an intense UV radiation field, where HNCO is photodissociated and CS production favored via ion reactions. We propose the relative HNCO to CS abundance ratio as a highly contrasted diagnostic tool to distinguish between the influence of shocks and/or the radiation field in the nuclear regions of galaxies and their relation to the evolutionary state of their nuclear star formation bursts.

We have studied the nonresonant streaming instability of charged energetic particles moving through a background plasma, discovered by Bell. We confirm his numerical results regarding a significant magnetic field amplification in the system. A detailed physical picture of the instability development and of the magnetic field evolution is given.

The TRACER instrument (Transition Radiation Array for Cosmic Energetic Radiation) has been developed for direct measurements of the heavier primary cosmic-ray nuclei at high energies. The instrument had a successful long-duration balloon flight in Antarctica in 2003. The detector system and measurement process are described, details of the data analysis are discussed, and the individual energy spectra of the elements O, Ne, Mg, Si, S, Ar, Ca, and Fe (nuclear charge Z = 8-26) are presented. The large geometric factor of TRACER and the use of a transition radiation detector make it possible to determine the spectra up to energies in excess of 10¹⁴ eV per particle. A power-law fit to the individual energy spectra above 20 GeV amu⁻¹ exhibits nearly the same spectral index (2.65 ± 0.05) for all elements, without noticeable dependence on the elemental charge Z.

We explore the importance of magnetic-field-oriented thermal conduction in the interaction of supernova remnant (SNR) shocks with radiative gas clouds and in determining the mass and energy exchange between the clouds and the hot surrounding medium. We perform 2.5-dimensional MHD simulations of a shock impacting on an isolated gas cloud, including anisotropic thermal conduction and radiative cooling; we consider the representative case of a Mach 50 shock impacting on a cloud 10 times denser than the ambient medium. We consider different configurations of the ambient magnetic field and compare MHD models with or without thermal conduction. The efficiency of thermal conduction in the presence of a magnetic field is, in general, reduced with respect to the unmagnetized case. The reduction factor strongly depends on the initial magnetic field orientation, and it is at a minimum when the magnetic field is initially aligned with the direction of the shock propagation. Thermal conduction contributes to the suppression of hydrodynamic instabilities, reducing the mass mixing of the cloud and preserving the cloud from complete fragmentation. Depending on the magnetic field orientation, the heat conduction may determine a significant energy exchange between the cloud and the hot surrounding medium which, while remaining always at levels less than those in the unmagnetized case, leads to a progressive heating and evaporation of the cloud. This additional heating may offset the radiative cooling of some parts of the cloud, preventing the onset of thermal instabilities.

We present a 3 yr series of observations at 24 μm with the Spitzer Space Telescope of the interstellar material in a 200' × 200' square area centered on Cassiopeia A. Interstellar dust heated by the outward light pulse from the supernova explosion emits in the form of compact, moving features. Their sequential outward movements allow us to study the complicated three-dimensional structure of the interstellar medium (ISM) behind and near Cassiopeia A. The ISM consists of sheets and filaments, with many structures on a scale of a parsec or less. The spatial power spectrum of the ISM appears to be similar to that of fractals with a spectral index of 3.5. The filling factor for the small structures above the spatial wavenumber k ∼ 0.5 cycles pc⁻¹ is only ~0.4%.

We report on the discovery of fast-moving X-ray-emitting ejecta knots in the Galactic oxygen-rich supernova remnant Puppis A from XMM-Newton observations. We find an X-ray knotty feature positionally coincident with an O-rich fast-moving optical filament with blueshifted line emission located in the northeast of Puppis A. We extract spectra from northern and southern regions of the feature. Applying a one-component nonequilibrium ionization model for the two spectra, we find high metal abundances relative to the solar values in both spectra. This fact clearly shows that the feature originates from metal-rich ejecta. In addition, we find that line emission in the two regions is blueshifted. The Doppler velocities derived in the two regions are different with each other, suggesting that the knotty feature consists of two knots that are close to each other along the line of sight. Since fast-moving O-rich optical knots/filaments are believed to be recoiled metal-rich ejecta, expelled to the opposite direction against the high-velocity central compact object, we propose that the ejecta knots disclosed here are also part of the recoiled material.

We report high angular resolution VLA observations of cyanopolyyne molecules HC₃N and HC₅N from the carbon rich circumstellar envelope of IRC+10216. The observed low-lying rotational transitions trace a much more extended emitting region than seen in previous observations at higher frequency transitions. We resolve the hollow quasi-spherical distribution of the molecular emissions into a number of clumpy shells. These molecular shells coincide spatially with dust arcs seen in deep optical images of the IRC+10216 envelope, allowing us to study for the first time the kinematics of these features. We find that the molecular and dust shells represent the same density enhancements in the envelope separated in time by ~120 to ~360 yr. From the angular size and velocity spread of the shells, we estimate that each shell typically covers about 10% of the stellar surface at the time of ejection. The distribution of the shells seems to be random in space. The good spatial correspondence between HC₃N and HC₅N emissions is in qualitative agreement with a recent chemical model that takes into account the presence of density-enhanced shells. The broad spatial distribution of the cyanopolyyne molecules, however, would necessitate further study on their formation.

The pure rotational spectra of phenanthridine, acridine, and 1,10-phenanthroline, small polycyclic aromatic nitrogen heterocycle molecules (PANHs), have been measured and assigned from 2 to 85 GHz. An initial spectral assignment, guided by ab initio molecular orbital predictions, employed broadband Stark modulated millimeter wave absorption spectroscopy of a supersonic rotationally cold molecular beam, yielding a preliminary set of rotational and centrifugal distortion constants. Subsequent spectral analysis employed Fourier transform microwave (FT-MW) spectroscopy of a supersonic rotationally cold molecular beam. The extremely high spectral resolution of the FT-MW instrument yielded improved rotational constants and centrifugal distortion constants, together with nitrogen quadrupole coupling constants, for all three species. Density functional theory (DFT) calculations at the B3LYP level of theory employing the cc-pVTZ and 6-311+G** basis sets are shown to closely predict rotational constants and to be useful in predicting quadrupole coupling constants and dipole moments for such PANH species. The data presented here will be useful for deep radio astronomical searches for PANHs employing large radio telescopes.

The mid-infrared spectra of large PAHs ranging from C₅₄H₁₈ to C₁₃₀H₂₈ are determined computationally using density functional theory. Trends in the band positions and intensities as a function of PAH size, charge, and geometry are discussed. Regarding the 3.3, 6.3, and 11.2 μm bands similar conclusions hold as with small PAHs. This does not hold for the other features. The larger PAH cations and anions produce bands at 7.8 μm and, as PAH sizes increases, a band near 8.5 μm becomes prominent and shifts slightly to the red. In addition, the average anion peak falls slightly to the red of the average cation peak. The similarity in behavior of the 7.8 and 8.6 μm bands with the astronomical observations suggests that they arise from large, cationic and anionic PAHs, with the specific peak position and profile reflecting the PAH cation to anion concentration ratio and relative intensities of PAH size. Hence, the broad astronomical 7.7 μm band is produced by a mixture of small and large PAH cations and anions, with small and large PAHs contributing more to the 7.6 and 7.8 μm components, respectively. For the CH out-of-plane vibrations, the duo hydrogens couple with the solo vibrations and produce bands that fall at wavelengths slightly different than their counterparts in smaller PAHs. As a consequence, previously deduced PAH structures are altered in favor of more compact and symmetric forms. In addition, the overlap between the duo and trio bands may reproduce the blue-shaded 12.8 μm profile.

We report on a recent spectral line survey of the planetary nebula (PN) NGC 7027 using the Arizona Radio Observatory (ARO) 12 m telescope and the Heinrich Hertz Submillimeter Telescope (SMT) at millimeter wavelengths. The spectra covering the frequency ranges 71-111, 157-161, and 218-267 GHz were obtained with a typical sensitivity of rms < 8 mK. A total of 67 spectral lines are detected, 21 of which are identified with 8 molecular species, 32 with recombination lines from hydrogen and helium, and 14 remain unidentified. As the widths of emission lines from CO, other neutral molecules, molecular ions, and recombination of H⁺ and He⁺ are found to be different from each other, the line strengths and profiles are used to investigate the physical conditions and chemical processes of the neutral envelope of NGC 7027. The column densities and fractional abundances relative to the H₂ of the observed molecular species are calculated and compared with predictions from chemical models. We found evidence for overabundance of N₂H⁺ and underabundance of CS and HNC in NGC 7027, suggesting that X-ray emission and shock waves may play an important role in the chemistry of the hot molecular envelope of the young PN.

We explore how the expulsion of gas from star cluster forming cloud cores due to supernova explosions affects the shape of the initial cluster mass function, that is, the mass function of star clusters when effects of gas expulsion are over. We demonstrate that if the radii of cluster-forming gas cores are roughly constant over the core mass range, as supported by observations, then more massive cores undergo slower gas expulsion. Therefore, for a given star formation efficiency, more massive cores retain a larger fraction of stars after gas expulsion. The initial cluster mass function may thus differ from the core mass function substantially, with the final shape depending on the star formation efficiency. A mass-independent star formation efficiency of ~20% turns a power-law core mass function into a bell-shaped initial cluster mass function, while mass-independent efficiencies of order 40% preserve the shape of the core mass function.

It has been argued that the observational limits on a supernova (SN) associated with GRB 060614 convincingly exclude a SN akin to SN 1998bw as its originator and provide evidence for a new class of long-duration GRBs. We discuss this issue in the contexts of indirect "redshift estimators" and of the fireball and cannonball models of GRBs. The latter explains the unusual properties of GRB 060614: at its debated but favored low redshift (0.125), they are predicted, as opposed to exceptional, if the associated core-collapse SN is of a recently discovered, very faint type. We take the occasion to discuss the association between GRBs and SNe.

We examine neutron star properties based on a model of dense matter composed of B = 1 skyrmions immersed in a mesonic mean field background. The model realizes spontaneous chiral symmetry breaking nonlinearly and incorporates scale breaking of QCD through a dilaton vacuum expectation value that also affects the mean fields. Quartic self-interactions among the vector mesons are introduced on grounds of naturalness in the corresponding effective field theory. Within a plausible range of the quartic couplings, the model generates neutron star masses and radii that are consistent with a preponderance of observational constraints, including recent ones that point to the existence of relatively massive neutron stars M ∼ 1.7 M_☉ and radii R ∼ 12–14 km. If the existence of neutron stars with such dimensions is confirmed, matter at supranuclear density is stiffer than extrapolations of most microscopic models suggest.

We report infrared observations of the microquasar GRS 1915+105 using the NICMOS instrument of the Hubble Space Telescope during nine visits in 2003 April-June. During epochs of high X-ray/radio activity near the beginning and end of this period, we find that the 1.87 μm infrared flux is generally low (~2 mJy) and relatively steady. However, during the X-ray/radio "plateau" state between these epochs, we find that the infrared flux is significantly higher (~4-6 mJy), and strongly variable. In particular, we find events with amplitudes ~20%-30% occurring on timescales of ~10-20 s (e-folding timescales of ~30 s). These flickering timescales are several times faster than any previously observed infrared variability in GRS 1915+105 and the IR variations exceed corresponding X-ray variations at the same (~8 s) timescale. These results suggest an entirely new type of infrared variability from this object. Based on the properties of this flickering, we conclude that it arises in the plateau-state jet outflow itself, at a distance <2.5 AU from the accretion disk. We discuss the implications of this work and the potential of further flickering observations for understanding jet formation around black holes.

We present optical and X-ray time series photometry of EI UMa that reveal modulation at 746 and 770 s, which we interpret as the white dwarf spin and spin-orbit sidebands. These detections, combined with previous X-ray studies, establish EI UMa as an intermediate polar. We estimate the mass accretion rate to be ~3.6 × 10¹⁷ g s⁻¹, which is close to, and likely greater than, the critical rate above which dwarf nova instabilities are suppressed. We also estimate the white dwarf to have a large magnetic moment μ > (3.4 ± 0.2) × 10³³ G cm³. The high mass accretion rate and magnetic moment imply the existence of an accretion ring rather than a disk, and along with the relatively long orbital period, these suggest that EI UMa is a rare example of a prepolar cataclysmic variable.

X-ray spectra from stellar coronae are reprocessed by the underlying photosphere through scattering and photoionization events. While reprocessed X-ray spectra reaching a distant observer are at a flux level of only a few percent of that of the corona itself, characteristic lines formed by inner shell photoionization of some abundant elements can be significantly stronger. The emergent photospheric spectra are sensitive to the distance and location of the fluorescing radiation and can provide diagnostics of coronal geometry and abundance. Here we present Monte Carlo simulations of the photospheric K α₁,α₂ doublet arising from quasi-neutral Fe irradiated by a coronal X-ray source. Fluorescent line strengths have been computed as a function of the height of the radiation source, the temperature of the ionizing X-ray spectrum, and the viewing angle. We also illustrate how the fluorescence efficiencies scale with the photospheric metallicity and the Fe abundance. Based on the results we make three comments: (1) fluorescent Fe lines seen from pre-main-sequence stars mostly suggest flared disk geometries and/or supersolar disk Fe abundances; (2) the extreme ≈1400 mÅ line observed from a flare on V1486 Ori could be explained entirely by X-ray fluorescence if the flare itself were partially eclipsed by the limb of the star; and (3) the fluorescent Fe line detected by Swift during a large flare on II Peg is consistent with X-ray excitation and does not require a collisional ionization contribution. There is no convincing evidence supporting the energetically challenging explanation of electron impact excitation for observed stellar Fe Kα lines.

We present an analysis of the atmospheric properties of the evolved, hydrogen-rich object GD 605 using FUSE, IUE, and optical spectra in conjunction with non-LTE (NLTE) model atmospheres and synthetic spectra. We also present an analysis of the interstellar medium along the line of sight toward this star. Our effective temperature determination relies on the constraints on the ionization balance O IV/O V imposed by the FUSE data, while the surface gravity relies on a match to the Balmer lines in the optical spectrum. Our analysis yields T_eff ∼ 85,000 K, log g ∼ 5.25, and a helium abundance close to the solar value. These parameters suggest that GD 605 is in a post-AGB evolutionary phase and belongs to the class of hydrogen-rich central star of planetary nebulae, subclass O(H). Apart from lines of hydrogen and helium, about two dozen photospheric lines are observed in the FUSE data, which are dominated by the O VI λλ1031.9 and 1037.6 transitions. In addition, we detect lines associated with the following ions: N IV, O IV, O V, Si IV, S VI, Ar VII, as well as Fe VII. Synthetic spectra based on NLTE line-blanketed model atmospheres reproduce most of the line profiles observed in what appears to be an atmosphere deficient in heavy elements. Our calculations do not fully reproduce the strength of the strongest ultraviolet lines seen, the O VI doublet, perhaps a sign that some contribution to this structure may arise in the interstellar medium or in a circumstellar environment. We discuss various scenarios to account for the absence of a visible nebula and the dearth of heavy elements in the atmosphere of GD 605.

We model the detailed time-evolution of discrete absorption components (DACs) observed in P Cygni profiles of the Si IV λ1400 resonance doublet lines of the fast-rotating supergiant HD 64760 (B0.5 Ib). We adopt the common assumption that the DACs are caused by corotating interaction regions (CIRs) in the stellar wind. We perform 3D radiative transfer calculations with hydrodynamic models of the stellar wind that incorporate these large-scale density- and velocity-structures. We develop the 3D transfer code Wind3D to investigate the physical properties of CIRs with detailed fits to the DAC shape and morphology. We constrain the properties of large-scale wind structures with detailed fits to DACs observed in HD 64760. A model with two spots of unequal brightness and size on opposite sides of the equator—20% ± 5% and 8% ± 5% brighter than the stellar surface and with opening angles of 20° ± 5° and 30° ± 5° diameter, respectively—provides the best fit to the observed DACs. The recurrence time of the DACs compared to the estimated rotational period corresponds to spot velocities that are 5 times slower than the rotational velocity. The mass-loss rate of the structured-wind model for HD 64760 does not exceed the rate of the spherically symmetric smooth-wind model by more than 1% . The fact that DACs are observed in a large number of hot stars constrains the clumping that can be present in their winds, as substantial amounts of clumping would tend to destroy the CIRs.

As part of an effort to understand the origin of open clusters, we present a statistical analysis of the currently observed Pleiades. Starting with a photometric catalog of the cluster, we employ a maximum likelihood technique to determine the mass distribution of its members, including single stars and both components of binary systems. We find that the overall binary fraction for unresolved pairs is 68%. Extrapolating to include resolved systems, this fraction climbs to about 76%, significantly higher than the accepted field star result. Both figures are sensitive to the cluster age, for which we have used the currently favored value of 125 Myr. The primary and secondary masses within binaries are correlated, in the sense that their ratios are closer to unity than under the hypothesis of random pairing. We map out the spatial variation of the cluster's projected and three-dimensional mass and number densities. Finally, we revisit the issue of mass segregation in the Pleiades. We find unambiguous evidence of segregation, and introduce a new method for quantifying it.

In order to establish Vega's absolute rotational velocity, v_e, as distinct from the inclination angle, i, we conducted a detailed profile study on a large number (~200) of weak lines based on very high signal-to-noise ratio spectrum data, making use of the fact that the lines' characteristic shapes may contain information about rotation-induced gravity darkening. To this end, we developed computer programs to simulate the surface of a rapid rotator with latitude-dependent atmospheric parameters, from which the spectral energy distribution (SED) and the detailed line profile can be computed. Having restricted the freedom of the parameters according to the requirements of the SED, we concluded by comparing the observed and theoretical profiles that v_e≃ 175 km s⁻¹ (with i≃ 7°) is the best solution. It was also found that the abundances derived from lines showing peculiar flat-bottom shapes (e.g., Fe I lines) tend to be overestimated by up to ~0.2 dex when the conventional method of analysis using classical model atmospheres is applied, although this effect is less significant for lines showing normal profiles (e.g., high-excitation Fe II lines).

We report a dynamical measurement of the mass of the brown dwarf GJ 802B using aperture-masking interferometry and astrometry. In addition, we report the discovery that GJ 802A is itself a close spectroscopic noneclipsing binary with a 19 hr period. We find the mass of GJ 802B to be 0.063 ± 0.005 M_☉. GJ 802 has kinematics inconsistent with a young star and more consistent with the thick-disk population, implying a system age of ~10 Gyr. However, model evolutionary tracks for GJ 802B predict system ages of ~2 Gyr, suggesting that brown dwarf evolutionary models may be underestimating luminosity for old brown dwarfs.

We describe a new radial velocity survey of T Tauri stars and present the first results. Our search is motivated by an interest in detecting massive young planets, as well as investigating the origin of the brown dwarf desert. As part of this survey, we discovered large-amplitude, periodic, radial velocity variations in the spectrum of the weak-line T Tauri star LkCa 19. Using line bisector analysis and a new simulation of the effect of starspots on the photometric and radial velocity variability of T Tauri stars, we show that our measured radial velocities for LkCa 19 are fully consistent with variations caused by the presence of large starspots on this rapidly rotating young star. These results illustrate the level of activity-induced radial velocity noise associated with at least some very young stars. This activity-induced noise will set lower limits on the mass of a companion detectable around LkCa 19 and similarly active young stars.

We investigate the tidal interaction between a low-mass planet and a self-gravitating protoplanetary disk by means of two-dimensional hydrodynamic simulations. We first show that considering a planet as freely migrating in a disk without self-gravity leads to a significant overestimate of the migration rate. The overestimate can reach a factor of 2 for a disk having 3 times the surface density of the minimum mass solar nebula. Unbiased drift rates may be obtained only by considering a planet and a disk orbiting within the same gravitational potential. In the second part, the disk self-gravity is taken into account. We confirm that the disk gravity enhances the differential Lindblad torque with respect to the situation where neither the planet nor the disk feels the disk gravity. This enhancement only depends on the Toomre parameter at the planet location. It is typically 1 order of magnitude smaller than the spurious one induced by assuming a planet migrating in a disk without self-gravity. We confirm that the torque enhancement due to the disk gravity can be entirely accounted for by a shift of Lindblad resonances and can be reproduced by the use of an anisotropic pressure tensor. We do not find any significant impact of the disk gravity on the corotation torque.

We have investigated the formation of close-in extrasolar giant planets through a coupling effect of mutual scattering, the Kozai mechanism, and tidal circularization, by orbital integrations. Close-in gas giants would have been originally formed at several AU beyond the ice lines in protoplanetary disks and migrated close to their host stars. Although type II migration due to planet-disk interactions may be a major channel for the migration, we show that this scattering process would also give a nonnegligible contribution. We carried out orbital integrations of three planets with Jupiter mass, directly including the effect of tidal circularization. We have found that in about 30% of the runs close-in planets are formed, which is much higher than suggested by previous studies. Three-planet orbit crossing usually results in the ejection of one or two planets. Tidal circularization often occurs during three-planet orbit crossing, but previous studies have monitored only the final stage after the ejection, significantly underestimating the formation probability. We have found that the Kozai mechanism in outer planets is responsible for the formation of close-in planets. During three-planet orbital crossing, Kozai excitation is repeated and the eccentricity is often increased secularly to values close enough to unity for tidal circularization to transform the inner planet to a close-in planet. Since a moderate eccentricity can retain for the close-in planet, this mechanism may account for the observed close-in planets with moderate eccentricities and without nearby secondary planets. Since these planets also remain a broad range of orbital inclinations (even retrograde ones), the contribution of this process would be clarified by more observations of Rossiter-McLaughlin effects for transiting planets.

Radio observations at 210 GHz taken by the Bernese Multibeam Radiometer for KOSMA (BEMRAK) are combined with hard X-ray and γ-ray observations from the SONG instrument on board CORONA-F and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to investigate high-energy particle acceleration during the energetic solar flare of 2003 October 28. Two distinct components at submillimeter wavelengths are found. The first is a gradual, long-lasting (>30 minutes) component with large apparent source sizes (~60''). Its spectrum below ~200 GHz is consistent with synchrotron emission from flare-accelerated electrons producing hard X-ray and γ-ray bremsstrahlung assuming a magnetic field strength of ≥200 G in the radio source and a confinement time of the radio-emitting electrons in the source of less than 30 s. The other component is impulsive and starts simultaneously with high-energy (>200 MeV nucleon⁻¹) proton acceleration and the production of pions. The derived radio source size is compact (≤10''), and the emission is cospatial with the location of precipitating flare-accelerated >30 MeV protons as seen in γ-ray imaging. The close correlation in time and space of radio emission with the production of pions suggests that synchrotron emission of positrons produced in charged-pion decay might be responsible for the observed compact radio source. However, order-of-magnitude approximations rather suggest that the derived numbers of positrons from charged-pion decay are probably too small to account for the observed radio emission. Synchrotron emission from energetic electrons therefore appears as the most likely emission mechanism for the compact radio source seen in the impulsive phase, although it does not account for its close correlation, in time and space, with pion production.

Prominence cavities in coronal helmet streamers are readily detectable in white-light coronagraph images, yet their interpretation may be complicated by projection effects. In order to determine a cavity's density structure, it is essential to quantify the contribution of noncavity features along the line of sight. We model the coronal cavity as an axisymmetric torus that encircles the Sun at constant latitude and fit it to observations of a white-light cavity observed by the Mauna Loa Solar Observatory (MLSO) MK4 coronagraph from 2006 January 25 to 30. We demonstrate that spurious noncavity contributions (including departures from axisymmetry) are minimal enough to be incorporated in a density analysis as conservatively estimated uncertainties in the data. We calculate a radial density profile for cavity material and for the surrounding helmet streamer (which we refer to as the "cavity rim") and find that the cavity density is depleted by a maximum of 40% compared to the surrounding helmet streamer at low altitudes (1.18 R_☉) but is consistently higher (double or more) than in coronal holes. We also find that the relative density depletion between cavity and surrounding helmet decreases as a function of height. We show that both increased temperature in the cavity relative to the surrounding helmet streamer and a magnetic flux rope configuration might lead to such a flattened density profile. Finally, our model provides general observational guidelines that can be used to determine when a cavity is sufficiently unobstructed to be a good candidate for plasma diagnostics.

Simultaneous measurement of line-of-sight (LOS) magnetic and velocity fields at the photosphere and chromosphere are presented. The Fe I line at λ6569 and Hα at λ6563 are used for deriving the physical parameters at photospheric and chromospheric heights, respectively. The LOS magnetic field obtained through the center-of-gravity method shows a linear relation between the photospheric and chromospheric fields for field strengths less than 700 G. But in strong field regions, the LOS magnetic field values derived from Hα are much weaker than what one gets from the linear relationship, and also from those expected from the extrapolation of the photospheric magnetic field. We discuss in detail the properties of the magnetic field observed in Hα from the point of view of observed velocity gradients. The bisector analysis of Hα Stokes I profiles shows larger velocity gradients in those places where strong photospheric magnetic fields are observed. These observations may support the view that the stronger fields diverge faster with height compared to weaker fields.

We investigate the distribution and evolution of existing and emerging magnetic network elements in the quiet-Sun photosphere. The ephemeral region emergence rate is found to depend primarily on the imbalance of magnetic flux in the area surrounding its emergence location, such that the rate of flux emergence is lower within strongly unipolar regions by at least a factor of 3 relative to flux-balanced quiet Sun. As coronal holes occur over unipolar regions, this also means that ephemeral regions occur less frequently there, but we show that this is an indirect effect—independent of whether the region is located within an open-field coronal hole or a closed-field quiet region. We discuss the implications of this finding for near-photospheric dynamo action and for the coupling between closed coronal and open heliospheric fields.

Nearby supernova explosions—within a few tens of pc of the solar system—have become a subject of intense scrutiny, due to the discovery of live undersea ⁶⁰Fe from an event 2.8 Myr ago. A key open question concerns the delivery of supernova ejecta to the Earth, in particular penetration of the heliosphere by the supernova remnant (SNR). We present the first systematic numerical hydrodynamical study of the interaction between a supernova blast and the solar wind. Our simulations explore dynamic pressure regimes that are factors ≥10 above those in other studies of the heliosphere under exotic conditions, for supernovae exploding at a range of distances through different interstellar environments, and interacting with solar winds of varying strengths. Our results are qualitatively consistent with the structure of the contemporary heliosphere modeled by previous work, but compressed to within the inner solar system. We demonstrate that key characteristics of the resulting heliospheric structure follow simple scaling laws that can be understood in terms of pressure-balance arguments, and which are in agreement with previous work. Our models show that a 10 pc supernova event, incident on a solar-wind outflow with the mean observed properties, compresses the heliopause to just beyond 1 AU. We also demonstrate scenarios where the supernova remnant compresses the heliopause to within 1 AU, in which cases supernova material will be directly deposited on Earth. Since 8 pc marks the nominal "kill radius" for severe biosphere damage, any extinction-level events should have left terrestrial deposits of supernova debris. We conclude with a brief discussion of the effect of our approximations and the impact of additional physics.

We have investigated deviation from the standard recombination process, using the ACBAR 2008 and the WMAP 3 year data. In this investigation, we have considered the possibility of accelerated recombination as well as delayed recombination. We find that accelerated recombination is as likely as delayed recombination, and that there is some degeneracy between _α and {n_s, log (10¹⁰A_s) , H₀}.

Many present-day galaxies are known to harbor supermassive, ≥10⁶M_☉, black holes. These central black holes must have grown through accretion from less massive seeds in the early universe. The molecules CO and H₂ can be used to trace this young population of accreting massive black holes through the X-ray irradiation of ambient gas. The X-rays drive a low-metallicity ion-molecule chemistry that leads to the formation and excitation of CO and H₂ in 100 K < T ≤ 1000 K gas. H₂ traces very low metallicity gas, ~10⁻³ solar or less, while some pollution by metals, ~10⁻² solar or more, must have taken place to form CO. Strong CO J > 15 and H₂S(0) and S(1) emission is found that allows one to constrain ambient conditions. Comparable line strengths cannot be produced by FUV or cosmic-ray irradiation. Weak, but perhaps detectable, H⁺₃ (2, 2) → (1, 1) emission is found and discussed. The models predict that black hole masses larger than 10⁵M_☉ can be detected with ALMA, over a redshift range of 5-20, provided that the black holes radiate close to Eddington.

Based on a new survey of AGN activity in compact groups of galaxies, we report a remarkable deficiency of broad-line AGNs as compared to narrow-line AGNs. The cause of such deficiency could be related to the average low luminosity of AGNs in CGs: 10³⁹ ergs s⁻¹. This result may imply lower accretion rates in CG AGNs, making broad-line regions (BLRs) undetectable, or may indicate a genuine absence of BLRs. Both phenomena are consistent with gas stripping through tidal interaction and dry mergers.

We report the discovery of superstrong, fading, high-ionization iron line emission in the galaxy SDSS J095209.56+214313.3 (SDSS J0952+2143 hereafter), which must have been caused by an X-ray outburst of large amplitude. SDSS J0952+2143 is unique in its strong multiwavelength variability; such a broadband emission-line and continuum response has not been observed before. The strong iron line emission is accompanied by unusual Balmer line emission with a broad base, narrow core, and double-peaked narrow horns, and strong He II emission. These lines, while strong in the SDSS spectrum taken in 2005, have faded away significantly in new spectra taken in 2007 December. Comparison of SDSS, 2MASS, GALEX, and follow-up GROND photometry reveals variability in the NUV, optical, and NIR band. Taken together, these unusual observations can be explained by a giant outburst in the EUV-X-ray band, detected even in the optical and NIR. The intense and variable iron, helium, and Balmer lines represent the "light echo" of the flare, as it traveled through circumnuclear material. The outburst may have been caused by the tidal disruption of a star by a supermassive black hole. Spectroscopic surveys such as SDSS are well suited to detect emission-line light echoes of such rare flare events. Reverberation-mapping of these light echoes can then be used as a new and efficient probe of the physical conditions in the circumnuclear material in nonactive or active galaxies.

IC 10 X-1 is a variable X-ray source in the Local Group starburst galaxy IC 10 whose optical counterpart is a Wolf-Rayet (WR) star. Prestwich and coworkers recently proposed that it contains the most massive known stellar-mass black hole (23-34 M_☉), but their conclusion was based on radial velocities derived from only a few optical spectra, the most important of which was seriously affected by a CCD defect. Here we present new spectra of the WR star, spanning 1 month, obtained with the Keck I 10 m telescope. The spectra show a periodic shift in the He II λ 4686 emission line as compared with IC 10 nebular lines such as [O III] λ 5007. From this, we calculate a period of 34.93 ± 0.04 hr (consistent with the X-ray period of 34.40 ± 0.83 hr reported by Prestwich) and a radial velocity semiamplitude of 370 ± 20 km s⁻¹. The resulting mass function is 7.64 ± 1.26 M_☉, consistent with that of Prestwich (7.8 M_☉). This, combined with the previously estimated (from spectra) mass of 35 M_☉ for the WR star, yields a minimum primary mass of 32.7 ± 2.6 M_☉. Even if the WR star has a mass of only 17 M_☉, the minimum primary mass is 23.1 ± 2.1 M_☉. Thus, IC 10 X-1 is indeed a WR/black-hole binary containing the most massive known stellar-mass black hole.

We present a study of the radial distribution of RR Lyrae variables, which present a range of photometric and pulsational properties, in the dwarf spheroidal galaxy Tucana. We find that the fainter RR Lyrae stars, having a shorter period, are more centrally concentrated than the more luminous, longer period RR Lyrae variables. Through comparison with the predictions of theoretical models of stellar evolution and stellar pulsation, we interpret the fainter RR Lyrae stars as a more metal-rich subsample. In addition, we show that they must be older than about 10 Gyr. Therefore, the metallicity gradient must have appeared very early on in the history of this galaxy.

We investigate the distribution of stars along the horizontal branch of the Galactic globular cluster NGC 1851, to shed light on the progeny of the two distinct subgiant-branch populations harbored by this cluster. On the basis of detailed synthetic horizontal-branch modeling, we conclude that the two subpopulations are distributed in different regions of the observed horizontal branch: the evolved stars belonging to the bright subgiant-branch component are confined to the red portion of the observed sequence, whereas the ones belonging to the faint subgiant-branch component are distributed from the blue to the red, populating also the RR Lyrae instability strip. Our simulations strongly suggest that it is not possible to reproduce the observations assuming that the two subpopulations lose the same amount of mass along the red giant branch. We warmly encourage empirical estimates of mass-loss rates in red giant stars belonging to this cluster.

We present the first far-ultraviolet (FUV; 1350-1750 Å) diffuse emission map of the Eridanus superbubble, obtained using the FIMS/SPEAR instrument. The features seen in the FUV image closely resemble those seen in the Hα map, including two prominent arcs identified earlier in Hα. While it has been argued that the FUV emission in this region is mostly due to the scattering of star light by dust, a close spectral examination reveals that one of the arcs is abundant in molecules and dust while the other is mainly composed of atomic species. Upon comparison to emission maps in other bands (X-ray, IR, optical Hα), we propose the most plausible geometrical structure of this region.

The detection of the radioactive decay line of ⁴⁴Ti provides unique evidence that the γ-ray source is a young (<1000 yr) supernova remnant because of the short ⁴⁴Ti lifetime of ~90 yr. Only two Galactic remnants, Cassiopeia A and RX J0852.0–4622, have hitherto been reported as ⁴⁴Ti line emitters, although the detection from the latter has been debated. Here we report on an expansion measurement of the northwestern rim of RX J0852.0–4622 obtained with X-ray observations separated by 6.5 yr. The expansion rate is derived to be 0.023% ± 0.006%, which is about 5 times lower than those of young historical remnants. Such a slow expansion suggests that RX J0852.0–4622 is not as young a remnant as has been expected. We estimate an age of 1700-4300 yr for this remnant, depending on its evolutionary stage. Assuming a high shock speed of ~3000 km s⁻¹, which is suggested by the detection of nonthermal X-ray radiation, the distance of ~750 pc to this remnant is also derived.

Geminga and B0656+14 are the closest pulsars with characteristic ages in the range of 100 kyr to 1 Myr. They both have spin-down powers of the order 3 × 10³⁴ ergs s⁻¹ at present. The winds of these pulsars had most probably powered pulsar wind nebulae (PWNe) that broke up less than about 100 kyr after the birth of the pulsars. Assuming that leptonic particles accelerated by the pulsars were confined in the PWNe and were released into the interstellar medium (ISM) on breakup of the PWNe, we show that, depending on the pulsar parameters, both pulsars make a nonnegligible contribution to the local cosmic ray (CR) positron spectrum, and they may be the main contributors above several GeV. The relatively small angular distance between Geminga and B0656+14 thus implies an anisotropy in the local CR positron flux at these energies. We calculate the contribution of these pulsars to the locally observed CR electron and positron spectra depending on the pulsar birth period and the magnitude of the local CR diffusion coefficient. We further give an estimate of the expected anisotropy in the local CR positron flux. Our calculations show that within the framework of our model, the local CR positron spectrum imposes constraints on pulsar parameters for Geminga and B0656+14, notably the pulsar period at birth, and also the local interstellar diffusion coefficient for CR leptons.

We present results from the archival Chandra observations of the 0.3 s X-ray pulsar PSR J1846–0258 associated with the supernova remnant (SNR) Kes 75. The pulsar has the highest spin-down luminosity ( = 8.3 × 10³⁶ ergs s⁻¹) among all the high magnetic field pulsars (HBPs) and has been classified as a Crab-like pulsar despite its magnetic field (5 × 10¹³ G) being above the quantum critical field. It is the only HBP described by a nonthermal Crab-like spectrum, powering a bright pulsar wind nebula (PWN). Our spectroscopic study shows evidence of spectral softening (photon index Γ = 1.32 ^{+ 0.08}_−0.09 to 1.97 ^{+ 0.05}_−0.07 ) and temporal brightening [unabsorbed flux F_unabs = (4.3 ± 0.2) × 10⁻¹² to 2.7_−0.2^+0.1 × 10⁻¹¹ ergs cm⁻² s⁻¹] of the pulsar by ~6 times from 2000 to 2006. The 0.5-10 keV luminosity of the pulsar at the revised distance of 6 kpc has also increased from L_X = (1.85 ± 0.08) × 10³⁴ to 1.16 ^{+ 0.03}_−0.07 × 10³⁵ ergs s⁻¹, and the X-ray efficiency increased from 0.2% ± 0.01% to 1.4^{+ 0.04}_−0.08% . The observed X-ray brightening and softening of the pulsar suggests for the first time that this HBP is revealing itself as a magnetar.

The so-called stationary Hα line of SS 433 is shown to consist of three components. A broad component is identified as emitted in that wind from the accretion disk that grows in speed with elevation above the plane of the disk. There are two narrow components, one permanently redshifted and the other permanently shifted to the blue. These are remarkably steady in wavelength and must be emitted from a circumbinary ring, orbiting the center of mass of the system rather than orbiting either the compact object or its companion: perhaps the inner rim of an excretion disk. The orbiting speed (approximately 200 km s⁻¹) of this ring material strongly favors a large mass for the enclosed system (around 40 M_☉), a large mass ratio for SS 433, a mass for the compact object plus accretion disk of ~16 M_☉, and hence the identity of the compact object as a rather massive stellar black hole.

We present the results of a search for pulsations in six of the recently discovered carbon-atmosphere white dwarf ("hot DQ") stars. On the basis of our theoretical calculations, the star SDSS J142625.71+575218.3 is the only object expected to pulsate. We observe this star to be variable, with significant power at 417.7 s and 208.8 s (first harmonic), making it a strong candidate as the first member of a new class of pulsating white dwarf stars, the DQVs. Its folded pulse shape, however, is quite different from that of other white dwarf variables and shows similarities with that of the cataclysmic variable AM CVn, raising the possibility that this star may be a carbon-transferring analog of AM CVn stars. In either case, these observations represent the discovery of a new and exciting class of object.

We present K-band spectra of the near infrared counterparts to IRS 2E and IRS 2W which is associated with the ultracompact H II region W51d, both of them embedded sources in the Galactic compact H II region W51 IRS 2. The high spatial resolution observations were obtained with the laser guide star facility and Near-infrared Integral Field Spectrograph (NIFS) mounted at the Gemini-North observatory. The spectrum of the ionizing source of W51d shows the photospheric features N III (21155 Å) in emission and He II (21897 Å) in absorption which lead us to classify it as a young O3 type star. We detected CO overtone in emission at 23000 Å in the spectrum of IRS 2E, suggesting that it is a massive young object still surrounded by an accretion disk, probably transitioning from the hot core phase to an ultracompact H II region.

CoKu Tau/4 has been labeled as one of the very few known transition disk objects—disks around young stars that have their inner disks cleared of dust, arguably as a result of planetary formation. We report aperture-masking interferometry and adaptive optics imaging observations showing that CoKu Tau/4 is in fact a near-equal binary star of projected separation ~53 mas (~8 AU). The spectral energy distribution of the disk is then naturally explained by the inner truncation of the disk through gravitational interactions with the binary star system. We discuss the possibility that such "unseen" binary companions could cause other circumbinary disks to be labeled as transitional.

The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) provides for the first time imaging spectroscopy of solar flares up to the γ-ray range. The three RHESSI flares with best counting statistics are analyzed in the 200-800 keV range revealing γ-ray emission produced by electron bremsstrahlung from footpoints of flare loops, but also from the corona. Footpoint emission dominates during the γ-ray peak, but as the γ-ray emission decreases the coronal source becomes more and more prominent. Furthermore, the coronal source shows a much harder spectrum (with power-law indices γ between 1.5 and 2) than the footpoints (with γ between 3 and 4). These observations suggest that flare-accelerated high-energy (~MeV) electrons stay long enough in the corona to lose their energy by collisions producing γ-ray emission, while lower energetic electrons precipitate more rapidly to the footpoints.

The solar active region 10938 has been observed from the disk center to the west limb with the Hinode EUV Imaging Spectrometer. In the disk-center observation, subsonic upflow motions of tens of km s⁻¹ and enhanced nonthermal velocities have been found near the footpoints of the active region loops assuming a single Gaussian approximation for the emission-line profiles. When the same part of the active region is observed near the limb, both upflows and enhanced nonthermal velocities essentially decrease. There is a strong correlation between Doppler velocity and nonthermal velocity. Significant deviations from a single Gaussian profile are found in the blue wing of the line profiles for the upflows. These suggest that there are unresolved high-speed upflows. We discuss the implications for coronal heating mechanisms.

Solar flares are large explosions on the Sun's surface caused by a sudden release of magnetic energy. They are known to cause local short-lived oscillations traveling away from the explosion like water rings. Here we show that the energy in the solar acoustic spectrum is correlated with flares. This means that the flares drive global oscillations in the Sun in the same way that the entire Earth is set ringing for several weeks after a major earthquake such as the 2004 December Sumatra-Andaman one. The correlation between flares and energy in the acoustic spectrum of disk-integrated sunlight is stronger for high-frequency waves than for ordinary p-modes which are excited by the turbulence in the near-surface convection zone immediately beneath the photosphere.