Table of contents

Constraints on cosmological parameters from upcoming measurements with the Mileura Wide-field Array Low Frequency Demonstrator (MWA LFD) of the redshifted 21 cm power spectrum are forecasted assuming a flat ΛCDM cosmology and assuming that the reionization of neutral hydrogen in the intergalactic medium occurs below a redshift of z = 8. We find that observations with the MWA LFD cannot constrain the underlying cosmology in this scenario. In principle, a similar experiment with a 10-fold increase in collecting area could provide useful constraints on the slope of the inflationary power spectrum, n_s, and the running of the spectral index, α_s, but these constraints are subject to the caveat that even a small reionization contribution could be confused with the cosmological signal. In addition to the redshifted 21 cm signal, we include two nuisance components in our analysis related to the systematics and astrophysical foregrounds present in low-frequency radio observations. These components are found to be well separated from the signal and contribute little uncertainty (<30%) to the measured values of the cosmological model parameters.

We use cosmological, chemodynamical, SPH simulations of Milky Way-analog galaxies to find the expected present-day distributions of both metal-free stars that formed from primordial gas and the oldest star populations. We find that metal-free stars continue to form until z ~ 4 in halos that are chemically isolated and located far away from the biggest progenitor of the final system. As a result, if the Population III initial mass function allows stars with low enough mass to survive until z = 0 (<0.8 M_☉), they would be distributed throughout the Galactic halo. On the other hand, the oldest stars form in halos that collapsed close to the highest density peak of the final system, and at z = 0 they are located preferentially in the central region of the Galaxy, i.e., in the bulge. According to our models, these trends are not sensitive to the merger histories of the disk galaxies or the implementation of supernova feedback. Furthermore, these full hydrodynamics results are consistent with our N-body results in Paper I and lend further weight to the conclusion that surveys of low-metallicity stars in the Galactic halo can be used to directly constrain the properties of primordial stars. In particular, they suggest that the current lack of detections of metal-free stars implies that their lifetimes were shorter than a Hubble time, placing constraints on the metal-free initial mass function.

X-ray and optical observations of quadruply lensed quasars can provide a microarcsecond probe of the lensed quasar, corresponding to scale sizes of ~10²-10⁴ gravitational radii of the central black hole. This high angular resolution is achieved by taking advantage of microlensing by stars in the lensing galaxy. In this paper we use X-ray observations of 10 lensed quasars recorded with the Chandra X-Ray Observatory as well as corresponding optical data obtained with either the Hubble Space Telescope or ground-based optical telescopes. These are analyzed in a systematic and uniform way with emphasis on the flux ratio anomalies that are found relative to the predictions of smooth lens models. A comparison of the flux ratio anomalies between the X-ray and optical bands allows us to conclude that the optical emission regions of the lensed quasars are typically larger than expected from basic thin-disk models by factors of ~3-30.

We present an analysis of the effects of luminosity on the shape of the mid-infrared spectral energy distributions (SEDs) of 234 radio-quiet quasars originally presented by Richards et al. In quasars without evident dust extinction, the spectrally integrated optical and infrared luminosities are linearly correlated over nearly three decades in luminosity. We find a significant (≳99.99% confidence) correlation between the 1.8-8.0 μm spectral index and infrared luminosity that indicates an enhancement of the mid-infrared continuum with increasing luminosity. Coupled with strong evidence for spectral curvature in more luminous quasars, we conclude that this trend is likely a manifestation of the "near-infrared (3-5 μm) bump" noted in earlier quasar SED surveys. The strength of this feature is indicative of the contribution of emission from the hottest (≳1000 K) dust to the mid-infrared spectrum; higher luminosity quasars tend to show more hot dust emission. Finally, the comparable distribution of bolometric corrections from the monochromatic 3 μm luminosity as well as its lack of sensitivity to dust extinction as compared to the standard bolometric correction from νL_{5100 Å} suggest that the former may be a more robust indicator of bolometric quasar luminosity. The close link between the power in the mid-infrared and optical and the effect of luminosity on the shape of the mid-infrared continuum indicate that considering mid-infrared emission independent of the properties of the quasar itself is inadequate for understanding the parsec-scale quasar environment.

We present the results from Fourier-resolved spectroscopy of archival XMM-Newton data of five AGNs, namely, Mrk 766, NGC 3516, NGC 3783, NGC 4051, and Ark 564. This work supplements an earlier study of MCG -6-30-15 and of several Galactic black hole candidate sources. Our results exhibit much larger diversity than for Galactic sources, a fact we attribute to the diversity of their masses. When we take into account this effect and combine our results with those for Cyg X-1, it seems reasonable to conclude that at high frequencies, the slope of the Fourier-resolved spectra in accreting black hole systems decreases with increasing frequency as ∝f^-0.25, irrespective of whether the system is in its high or low state. This result implies that the flux variations in AGNs are accompanied by complex spectral slope variations as well. We also find that the Fe Kα line in Mrk 766, NGC 3783, and NGC 4051 is variable on timescales of ~1 day to 1 hr. The iron fluorescence line is absent in the spectra of the highest frequencies, and there is an indication that, just like in Cyg X-1, the equivalent width of the line in the Fourier-resolved AGNs decreases with increasing frequency.

Explicit 2D axisymmetric solutions are found to the hydrostatic equilibrium, energy balance, and photon diffusion equations within obscuring tori around active galactic nuclei (AGNs). These solutions demonstrate that infrared radiation pressure can support geometrically thick structures in AGN environments subject to certain constraints: the bolometric luminosity must be roughly ~0.03-1 times the Eddington luminosity; and the Compton optical depth of matter in the equatorial plane should be ~1, with a tolerance of about an order of magnitude up or down. Both of these constraints are at least roughly consistent with observations. In addition, angular momentum must be redistributed so that the fractional rotational support against gravity rises from the inner edge of the torus to the outer in a manner specific to the detailed shape of the gravitational potential. This model also predicts that the column densities observed in obscured AGNs should range from ~10²² to ~10²⁴ cm^-2.

We monitored 13 moderate-luminosity active galactic nuclei at z = 0.36 to measure flux variability, explore feasibility of reverberation mapping, and determine uncertainties on estimating black hole mass from single-epoch data. Spectra and images were obtained with approximately weekly cadence for up to 4 months, using the Kast spectrograph on the 3 m Shane Telescope. In broad band we detect peak-to-peak variations of 9%-37% and rms variations of 2%-10%. The observed flux variability in the g' band (rest frame 2800-4000 Å) is consistent with that in the r' band (rest frame 4000-5200 Å), but with larger amplitude. However, after correcting for stellar light dilution using Hubble Space Telescope images, we find nuclear variability of 3%-24% (rms variation) with similar amplitudes in the g' and r' bands within the errors. Intrinsic flux variability of the Hβ line is also detected at the 3%-13% level, after accounting for systematic errors on the spectrophotometry. This demonstrates that a reverberation mapping campaign beyond the local universe can be carried out with a 3 m class telescope, provided that sufficiently long light curves are obtained. Finally, we compare the Hβ FWHM measured from mean spectra with that measured from single-epoch data, and find no bias but an rms scatter of 14%, mostly accounted for by the uncertainty on FWHM measurements. The propagated uncertainty on black hole mass estimates, due to the FWHM measurement errors using low S/N (10-15 pixel^-1) single-epoch spectra, is 30%.

In order to identify the dominant nuclear outflow mechanisms in active galactic nuclei, we have undertaken deep, high-resolution observations of two compact radio sources (PKS 1549-79 and PKS 1345+12) with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope. Not only are these targets known to have powerful emission-line outflows, but they also contain all the potential drivers for the outflows: relativistic jets, quasar nuclei, and starbursts. ACS allows the compact nature (<0.15'') of these radio sources to be optically resolved for the first time. Through comparison with existing radio maps, we have seen consistency in the nuclear position angles of both the optical emission-line and radio data. There is no evidence for biconical emission-line features on the large scale, and there is a divergence in the relative position angles of the optical and radio structure. This enables us to exclude starburst-driven outflows. However, we are unable to clearly distinguish between radiative AGN wind-driven outflows and outflows powered by relativistic radio jets. The small-scale biconical features, indicative of such mechanisms, could be below the resolution limit of ACS, especially if aligned close to the line of sight. In addition, there may be offsets between the radio and optical nuclei induced by heavy dust obscuration, nebular continuum, or scattered light from the AGN.

We present multifrequency VLBA observations of two polarized Compact Symmetric Objects (CSOs), J0000+4054 and J1826+1831, and a polarized CSO candidate, J1915+6548. Using the wavelength-squared dependence of Faraday rotation, we obtained rotation measures (RMs) of -180 ± 10 rad m^-2 and 1540 ± 7 rad m^-2 for the latter two sources. These are lower than what is expected of CSOs (several 1000 rad m^-2) and, depending on the path length of the Faraday screens, require magnetic fields from 0.03 to 6 μG. These CSOs may be more heavily affected by Doppler boosting than their unpolarized counterparts, suggesting that a jet-axis orientation more inclined toward the line of sight is necessary to detect any polarization. This allows for low RMs if the polarized components are oriented away from the depolarizing circumnuclear torus. These observations also add a fourth epoch to the proper-motion studies of J0000+4054 and J1826+1831, constraining their kinematic age estimates to >610 and 2600 ± 490 yr, respectively. The morphology, spectrum, and component motions of J1915+6548 are discussed in light of its new classification as a CSO candidate, and its angle to the line of sight (~50°) is determined from relativistic beaming arguments.

Observations of low mean metallicity of damped Lyα (DLA) quasar absorbers at all redshifts studied appear to contradict the predictions for the global mean interstellar metallicity in galaxies from cosmic chemical evolution models. On the other hand, a number of metal-rich sub-DLA systems have been identified recently, and the fraction of metal-rich sub-DLAs appears to be considerably larger than that of metal-rich DLAs, especially at z < 1.5. In view of this, here we investigate the evolution of metallicity in sub-DLAs. We find that the mean Zn metallicity of the observed sub-DLAs may be higher than that of the observed DLAs, especially at low redshifts, reaching a near-solar level at z ≲ 1. This trend does not appear to be an artifact of sample selection, the use of Zn, the use of N_{H i} weighting, or observational sensitivity. While a bias against very low metallicity could be present in the sub-DLA sample in some situations, this cannot explain the difference between the DLA and sub-DLA metallicities at low z. The primary reason for the difference between the DLAs and sub-DLAs appears to be the dearth of metal-rich DLAs. We estimate the sub-DLA contribution to the total metal budget using measures of their metallicity and comoving gas density. These calculations suggest that at z ≲ 1, the contribution of sub-DLAs to the total metal budget may be several times that of DLAs. At higher redshifts also, there are indications that the sub-DLAs may contribute significantly to the cosmic metal budget.

We compare the luminosity functions for red galaxies lying on the rest-frame (U - V) color-magnitude sequence in a homogeneous sample of 10 X-ray-luminous clusters from the MACS survey at z ~ 0.5 to a similarly selected X-ray cluster sample at z ~ 0.1. We exploit deep Hubble Space Telescope ACS imaging in the F555W and F814W passbands of the central 1.2 Mpc diameter regions of the distant clusters to measure precise colors for the galaxies in these regions and statistically correct for contamination by field galaxies using observations of blank fields. We apply an identical analysis to ground-based photometry of the z ~ 0.1 sample. This comparison demonstrates that the number of faint, M_V ~ -19, red galaxies relative to the bright population seen in the central regions of massive clusters has roughly doubled over the 4 Gyr between z ~ 0.5 and z ~ 0.1. We quantify this difference by measuring the dwarf-giant ratio on the red sequence, which increases by a factor of at least 2.2 ± 0.4 since z ~ 0.5. This is consistent with the idea that many faint, blue, star-forming galaxies in high-density environments are transforming onto the red sequence in the last half of the Hubble time.

We present observations of the dust and atomic gas phase in seven dwarf irregular galaxies of the M81 group from the Spitzer SINGS and VLA THINGS surveys. The Spitzer observations provide a first glimpse of the nature of the nonatomic ISM in these metal-poor (Z ~ 0.1 Z_☉), quiescent (SFR ~ 0.001-0.1 M_☉ yr^-1) dwarf galaxies. Most detected dust emission is restricted to H I column densities >1 × 10²¹ cm^-2, and almost all regions of high H I column density (>2.5 × 10²¹ cm^-2) have associated dust emission. Spitzer spectroscopy of two regions in the brightest galaxies (IC 2574 and Holmberg II) show distinctly different spectral shapes and aromatic features, although the galaxies have comparable gas-phase metallicities. This result emphasizes that the strength of the aromatic features is not a simple linear function of metallicity. We estimate dust masses of ~10⁴-10⁶M_☉ for the M81 dwarf galaxies, resulting in an average dust-to-gas ratio (M_dust/M_{H I}) of ~3 × 10^-4 (1.5 × 10^-3 if only the H I that is associated with dust emission is considered); this is an order of magnitude lower than the typical value derived for the SINGS spirals. The dwarf galaxies are underluminous per unit star formation rate at 70 μm as compared to the more massive galaxies in SINGS by a factor of ~2. However, the average 70/160 μm ratio in the sample dwarf galaxies is higher than what is found in the other galaxies of the SINGS sample. This can be explained by a combination of a lower dust content in conjunction with a higher dust temperature in the dwarfs.

We present the results from the analysis of optical spectra of 31 Hα-selected regions in the extended UV (XUV) disks of M83 (NGC 5236) and NGC 4625 recently discovered by GALEX. The spectra were obtained using IMACS at the Las Campanas Observatory 6.5 m Magellan I telescope and COSMIC at the Palomar 200 inch (5 m) telescope, respectively, for M83 and NGC 4625. The line ratios measured indicate nebular oxygen abundances (derived from the R23 parameter) of the order of Z_☉/5-Z_☉/10. For most emission-line regions analyzed the line fluxes and ratios measured are best reproduced by models of photoionization by single stars with masses in the range 20-40 M_☉ and oxygen abundances comparable to those derived from the R23 parameter. We find indications for a relatively high N/O abundance ratio in the XUV disk of M83. Although the metallicities derived imply that these are not the first stars formed in the XUV disks, such a level of enrichment could be reached in young spiral disks only 1 Gyr after these first stars would have formed. The amount of gas in the XUV disks allows maintaining the current level of star formation for at least a few Gyr.

We present the X-ray luminosity functions (XLFs) of the X-ray source population detected in the Chandra monitoring observations of NGC 4038/4039 (the Antennae). The seven individual XLFs are well described by a flat power law with a cumulative slope α ~ 0.5-0.8. A similar slope (α = 0.48) is measured for the sources detected in the co-added observation, which reaches a limiting luminosity of ~10³⁷ erg s^-1. In our analysis we account for observational biases by deriving incompleteness functions and including them in the fitting process. We do not detect significant variations between the shape of the XLF of the seven observations. The two shorter exposures appear to have steeper XLFs, but these are still consistent with the other observations. These results indicate that the XLFs of star-forming galaxies are indeed flatter than those of more evolved stellar populations, even down to the typical luminosities of X-ray binaries. Based on this, as well as the X-ray variability and spectral properties of the X-ray sources, we suggest that the observed population down to our detection limit consists predominantly of X-ray binaries accreting close to their Eddington limit, similar to the high or very high states of Galactic X-ray binaries. In the case of ultraluminous X-ray sources (L_X > 10³⁹ erg s^-1), we cannot rule out the contribution of a beamed component (because of either mechanical focusing or Doppler boosting) in their observed emission. However, even without beaming, we estimate that the maximum observed luminosity (L_X ~ 10⁴⁰ erg s^-1) could be produced by a ~ 80 M_☉ black hole accreting at its Eddington limit; such black holes can be the result of regular stellar evolution of double stellar systems.

We investigate the stellar populations in the star-forming ring of the luminous infrared galaxy NGC 7469. We use HST multiwavelength (UV through NIR) imaging complemented with new K-band ground-based long-slit spectroscopy, and mid-IR and radio maps from the literature. SEDs and evolutionary synthesis models have been used to characterize the star formation at different scales, from those of individual star clusters (tens of pc) to that of the entire star-forming ring (kpc scale). At the smallest scales two different populations of massive (1-10 × 10⁶M_☉) clusters are identified. About 25% of the clusters are young (1-3 Myr) and extincted (A_V ≈ 3 mag), whereas the vast majority are of intermediate age (~9-20 Myr) and less obscured (A_V ≈ 1 mag). At larger (hundreds of pc) scale, an analysis of the integrated SED and spectroscopic data of the ring indicates the presence of two stellar populations. The young (5-6 Myr) and obscured stellar population accounts for the Brγ emission and most of the IR luminosity, and for about one-third of the stellar mass of the ring. The much less obscured intermediate-age population has properties similar to those of the majority of the (older) 1.1 μm-selected star clusters. The distribution of these two populations is clearly different and even spatially anticorrelated. The UV-optical-NIR continuum (including the majority of the clusters) of the ring traces mostly the mildly obscured intermediate-age population, while the MIR and radio peaks mark the location of the youngest and obscured star-forming regions. Moreover, the two brightest MIR and radio peaks are spatially coincident with the ends of the nuclear molecular gas bar. This study emphasizes the need for multiwavelength, high angular resolution observations to characterize the star formation in the dust-obscured regions commonly present in LIRGs.

NGC 1313 X-2 was among the first ultraluminous X-ray sources discovered, and has been a frequent target of X-ray and optical observations. Using the HST ACS multiband observations, this source is identified with a unique counterpart within an error circle of 0.2''. The counterpart is a blue star on the edge of a young cluster of ≤10⁷ yr amid a dominant old stellar population. Its spectral energy distribution is consistent with that for a Z = 0.004 star with 8.5 M_☉ about 5 × 10⁶ yr old, or for an O7 V star at solar metallicity. The counterpart exhibited significant variability of Δm = 0.153 ± 0.033 mag between two F555W observations separated by 3 months, reminiscent of the ellipsoidal variability due to the orbital motion of this ULX binary.

VLA observations of large-scale H I and OH absorption in the merging galaxy of NGC 6240 are presented with 1'' resolution. H I absorption is found across large areas of the extended radio continuum structure with a strong concentration toward the double nucleus. The OH absorption is confined to the nuclear region. The H I and OH observations identify fractions of the gas disks of the two galaxies and confirm the presence of central gas accumulation between the nuclei. The data clearly identify the nucleus of the southern galaxy as the origin of the symmetric superwind outflow and also reveal blueshifted components resulting from a nuclear starburst. Various absorption components are associated with large-scale dynamics of the system including a foreground dust lane crossing the radio structure in the northwest region.

Perpendicular spatial diffusion of cosmic rays is particularly important in highly flattened astrophysical objects such as the Galactic disk and in the outer heliosphere. By maintaining perpendicular Fokker-Planck coefficients in the Fokker-Planck equation for the gyrotropic cosmic-ray phase-space density, it is shown that the diffusion approximation gives rise to a new, third contribution to the cosmic-ray anisotropy due to perpendicular spatial diffusion, adding to the well-known streaming anisotropy due to pitch-angle scattering and parallel spatial gradients and the Compton-Getting anisotropy due to momentum gradients. This new anisotropy modifies the transport parameters of the diffusion-convection transport equation for the isotropic gyrotropic part of the cosmic-ray phase-space density; in general, one obtains two new perpendicular rates of adiabatic deceleration and nine nonzero elements of the full cosmic-ray spatial diffusion tensor.

The process of electron injection at high Mach number, collisionless, quasi-perpendicular shock waves is investigated by means of one-dimensional electromagnetic particle-in-cell simulations. We find that energetic electrons are generated in two steps: (1) electrons are accelerated nearly perpendicular to the local magnetic field by shock surfing acceleration at the leading edge of the shock transition region, and (2) these preaccelerated electrons are further accelerated by shock drift acceleration. As a result, energetic electrons are preferentially reflected back upstream. Shock surfing acceleration provides sufficient energy for the reflection. Therefore, it is important not only for the energization process itself, but also for triggering the secondary acceleration. We also present a theoretical model of the two-step acceleration mechanism, based on the simulation results, that can predict the injection efficiency for a subsequent diffusive shock acceleration process. We show that the injection efficiency obtained in the present model agrees well with the value obtained from Chandra X-ray observations of SN 1006. At typical supernova remnant shocks, energetic electrons injected by this mechanism can self-generate upstream Alfvén waves, which scatter the energetic electrons themselves.

Irradiation of a stellar atmosphere by an external source (e.g., an AGN) changes its structure and therefore its spectrum. Using a state-of-the-art stellar atmosphere code, we calculate the infrared spectra of such irradiated and transformed stars. We show that the original spectrum of the star, which is dominated by molecular bands, changes dramatically when irradiated even by a low-luminosity AGN (L_X = 10³³ ergs s^-1), becoming dominated by atomic lines in absorption. We study the changes in the spectrum of low-mass carbon- and oxygen-rich giant stars as they are irradiated by a modest AGN, similar to the one at the Galactic center (GC). The resulting spectra are similar to those of the faintest S-cluster stars observed in the GC. The spectrum of a star irradiated by a much brighter AGN, like that powered by a tidally disrupted star, is very different from that of any star currently observed near the GC. For the first time, we have discovered that the structure of the atmosphere of an irradiated giant changes dramatically and induces a double inversion layer. We show that irradiation at the current level can explain the observed trend of CO-band intensities decreasing as a function of increasing proximity to Sgr A*. This may indicate that (contrary to previous claims) there is no paucity of old giants in the GC, which coexist simultaneously with young massive stars.

The formation of blue stragglers is still not completely understood, particularly the relationship between formation environment and mechanism. We use a large, homogeneous sample of blue stragglers in the cores of 57 globular clusters to investigate the relationships between blue straggler populations and their environments. We use a consistent definition of "blue straggler" based on position in the color-magnitude diagram and normalize the population relative to the number of red giant branch stars in the core. We find that the previously determined anticorrelation between blue straggler frequency and total cluster mass is present in the purely core population. We find some weak anticorrelations with central velocity dispersion and with half-mass relaxation time. The blue straggler frequency does not show any trend with any other cluster parameter. Even though collisions may be expected to be a dominant blue straggler formation process in globular cluster cores, we find no correlation between the frequency of blue stragglers and the collision rate in the core. We also investigated the blue straggler luminosity function shape and found no relationship between any cluster parameter and the distribution of blue stragglers in the color-magnitude diagram. Our results are inconsistent with some recent models of blue straggler formation that include collisional formation mechanisms and may suggest that almost all observed blue stragglers are formed in binary systems.

In order to determine the circumstances under which isolated SNRs are capable of rising into and enriching the thick disk and Galactic halo, simulations of supernova remnants are performed with the FLASH magnetohydrodynamic code. We performed simulations in which the interstellar magnetic field is parallel to or perpendicular to the Galactic plane, as well as a simulation without a magnetic field. The ambient gas density distribution and gravitational potential are based on observations of our Galaxy. We evolve the remnants to ages of roughly 10⁷ yr. For our simulation without a magnetic field, we compare the evolution of the hot bubble's velocity to the velocity evolution calculated from the buoyant and drag accelerations. We found surprisingly small vertical velocities for the hot gas, from which we estimated the drag coefficient to be 10 for the nonmagnetic simulation. Although we found little buoyant motion of the hot gas during the remnant's lifetime, we found rapid vertical motion of the associated cool dense gas near the end of the remnants life. This motion deformed the remnant into a mushroom-cloud structure similar to those found in previous simulations. The simulation in which we have a 4 μG magnetic field parallel to the Galactic midplane shows a dramatically elongated bubble parallel to the magnetic field. The magnetic field pins the supernova remnant, preventing it from rising. In the simulation with the 4 μG magnetic field perpendicular to the midplane, the hot bubble rises more, indicating that having the magnetic field in the same direction as the gravitational force enhances the rise of the bubble.

The shell-type supernova remnant RX J0852.0-4622 was observed with the High Energy Stereoscopic System (H.E.S.S.) of atmospheric Cerenkov telescopes between 2004 December and 2005 May for a total observation time of 33 hr, above an average gamma-ray energy threshold of 250 GeV. The angular resolution of ~0.06° (for events triggering three or four telescopes) and the large field of view of H.E.S.S. (5° diameter) are well adapted to studying the morphology of the object in very high energy gamma rays, which exhibits a remarkably thin shell very similar to the features observed in the radio range and in X-rays. The spectral analysis of the source from 300 GeV to 20 TeV is also presented. Finally, the possible origins of the very high energy gamma-ray emission (inverse Compton scattering by electrons or the decay of neutral pions produced by proton interactions) are discussed, on the basis of morphological and spectral features obtained at different wavelengths.

We present the complete data set, model, and line identification of a survey of the emission from the C-rich proto-planetary nebula CRL 618 performed with the IRAM 30 m telescope in the frequency ranges 80.25-115.75 GHz, 131.25-179.25 GHz, and 204.25-275.250 GHz. A selection of lines from different species has been used in previous works to derive the structure of the source, its physical conditions, and the chemical abundances in the different gas regions. In this work, we have used this information to run a global simulation of the spectrum in order to check the consistency of the model and to ease the task of line identification. The total number of lines that have a correspondence in both data and model is ~3100, although quite often in this object many lines blend into complex features, so that the model, which takes into account line blending, is a key tool at this stage of the analysis. Of all the lines that we have been able to label, ~55% of them belong to the different forms of HC₃N, and ~18% to HC₅N. The density of remaining unidentified features above the 3 σ limit is only 1 per ~2.1 GHz (74 features), which is unprecedented in the analysis of this type of large millimeter-wave line surveys.

We study the instantaneous virial balance of CCs in numerical models of MCs. The models represent a range of magnetic field strengths in MCs from subcritical to nonmagnetic regimes. We identify CCs at different density thresholds and calculate, for each object, the terms that enter the EVT. A CC is gravitationally bound when the gravitational term in the EVT is larger than the amount for the system to be virialized, which is more stringent than the condition that it be large enough to make the total volume energy negative. We also calculate other quantities used to indicate the state of gravitational boundedness: Jeans number J_c, mass-to-magnetic flux ratio μ_c, and virial parameter α_vir. Our results suggest the following: (1) CCs are dynamical out-of-equilibrium structures. (2) The surface energies are of the same order as their volume counterparts. (3) CCs are either in the process of being compressed or dispersed by the velocity field. Yet, not all CCs that have a compressive net kinetic energy are gravitationally bound. (4) There is no one-to-one correspondence between the states of gravitational boundedness as described by the virial analysis or by the other indicators. In general, in the virial analysis, only the inner regions of the objects are gravitationally bound, whereas J_c, α_vir, and μ_c estimates tend to show that they are more bound at the lowest threshold levels and more magnetically supercritical. (5) We observe, in the nonmagnetic simulation, the existence of a bound core with structural and dynamical properties that resemble those of Barnard 68. This suggests that such cores can form in a larger MC and then be confined by the warm gas of a newly formed, nearby H II region.

The identification of expanding H I shells is difficult because of their variable morphology. In this paper we present an automatic detector for H I shells, based on the more stable dynamical characteristics of expanding bubbles with radii <40 pc. The detection is performed in two stages. First, artificial neural networks are trained to recognize the dynamical signature of an expanding bubble in the velocity spectra of 21 cm data. The second stage consists of subsequent validations based on the potential bubble's morphology. The technique is tested on 11 known bubbles, and 10 of them are successfully detected. Conducting a systematic detection on a 48° × 9° region in the Perseus arm, we obtain 7100 detections with spatial distribution following the stellar distribution of the Galactic disk. The estimated radius and expansion velocity distributions for objects with R ≤ 10 pc agree with the distributions predicted by models of adiabatically expanding bubble populations. The fraction of the Perseus arm volume occupied by the detected objects, which can be interpreted as the small bubbles' contribution to the Galactic porosity Q, is calculated to Q_{R<40 pc} = 0.007. Four new bubble cases and eight serious candidates, related to known progenitors, are proposed.

We present an analysis of the X-ray spectrum of the Local Bubble, obtained by simultaneously analyzing spectra from two XMM-Newton pointings on and off an absorbing filament in the southern Galactic hemisphere (b ≈ -45°). We use the difference in the Galactic column density in these two directions to deduce the contributions of the unabsorbed foreground emission due to the Local Bubble, and the absorbed emission from the Galactic halo and the extragalactic background. We find the Local Bubble emission is consistent with emission from a plasma in collisional ionization equilibrium with a temperature log(T_LB/K) = 6.06 and an emission measure ndl = 0.018 cm^-6 pc. Our measured temperature is in good agreement with values obtained from ROSAT All-Sky Survey data, but is lower than that measured by other recent XMM-Newton observations of the Local Bubble, which find log(T_LB/K) ≈ 6.2 (although for some of these observations it is possible that the foreground emission is contaminated by non-Local Bubble emission from Loop I). The higher temperature observed toward other directions is inconsistent with our data when combined with a FUSE measurement of the Galactic halo O VI intensity. This therefore suggests that the Local Bubble is thermally anisotropic. Our data are unable to rule out a nonequilibrium model in which the plasma is underionized. However, an overionized recombining plasma model, while observationally acceptable for certain densities and temperatures, generally gives an implausibly young age for the Local Bubble (≲6 × 10⁵ yr).

We carry out numerical simulations of dust aggregate collisions to study the compression and disruption processes of aggregates in their growth. To compare with the pioneering studies of Dominik & Tielens, we focus on two-dimensional head-on collisions, in which we obtain similar results for compression and disruption to theirs. In addition to the similarities, we examine the dependence of the collisional outcomes on the aggregate size and the parameters relevant to the particle interaction in detail by treating large aggregates that consist of up to 2000 particles. Compression of aggregates by collisions reduces the radius of gyration and increases the number of contacts between the constituent particles. Our results show that the changes in the gyration radius and the number of contacts after impact depend on the impact energy and that the dependence is scaled by the energy necessary to roll all contacts. We provide empirical formulae for the changes in the gyration radius and the number of contacts. Furthermore, we find that the degree of maximum compression is determined by the ratio of rolling energy to breaking energy. This indicates that ice aggregates become more compact than quartz aggregates in the same impact conditions. Any aggregates are catastrophically disrupted when the impact energy exceeds approximately 10 times the energy necessary to break all contacts. Our results, however, suggest that it becomes harder to disrupt the aggregates with an increasing number of particles.

Detailed models for the density and temperature profiles of gas and dust in protoplanetary disks are constructed by taking into account X-ray and UV irradiation from a central T Tauri star, as well as dust size growth and settling toward the disk midplane. The spatial and size distributions of dust grains are numerically computed by solving the coagulation equation for settling dust particles, with the result that the mass and total surface area of dust grains per unit volume of the gas in the disks are very small, except at the midplane. The H₂ level populations and line emission are calculated using the derived physical structure of the disks. X-ray irradiation is the dominant heating source of the gas in the inner disk and in the surface layer, while the UV heating dominates otherwise. If the central star has strong X-ray and weak UV radiation, the H₂ level populations are controlled by X-ray pumping, and the X-ray-induced transition lines could be observable. If the UV irradiation is strong, the level populations are controlled by thermal collisions or UV pumping, depending on the dust properties. As the dust particles evolve in the disks, the gas temperature at the disk surface drops because the grain photoelectric heating becomes less efficient. This makes the level populations change from LTE to non-LTE distributions, which results in changes to the line ratios. Our results suggest that dust evolution in protoplanetary disks could be observable through the H₂ line ratios. The emission lines are strong from disks irradiated by strong UV and X-rays and possessing small dust grains; such disks will be good targets in which to observe H₂ emission.

We present detailed modeling of the disk around the TW Hydrae Association (TWA) brown dwarf 2MASSW J1207334-393254 (2M1207), using Spitzer observations from 3.6 to 24 μm. The spectral energy distribution (SED) does not show a high amount of flaring. We have obtained a good fit using a flat disk of mass between 10^-4 and 10 ^-6M_☉, ≲ 10^-11M_☉ yr^-1, and a large inclination angle between 60° and 70°. We have used three different grain models to fit the 10 μm Si emission feature, and have found the results to be consistent with ISM-like dust. In comparison with other TWA members, this suggests lesser dust processing for 2M1207, which could be explained by mechanisms such as aggregate fragmentation and/or turbulent mixing. We have found a good fit using an inner disk radius equal to the dust sublimation radius, which indicates the absence of an inner hole in the disk. This suggests the presence of a small K - L' excess, similar to the observed K - [3.6] excess.

We announce the discovery of SST-Lup3-1, a very low mass star close to the brown dwarf boundary in Lupus III with a circum(sub)stellar disk, discovered by the "Cores to Disks" Spitzer Legacy Program from mid-infrared, with very conspicuous crystalline silicate features in its spectrum. It is the first of such objects with a full 5-35 μm spectrum taken with the IRS, and it shows strong 10 and 20 μm silicate features with high feature-to-continuum ratios and clear crystalline features out to 33 μm. The dust in the disk upper layer has a crystalline silicate grain fraction between 15% and 33%, depending on the assumed dust continuum. The availability of the full Spitzer infrared spectrum allows an analysis of the dust composition as a function of temperature and position in the disk. The hot (~300 K) dust responsible for the 10 μm feature consists of a roughly equal mix of small (~0.1 μm) and large (~1.5 μm) grains, whereas the cold (~70 K) dust responsible for the longer wavelength silicate features contains primarily large grains (>1 μm). Since the cold dust emission arises from deeper layers in the inner (<3 AU) disk as well as from the surface layers of the outer (3-5 AU) disk, this provides direct evidence for combined grain growth and settling in the disk. The inferred crystalline mass fractions in the two components are comparable. Since only the inner 0.02 AU of the disk is warm enough to anneal the amorphous silicate grains, even the lowest fraction of 15% of crystalline material requires either very efficient mixing or other formation mechanisms.

Subarcsecond scale Keck images of the young A1 V star, 49 Ceti, resolve emission at λ = 12.5 and 17.9 μm from a disk with long axis at position angle (P.A.) 125° ± 10° and inclination ϕ = 60° ± 15°. At 17.9 μm, the emission is brighter and more extended toward the northwest (NW) than the southeast (SE). Modeling of the mid-infrared images combined with flux densities from the literature indicate that the bulk of the mid-infrared emission comes from very small grains (a ~ 0.1 μm) confined between 30 and 60 AU from the star. This population of dust grains contributes negligibly to the significant excess observed in the spectral energy distribution. Most of the nonphotospheric energy is radiated at longer wavelengths by an outer disk of larger grains (a ~ 15 μm), inner radius ~60 AU, and outer radius ~900 AU. Global properties of the 49 Cet disk show more affinity with the β Pic and HR 4796A disks than with other debris disks. This may be because they are all very young (t < 20 Myr), adding strength to the argument that they are transitional objects between Herbig Ae and "Vega-like" A stars with more tenuous circumstellar disks.

Dust-grain growth and settling are the first steps toward planet formation. An understanding of dust physics is therefore integral to a complete theory of the planet formation process. In this paper we explore the possibility of using the dust evaporation front in YSO disks ("the inner rim") as a probe of the dust physics operating in circumstellar disks. The geometry of the rim depends sensitively on the composition and spatial distribution of dust. Using radiative transfer and hydrostatic equilibrium calculations we demonstrate that dust growth and settling can curve the evaporation front dramatically (from a cylindrical radius of about 0.5 AU in the disk midplane to 1.2 AU in the disk upper layers for an A0 star). We compute synthetic images and interferometric visibilities for our representative rim models and show that the current generation of near-IR long-baseline interferometers (VLTI, CHARA) can strongly constrain the dust properties of circumstellar disks, shedding light on the relatively poorly understood processes of grain growth, settling and turbulent mixing.

We present a self-similar solution to describe the propagation of a shock wave whose energy is deposited or lost at the front. The propagation of the shock wave in a medium having a power-law density profile and the expansion of the medium to a vacuum after the shock breakout are both described with a Lagrangian coordinate. The Chapman-Jouguet detonation is found to accelerate the medium most effectively. The results are compared with some numerical simulations in the literature. We derive the fractions of the deposited/lost energy at the shock front in some specific cases, which will be useful when applying this solution to actual phenomena.

A simple physical model for long-duration gamma-ray bursts (GRBs) is used to fit the redshift (z) and the jet opening angle distributions measured with earlier GRB missions and with Swift. The effect of different sensitivities for GRB triggering is sufficient to explain the difference in the z distributions of the pre-Swift and Swift samples, with mean redshifts of ⟨z⟩ ≅ 1.5 and ⟨z⟩ ≅ 2.7, respectively. Assuming that the emission properties of GRBs do not change with time, we find that the data can only be fitted if the comoving rate density of GRB sources exhibits positive evolution to z ≳ 3-5. The mean intrinsic beaming factor of GRBs is found to range from ≈34 to 42, with the Swift average opening half-angle ⟨θ_j⟩ ~ 10°, compared to the pre-Swift average of ⟨θ_j⟩ ~ 7°. Within the uniform jet model, the GRB luminosity function is ∝L, as inferred from our best fit to the opening angle distribution. Because of the unlikely detection of several GRBs with z ≲ 0.25, our analysis indicates that low-redshift GRBs represent a different population of GRBs than those detected at higher redshifts. Neglecting possible metallicity effects on GRB host galaxies, we find that ≈1 GRB occurs every 600,000 yr in a local L_* spiral galaxy like the Milky Way. The fraction of high-redshift GRBs is estimated at 8%-12% and 2.5%-6% at z ≥ 5 and z ≥ 7, respectively, assuming continued positive evolution of the GRB rate density to high redshifts.

The distribution function describing the acceleration of relativistic particles in an advection-dominated accretion disk is analyzed using a transport formalism that includes first-order Fermi acceleration, advection, spatial diffusion, and the escape of particles through the upper and lower surfaces of the disk. When a centrifugally supported shock is present in the disk, the concentrated particle acceleration occurring in the vicinity of the shock channels a significant fraction of the binding energy of the accreting gas into a population of relativistic particles. These high-energy particles diffuse vertically through the disk and escape, carrying away both energy and entropy and allowing the remaining gas to accrete. The dynamical structure of the disk/shock system is computed self-consistently using a model previously developed by the authors that successfully accounts for the production of the observed relativistic outflows (jets) in M87 and Sgr A*. This ensures that the rate at which energy is carried away from the disk by the escaping relativistic particles is equal to the drop in the radial energy flux at the shock location, as is required for energy conservation. We investigate the influence of advection, diffusion, and acceleration on the particle distribution by computing the nonthermal Green's function, which displays a relatively flat power-law tail at high energies. We also obtain the energy distribution for the particles escaping from the disk, and we conclude by discussing the spectrum of the observable secondary radiation produced by the escaping particles.

The inspiraling and merger of binary black holes will likely involve black holes with not only unequal masses but also arbitrary spins. The gravitational radiation emitted by these binaries will carry angular as well as linear momentum. A net flux of emitted linear momentum implies that the black hole produced by the merger will experience a recoil or kick. Previous studies have focused on the recoil velocity from unequal-mass, nonspinning binaries. We present results from simulations of equal-mass but spinning black hole binaries and show how a significant gravitational recoil can also be obtained in these situations. We consider the case of black holes with opposite spins of magnitude a aligned and antialigned with the orbital angular momentum, with a the dimensionless spin parameter of the individual holes. For the initial setups under consideration, we find a recoil velocity of V = 475a km s^-1. Supermassive black hole mergers producing kicks of this magnitude could result in the ejection of the final hole produced by the collision from the core of a dwarf galaxy.

Using archival X-ray data, we find that the catalog location of the X-ray binary Scutum X-1 (Sct X-1) is incorrect and that the correct location is that of the X-ray source AX J183528-0737, which is 15' to the west. Our identification is made on the basis of the 112 s pulse period for this object detected in an XMM-Newton observation, as well as spatial coincidence between AX J183528-0737 and previous X-ray observations. Based on the XMM-Newton data and archival RXTE data, we confirm secular spin-down over 17 yr with period derivative ≈ 3.9 × 10^-9 s s^-1, but do not detect a previously reported X-ray iron fluorescence line. We identify a bright (K_s = 6.55) red (J - K_s = 5.51) optical and infrared counterpart to AX J183528-0737 from 2MASS, a number of mid-IR surveys, and deep optical observations, which we use to constrain the extinction to and distance of Sct X-1. From these data, as well as limited near-IR spectroscopy, we conclude that Sct X-1 is most likely a binary system composed of a late-type giant or supergiant and a neutron star.

With recent and archival Rossi X-Ray Timing Explorer (RXTE) X-ray measurements of the heavily obscured X-ray pulsar EXO 1722-363 (IGR J17252-3616), we carried out a pulse-timing analysis to significantly improve the accuracy of the orbital parameters. The binary system is characterized by a_x sin i = 101 ± 3 lt-s and P_orb = 9.7403 ± 0.0004 days (90% confidence), with the precision of the orbital period obtained by connecting data sets separated by more than 7 yr (272 orbital cycles). The orbit is consistent with circular, and e < 0.19 at the 90% confidence level. The orbital solution also shows that a torque reversal occurred sometime between 1998 November and 2006 February. The mass function is 11.7 ± 1.2 M_☉ and confirms that this source is a high-mass X-ray binary (HMXB) system. Using previous eclipse time measurements by Corbet et al. and our orbital solution, combined with the assumption that the primary underfills its Roche lobe, we find that i > 61° at the 99% confidence level, the radius of the primary is between 21 and 37 R_☉, and its mass is less than about 22 M_☉. The acceptable range of radius and mass shows that the primary is probably a supergiant of spectral type B0 I-B5 I. Photometric measurements of its likely counterpart are consistent with the spectral type and luminosity if the distance to the system is between 5.3 and 8.7 kpc. Spectral analysis of the pulsar as a function of orbital phase reveals an evolution of the hydrogen column density suggestive of dense filaments of gas in the downstream wake of the pulsar, with higher levels of absorption seen at orbital phases 0.5-1.0, as well as a variable Fe Kα line.

The giant flare of 2004 December 27 from SGR 1806-20 represents one of the most extraordinary events captured in over three decades of monitoring the γ-ray sky. One measure of the intensity of the main peak is its effect on X- and γ-ray instruments. RHESSI, an instrument designed to study the brightest solar flares, was completely saturated for ~0.5 s following the start of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the time of the giant flare, however, allowed RHESSI a unique view of the giant flare event, including the precursor, the main-peak decay, and the pulsed tail. Since RHESSI was saturated during the main peak, we augment these observations with Wind and RHESSI particle detector data in order to reconstruct the main peak as well. Here we present detailed spectral analysis and evolution of the giant flare. We report the identification of a relatively soft fast peak just milliseconds before the main peak, whose timescale and size scale indicate a magnetospheric origin. We present the novel detection of emission extending up to 17 MeV immediately following the main peak, perhaps revealing a highly extended corona driven by the hyper-Eddington luminosities. The spectral evolution and pulse evolution during the tail are presented, demonstrating significant magnetospheric twist during this phase, but no apparent magnetospheric evolution. Blackbody radii are derived for every stage of the flare, which show remarkable agreement despite the range of luminosities and temperatures covered. Finally, we confirm the existence of a hard afterglow emission extending up to 2.5 MeV in the hundreds of seconds following the giant flare.

We use the two-zone model of Cooper & Narayan to study the onset and time evolution of hydrogen-triggered type I X-ray bursts on accreting neutron stars. At the lowest accretion rates, thermally unstable hydrogen burning ignites helium as well and produces a mixed hydrogen and helium burst. For somewhat higher accretion rates, thermally unstable hydrogen burning does not ignite helium and thus triggers only a weak hydrogen flash. For our choice of model parameters, these weak hydrogen flashes occur for 10^-3 ≲ /_Edd ≲ 3 × 10^-3. The peak luminosities of weak hydrogen flashes are typically much lower than the accretion luminosity. These results are in accord with previous theoretical work. We find that a series of weak hydrogen flashes generates a massive layer of helium that eventually ignites in an energetic pure helium flash. Although previously conjectured, this is the first time such bursting behavior has been actually demonstrated in a theoretical model. For yet higher accretion rates, hydrogen burning is thermally stable and thus steadily generates a layer of helium that ultimately ignites in a pure helium flash. We find that, for a narrow range of accretion rates between the mixed hydrogen and helium burst and weak hydrogen flash regimes, unstable hydrogen burning ignites helium only after a short series of weak hydrogen flashes has generated a sufficiently deep layer of helium. These bursts have fluences that are intermediate between those of normal mixed hydrogen and helium bursts and energetic pure helium flashes.

Metals in the photospheres of white dwarfs with T_eff between 12,000 and 25,000 K should gravitationally settle out of these atmospheres in 1-2 weeks. Temporal variations in the line strengths of these metals could provide a direct measurement of episodic metal accretion. Using archival VLT and Keck spectroscopy, we find evidence that the DAZd white dwarf G29-38 shows significant changes in its Ca II K line strength. At the two best-observed epochs, we find that the Ca line equivalent width (EW) = 165 ± 4 mÅ (in 1996.885) and 280 ± 8 mÅ (in 1999.653), which is an increase of 70%. We consider the effect that pulsation has on the Ca EWs for this known variable star, and find that it adds an error of <1 mÅ to these measurements. Calcium line strengths at other observational epochs support variations with timescales as short as 2 weeks. These Ca EW variations indicate that the metal accretion process in G29-38, presumably from its debris disk, is episodic on timescales of a few weeks or less, and thus, the accretion is not dominated by Poynting-Robertson drag from an optically thin, continuous disk, which has a timescale of ~1 yr.

We show that the rapid and large decrease in the intensity of high-ionization emission lines from the η Carinae massive binary system can be explained by the accretion model. These emission lines are emitted by material in the nebula around the binary system that is being ionized by radiation from the hot secondary star. The emission lines suffer 3 months long deep fading every 5.54 yr, assumed to be the orbital period of the binary system. In the accretion model, for ~70 days the less massive secondary star is accreting mass from the primary wind instead of blowing its fast wind. The accretion event has two effects that substantially reduce the high-energy ionizing radiation flux from the secondary star: (1) The accreted mass absorbs a larger fraction of the ionizing flux. (2) The accreted mass forms a temporary blanket around the secondary star that increases its effective radius, hence lowering its effective temperature and the flux of high-energy photons. This explanation is compatible with the fading of the emission lines at the same time the X-ray is declining to its minimum and with the fading being less pronounced in the polar directions.

I compare the structures of the bipolar nebulae around the massive binary system η Carinae and around the low-mass binary system HD 44179. While η Car is on its way to becoming a supernova, the Red Rectangle is on its way to forming a planetary nebula. Despite the 2 orders of magnitude difference in mass, these two systems show several similarities, both in the properties of the stellar binary systems and in the nebulae. From this comparison and further analysis of the accretion process during the 20 yr Great Eruption of η Car, I strengthen the binary model for the formation of its bipolar nebula—the Homunculus. In the binary model a large fraction of the mass lost by the primary star during the Great Eruption was transferred to the secondary star (the companion). An accretion disk was formed around the companion, and the companion launched two opposite jets. I show that the gravitational energy of the mass accreted onto the secondary star during the Great Eruption can account for the extra energy of the Great Eruption, both the radiated energy and the kinetic energy in the Homunculus. I also conclude that neither the proximity of the primary star in η Car to the Eddington luminosity nor the rotation of the primary star are related directly to the shaping of the Homunculus. I speculate that the Great Eruption of η Car was triggered by disturbance in the outer boundary of the convective region, most likely by magnetic activity, that expelled the outer radiative zone.

We have used aperture masking interferometry and adaptive optics (AO) at the Palomar 200 inch telescope to obtain precise mass measurements of the binary M dwarf GJ 623. AO observations spread over 3 yr combined with a decade of radial velocity measurements constrain all orbital parameters of the GJ 623 binary system accurately enough to critically challenge the models. The dynamical masses measured are m₁ = 0.371 ± 0.015 M_☉ (4%) and m₂ = 0.115 ± 0.0023 M_☉ (2%) for the primary and the secondary, respectively. Models are not consistent with color and mass, requiring very low metallicities.

We calculate the theoretical evolution of the radii of all 14 of the known transiting extrasolar giant planets (EGPs) for a variety of assumptions concerning atmospheric opacity, dense inner core masses, and possible internal power sources. We incorporate the effects of stellar irradiation and customize such effects for each EGP and star. Looking collectively at the family as a whole, we find that there are in fact two radius anomalies to be explained. Not only are the radii of a subset of the known transiting EGPs larger than expected from previous theory, but many of the other objects are smaller than the default theory would allow. We suggest that the larger EGPs can be explained by invoking enhanced atmospheric opacities that naturally retain internal heat. This explanation might obviate the necessity for an extra internal power source. We explain the smaller radii by the presence in perhaps all the known transiting EGPs of dense cores, such as have been inferred for Saturn and Jupiter. Importantly, we derive a rough correlation between the masses of our "best-fit" cores and the stellar metallicity that seems to buttress the core-accretion model of their formation. Although many caveats and uncertainties remain, the resulting comprehensive theory that incorporates enhanced-opacity atmospheres and dense cores is in reasonable accord with all the current structural data for the known transiting giant planets.

Several models of extrasolar giant planet (EGP) atmospheres have been developed recently. Many of them are one-dimensional or concentrate on the lower or middle atmosphere. Three-dimensional hydrodynamic models are needed to study the horizontal variations in temperature and composition of EGP atmospheres. Circulation models for the upper atmosphere are particularly important as they can be used to study the thermal structure due to stellar irradiation, radiative cooling, and atmospheric circulation in the thermospheres of close-in EGPs and hence the rate of evaporation of their atmospheres. We present a generic gas giant model that is capable of generating three-dimensional, self-consistent global simulations of stable EGP thermospheres at different orbital distances. Calculations performed by this model indicate that IR emissions from H ions may play a significant role in cooling the thermospheres of EGPs at least in the range of 0.2-1 AU from a solar-type host star. In this range thermal dissociation of H₂ is negligible and ion densities are small compared to the overall neutral density. Inside 0.2 AU thermal dissociation and dissociative photoionization of H₂ may prevent the effective formation of H. In the absence of radiative cooling from H the upper atmospheres reach temperatures well above 10,000 K within ~0.5 AU. In this case the upper thermospheres are entirely converted into atomic hydrogen and the temperatures are high enough for significant atmospheric loss to take place. Our model is capable of calculating the IR signal strengths for various vibrational transitions of H based on the thermal state and the composition of the atmosphere. Potential detection of such signals would thus provide a validation of some of our results.

We report the detection of an extrasolar planet orbiting Tau, one of the giant stars in the Hyades open cluster. This is the first planet ever discovered in an open cluster. Precise Doppler measurements of this star from Okayama Astrophysical Observatory have revealed Keplerian velocity variations with an orbital period of 594.9 ± 5.3 days, a semiamplitude of 95.9 ± 1.8 m s^-1, and an eccentricity of 0.151 ± 0.023. The minimum mass of the companion is 7.6 ± 0.2 M_J, and the semimajor axis is 1.93 ± 0.03 AU adopting a stellar mass of 2.7 ± 0.1 M_☉. The age of 625 Myr for the cluster sets the most secure upper limit ever on the timescale of giant planet formation. The mass of 2.7 M_☉ for the host star is robustly determined by isochrone fitting, which makes the star the heaviest among planet-harboring stars. Putting together the fact that no planets have been found around about 100 low-mass dwarfs in the cluster, the frequency of massive planets is suggested to be higher around high-mass stars than around low-mass ones.

Fine-scale structure in the corona appears not to be well resolved by current imaging instruments. Assuming this to be true offers a simple geometric explanation for several current puzzles in coronal physics, including the apparent uniform cross section of bright threadlike structures in the corona, the low EUV contrast (long apparent scale height) between the top and bottom of active region loops, and the inconsistency between loop densities derived by spectral and photometric means. Treating coronal loops as a mixture of diffuse background and very dense, unresolved filamentary structures addresses these problems with a combination of high plasma density within the structures, which greatly increases the emissivity of the structures, and geometric effects that attenuate the apparent brightness of the feature at low altitudes. It also suggests a possible explanation for both the surprisingly high contrast of EUV coronal loops against the coronal background, and the uniform "typical" height of the bright portion of the corona (about 0.3 R_☉) in full-disk EUV images. Some ramifications of this picture are discussed, including an estimate (10-100 km) of the fundamental scale of strong heating events in the corona.

A streamer puff is a recently identified variety of coronal mass ejection (CME) of narrow to moderate width. It (1) travels out along a streamer, transiently inflating the streamer but leaving it largely intact, and (2) occurs in step with a compact ejective flare in an outer flank of the base of the streamer. These aspects suggest the following magnetic-arch-blowout scenario for the production of these CMEs: the magnetic explosion that produces the flare also produces a plasmoid that explodes up the leg of an outer loop of the arcade base of the streamer, blows out the top of this loop, and becomes the core of the CME. In this paper, we present a streamer-puff CME that produced a coronal dimming footprint. The coronal dimming, its magnetic setting, and the timing and magnetic setting of a strong compact ejective flare within the dimming footprint nicely confirm the magnetic-arch-blowout scenario. From these observations, together with several published cases of a transequatorial CME produced in tandem with an ejective flare or filament eruption that was far offset from directly under the CME, we propose the following. Streamer-puff CMEs are a subclass (one variety) of a broader class of "over-and-out" CMEs that are often much larger than streamer puffs but are similar to them in that they are produced by the blowout of a large quasi-potential magnetic arch by a magnetic explosion that erupts from one foot of the large arch, where it is marked by a filament eruption and/or an ejective flare.

We consider the potential magnetic field associated with a helical electric line current flow, idealizing the near-potential coronal field within which a highly localized twisted current structure is embedded. It is found that this field has a significant axial component off the helical magnetic axis where there is no current flow, such that the flux winds around the axis. The helical line current field, in including the effects of flux rope writhe, is therefore more topologically complex than the straight line and ring current fields sometimes used in solar flux rope models. The axial flux in magnetic fields around confined current structures may be affected by the writhe of these current structures such that the field twists preferentially with the same handedness as the writhe. This property of fields around confined current structures with writhe may be relevant to classes of coronal magnetic flux ropes, including structures observed to have sigmoidal forms in soft X-rays and prominence magnetic fields. For example, "bald patches" and the associated heating by Parker current sheet dissipation seem likely. Thus, some measurements of flux rope magnetic helicities may derive from external, near-potential fields. The predicted hemispheric preference for positive and negative magnetic helicities is consistent with observational results for prominences and sigmoids and past theoretical results for flux rope internal fields.

Using Doppler velocity data from the SOI/MDI instrument on board the SOHO spacecraft, we do time-distance helioseismic inversions for sound-speed perturbations beneath 16 sunspots observed in high-resolution mode. We clearly detect ringlike regions of enhanced sound speed beneath most sunspot penumbrae, extending from near the surface to depths of about 3.5 Mm. Due to their location and dependence on frequency bands of p-modes used, we believe these rings to be artifacts produced by a surface signal probably associated with the sunspot magnetic field.

Groups of linear g-modes can sum to create nonlinear motion in small "hot volumes" (diameter ~10 Mm) near the solar center that help drive the modes. We explore the consequences of coupling only in the hot volumes where the time-averaged rate of ³He burning can double if temperature oscillations exceed 10%. Anticipating large local motions in the core, we impose a mixed shell r = (0.10 ± 0.03) R_☉ on an otherwise standard solar model before computing g-mode solutions. Mixing is rapid (≪10⁶ yr) in this shell, with slower mixing somewhat beyond. If l is the principal spherical harmonic index, a set of g-modes for any single l ≤ 5 with six consecutive radial harmonics can be excited with nearly linear thermal amplitudes A_T ≤ 0.05 throughout the star. But far smaller thresholds for excitation are actually expected when sets for many values of l can be computed simultaneously. This is a new kind of stellar instability whose effectiveness rises with the number of active modes. Each set rotates rigidly and maximizes the release of nuclear energy from its hot volumes. There is some evidence for their rotation rates in the long solar activity record. The upward wave flux powered by the hot volumes has also been suggested to explain the 1.3 yr reversing flows tentatively detected below the Sun's convective envelope. An analog using uncoupled modes is also investigated based on an observation that indicates g-mode activity up to l ≊ 20. If all modes in that range had linear amplitudes of only A_T ≊ 0.0015, their combined effect would give positive growth rates to dozens of low harmonic g-modes.

There is strong observational evidence for solar p-modes being scattered by sunspots. Understanding and comparing phase shifts or travel time delays of scattered waves can present an opportunity to deduce the subsurface structure of sunspots from the observations of p-modes. We study the scattering of acoustic waves by magnetic flux tubes embedded in the stratified atmosphere, taking into account magnetic field perturbations. For this purpose, we solve a set of linearized MHD equations using the Born approximation approach. It is shown that convergence of the magnetic field may substantially affect the phase shifts of the scattered waves.

The interplay between the proton-alpha-particle differential flow speed, v_αp, and angular momentum transport in the solar wind is explored by using a three-fluid model. The force introduced by the azimuthal components is found to play an important role in the force balance for ions in interplanetary space, bringing the radial flow speeds of protons and alpha particles closer to each other. For the fast solar wind, the model cannot account for the decrease of v_αp observed by Helios between 0.3 and 1 AU. However, it can reproduce the v_αp profile measured by Ulysses beyond 2 AU, if the right value for v_αp is imposed at that distance. In the slow wind, the effect of solar rotation is more pronounced if one starts with the value measured by Helios at 0.3 AU: a relative change of 10%-16% is introduced in the radial speed of the alpha particles between 1 and 4 AU. The model calculations show that, although alpha particles consume only a small fraction of the energy and linear momentum fluxes of protons, they cannot be neglected when considering the proton angular momentum flux L_p. In most examples, it is found that L_p is determined by v_αp for both the fast and the slow wind. In the slow solar wind, the proton and alpha particle angular momentum fluxes L_p and L_α can be several times larger in magnitude than the flux carried by the magnetic stresses L_M. While the sum L_P = L_p + L_α is smaller than L_M, for the modeled fast and slow wind alike, this result is at variance with the Helios measurements.

As many as five ice giants—Neptune-mass planets composed of ~90% ice and rock and ~10% hydrogen—are thought to form at heliocentric distances of ~10-25 AU on closely packed orbits spaced ~5 Hill radii apart. Such oligarchies are ultimately unstable. Once the parent disk of planetesimals is sufficiently depleted, oligarchs perturb one another onto crossing orbits. We explore both the onset and outcome of the instability through numerical integrations, including dynamical friction cooling of planets by a planetesimal disk whose properties are held fixed. To trigger instability and the ejection of the first ice giant in systems having an original surface density in oligarchs of Σ ~ 1 g cm^-2, the disk surface density σ must fall below ~0.1 g cm^-2. Ejections are predominantly by Jupiter and occur within ~10⁷ yr. To eject more than one oligarch requires σ ≲ 0.03 g cm^-2. For certain choices of σ and initial semimajor axes of planets, systems starting with up to four oligarchs in addition to Jupiter and Saturn can readily yield solar system-like outcomes in which two surviving ice giants lie inside 30 AU and have their orbits circularized by dynamical friction. Our findings support the idea that planetary systems begin in more crowded and compact configurations, like those of shear-dominated oligarchies. In contrast to previous studies, we identify σ ≲ 0.1Σ as the regime relevant for understanding the evolution of the outer solar system, and we encourage future studies to concentrate on this regime while relaxing our assumption of a fixed planetesimal disk. Whether evidence of the instability can be found in Kuiper Belt objects (KBOs) is unclear, since in none of our simulations do marauding oligarchs excite as large a proportion of KBOs having inclinations ≳20° as is observed.

We have designed and constructed a "dispersed Fourier transform spectrometer" (dFTS), consisting of a conventional FTS followed by a grating spectrometer. By combining these two devices, we negate a substantial fraction of the sensitivity disadvantage of a conventional FTS for high-resolution, broadband, optical spectroscopy, while preserving many of the advantages inherent to interferometric spectrometers. In addition, we have implemented a simple and inexpensive laser metrology system, which enables very precise calibration of the interferometer wavelength scale. The fusion of interferometric and dispersive technologies with a laser metrology system yields an instrument well suited to stellar spectroscopy, velocimetry, and extrasolar planet detection, which is competitive with existing high-resolution, high-accuracy stellar spectrometers. In this paper we describe the design of our prototype dFTS, explain the algorithm we use to efficiently reconstruct a broadband spectrum from a sequence of narrowband interferograms, and present initial observations and resulting velocimetry of stellar targets.

We propose a method for a differential imager that makes use of a variable channeled spectrum. The channeled spectrum is an oscillating spectrum with respect to wavelength, generated by two polarizers with a retarder between them. A variable retarder, inserted between these polarizers, modulates the channeled spectrum. By restricting a spectral band with a bandpass filter, the whole optical system acts as a common-path variable bandpass filter with two specific central wavelengths. Provided that the temporal modulation of the channeled spectrum is fast enough compared to that of the evolution of phase aberrations, it is possible to obtain spectral differential images free from non-common-path aberrations. Furthermore, polarization differential images are also available by inserting another variable retarder in front of this optical system. Thus, both common-path spectral and polarization differential images can be obtained. We propose using a liquid-crystal variable retarder, which has been successfully demonstrated in a polarization differential imager equipped with a four-quadrant polarization mask coronagraph. In order to achieve a higher system throughput, we suggest using Wollaston prisms instead of polarizers. In this paper, we describe the principle of this technique. Results of a preliminary laboratory experiment and a numerical simulation of the spectral differential imager are also given.

We introduce a new technique to detect the discrete temperature steps that cosmic strings might have left in the cosmic microwave background (CMB) anisotropy map. The technique provides a validity test on the pattern search of cosmic strings that could serve as the groundwork for future pattern searches. The detecting power of the technique is only constrained by two unavoidable features of CMB data: (1) the finite pixelization of the sky map and (2) the Gaussian fluctuation from instrumental noise and primordial anisotropy. We set the upper limit on the cosmic string parameter as Gμ ≲ 1.6 × 10^-5 at the 95% confidence level (CL) and find that the amplitude of the temperature step has to be greater than 44 μK in order to be detected for the Wilkinson Microwave Anisotropy Probe (WMAP) 3 year data.

We observe a sharp transition from a singular, high-mass mode of star formation to a low-mass-dominated mode, in numerical simulations, at a metallicity of 10^-3Z_☉. We incorporate a new method for including the radiative cooling from metals into adaptive mesh refinement hydrodynamic simulations. Our results illustrate how metals, produced by the first stars, led to a transition from the high-mass star formation mode of Population III stars to the low-mass mode that dominates today. We ran hydrodynamic simulations with cosmological initial conditions in the standard ΛCDM model, with metallicities, from zero to 10^-2Z_☉, beginning at redshift z = 99. The simulations were run until a dense core forms at the center of a 5 × 10⁵M_☉ dark matter halo, at z ~ 18. Analysis of the central 1 M_☉ core reveals that the two simulations with the lowest metallicities, Z = 0 and 10^-4Z_☉, contain one clump with 99% of the mass, while the two with metallicities Z = 10^-3 and 10^-2Z_☉ each contain two clumps that share most of the mass. The Z = 10^-3Z_☉ simulation also produced two low-mass protostellar objects with masses between 10^-2 and 10^-1M_☉. Gas with Z ≥ 10^-3Z_☉ is able to cool to the temperature of the cosmic microwave background (CMB), which sets a lower limit to the minimum fragmentation mass. This suggests that the second-generation stars produced a spectrum of lower mass stars but were still more massive, on average, than stars formed in the local universe.

We investigate the dust extinction properties in the host galaxy of GRB 050904 at z = 6.29 by analyzing simultaneous broadband observations of the optical and UV afterglow at three different epochs. We show that the peculiar afterglow spectral energy distribution (SED) observed at 0.5 days and at 1 day after the burst (1.6 and 3 hr rest frame) cannot be explained by dust reddening with any of the extinction curves observed at low redshift. Yet, the extinction curve recently inferred for the most distant BAL QSO at z = 6.2 nicely reproduces the SED of GRB 050904 at both epochs. Our result provides an additional, independent indication that the properties of dust evolve beyond z ~ 6. We discuss the implications of this finding within the context of the dust production mechanisms through the cosmic ages.

The uncertainty surrounding the nature of the heating mechanism for the dust that emits at mid- to far-IR (MFIR) wavelengths in active galaxies limits our understanding of the links between active galactic nuclei (AGNs) and galaxy evolution, as well as our ability to interpret the prodigious infrared and submillimeter emission of some of the most distant galaxies in the universe. Here we report deep Spitzer observations of a complete sample of powerful, intermediate-redshift (0.05 < z < 0.7) radio galaxies and quasars. We show that AGN power, as traced by [O III] λ5007 emission, is strongly correlated with both the mid-IR (24 μm) and the far-IR (70 μm) luminosities, but with increased scatter in the 70 μm correlation. A major cause of this increased scatter is a group of objects that falls above the main correlation and shows signs of prodigious recent star formation activity at optical wavelengths, along with relatively cool MFIR colors. These results provide evidence that illumination by the AGNs is the primary heating mechanism for the dust emitting at both 24 and 70 μm, with starbursts dominating the heating of the cool dust in only 20%-30% of objects. This implies that powerful AGNs are not always accompanied by the type of luminous starbursts that are characteristic of the peak of activity in major gas-rich mergers.

We investigate a magnetized plasma in which injected high-energy γ-rays annihilate on a soft photon field that is provided by the synchrotron radiation of the created pairs. For a very wide range of magnetic fields, this process involves γ-rays between 0.3 GeV and 30 TeV. We derive a simple dynamical system for this process, analyze its stability to runaway production of soft photons and pairs, and find conditions for it to automatically quench by reaching a steady state with an optical depth to photon-photon annihilation larger than unity. We discuss applications to broadband γ-ray emitters, in particular supermassive black holes. Automatic quenching limits the γ-ray luminosity of these objects and predicts substantial pair loading of the jets of less active sources.

This Letter reports on three-dimensional (3D) simulations of Kerr black hole magnetospheres that obey the general relativistic equations of perfect magnetohydrodynamics (MHD). In particular, we study powerful Poynting flux-dominated jets that are driven from dense gas in the equatorial plane in the ergosphere, the physics of which has been previously studied in the simplified limit of an ergospheric disk. For high-spin black holes, a/M > 0.95, the ergospheric disk is prominent in the 3D simulations and is responsible for greatly enhanced Poynting flux emission. Any large-scale poloidal magnetic flux that is trapped in the equatorial region leads to an enormous release of electromagnetic energy that dwarfs the jet energy produced by magnetic flux threading the event horizon. The implication is that magnetic flux threading the equatorial plane of the ergosphere is a likely prerequisite for the central engine of powerful FR II quasars.

We report the first detection of the 6.2 and 7.7 μm infrared polycyclic aromatic hydrocarbon (PAH) emission features in the spectrum of a high-redshift QSO, from the Spitzer IRS spectrum of the Cloverleaf lensed QSO (H1413+117, z ~ 2.56). The ratio of PAH features and rest-frame far-infrared emission is the same as in lower luminosity star-forming ULIRGs and in local PG QSOs, supporting a predominantly starburst nature of the Cloverleaf's huge far-infrared luminosity (5.4 × 10¹²L_☉, corrected for lensing). The Cloverleaf's period of dominant QSO activity (L_Bol ~ 7 × 10¹³L_☉) is coincident with an intense (star formation rate ~1000 M_☉ yr^-1) and short (gas exhaustion time ~3 × 10⁷ yr) star-forming event.

We present diffraction-limited, 10 μm imaging polarimetry data for the central regions of the archetypal Seyfert active galactic nucleus NGC 1068. The position angle of polarization is consistent with three dominant polarizing mechanisms. We identify three distinct regions of polarization: (1) north of the nucleus, arising from aligned dust in the narrow emission line region, (2) south, east, and west of the nucleus, consistent with dust being channeled toward the central engine, and (3) a central minimum of polarization consistent with a compact (≤22 pc) torus. These observations provide continuity between the geometrically and optically thick torus and the host galaxy's nuclear environments. These images represent the first published mid-IR polarimetry from an 8 m-class telescope and illustrate the potential of such observations.

We present the statistically complete and cosmologically most relevant subset of the 12 most distant galaxy clusters detected at z > 0.5 by the Massive Cluster Survey (MACS). Ten of these systems are new discoveries; only two (MACS J0018.5+1626, aka Cl 0016+1609, and MACS J0454.1-0300, aka MS 0451.6-0305) were previously known. We provide fundamental cluster properties derived from our optical and X-ray follow-up observations as well as the selection function in tabulated form to facilitate cosmological studies using this sample.

We investigate the correlation between the bulge effective radius (r_e) and disk scale length (r_d) in the near-infrared K band for lenticular galaxies in the field and in clusters. We find markedly different relations between the two parameters as a function of luminosity. Lenticulars with total absolute magnitude fainter than M_T = -24.5 show a positive correlation, in line with predictions of secular formation processes for the pseudobulges of late-type disk galaxies. But brighter lenticulars with M_T < -24.5 show an anticorrelation, indicating that they formed through a different mechanism. The available data are insufficient to reliably determine the effect of galaxy environment on these correlations.

We combine Spitzer 24 μm observations with data from the COMBO-17 survey for ~15,000 0.2 < z ≤ 1 galaxies to determine how the average star formation rates (SFRs) have evolved for galaxy subpopulations of different stellar masses. In the determination of , we consider both the ultraviolet (UV) and the infrared (IR) luminosities, and account for the contributions of galaxies that are individually undetected at 24 μm through image stacking. For all redshifts, we find that higher mass galaxies have a substantially lower specific SFR, ⟨SFR⟩/⟨M_*⟩, than lower mass ones. However, we find the striking result that the rate of decline in cosmic SFR with redshift is nearly the same for massive and low mass galaxies, i.e., not a strong function of stellar mass. This analysis confirms one version of what has been referred to as "downsizing," namely, that the epoch of major mass buildup in massive galaxies is substantially earlier than the epoch of mass buildup in low-mass galaxies. Yet it shows that star formation activity is not becoming increasingly limited to low-mass galaxies toward the present epoch. We argue that this suggests that heating by AGN-powered radio jets is not the dominant mechanism responsible for the decline in cosmic SFR since z ~ 1, which is borne out by comparison with semianalytic models that include this effect.

The Angstrom Project is undertaking an optical survey of stellar microlensing events across the bulge region of the Andromeda galaxy (M31) using a distributed network of 2 m class telescopes. The Angstrom Project Alert System (APAS) has been developed to identify candidate microlensing and transient events in real time, using data from the Liverpool and Faulkes North robotic telescopes. This is the first time that real-time microlensing discovery has been attempted outside of the Milky Way and its satellite galaxies. The APAS is designed to enable follow-up studies of M31 microlensing systems, including searches for gas giant planets in M31. Here we describe the APAS, and we present a few example light curves obtained during its commissioning phase that clearly demonstrate its real-time capability to identify microlensing candidates as well as other transient sources.

About 25% of the Milky Way globular clusters (GCs) exhibit unusually extended color distribution of stars in the core helium-burning horizontal-branch (HB) phase. This phenomenon is now best understood as due to the presence of helium-enhanced second-generation subpopulations, which has raised the possibility that these peculiar GCs might have a unique origin. Here we show that these GCs with extended HB are clearly distinct from other normal GCs in kinematics and mass. The GCs with extended HB are more massive than normal GCs and are dominated by random motion with no correlation between kinematics and metallicity. Surprisingly, however, when they are excluded, most normal GCs in the inner halo show clear signs of dissipational collapse that apparently led to the formation of the disk. Normal GCs in the outer halo share their kinematic properties with the extended HB GCs, which is consistent with the accretion origin. Our result further suggests heterogeneous origins of GCs, and we anticipate this to be a starting point for more detailed investigations of Milky Way formation, including early mergers, collapse, and later accretion.

Accurate photometry with HST's ACS shows that the main sequence (MS) of the globular cluster NGC 2808 splits into three separate branches. The three MS branches may be associated with the complexities of the cluster's horizontal branch and of its abundance distribution. We attribute the MS branches to successive rounds of star formation, with different helium abundances; we discuss possible sources of helium enrichment. Some other massive globulars also appear to have complex populations; we compare them with NGC 2808.

We present FUSE observations of the line of sight to WD 0439+466 (LS V +46 21), the central star of the old planetary nebula Sh 2-216. The FUSE data show absorption by many interstellar and stellar lines, in particular D I, H₂ (J = 0-9), HD J = 0-1, and CO. Many other stellar and ISM lines are detected in the STIS E140M HST spectra of this sight line, which we use to determine N(H I). We derive, for the neutral gas, D/H = 0.76 × 10^-5, O/H = 0.89 × 10^-4, and N/H = 3.24 × 10^-5. We argue that most of the gas along this sight line is associated with the planetary nebula. The low D/H ratio is likely the result of this gas being processed through the star (astrated) but not mixed with the ISM. This would be the first time that the D/H ratio has been measured in predominantly astrated gas. The O/H and N/H ratios derived here are lower than typical values measured in other planetary nebulae likely due to unaccounted for ionization corrections.

Radio astronomical data of C₆H^- observed with the Nobeyama 45 m and IRAM 30 m telescopes have been analyzed by the local thermodynamic equilibrium approximation to give the column density of (6.1-8.0) × 10¹² cm^-2 and the excitation temperature of 32 ± 3 K, with an assumed source size of 30'' ± 3''. The abundance of C₆H^- was estimated to be 8.6% of C₆H. The observed line shapes of C₆H ^- and C₆H indicate that the distribution of C ₆H^- is more present in the inner region than C₆H. The production mechanism of C₆H^- is discussed.

We present an empirical spectral modeling of the high-energy emission of the anomalous X-ray pulsar 4U 0142+614 based on simultaneous Swift and INTEGRAL observations from X- to γ-ray energies. We adopted models contained in the XSPEC analysis package, as well as models based on recent theoretical studies, and restricted ourselves to those combinations of up to three components that produced a good fit while requiring the lowest number of free parameters. Only three models were found to satisfactorily fit the 0.5-250 keV spectrum of 4U 0142+614: (1) a ~0.4 keV blackbody and two power laws, (2) a resonant cyclotron scattering model plus a power law, and (3) two log-parabolic functions. We found that only the latter two models do not overpredict the infrared/optical emission observed simultaneously from this AXP, and only the log-parabolic functions can naturally account for the upper limits set by COMPTEL in the γ-ray range. A possible interpretation of the two log parabolae in terms of inverse Compton scattering of soft X-ray photons by very energetic particles is discussed.

We present observations of pure rotational molecular hydrogen emission from the Herbig Ae star, AB Aur. Our observations were made using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for H₂ emission in the S(1), S(2), and S(4) lines at high spectral resolution and detected all three. By fitting a simple model for the emission in the three transitions, we derive T = 670 ± 40 K and M = 0.52 ± 0.15 M_⊕ for the emitting gas. On the basis of the 8.5 km s^-1 FWHM of the S(2) line, assuming the emission comes from the circumstellar disk, and with an inclination estimate of the AB Aur system taken from the literature, we place the location for the emission near 18 AU. Comparison of our derived temperature to a disk structure model suggests that UV and X-ray heating are important in heating the disk atmosphere.

Disk instability is an attractive yet controversial means for the rapid formation of giant planets in our solar system and elsewhere. Recent concerns regarding the first adiabatic exponent of molecular hydrogen gas are addressed and shown not to lead to spurious clump formation in the author's disk instability models. A number of disk instability models have been calculated in order to further test the robustness of the mechanism, exploring the effects of changing the pressure equation of state, the vertical temperature profile, and other parameters affecting the temperature distribution. Possible reasons for differences in results obtained by other workers are discussed. Disk instability remains as a plausible formation mechanism for giant planets.

We report on the results of the first 3D SPH simulations of gravitationally unstable protoplanetary disks with radiative transfer. We adopt a flux-limited diffusion scheme justified by the high opacity of most of the disk. The optically thin surface of the disk cools as a blackbody. We find that gravitationally bound clumps with masses close to a Jupiter mass can arise. Fragmentation appears to be driven by vertical convective-like motions capable of transporting the heat from the disk midplane to its surface on a timescale of only about 40 years at 10 AU. A larger or smaller cooling efficiency of the disk at the optically thin surface can promote or stifle fragmentation by affecting the vertical temperature profile, which determines whether convection can happen or not, and by regulating accretion from optically thin regions toward overdense regions. We also find that the chances of fragmentation increase for a higher mean molecular weight, μ, since compressional heating is reduced. Only disks with masses >0.12 M_☉ and with μ ≥ 2.4, as expected for gas with a metallicity comparable to solar or higher, can fragment.

We explore the possibility that large-scale convection is inhibited over some regions of giant planet interiors, as a consequence of a gradient of composition inherited from either their formation history or particular events like giant impacts or core erosion during their evolution. Under appropriate circumstances, the redistribution of the gradient of molecular weight can lead to double diffusive layered or overstable convection. This leads to much less efficient heat transport and compositional mixing than large-scale adiabatic convection. We show that this process can explain the abnormally large radius of the transit planet HD 209458b and similar objects and may be at play in some giant planets, with short-period planets offering the most favorable conditions. Observational signatures of this transport mechanism are a large radius and a reduced heat flux output compared with uniformly mixed objects. If our suggestion is correct, it bears major consequences on our understanding of giant planet formation, structure and evolution, including possibly our own Jovian planets.

We report the first scattered light detection of a dusty debris disk surrounding the F2 V star HD 15115 using the Hubble Space Telescope in the optical and Keck adaptive optics in the near-infrared. The most remarkable property of the HD 15115 disk relative to other debris disks is its extreme length asymmetry. The east side of the disk is detected to ~315 AU radius, whereas the west side of the disk has radius >550 AU. We find a blue optical to near-infrared scattered light color relative to the star that indicates grain scattering properties similar to the AU Mic debris disk. The existence of a large debris disk surrounding HD 15115 adds further evidence for membership in the β Pic moving group, which was previously argued based on kinematics alone. Here we hypothesize that the extreme disk asymmetry is due to dynamical perturbations from HIP 12545, an M star east of HD 15115 that shares a common proper motion vector, heliocentric distance, galactic space velocity, and age.

Cometary nuclei are believed to contain important information on the condition of the solar nebula, but there is little observational data available on their interior structure. Our ground-based observations of NASA's Deep Impact event show that comet 9P/Tempel 1 has a surface layer consisting of small (submicron-sized) carbonaceous grains whose thickness is several tens of centimeters. This suggests that comet 9P/Tempel 1 contains, at several tens of centimeters of depth, material that has not metamorphosed since this comet left the trans-Neptunian region. This further implies that many short-period comets may maintain the components they had upon leaving the trans-Neptunian region at ~1 m of depth from the surface, even after numerous perihelion passages.

In 2007 January, at the heliocentric distance r < 0.3 AU, comet McNaught 2006P1 became the brightest comet since C/Ikeya-Seki 1965S1 and was continuously monitored by space-based solar observatories. We provide strong evidence that an archlike tail observed by the Heliospheric Imager aboard the STEREO spacecraft is the first ever detected tail composed of neutral Fe atoms. We obtain an Fe lifetime τ = (4.1 ± 0.4) × 10⁴ s at r = 0.25 AU, in agreement with theoretical predictions of the photoionization lifetime. The expected dust temperature is inconsistent with iron sublimation, suggesting that Fe atoms are coming from troilite evaporation.

We present spatially resolved measurements of the rotational temperature and ortho-para ratio for H₂O in the inner coma of the Oort Cloud comet C/2004 Q2 (Machholz). Our results are based on direct simultaneous detections of ortho-H₂O and para-H₂O via "hot-band" fluorescence near 2.9 μm. We find a well-defined decline in rotational temperature with increasing nucleocentric distance (up to ~1000 km). The ortho-para ratio remains constant (within stochastic uncertainty) with increasing nucleocentric distance and is close to the statistical equilibrium value of 3.0 (2.86 ± 0.06 [0.17], including, respectively, stochastic [systematic] uncertainty), resulting in spin temperature T_spin ≥ 34 K. We compare the present results with those reported for other comets and discuss the difficulties in interpreting spin temperatures deduced from measured ortho-para ratios. Improved understanding of the special conditions that enable nuclear spin conversion would test the extent to which derived spin temperatures reflect the formative history or the processing record of cometary ices.

Carbon monoxide emission was targeted in fragment C of the recently split Jupiter-family comet 73P/Schwassmann-Wachmann 3 during its 2006 apparition, using the Cryogenic Echelle Spectrograph (CSHELL) at the NASA IRTF on Mauna Kea, Hawaii. Simultaneous sounding with H₂O near 4.65 μm revealed highly depleted CO, consistent with a mixing ratio of 0.5% ± 0.13%. Along with depleted CH₃OH but nearly normal HCN, this may indicate that this comet formed in the inner giant planets' region or, alternatively, that it formed relatively late, after significant clearing of the protosolar nebula.

Two scenarios of possible ion heating due to finite amplitude parallel propagating Alfvén waves in the solar atmosphere are investigated using a one-dimensional test particle approach. (1) A finite amplitude Alfvén wave is instantly introduced into a plasma (or equivalently, new ions are instantly created). (2) New ions are constantly created. In both scenarios, ions will be picked up by the Alfvén wave. In case 1, the wave scatters ions in the transverse direction leading to a randomization (or heating) process. This process is complete when a phase shift of ±π in the ion gyrospeed is produced between particles with characteristic parallel thermal speed and particles with zero parallel speed. This corresponds to t = π/kv_th (k is the wavenumber, and v_th is the ion thermal speed). A ring velocity distribution can be produced for a large wave amplitude. The process yields a mass-proportional heating in the transverse direction, a temperature anisotropy, and a bulk flow along the background magnetic field. In case 2, continuous ion creation represents a continuing phase shift in the ion gyrospeed leading to heating. New particles are picked up by the Alfvén wave within one ion gyroperiod. It is speculated that the mechanism may operate in the chromosphere and active regions where transient events may generate finite amplitude Alfvén waves.

We define the effective connected magnetic field, B_eff, a single metric of the flaring potential in solar active regions. We calculated B_eff for 298 active regions (93 X- and M-flaring, 205 nonflaring) as recorded by SOHO/MDI during a 10 yr period covering much of solar cycle 23. We find that B_eff is a robust criterion for distinguishing flaring from nonflaring regions. A well-defined 12 hr conditional probability for major flares depends solely on B_eff. This probability exceeds 0.95 for M-class and X-class flares if B_eff > 1600 G and B_eff > 2100 G, respectively, while the maximum calculated B_eff-values are near 4000 G. Active regions do not give M-class and X-class flares if B_eff < 200 G and B_eff < 750 G, respectively. We conclude that B_eff is an efficient flare-forecasting criterion that can be computed on a timely basis from readily available data.