Table of contents

We present a cosmologically motivated model for the hierarchical formation of the stellar halo that includes a semianalytic treatment of galactic chemical enrichment coupled to numerical simulations that track the orbital evolution and tidal disruption of satellites. A major motivating factor in this investigation is the observed systematic difference between the chemical abundances of stars in satellite galaxies and those in the Milky Way halo. Specifically, for the same [Fe/H] values, stars in neighboring satellite galaxies display significantly lower [α/Fe] ratios than stars in the halo. We find that the observed chemical abundance patterns are a natural outcome of the process of hierarchical assembly of the Galaxy. This result follows because the stellar halo in this context is built up from satellite galaxies accreted early on (more than 8-9 Gyr ago) and enriched in α-elements produced in Type II supernovae. In contrast, satellites that still survive today are typically accreted late (within the last 4-5 Gyr) with nearly solar [α/Fe] values as a result of contributions from both Type II and Type Ia supernovae. We use our model to investigate the abundance distribution functions (using both [Fe/H] and [α/Fe] ratios) for stars in the halo and in surviving satellites. Our results suggest that the shapes and peaks in the abundance distribution functions provide a direct probe of the accretion histories of galaxies.

We present results from optical narrowband (λ_c = 8150 Å and Δλ = 120 Å) observations of the Great Observatories Origins Deep Survey (GOODS) fields, using Suprime-Cam on the Subaru Telescope. Using these narrowband data, we then perform a survey of Lyα emitters (LAEs) at z ~ 5.7. The LAE survey covers an area of ≈320 arcmin² and a comoving volume of ≃8.0 × 10⁴ Mpc³. We found a total of 10 (GOODS-N) and 4 (GOODS-S) LAE candidates at z ~ 5.7. We perform a study of the spatial distribution, space density, and star formation properties of the LAEs at z ~ 5.7.

This is the first paper in a series presenting and analyzing data for a K-selected sample of galaxies collected in order to identify and study galaxies at moderate to high redshift in rest-wavelength optical light. The sample contains 842 objects over six separate fields covering 75.6 arcmin² down to K = 20-20.5. We combine the K band with UBVRIzJH multiband imaging, reaching depths of R ~ 26. Two of the fields studied also have deep HST WFPC2 imaging, totaling more than 60 hr in the F300W, F450W, F606W, and F814W filters. Using artificial galaxy modeling and extraction, we measure 85% completeness limits down to K = 19.5-20, depending on the field examined. The derived K-band number counts are in good agreement with previous studies. We find a density for extremely red objects (EROs; R - K > 5) of 1.55 ± 0.16 arcmin^-2 for K < 19.7, dominated by the 1714+5015 field (centered on 53w002), with an ERO number density more than 3 times that of the other sample fields. If we exclude the counts for 1714+5015, our density is 0.95 ± 0.14 arcmin^-2. Both ERO densities are consistent with previous measurements due to the significant known cosmic variance of these red sources. Keck spectroscopic redshifts were obtained for 18 of the EROs, all but one of which are emission galaxies. None of the EROs in the 1714+5015 field for which we obtained spectroscopic redshifts are associated with the known z = 2.39 overdensity, although there are three different galaxy redshift pairs (z = 0.90, 1.03, and 1.22).

Spectra have been obtained with the Infrared Spectrograph on the Spitzer Space Telescope for 18 optically faint sources (R ≳ 23.9 mag) having f_ν(24 μm) > 1.0 mJy and having radio detections at 20 cm to a limit of 115 μJy. The sources are within the Spitzer First Look Survey. Redshifts are determined for 14 sources from strong silicate absorption features (12 sources) or strong PAH emission features (two sources), with median redshift of 2.1. Results confirm that optically faint sources of ~1 mJy at 24 μm are typically at redshifts z ~ 2, verifying the high efficiency in selecting high-redshift sources based on extreme infrared-to-optical flux ratio, and indicate that 24 μm sources that also have radio counterparts are not systematically different than samples chosen only by their infrared-to-optical flux ratios. Using the parameter q = log[f_ν(24 μm)/f_ν(20 cm)], 17 of the 18 sources observed have values of 0 < q < 1, in the range expected for starburst-powered sources, but only a few of these show strong PAH emission as expected from starbursts, with the remainder showing absorbed or power-law spectra consistent with an AGN luminosity source. This confirms previous indications that optically faint Spitzer sources with f_ν(24 μm) ≳ 1.0 mJy are predominately AGNs and represent the upper end of the luminosity function of dusty sources at z ~ 2. Based on the characteristics of the sources observed so far, we predict that the nature of sources selected at 24 μm will change for f_ν(24 μm) ≲ 0.5 mJy to sources dominated primarily by starbursts.

We present new measurements of the quasar angular autocorrelation function from a sample of ~80,000 photometrically classified quasars taken from the First Data Release of the Sloan Digital Sky Survey. We find a best-fit model of ω(θ) = (0.066)θ^{-(0.98±0.15)} for the angular correlation function, consistent with estimates of the slope from spectroscopic quasar surveys. We show that only models with little or no evolution in the clustering of quasars in comoving coordinates since a median redshift of z ~ 1.4 can recover a scale length consistent with local galaxies and active galactic nuclei (AGNs). A model with little evolution of quasar clustering in comoving coordinates is best explained in the current cosmological paradigm by rapid evolution in quasar bias. We show that quasar biasing must have changed from b_Q ~ 3 at a (photometric) redshift of _phot = 2.2 to b_Q ~ 1.2-1.3 by _phot = 0.75. Such a rapid increase with redshift in biasing implies that quasars at z ~ 2 cannot be the progenitors of modern L* objects; rather they must now reside in dense environments, such as clusters. Similarly, the duration of the UVX (ultraviolet-excess) quasar phase must be short enough to explain why local UVX quasars reside in essentially unbiased structures. Our estimates of b_Q are in good agreement with recent spectroscopic results (Croom et al. 2005), which demonstrate that the implied evolution in b_Q is consistent with quasars inhabiting halos of similar mass at every redshift. Treating quasar clustering as a bivariate function of both redshift and luminosity, we find no evidence for luminosity dependence in quasar clustering, and that redshift evolution thus affects quasar clustering more than changes in quasars' luminosity. Our results are robust against a range of systematic uncertainties. We provide a new method for quantifying stellar contamination in photometrically classified quasar catalogs via the correlation function.

We report Chandra ACIS observations of the fields of four QSOs showing strong extended optical emission-line regions. Two of these show no evidence for significant extended X-ray emission. The remaining two fields, those of 3C 249.1 and 4C 37.43, show discrete (but resolved) X-ray sources at distances ranging from ~10 to ~40 kpc from the nucleus. In addition, 4C 37.43 also may show a region of diffuse X-ray emission extending out to ~65 kpc and centered on the QSO. It has been suggested that extended emission-line regions such as these may originate in the cooling of a hot intragroup medium. We do not detect a general extended medium in any of our fields, and the upper limits we can place on its presence indicate cooling times of at least a few 10⁹ yr. The discrete X-ray emission sources we detect cannot be explained as the X-ray jets frequently seen associated with radio-loud quasars, nor can they be due to electron scattering of nuclear emission. The most plausible explanation is that they result from high-speed shocks from galactic superwinds resulting either from a starburst in the QSO host galaxy or from the activation of the QSO itself. Evidence from the densities and velocities found in studies of the extended optical emission around QSOs also supports this interpretation.

We present the first INTEGRAL AGN catalog, based on observations performed from launch of the mission in 2002 October until 2004 January. The catalog includes 42 AGNs, of which 10 are Seyfert 1, 17 are Seyfert 2, and 9 are intermediate Seyfert 1.5. The fraction of blazars is rather small, with five detected objects, and only one galaxy cluster and no starburst galaxies have been detected so far. A complete subset consists of 32 AGNs with a significance limit of 7 σ in the INTEGRAL ISGRI 20-40 keV data. Although the sample is not flux limited, the distribution of sources shows a ratio of obscured to unobscured AGNs of 1.5-2.0, consistent with luminosity-dependent unified models for AGNs. Only four Compton-thick AGNs are found in the sample. Based on the INTEGRAL data presented here, the Seyfert 2 spectra are slightly harder (Γ = 1.95 ± 0.01) than Seyfert 1.5 (Γ = 2.10 ± 0.02) and Seyfert 1 (Γ = 2.11 ± 0.05).

H1426+428 is one of many blazars that are observed by γ-rays in the TeV region. Because TeV γ-rays from distant sources are subject to attenuation by the extragalactic background light (EBL) via electron-positron pair production, the intrinsic spectrum of the TeV γ-rays should be inferred by using the models of radiation processes and EBL spectrum. We set constraints on the physical condition of H1426+428 with the synchrotron self-Compton model applying several EBL models. We find that the emission region of H1426+428 is moving toward us with a bulk Lorentz factor of ~20 and that its magnetic field strength is ~0.1 G. These properties are similar to other TeV blazars such as Mrk 421 and Mrk 501. However, the ratio of the energy density of nonthermal electrons to that of the magnetic fields is about 190 and fairly larger than those of Mrk 421 and Mrk 501, which are about 5-20. We also find that the intensity of EBL in the mid- and near-infrared wavelengths should be low; i.e., the intensity at 10 μm is about 1 nW m^-2 sr^-1 to account for the observed TeV γ-ray flux. Because the spectral data of H1426+428 in X-rays and γ-rays used in our analysis were not obtained simultaneously, further observations of TeV blazars are necessary to make the constraints on EBL more stringent.

The discovery of extended, approximately spherical weak shock waves in the hot intercluster gas in Perseus and Virgo has precipitated the notion that these waves may be the primary heating process that explains why so little gas cools to low temperatures. This type of heating has received additional support from recent gasdynamical models. We show here that outwardly propagating, dissipating waves deposit most of their energy near the center of the cluster atmosphere. Consequently, if the gas is heated by (intermittent) weak shocks for several Gyr, the gas within 30-50 kpc is heated to temperatures that far exceed observed values. This heating can be avoided if dissipating shocks are sufficiently infrequent or weak so as not to be the primary source of global heating. Local PV and viscous heating associated with newly formed X-ray cavities are likely to be small, which is consistent with the low gas temperatures generally observed near the centers of groups and clusters where the cavities are located.

We study the clustering properties of groups and of galaxies in groups in the DEEP2 Galaxy Redshift Survey data set at z ~ 1 in three separate fields covering a total of 2 deg². Four measures of two-point clustering in the DEEP2 data are presented: (1) the group correlation function for 460 groups with estimated velocity dispersions of σ ≥ 200 km s^-1; (2) the galaxy correlation for the full DEEP2 galaxy sample, using a flux-limited sample of 9800 objects in the range 0.7 ≤ z ≤ 1.0; (3) the galaxy correlation for galaxies in groups or in the field; and (4) the group-galaxy cross-correlation function. Our results are compared with mock group and galaxy catalogs produced from ΛCDM simulations. Using the observed number density and clustering amplitude of the DEEP2 groups, the estimated minimum group dark matter halo mass is M_min ~ 6 × 10¹²h^-1M_☉. Groups are more clustered than galaxies in the DEEP2 data, with a relative bias of b = 1.17 ± 0.04 on scales r_p = 0.5-15 h^-1 Mpc. The correlation function of galaxies in groups has a steeper slope (γ ~ 2.12 ± 0.06) than for the full galaxy sample (γ ~ 1.74 ± 0.03), and both samples can be fit by a power law on scales r_p = 0.05-20 h^-1 Mpc. We empirically measure the contribution to the projected correlation function, w_p(r_p), for galaxies in groups from a "one-halo" term and a "two-halo" term. The projected cross-correlation between group centers and the full galaxy sample shows that red galaxies are more centrally concentrated in groups than blue galaxies at z ~ 1. DEEP2 galaxies in groups appear to have a shallower radial distribution than that of mock galaxy catalogs made from N-body simulations, which assume a central galaxy surrounded by satellite galaxies with an NFW profile. Using simulations with different halo model parameters, we show that the clustering of galaxies in groups can be used to place tighter constraints on the halo model than can be gained from using the usual galaxy correlation function alone.

We present the results of a series of empirical computations regarding the role of major mergers in forming the stellar masses of modern galaxies based on measured galaxy merger and star formation histories from z ~ 0.5 to 3. We reconstruct the merger history of normal field galaxies from z ~ 3 to z ~ 0 as a function of initial mass using published pair fractions and merger fractions from structural analyses. We calibrate the observed merger timescale and mass ratios for galaxy mergers using self-consistent N-body models of mergers with mass ratios from 1 : 1 to 1 : 5 at various orbital properties and viewing angles. We use these simulations to determine the timescales and mass ratios that produce structures that would be identified as major mergers. Based on these calculations, we argue that a typical massive galaxy at z ~ 3 with M_* > 10¹⁰M_☉ undergoes 4.4 major mergers at z > 1. We find that by z ~ 1.5 the stellar mass of an average massive galaxy is relatively established, a scenario qualitatively favored in a Λ-dominated universe. We argue that the final masses of these systems increase by as much as a factor of 100, allowing Lyman break galaxies, which tend to have low stellar masses, to become the most massive galaxies in today's universe with M > M*. Induced star formation, however, only accounts for 10%-30% of the stellar mass formed in these galaxies at z < 3. A comparison to semianalytic models of galaxy formation shows that cold dark matter (CDM) models consistently underpredict the merger fraction, and rate of merging, of massive galaxies at high redshift. This suggests that massive galaxy formation occurs through more merging than predicted in CDM models, rather than a rapid early collapse.

The Sloan Lens ACS (SLACS) Survey is an efficient Hubble Space Telescope (HST) Snapshot imaging survey for new galaxy-scale strong gravitational lenses. The targeted lens candidates are selected spectroscopically from the Sloan Digital Sky Survey (SDSS) database of galaxy spectra for having multiple nebular emission lines at a redshift significantly higher than that of the SDSS target galaxy. The SLACS survey is optimized to detect bright early-type lens galaxies with faint lensed sources in order to increase the sample of known gravitational lenses suitable for detailed lensing, photometric, and dynamical modeling. In this paper, the first in a series on the current results of our HST Cycle 13 imaging survey, we present a catalog of 19 newly discovered gravitational lenses, along with nine other observed candidate systems that are either possible lenses, nonlenses, or nondetections. The survey efficiency is thus ≥68%. We also present Gemini 8 m and Magellan 6.5 m integral-field spectroscopic data for nine of the SLACS targets, which further support the lensing interpretation. A new method for the effective subtraction of foreground galaxy images to reveal faint background features is presented. We show that the SLACS lens galaxies have colors and ellipticities typical of the spectroscopic parent sample from which they are drawn (SDSS luminous red galaxies and quiescent MAIN sample galaxies), but are somewhat brighter and more centrally concentrated. Several explanations for the latter bias are suggested. The SLACS survey provides the first statistically significant and homogeneously selected sample of bright early-type lens galaxies, furnishing a powerful probe of the structure of early-type galaxies within the half-light radius. The high confirmation rate of lenses in the SLACS survey suggests consideration of spectroscopic lens discovery as an explicit science goal of future spectroscopic galaxy surveys.

We present a unifying empirical description of the structural and kinematic properties of all spheroids embedded in dark matter halos. We find that the intracluster stellar spheroidal components of galaxy clusters, which we call cluster spheroids (CSphs) and which are typically 100 times the size of normal elliptical galaxies, lie on a "fundamental plane" as tight as that defined by elliptical galaxies (rms in effective radius of ~0.07) but having a different slope. The slope, as measured by the coefficient of the log σ term, declines significantly and systematically between the fundamental planes of ellipticals, brightest cluster galaxies (BCGs), and CSphs. We attribute this decline primarily to a continuous change in M_e/L_e, the mass-to-light ratio within the effective radius r_e, with spheroid scale. The magnitude of the slope change requires that it arise principally from differences in the relative distributions of luminous and dark matter, rather than from stellar population differences such as in age and metallicity. By expressing the M_e/L_e term as a function of σ in the simple derivation of the fundamental plane and requiring the behavior of that term to mimic the observed nonlinear relationship between log M_e/L_e and log σ, we simultaneously fit a two-dimensional manifold to the measured properties of dwarf elliptical and elliptical galaxies, BCGs, and CSphs. The combined data have an rms scatter in log r_e of 0.114 (0.099 for the combination of ellipticals, BCGs, and CSphs), which is modestly larger than each fundamental plane has alone, but which includes the scatter introduced by merging different studies done in different filters by different investigators. This "fundamental manifold" fits the structural and kinematic properties of spheroids that span a factor of 100 in σ and 1000 in r_e. While our mathematical form is neither unique nor derived from physical principles, the tightness of the fit leaves little room for improvement by other unification schemes over the range of observed spheroids.

In this paper we compare the predictions of a detailed multizone chemical evolution model for elliptical galaxies with the very recent observations of the galaxy NGC 4697. In particular, the model allows for an initial gas infall and a subsequent galactic wind; it takes into account detailed nucleosynthesis prescriptions of both Types II and Ia supernovae and reproduces the main photochemical properties of normal elliptical galaxies. As a consequence of the earlier development of the wind in the outer regions with respect to the inner ones, we predict an increase of the mean stellar [⟨Mg/Fe⟩] ratio with radius, in very good agreement with the data for NGC 4697. This finding strongly supports the proposed outside-in formation scenario for elliptical galaxies. We also calculate the theoretical "G-dwarf" distributions of stars as functions of both metallicity ([Z/H]) and [Fe/H], showing that they are broad and asymmetric so that a SSP cannot correctly mimic the mixture of stellar populations at any given radius. We also compute the stellar distribution as a function of the [Mg/Fe] ratio, which has negligible "skewness" and is narrower than functions of [Z/H] and [Fe/H] and hence can be better represented by a SSP with an abundance ratio given by the average [⟨Mg/Fe⟩] ratio. Moreover, we compute the luminosity distributions of stars for a typical elliptical galaxy as functions of the [Z/H], [Fe/H], and [Mg/Fe] ratios. We find that these distributions differ from the G-dwarf distributions especially at large radii, except for that as a function of [Mg/Fe]. Therefore, we conclude that in elliptical galaxies the [Mg/Fe] ratio is the most reliable quantity to be compared with observations and is the best estimator of the star formation timescale at each radius.

We present first results from our Very Large Telescope large program to study the dynamical evolution of ultraluminous infrared galaxies (ULIRGs), which are the products of mergers of gas-rich galaxies. The full data set consists of high-resolution long-slit H- and K-band spectra of 38 ULIRGs and 12 QSOs (in the range 0.042 < z < 0.268). In this paper, we present the sources that have not fully coalesced and therefore have two distinct nuclei. This subsample consists of 21 ULIRGs, the nuclear separation of which varies between 1.6 and 23.3 kpc. From the CO band heads that appear in our spectra, we extract the stellar velocity dispersion, σ, and the rotational velocity, V_rot. The stellar dispersion equals 142 km s^-1 on average, while V_rot is often of the same order. We combine our spectroscopic results with high-resolution infrared (IR) imaging data to study the conditions for ULIRG activity in interacting pairs. We find that the majority of ULIRGs are triggered by almost equal-mass major mergers of 1.5 : 1 average ratio. Less frequently, 3 : 1 encounters are also observed in our sample. However, less violent mergers of mass ratio >3 : 1 typically do not force enough gas into the center to generate ULIRG luminosities.

Silicates are an important component of interstellar dust, and the structure of these grains (amorphous or crystalline) is sensitive to the local physical conditions. We have studied the infrared spectra of a sample of ultraluminous infrared galaxies (ULIRGs). Here we report the discovery of weak, narrow absorption features at 11, 16, 19, 23, and 28 μm, characteristic of crystalline silicates, superimposed on the broad absorption bands at 10 and 18 μm due to amorphous silicates in a subset of this sample. These features betray the presence of forsterite (Mg₂SiO₄), the magnesium-rich end member of the olivines. Previously, crystalline silicates have only been observed in circumstellar environments. The derived fraction of forsterite to amorphous silicates is typically 0.1 in these ULIRGs. This is much larger than the upper limit for this ratio in the interstellar medium of the Milky Way, 0.01. These results suggest that the timescale for injection of crystalline silicates into the ISM is short in a merger-driven starburst environment (e.g., as compared to the total time to dissipate the gas), pointing toward massive stars as a prominent source of crystalline silicates. Furthermore, amorphization due to cosmic rays, which is thought to be of prime importance for the local ISM, lags in vigorous starburst environments.

We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to obtain deep photometry of the NGC 300 spiral galaxy in the Sculptor group. The results have been used to derive an accurate distance determination based on the tip of the red giant branch distance estimator. Both edge-detection and maximum likelihood methods have been applied to derive a distance modulus (m - M)₀ = 26.30 ± 0.03 ± 0.12 for edge detection and (m - M)₀ = 26.36 ± 0.02 ± 0.12 for maximum likelihood. These results are fully consistent with the recent distance estimate derived from near-IR photometry of Cepheids variable stars in the context of the Araucaria project, (m - M)₀ = 26.37 ± 0.05 ± 0.03 (Gieren and coworkers).

We present a measurement of the systemic proper motion of the Large Magellanic Cloud (LMC) from astrometry with the High Resolution Camera (HRC) of the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). We observed LMC fields centered on 21 background QSOs that were discovered from their optical variability in the MACHO database. The QSOs are distributed homogeneously behind the central few degrees of the LMC. With two epochs of HRC data and a ~2 yr baseline, we determine the proper motion of the LMC to better than 5% accuracy: μ_W = -2.03 ± 0.08 mas yr^-1, and μ_N = 0.44 ± 0.05 mas yr^-1. This is the most accurate proper-motion measurement for any Milky Way satellite thus far. When combined with H I data from the Magellanic Stream, this should provide new constraints on both the mass distribution of the Galactic halo and models of the Stream.

Using a combination of one-dimensional and three-dimensional hydrodynamic simulations, we have carried out the first in-depth analysis of the remnant's evolution and its various interactions: with the stellar winds flowing out from the inner ~2 pc; with the supermassive black hole, Sgr A*; and with the 50 km s^-1 molecular cloud behind and to the east of the nucleus. We have found that, unlike previous estimates, a rather "standard" supernova explosion with energy ~1.5 × 10⁵¹ ergs would have been sufficient to create the remnant we see today and that the latter is probably only ~1700 yr old. We have found that the passage of the remnant across the black hole would have enhanced the accretion rate onto the central object by less than a factor of 2. Such a small increase cannot explain the current Fe fluorescence observed from the molecular cloud Sgr B2; this fluorescence would have required an increase in the luminosity of Sgr A* by 6 orders of magnitude several hundred years ago. Instead, we have uncovered what appears to be a more plausible scenario for this transient irradiation: the interaction between the expanding remnant and the 50 km s^-1 molecular cloud. The first impact would have occurred about 1200 yr after the explosion, producing a 2-200 keV luminosity of ~10³⁹ ergs s^-1. During the intervening 300-400 yr, the dissipation of kinetic energy subsided considerably, leading to the much lower luminosity (~10³⁶ ergs s^-1 at 2-10 keV) we see today.

We investigate the relationship between the velocity dispersion of the gas and the supernova (SN) rate and feedback efficiency with three-dimensional numerical simulations of SN-driven turbulence in the interstellar medium (ISM). Our simulations aim to explore the constancy of the velocity dispersion profiles in the outer parts of galactic disks at ~6-8 km s^-1 and the transition to the starburst regime, i.e., high star formation rates (SFRs) associated with high velocity dispersions. With our fiducial value of the SN feedback efficiency (i.e., = 0.25, corresponding to an injected energy per SN of 0.25 × 10⁵¹ ergs), our results show that (1) SN driving leads to constant velocity dispersions of σ ~ 6 km s^-1 for the total gas and σ ~ 3 km s^-1 for the H I gas, independent of the SN rate, for values of the rate between 0.01 and 0.5 the Galactic value (η_G); (2) the position of the transition to the starburst regime (i.e., location of sharp increase in the velocity dispersion) at around SFR/area ≃ 5 × 10^-3 to 10^-2M_☉ yr^-1 kpc^-2 observed in the simulations is in good agreement with the transition to the starburst regime in the observations (e.g., NGC 628 and NGC 6949); (3) for the high SN rates, no H I gas is present in the simulations box; however, for the total gas velocity dispersion, there is good agreement between the models and the observations; (4) at the intermediate SN rates (η/η_G ~ 0.5-1), taking into account the thermal broadening of the H I line helps reach a good agreement in that regime between the models and the observations; and (5) for η/η_G < 0.5, σ and σ fall below the observed values by a factor of ~2. However, a set of simulations with different values of indicates that, for larger values of the SN feedback efficiencies, velocity dispersions of the H I gas of the order of 5-6 km s^-1 can be obtained, in closer agreement with the observations. The fact that for η/η_G < 0.5, the H I gas velocity dispersions are a factor of ~2 smaller than the observed values could result from the fact that we might have underestimated the SN feedback efficiency. On the other hand, it might also be an indication that other physical processes couple to the stellar feedback in order to produce the observed level of turbulence in galactic disks.

Recent advances in understanding of MHD turbulence call for revisions in the picture of particle acceleration. We make use of the recently established scaling of slow and fast MHD modes in strong and weak MHD turbulence to provide a systematic study of particle acceleration in magnetic pressure-dominated (low β) and gaseous pressure-dominated (high β) plasmas. We consider the acceleration by large-scale compressions in both slow and fast particle diffusion limits. We compare the results with the acceleration rate that arises from resonance scattering and transit-time damping (TTD). We establish that fast modes accelerate particles more efficiently than slow modes. We find that particle acceleration by pitch-angle scattering and TTD dominates acceleration by slow or fast modes when the spatial diffusion rate is small. When the rate of spatial diffusion of particles is high, we establish an enhancement of the efficiency of particle acceleration by slow and fast modes in weak turbulence. We show that highly supersonic turbulence is an efficient agent for particle acceleration. We find that even incompressible turbulence can accelerate particles on the scales comparable with the particle mean free path.

We construct models of molecular clouds that are considered ensembles of transient cores. Each core is assumed to develop in the background gas of the cloud, grow to high density, and decay into the background. The chemistry in each core responds to the dynamical state of the gas and to the gas-dust interaction. Ices are deposited on the dust grains in the core's dense phase, and this material is returned to the gas as the core expands to low density. The cores of the ensemble typically number 1000, are placed randomly in position within the cloud, and are assigned a random evolutionary phase. The models are used to generate molecular line contour maps of a typical dark cloud. These maps are found to represent extremely well the characteristic features of observed maps of the dark cloud L673, which has been observed at both low and high resolution. The computed maps are found to exhibit the general morphology of the observed maps and to generate similar sizes of emitting regions, molecular column densities, and the separations between peaks of emissions of various molecular species. The models give insight into the nature of molecular clouds and the dynamical processes occurring within them and significantly constrain dynamical and chemical processes in the interstellar medium.

We have determined the ratios of total to selective extinction in the near-infrared bands (J,H,K_s) toward the Galactic center from the observations of the region ∣ l ∣ ≲ 20 and 05 ≲ ∣ b ∣ ≲ 10 with the IRSF telescope and the SIRIUS camera. Using the positions of red clump stars in color-magnitude diagrams as a tracer of the extinction and reddening, we determine the average of the ratios of total to selective extinction to be A/E = 1.44 ± 0.01, A/E = 0.494 ± 0.006, and A_H/E_J-H = 1.42 ± 0.02, which are significantly smaller than those obtained in previous studies. From these ratios, we estimate that A_J : A_H : A = 1 : 0.573 ± 0.009 : 0.331 ± 0.004 and E_J-H/E = 1.72 ± 0.04, and we find that the power law A_λ ∝ λ^-1.99±0.02 is a good approximation over these wavelengths. Moreover, we find a small variation in A/E across our survey. This suggests that the infrared extinction law changes from one line of sight to another, and the so-called universality does not necessarily hold in the infrared wavelengths.

We present measurements of the column densities of interstellar D I, O I, N I, and H₂ made with the Far Ultraviolet Spectroscopic Explorer (FUSE) and of H I made with the International Ultraviolet Explorer (IUE) toward the sdO star LSE 44 [(l, b) = (31337, + 1349); d = 554 ± 66 pc; z = +129 ± 15 pc]. This target is among the seven most distant Galactic sight lines for which these abundance ratios have been measured. The H I column density was estimated by fitting the damping wings of interstellar Lyα. The column densities of the remaining species were determined with profile fitting analyses and supplemented with curve-of-growth analyses for O I and H₂. We find log N(D ) = 15.87 ± 0.08, log N(O ) = 17.57, log N(N ) = 16.43 ± 0.14, and log N(H ) = 20.52 (all errors 2 σ). This implies that D/H = (2.24) × 10^-5, D/O = (1.99) × 10^-2, D/N = (2.75) × 10^-1, and O/H = (1.13) × 10^-3. Of the most distant Galactic sight lines for which the deuterium abundance has been measured, LSE 44 is one of the few with D/H higher than the Local Bubble value, but D/O toward all these targets is below the Local Bubble value and more uniform than the D/H distribution.

Chandra ACIS-I data of the molecular cloud and H II region complex NGC 6334 were analyzed. The hard X-ray clumps detected with ASCA (Sekimoto and coworkers) were resolved into 792 point sources. After removing the point sources, an extended X-ray emission component was detected over a 5 × 9 pc² region, with the 0.5-8 keV absorption-corrected luminosity of 2 × 10³³ ergs s^-1. The contribution from faint point sources to this extended emission was estimated as at most ~20%, suggesting that most of the emission is diffuse in nature. The X-ray spectrum of the diffuse emission was observed to vary from place to place. In tenuous molecular cloud regions with hydrogen column density of (0.5-1) × 10²² cm^-2, the spectrum can be represented by a thermal plasma model with temperatures of several keV. The spectrum in dense cloud cores exhibits harder continuum, together with higher absorption of more than ~3 × 10²² cm^-2. In some of such highly obscured regions, the spectra show extremely hard continua equivalent to a photon index of ~1, and favor a nonthermal interpretation. These results are discussed in the context of thermal and nonthermal emission, both powered by fast stellar winds from embedded young early-type stars through shock transitions.

Using high angular resolution (~025-005) Very Large Array (VLA) observations made at 3.6 cm, 1.3 cm, and 7 mm during the period 1991-2004, we report the detection of large proper motions in the components of the radio continuum jet associated with the high-mass young stellar object (YSO) HW2 in the star-forming region Cepheus A. The relative proper motions observed for the two main components of the outflow, moving away from the central source in nearly opposite directions, are of the order of 140 mas yr^-1, or ~480 km s^-1 at a distance of 725 pc. The proper motions observed in the northeast and southwest lobes are not completely antiparallel, and the central elongated source seems to be changing orientation. We discuss possible scenarios to account for these and other observed characteristics. We also report the detection of a 7 mm compact continuum condensation of emission near the center of the thermal radio continuum jet, which we propose as the location of the exciting star.

We assess the constraints on the evolutionary models of young low-mass objects that are provided by the measurements of the companion AB Dor C by Close and coworkers and by a new comparison of model-derived IMFs of star-forming regions to the well-calibrated IMF of the solar neighborhood. After performing an independent analysis of all of the imaging and spectroscopic data for AB Dor C that were obtained by Close, we find that AB Dor C (which has no methane) is not detected at a significant level (S/N ~ 1.2) in the SDI data when one narrowband image is subtracted from another but that it does appear in the individual SDI frames, as well as the images at J, H, and K_s. Although our broadband photometry for AB Dor C is consistent with that of Close, the uncertainties that we measure are larger. Using the age of τ = 75-150 Myr recently estimated for AB Dor by Luhman and coworkers, the luminosity predicted by the models of Chabrier and Baraffe is consistent with the value that we estimate from the photometry for AB Dor C. We measure a spectral type of M6 ± 1 from the K-band spectrum of AB Dor C, which is earlier than the value of M8 ± 1 reported by Close and is consistent with the model predictions when a dwarf temperature scale is adopted. In a test of these evolutionary models at much younger ages, we show that the low-mass IMFs that they produce for star-forming regions are similar to the IMF of the solar neighborhood. If the masses of the low-mass stars and brown dwarfs in these IMFs of star-forming regions were underestimated by a factor of 2 as suggested by Close, then the IMF characterizing the current generation of Galactic star formation would have to be radically different from the IMF of the solar neighborhood.

We present the results of an infrared imaging survey of two clusters in the Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelength range from 3.6 to 24 μm, allowing us to detect disk emission over a typical range of radii ~0.1 to ~20 AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be "transition" objects, with essentially photospheric fluxes at wavelengths ≤4.5 μm but with excesses at longer wavelengths, indicating an optically thin inner disk. The median optically thick disk emission in Tr 37 is lower than the corresponding median for stars in the younger Taurus region; the decrease in infrared excess is larger at 6-8 μm than at 24 μm, suggesting that grain growth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24 μm excesses around a few intermediate-mass stars, which may represent so-called "debris disk" systems. Our observations provide new constraints on disk evolution through an important age range.

This paper discusses Swift observations of the γ-ray burst GRB 050315 (z = 1.949) from 80 s to 10 days after the onset of the burst. The X-ray light curve displayed a steep early decay (t^-5) for ~200 s and several breaks. However, both the prompt hard X-ray/γ-ray emission (observed by the BAT) and the first ~300 s of X-ray emission (observed by the XRT) can be explained by exponential decays, with similar decay constants. Extrapolating the BAT light curve into the XRT band suggests that the rapidly decaying, early X-ray emission was simply a continuation of the fading prompt emission; this strong similarity between the prompt γ-ray and early X-ray emission may be related to the simple temporal and spectral character of this X-ray-rich GRB. The prompt (BAT) spectrum was steep down to ~15 keV and appeared to continue through the XRT bandpass, implying a low peak energy, inconsistent with the Amati relation. Following the initial steep decline, the X-ray afterglow did not fade for ~1.2 × 10⁴ s, after which time it decayed with a temporal index of α ≈ 0.7, followed by a second break at ~2.5 × 10⁵ s to a slope of α ~ 2. The apparent "plateau" in the X-ray light curve, after the early rapid decay, makes this one of the most extreme examples of the steep-flat-steep X-ray light curves revealed by Swift. If the second afterglow break is identified with a jet break, then the jet opening angle was θ₀ ~ 5^°, implying E_γ ≳ 10⁵⁰ ergs.

We present late-time radio observations of 68 local Type Ibc supernovae, including six events with broad optical absorption lines ("hypernovae"). None of these objects exhibit radio emission attributable to off-axis gamma-ray burst jets spreading into our line of sight. Comparison with our afterglow models reveals the following conclusions. (1) Less than ~10% of Type Ibc supernovae are associated with typical gamma-ray bursts initially directed away from our line of sight; this places an empirical constraint on the GRB beaming factor of ⟨f⟩ ≲ 10⁴, corresponding to an average jet opening angle, θ_j ≳ 08. (2) This holds in particular for the broad-lined supernovae (SNe 1997dq, 1997ef, 1998ey, 2002ap, 2002bl, and 2003jd), which have been argued to host GRB jets. Our observations reveal no evidence for typical (or even subenergetic) GRBs and rule out the scenario in which every broad-lined SN harbors a GRB at the 84% confidence level. Their large photospheric velocities and asymmetric ejecta (inferred from spectropolarimetry and nebular spectroscopy) appear to be characteristic of the nonrelativistic SN explosion and do not necessarily imply the existence of associated GRB jets.

SN 2000ft is the first radio supernova detected in the circumnuclear starburst of a luminous infrared Seyfert 1 galaxy. It is located at a distance of 600 pc from the QSO-like nucleus of NGC 7469. We report the temporal evolution of SN 2000ft during the 3 years after its discovery. Although SN 2000ft has exploded in the dusty and very dense environment that exists in the nuclear regions of luminous infrared galaxies, it shows the radio evolution properties characteristic of radio supernovae identified as Type II supernovae, aside from some foreground free-free absorption. The peak luminosity and circumstellar matter opacity of SN 2000ft are similar to other compact radio sources detected in luminous infrared galaxies such as NGC 6240, Arp 299, and Arp 220 and identified as Type II supernovae.

A brief description of the deformed spectra of microlensed SNe Ia is presented. We show that microlensing amplification can have significant effects on line profiles. The resonance-scattering code SYNOW is used to compute the intensity profile in the rest frame of the supernova. The observed (lensed) spectra are predicted assuming a simple stellar-size deflector, and are compared to unlensed cases to show what effects microlensing can have on spectral lines. We limit our work to spherically symmetric deflectors.

I present an analysis of the XMM-Newton observations of four millisecond pulsars, J0437-4715, J2124-3358, J1024-0719, and J0034-0534. The new data provide strong evidence of thermal emission in the X-ray flux detected from the first three objects. This thermal component is best interpreted as radiation from pulsar polar caps covered with a nonmagnetic hydrogen atmosphere. A nonthermal power-law component, dominating at energies E ≳ 3 keV, can also be present in the detected X-ray emission. For PSR J0437-4715, the timing analysis reveals that the shape and pulsed fraction of the pulsar light curves are energy dependent. This, together with the results obtained from the phase-resolved spectroscopy, supports the two-component (thermal plus nonthermal) interpretation of the pulsar's X-ray radiation. Highly significant pulsations have been found in the X-ray flux of PSRs J2124-3358 and J1024-0719. For PSR J0034-0534, a possible X-ray counterpart of the radio pulsar has been suggested. The inferred properties of the detected thermal emission are compared with predictions of radio pulsar models.

We report the discovery of millisecond pulsations from the low-mass X-ray binary HETE J1900.1-2455, which was discovered by the detection of a type I X-ray burst by the High Energy Transient Explorer 2 (HETE-2). The neutron star emits coherent pulsations at 377.3 Hz and is in an 83.3 minute circular orbit with a companion of mass greater than 0.016 M_☉ and likely less than 0.07 M_☉. The companion star's Roche lobe could be filled by a brown dwarf with no need for heating or nonstandard evolution. During one interval with an unusually high X-ray flux, the source produced quasi-periodic oscillations with a single peak at 883 Hz, and on subsequent days the pulsations were suppressed. We consider the distribution of spin versus orbital period in neutron star low-mass X-ray binaries.

We calculate the photon energy dependence of the pulsed amplitude of neutron star (NS) surface modes. Simple approximations demonstrate that it depends most strongly on the bursting NS surface temperature. This result compares well to full integrations that include Doppler shifts from rotation and general relativistic corrections to photon propagation. We show that the energy dependence of type I X-ray burst oscillations agrees with that of a surface mode, lending further support to the hypothesis that they originate from surface waves. The energy dependence of the pulsed emission is rather insensitive to the NS inclination, mass, and radius, or type of mode, thus hindering constraints on these parameters. We also show that, for this energy-amplitude relation, the majority of the signal (relative to the noise) comes in the ≈2-25 keV band, so that the current burst oscillation searches with the Rossi X-Ray Timing Explorer are close to optimal. The critical test of the mode hypothesis for X-ray burst oscillations would be a measurement of the energy dependence of burst oscillations from an accreting millisecond pulsar.

XTE J1739-302 is a transient X-ray source with unusually short outbursts, lasting on the order of hours. Here we give a summary of X-ray observations we have made of this object in outburst with the Rossi X-Ray Timing Explorer (RXTE) and at a low level of activity with the Chandra X-Ray Observatory, as well as observations made by other groups. Visible and infrared spectroscopy of the mass donor of XTE J1739-302 are presented in a companion paper. The X-ray spectrum is hard both at low levels and in outburst, but somewhat variable, and there is strong variability in the absorption column from one outburst to another. Although no pulsation has been observed, the outburst data from multiple observatories show a characteristic timescale for variability on the order of 1500-2000 s. The Chandra localization (R.A. = 17^h39^m1158, decl. = -30^°20'376, J2000.0) shows that despite being located less than 2° from the Galactic center and highly absorbed, XTE J1739-302 is actually a foreground object with a bright optical counterpart. The combination of a very short outburst timescale and a supergiant companion is shared with several other recently discovered systems, forming a class we designate as supergiant fast X-ray transients (SFXTs). Three persistently bright X-ray binaries with similar supergiant companions have also produced extremely short, bright outbursts: Cyg X-1, Vela X-1, and 1E 1145.1-6141.

The weak X-ray transient XTE J1739-302, characterized by extremely short outbursts, has recently been identified with a reddened star. Here we present spectroscopy and photometry of the counterpart, identifying it as a O8 Iab(f) supergiant at a distance of ~2.3 kpc. XTE J1739-302 thus becomes the prototype of the new class of supergiant fast X-ray transients (SFXTs). The optical and infrared spectra of the counterpart to XTE J1739-302 do not reveal any obvious characteristics setting it apart from other X-ray binaries with supergiant companions, which display a very different type of X-ray light curve.

We report on an unprecedented infrared time series of spectra of V1187 Sco, a very fast ONeMg nova. The observations covered a 56 day period (2004 August 6-September 30) starting 2 days after the nova's peak brightness. Time evolution of the spectra revealed changing line strengths and profiles on timescales of less than a day to weeks as the nova evolved from early postmaximum to early coronal phases. When our ground-based optical and Spitzer Space Telescope data were combined, the wavelength coverage of 0.38-36 μm allowed an accurate spectral energy distribution to be derived when it was about 6 weeks after outburst. Developing double structure in the He I lines showed them changing from narrow to broad in only a few days. Using the O I lines in combination with the optical spectra, we derived a reddening of E(B - V) = 1.56 ± 0.08 and a distance of 4.9 ± 0.5 kpc. Modeling of the ejected material strongly suggested that it was geometrically thick with ΔR/R = 0.8-0.9 (more of a wind than a shell) and a low filling factor of order a few percent. The line shapes were consistent with a cylindrical jet, bipolar, or spherical Hubble flow expansion with a maximum speed of about -3000 km s^-1. The central peak appeared to be more associated with the spherical component, while the two peaks (especially in Hβ) suggested a ring with either a lower velocity component or with its axis inclined to the line of sight.

We have created specialized target lists for radial velocity surveys that are biased toward stars that (1) possess planets and (2) are easiest to observe with current detection techniques. We use a procedure that uniformly estimates fundamental stellar properties of Tycho 2 stars, with errors, using spline functions of broadband photometry and proper motion found in Hipparcos/Tycho 2 and 2MASS. We provide estimates of T_eff and distance for 2.4 × 10⁶ Tycho 2 stars that lack trigonometric distances. For stars that appear to be FGK dwarfs, we also derive [Fe/H] and identify unresolved binary systems with mass ratios 1.25 < M₁/M₂ < 3.0. For FGK dwarfs with photometric error σ_V < 0.05, or V < 9, our temperature model gives a 1 σ error of σ_T = +58.7/ - 65.9 K and our metallicity model gives a 1 σ error of σ_[Fe/H] = +0.13/ - 0.14 dex. The binarity model can be used to remove 70% of doubles with 1.25 < M₁/M₂ < 3.0 from a magnitude-limited sample of dwarfs at a cost of cutting 20% of the sample. Our estimates of distance and spectral type enable us to isolate 354,822 Tycho 2 dwarfs, 321,996 absent from Hipparcos, with giant contamination of 2.6% and 7.2%, respectively. Roughly 100,000 of these stars, not in Hipparcos, have sufficiently low photometric errors to retain 0.13-0.3 dex [Fe/H] accuracy and 80-100 K temperature accuracy (1 σ). Our metallicity estimates have been used to identify targets for N2K, a large-scale radial velocity search for hot jupiters, which has verified the errors presented here. The catalogs that we publish can be used to further large-scale studies of Galactic structure and chemical evolution and to provide potential reference stars for narrow-angle astrometry programs such as the Space Interferometry Mission and large-aperture optical interferometry.

We present Mg isotope ratios in four red giants of the globular cluster M13 and one red giant of the globular cluster M71 based on high-resolution, high signal-to-noise ratio spectra obtained with HDS on the Subaru Telescope. We confirm earlier results by Shetrone that for M13 the ratio varies from /²⁴Mg ≃ 1 in stars with the highest Al abundance to /²⁴Mg ≃ 0.2 in stars with the lowest Al abundance. However, we separate the contributions of all three isotopes and find a considerable spread in the ratio ²⁴Mg : ²⁵Mg : ²⁶Mg, with values ranging from 48 : 13 : 39 to 78 : 11 : 11. As in NGC 6752, we find a positive correlation between ²⁶Mg and Al, an anticorrelation between ²⁴Mg and Al, and no correlation between ²⁵Mg and Al. In M71, our one star has a Mg isotope ratio of 70 : 13 : 17. For both clusters, even the lowest ratios of ²⁵Mg/²⁴Mg and ²⁶Mg/²⁴Mg exceed those observed in field stars at the same metallicity, a result also found in NGC 6752. The contribution of ²⁵Mg to the total Mg abundance is constant within a given cluster and between clusters with ²⁵Mg/ ≃ 0.13. For M13 and NGC 6752, the ranges of the Mg isotope ratios are similar and both clusters show the same correlations between Al and Mg isotopes, suggesting that the same process is responsible for the abundance variations in these clusters. While existing models fail to reproduce all the observed abundances, we continue to favor the scenario in which two generations of asymptotic giant branch (AGB) stars produce the observed abundances. A first generation of metal-poor AGB stars pollutes the entire cluster and is responsible for the large ratios of ²⁵Mg/²⁴Mg and ²⁶Mg/²⁴Mg observed in cluster stars with compositions identical to field stars at the same metallicity. Differing degrees of pollution by a second generation of AGB stars of the same metallicity as the cluster provides the star-to-star scatter in Mg isotope ratios.

The stars β Cet, 31 Com, and μ Vel represent the main stages through which late-type giants evolve during their lifetime (the Hertzsprung gap [31 Com], the rapid braking zone [μ Vel], and the core helium burning "clump" phase [β Cet]). An analysis of their high-resolution Chandra X-ray spectra reveals similar coronal characteristics in terms of both temperature structure and element abundances for the more evolved stars (μ Vel and β Cet), with slight differences for the "younger" giant (31 Com). The coronal temperature structure of 31 Com is significantly hotter, showing a clear peak, while β Cet and μ Vel show a plateau. β Cet and μ Vel show evidence for a FIP effect in which coronae are depleted in high-FIP elements relative to their photospheres by a factor of ~2. In contrast, 31 Com is characterized by a lack of FIP effect. In other words, neither depletion nor enhancement relative to stellar photospheric values is found. We conclude that the structural changes during the evolution of late-type giants could be responsible for the observed differences in coronal abundances and temperature structure. In particular, the size of the convection zone coupled with the rotation rate seem obvious choices for playing a key role in determining coronal characteristics.

This paper reports the results of a Berkeley-Illinois-Maryland Association array interferometric observation of EP Aqr, a semiregular pulsating star with a double-component line profile in the CO J = 1-0 line. The broad component shows a flat-topped profile, and the narrow component shows a spiky strong peak. Although previous single-dish observations suggested that the CO J = 2-1 line exhibits a Gaussian-like profile, the CO J = 1-0 line does not. The spatial distributions of both the narrow and the broad components appear to be roughly round with the same peak positions. No significant velocity gradient is seen. The spatial-kinetic properties of the molecular envelope of EP Aqr are reminiscent of a multiple-shell structure model rather than of a bipolar flow and disk model. A problem with this interpretation is that no evidence of interaction between the narrow- and broad-component regions is seen. A Gaussian-like feature seen in the CO J = 2-1 line might play a key role in understanding the spatiokinetic properties of the molecular envelope of EP Aqr.

We present moderate- and high-dispersion 1-2.5 μm spectra of the ~10'' radius nebula around P Cygni, dominated by bright emission lines of [Fe II]. Observed [Fe II] line ratios disagree with theoretical transition rates in the literature, so we use the spectrum of P Cyg's nebula to constrain the atomic data for low-lying levels of [Fe II]. Of particular interest is the ratio [Fe II] λ12567/λ16435, often used as a reddening indicator, for which we empirically derive an intrinsic value of 1.49, which is 10%-40% higher than previous estimates. High-dispersion spectra of [Fe II] λ16435 constrain the geometry, detailed structure, and kinematics of P Cyg's nebula, which is the major product of P Cyg's outburst in 1600 AD. We use the [N II]/[N I] line ratio to conclude that the nebula is mostly ionized, with a total mass of ~0.1 M_☉, more than the mass lost by the stellar wind since the eruption. For this mass, we would expect a larger infrared excess than observed. We propose that the dust that obscured the star after the outburst has since been largely destroyed, releasing Fe into the gas phase to produce the bright [Fe II] emission. The kinetic energy of this shell is ~10^46.3 ergs, far less than the kinetic energy released during the giant eruption of η Car in the 1840s, but close to the value for η Car's smaller 1890 outburst. In this respect, it is interesting that the infrared spectrum of P Cyg's nebula resembles that of the "Little Homunculus" around η Car, ejected in that star's 1890 eruption. The mass and kinetic energy in the nebulae of η Car and P Cyg give insight into the range of parameters expected for extragalactic η Car-like eruptions.

We report on the first dedicated monitoring campaign of spectroscopic variability in young brown dwarfs. High-resolution optical spectra of six targets in nearby star-forming regions were obtained over 11 nights between 2005 January and March on the Magellan 6.5 m telescope. We find significant variability in Hα and a number of other emission lines related to accretion and outflow processes on a variety of timescales ranging from hours to weeks to years. The most dramatic changes are seen for 2MASS J1207334-393254 (2M 1207), 2MASS J11013205-7718249 (2M 1101), and Cha I-ISO 217. We observe possible accretion rate changes by about an order of magnitude in two of these objects, over timescales of weeks (2M 1207) or hours (2M 1101). The accretion "burst" seen in 2M 1101 could be due to a "clumpy" flow. We also see indications for changes in the outflow rate in at least three objects. In one case (ISO 217), there appears to be a ~1 hr time lag between outflow and accretion variations, consistent with a scenario in which the wind forms at the inner disk edge. For some objects there is evidence for emission-line variability induced by rotation. Our variability study supports an inclination that is close to edge-on for the brown dwarf LS-RCrA 1. The fact that all targets in our sample show variations in accretion and/or outflow indicators suggests that studies of young brown dwarf properties should be based either on large samples or on time series. As an example, we demonstrate that the large scatter in the recently found accretion rate versus mass relationship can be explained primarily by variability. The observed profile variations imply asymmetric accretion flows in brown dwarfs, which, in turn, is evidence for magnetic funneling by large-scale fields. We show that accreting substellar objects may harbor magnetic fields with approximately kilogauss strength.

We report the discovery of a debris system associated with the ~30 Myr old G3/5V star HD 12039 using Spitzer Space Telescope observations from 3.6-160 μm. An observed infrared excess (L_IR/L_* = 1 × 10^-4) above the expected photosphere for λ ≳ 14 μm is fit by thermally emitting material with a color temperature of T ~ 110 K, warmer than the majority of debris disks identified to date around Sun-like stars. The object is not detected at 70 μm with a 3 σ upper limit 6 times the expected photospheric flux. The spectrum of the infrared excess can be explained by warm, optically thin material comprised of blackbody-like grains of size ≳7 μm that reside in a belt orbiting the star at 4-6 AU. An alternate model dominated by smaller grains, near the blowout size a ~ 0.5 μm, located at 30-40 AU is also possible but requires the dust to have been produced recently, since such small grains will be expelled from the system by radiation pressure in approximately a few times 10² yr.

Although some of the properties of the caustics in planetary microlensing have been known, our understanding of them is mostly from scattered information based on numerical approaches. In this paper, we conduct a comprehensive and analytic analysis of the properties of the planetary caustics, which are one of the two sets of caustics in planetary microlensing, those located away from the central star. Under the perturbative approximation, we derive analytic expressions for the location, size, and shape of the planetary caustic as a function of the star-planet separation and the planet/star mass ratio. Based on these expressions combined with those for the central caustic, which is the other set of caustics and is located close to the central star, we compare the similarities and differences between the planetary and central caustics. We also present the expressions for the size ratio between the two types of caustics and for the condition of the merging of the two types of caustics. These analytic expressions will be useful in understanding the dependence of the planetary lensing behavior on the planet parameters and thus in interpreting the planetary lensing signals.

A forward model is described in which we synthesize spectra from an ab initio three-dimensional MHD simulation of an outer stellar atmosphere, where the coronal heating is based on braiding of magnetic flux due to photospheric footpoint motions. We discuss the validity of assumptions such as ionization equilibrium and investigate the applicability of diagnostics like the differential emission measure inversion. We find that the general appearance of the synthesized corona is similar to the solar corona and that, on a statistical basis, integral quantities such as average Doppler shifts or differential emission measures are reproduced remarkably well. The persistent redshifts in the transition region, which have puzzled theorists since their discovery, are explained by this model as caused by the flows induced by the heating through braiding of magnetic flux. While the model corona is only slowly evolving in intensity, as is observed, the amount of structure and variability in Doppler shift is very large. This emphasizes the need for fast coronal spectroscopic observations, as the dynamical response of the corona to the heating process manifests itself in a comparably slow evolving coronal intensity but rapid changes in Doppler shift.

I investigate the processes at work in the cancellation of normal magnetic flux in solar magnetograms and study the relationships between cancellation and the budget of free magnetic energy in the coronal magnetic field that can power solar flares and CMEs. After defining cancellation mathematically, I derive equations that quantify the evolution of free magnetic energy in response to arbitrary plasma flows on the boundary, including flows consistent with cancellation. While cancellation can reduce the magnetic energy in both the actual coronal field and the potential field matching the same normal field boundary condition, cancellation can, in the process, increase the difference between the two; i.e., cancellation can increase the free magnetic energy in the corona. By making simple assumptions based upon typical observed field configurations in filament channels, I show that cancellation tends to add free energy to these fields. Finally, I discuss the implications of this fact, as well as wider applications of the free energy flux formalism developed here. I also briefly address related issues, including the relationship of cancellation to Taylor's hypothesis.

We report on SOHO UVCS observations of the coronal restructuring following a coronal mass ejection (CME) on 2002 November 26, at the time of a SOHO-Ulysses quadrature campaign. Starting about 1.5 hr after a CME in the northwest quadrant, UVCS began taking spectra at 1.7 R_☉, covering emission from both cool and hot plasma. Observations continued, with occasional gaps, for more than 2 days. Emission in the 974.8 Å line of [Fe XVIII], indicating temperatures above 6 × 10⁶ K, was observed throughout the campaign in a spatially limited location. Comparison with EIT images shows the [Fe XVIII] emission to overlie a growing post-flare loop system formed in the aftermath of the CME. The emission most likely originates in a current sheet overlying the arcade. Analysis of the [Fe XVIII] emission allows us to infer the evolution of physical parameters in the current sheet over the entire span of our observations: in particular, we give the temperature versus time in the current sheet and estimate its density. At the time of the quadrature, Ulysses was directly above the location of the CME and intercepted the ejecta. High ionization state Fe was detected by the Ulysses SWICS throughout the magnetic cloud associated with the CME, although its rapid temporal variation suggests bursty, rather than smooth, reconnection in the coronal current sheet. The SOHO-Ulysses data set provided us with the unique opportunity of analyzing a current sheet structure from its lowest coronal levels out to its in situ properties. Both the remote and in situ observations are compared with predictions of theoretical CME models.

We have performed the first polarimetry of solar flare emission at γ-ray energies (0.2-1 MeV). These observations were performed with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) for two large flares: the GOES X4.8-class solar flare of 2002 July 23 and the X17-class flare of 2003 October 28. We have marginal polarization detections in both flares, at levels of 21% ± 9% and -11% ± 5%, respectively. These measurements significantly constrain the levels and directions of solar flare γ-ray polarization and begin to probe the underlying electron distributions.

We present a study of the spatial and spectral evolution of the loop-top (LT) sources in a sample of six flares near the solar limb observed by RHESSI. A distinct coronal source, which we identify as the LT source, was seen in each of these flares from the early "preheating" phase through the late decay phase. Spectral analyses reveal an evident steep power-law component in the preheating and impulsive phases, suggesting that the particle acceleration starts upon the onset of the flares. In the late decay phase the LT source has a thermal spectrum and appears to be confined within a small region near the top of the flare loop and does not spread throughout the loop, as is observed at lower energies. The total energy of this source decreases usually faster than expected from the radiative cooling but much slower than that due to the classical Spitzer conductive cooling along the flare loop. These results indicate the presence of a distinct LT region, where the thermal conductivity is suppressed significantly and/or there is a continuous energy input. We suggest that plasma wave turbulence could play important roles in both heating the plasma and suppressing the conduction during the decay phase of solar flares. With a simple quasi-steady loop model we show that the energy input in the gradual phase can be comparable to that in the impulsive phase and demonstrate how the observed cooling and confinement of the LT source can be used to constrain the wave-particle interaction.

The temperature dependence of the Si XII n = 3 and 4 dielectronic satellite line features at 5.82 and 5.56 Å, respectively, near the Si XIII 1s²-1s3p and 1s²-1s4p lines (5.681 and 5.405 Å), is calculated using atomic data presented here. The resulting theoretical spectra are compared with solar flare spectra observed by the RESIK spectrometer on the CORONAS-F spacecraft. The satellites, like the more familiar n = 2 satellites near the Si XIII 1s²-1s2p lines, are formed mostly by dielectronic recombination, but unlike the n = 2 satellites, are unblended. The implications for similar satellite lines in flare Fe spectra are discussed.

A direct boundary integral formulation for a force-free magnetic field with finite energy content in the semispace above the Sun is presented. This is a new formulation for a three-dimensional nonlinear force-free field in which the boundary data can easily be incorporated. We have proposed an optimal method to numerically find the non-constant-α force-free field solution at any position in semispace. Therefore, the present computational procedure for the new representation is actually a pointwise method, so that no volume integration is needed. A test case study has been carried out to demonstrate the convergence, accuracy, and efficiency of the numerical procedures. The computational procedure is quite robust. The agreement between numerical and exact results validates the correctness and merits of the new formulation and computational procedure proposed. The application of the new direct boundary integral formulation to practical solar magnetic field problems is straightforward and prospective.

Observations with Wind and other spacecraft of the solar wind have shown that the velocity distributions of energetic electrons associated with solar impulsive electron events are often nearly isotropic, as shown by Lin et al. and Ergun et al. It is believed that in each case the electrons are originally associated with a flare-associated beam. The issue of interest is: how can a beam of fast electrons evolve into such a final state? This paper offers a theoretical explanation. We attribute the isotropization of the electron beam distribution to nonresonant pitch-angle scattering by enhanced Alfvén waves in the corona and the solar wind. The proposed scenario is demonstrated with test particle calculations.

High-definition TV spectra in the ultraviolet to visible region were obtained during the 2002 Leonid aircraft campaign. We analyzed 20 meteor spectra obtained from the 1767 (seven revolution) and 1866 (four revolution) trails on 2002 November 19 and identified neutral atoms, mainly Mg I, Fe I, Ca I, and Na I, in the observed wavelengths between 300 and 650 nm. The singly ionized atomic emissions, Ca II and Mg II lines, also appeared in the spectrum. The abundances of the metallic atoms, the electronic excitation temperature, and the electron density are obtained for each spectrum, assuming the Boltzmann distribution for the number at each energy level. The metallic abundances of Fe, Ca, and Na relative to Mg are slightly lower than solar abundances on average. We could not find any evidence of the solar heating effect on Leonid meteoroids between the 1767 and 1866 trails on orbit with their perihelion (q ~ 1 AU). We can support the idea that silicate and carbon-mixed silicate are preserved in interplanetary space for at least several hundred years. Bands of CHON-related molecules, such as OH and CN, are not detected in this study.

In order to examine the "giant impact hypothesis" for the formation of the Moon, we run the first grid-based, high-resolution hydrodynamic simulations of an impact between proto-Earth and a protoplanet. The spatial resolution for the impact-generated disk is greatly improved from previous particle-based simulations. This allows us to explore the fine structures of a circumterrestrial debris disk and its long-term evolution. We find that in order to form a debris disk from which a lunar-sized satellite can be accumulated, the impact must result in a disk of mostly liquid or solid debris, where pressure is not effective, well before the accumulation process starts. If the debris is dominated by vapor gas, strong spiral shocks are generated, and therefore the circumterrestrial disk cannot survive more than several days. This suggests that there could be an appropriate mass range for terrestrial planets to harbor a large moon as a result of giant impacts, since vaporization during an impact depends on the impact energy.

The abundance of cosmological data becoming available means that a wider range of cosmological models are testable than ever before. However, an important distinction must be made between parameter fitting and model selection. While parameter fitting simply determines how well a model fits the data, model selection statistics, such as the Bayesian evidence, are now necessary to choose between these different models, and in particular to assess the need for new parameters. We implement a new evidence algorithm known as nested sampling, which combines accuracy, generality of application, and computational feasibility, and we apply it to some cosmological data sets and models. We find that a five-parameter model with a Harrison-Zel'dovich initial spectrum is currently preferred.

We study the dependence of the cross-correlation between galaxies and galaxy groups on group properties. Confirming previous results, we find that the correlation strength is stronger for more massive groups, in good agreement with the expected mass dependence of halo bias. We also find, however, that for groups of the same mass, the correlation strength depends on the star formation rate (SFR) of the central galaxy: at fixed mass, the bias of galaxy groups decreases as the SFR of the central galaxy increases. We discuss these findings in light of the recent findings by Gao et al. that halo bias depends on halo formation time, in that halos that assemble earlier are more strongly biased. We also discuss the implication for galaxy formation and address a possible link to galaxy conformity, the observed correlation between the properties of satellite galaxies and those of their central galaxy.

Using the deep multiwavelength MUSYC, GOODS, and FIRES surveys we construct a stellar mass-limited sample of galaxies at 2 < z < 3. The sample comprises 294 galaxies with M > 10¹¹M_☉ distributed over four independent fields with a total area of almost 400 arcmin². The mean number density of massive galaxies in this redshift range ρ(M > 10¹¹M_☉) = (2.2 ± 0.6) × 10^-4h Mpc^-3. We present median values and 25th and 75th percentiles for the distributions of observed R_AB magnitudes, observed J - K_s colors, and rest-frame ultraviolet continuum slopes, M/L_V ratios, and U - V colors. The galaxies show a large range in all these properties. The "median galaxy" is faint in the observer's optical (R_AB = 25.9), red in the observed near-IR (J - K_s = 2.48), has a rest-frame UV spectrum that is relatively flat in F_λ (β = -0.4), and rest-frame optical colors resembling those of nearby spiral galaxies (U - V = 0.62). We determine which galaxies would be selected as Lyman break galaxies (LBGs) or distant red galaxies (DRGs, having J - K_s > 2.3) in this mass-limited sample. By number DRGs make up 69% of the sample, and LBGs 20%, with a small amount of overlap. By mass DRGs make up 77%, and LBGs 17%. Neither technique provides a representative sample of massive galaxies at 2 < z < 3 as they only sample the extremes of the population. As we show here, multiwavelength surveys with high-quality photometry are essential for an unbiased census of massive galaxies in the early universe. The main uncertainty in this analysis is our reliance on photometric redshifts; confirmation of the results presented here requires extensive near-infrared spectroscopy of optically faint samples.

We explore the consequences of new observational and theoretical evidence that long gamma-ray bursts (GRBs) prefer low-metallicity environments. Using recently derived mass-metallicity correlations and the mass function from SDSS studies, and adopting an average cosmic metallicity evolution from Kewley & Kobulnicky and Savaglio et al., we derive expressions for the relative number of massive stars formed below a given fraction of solar metallicity, , as a function of redshift. We demonstrate that about 1/10 of all stars form with < 0.1. Therefore, a picture in which the majority of GRBs form with < 0.1 is not inconsistent with an empirical global SN/GRB ratio of 1/1000. It implies that (1) GRBs peak at a significantly higher redshift than supernovae; (2) massive star evolution at low metallicity may be qualitatively different; and (3) the larger the low-metallicity bias of GRBs, the less likely binary evolution channels can be significant GRB producers.

We present the results of a systematic analysis of the intrinsic optical afterglow light curves for a complete sample of gamma-ray bursts (GRBs) observed in the period from 1997 February to 2005 August. These light curves are generally well sampled, with at least four detections in the R band. The redshifts of all the bursts in the sample are available. We derive the intrinsic R-band afterglow light curves (luminosity vs. time within the cosmic proper rest frame) for these GRBs and discover that they essentially follow two universal tracks beyond 2 hours after the GRBs triggered. The optical luminosities at 1 day show a clear bimodal distribution, peaking at 1.4 × 10⁴⁶ ergs s^-1 for the luminous group and 5.3 × 10⁴⁴ ergs s^-1 for the dim group. About 75% of the GRBs are in the luminous group, and the other 25% belong to the dim group. While the luminous group has a widely distributed range of redshifts, the bursts in the dim group all appear at redshifts lower than 1.1.

In this Letter, we discuss the flux and the behavior of the bright optical flare emission detected by the 25 cm TAROT robotic telescope during the prompt high-energy emission and the early afterglow. We combine our data with simultaneous observations performed in X-rays, and we analyze the broadband spectrum. These observations lead us to emphasize the similarity of GRB 050904 with GRB 990123, a remarkable gamma-ray burst whose optical emission reached 9th magnitude. While GRB 990123 was, until now, considered as a unique event, this observation suggests the existence of a population of GRBs that have very large isotropic equivalent energies and extremely bright optical counterparts. The luminosity of these GRBs is such that they are easily detectable through the entire universe. Since we can detect them to very high redshift even with small-aperture telescopes like TAROT, they will constitute powerful tools for the exploration of the high-redshift universe and might be used to probe the first generation of stars.

The quasar HE 0450-2958 was recently discovered to reside ~7 kpc away from a galaxy that was likely disturbed by a recent merger. The lack of a massive spheroid of stars around the quasar raised the unlikely suggestion that it may have formed in a dark galaxy. Here we explain this discovery as a natural consequence of a dynamical kick imparted to the quasar as it interacted with a binary black hole system during a galaxy merger event. The typical binary stalling radius provides a kick on the order of the escape velocity of the stellar spheroid, bringing the quasar out to around the observed radius before it reverses direction. This is consistent with the observed low relative velocity between the quasar and the merger-remnant galaxy. The gas carried with the black hole throughout the three-body interaction fuels the quasar for the duration of its journey, ~2 × 10⁷ years. Gravitational radiation recoil could not have produced the required kick.

We present a study of a faint fuzzy star cluster system in the nearby SB0 galaxy NGC 5195 interacting with the famous spiral galaxy NGC 5194 (M51), based on HST ACS BVI mosaic images taken by the Hubble Heritage Team. We have found about 50 faint fuzzy star clusters around NGC 5195 that are larger than typical globular clusters with effective radii r_eff > 7 pc and that are red with (V - I) > 1.0. They are mostly fainter than M_V ≈ -8.3 mag. From the comparison of BVI photometry of these clusters with the simple stellar population models, we find that they are as massive as ≈10⁵M_☉ and older than 1 Gyr. Strikingly, most of these clusters are found to be scattered in an elongated region almost perpendicular to the northern spiral arm of NGC 5194, and the center of the region is slightly north of the NGC 5195 center, while normal compact red clusters of NGC 5195 are located around the bright optical body of the host galaxy. This is in contrast to the cases of NGC 1023 and NGC 3384 where the spatial distribution of faint fuzzy clusters shows a ring structure around the host galaxy. We suggest that at least some faint fuzzy clusters are experiencing tidal interactions with the companion galaxy NGC 5194 and must be associated with the tidal debris in the western halo of NGC 5195.

The results of a spectral analysis, using XMM-Newton and Chandra data of the brightest ultraluminous X-ray source in the nearby galaxy M82, are presented. The spectrum of M82 X-1 was found to be unusually hard (photon spectral index Γ ≈ 1) with a sharp cutoff at ≈6 keV. The disk blackbody emission model requires a nonphysically high temperature. Instead, the spectrum is better described, with a lower reduced χ², as emission due to the nearly saturated Comptonization of photons in an optically thick (τ ≈ 10-30, depending on the geometry) plasma having a temperature kT ≈ 2 keV. This is in contrast to the high-energy spectra of other black hole systems, which are relatively steeper (Γ > 1.5) and hence are modeled as the unsaturated thermal and/or nonthermal Comptonization of soft photons, in an optically thin (τ ≈ 1) high-temperature plasma. An iron line emission that is marginally resolved (σ ~ 0.2 keV) is required to fit the data. We argue that the standard geometry for the X-ray-producing region, which consists of an optically thin inner disk or a uniform/patchy corona on top of a cold disk, is not applicable to this source. Alternatively, the geometry of the X-ray-producing region could be a large sphere surrounding a cold accretion disk or an optically thick inner disk region that cools by bremsstrahlung self-Comptonization. For the latter scenario, such an inner disk region, whose effective optical depth to absorption is less than unity, is expected in the standard accretion disk theory for near-Eddington accretion rates.

New images of M31 at 24, 70, and 160 μm taken with the Multiband Imaging Photometer for Spitzer (MIPS) reveal the morphology of the dust in this galaxy. This morphology is well represented by a composite of two logarithmic spiral arms and a circular ring (radius ~10 kpc) of star formation offset from the nucleus. The two spiral arms appear to start at the ends of a bar in the nuclear region and extend beyond the star-forming ring. As has been found in previous work, the spiral arms are not continuous, but composed of spiral segments. The star-forming ring is very circular except for a region near M32 where it splits. The lack of well-defined spiral arms and the prominence of the nearly circular ring suggest that M31 has been distorted by interactions with its satellite galaxies. Using new dynamical simulations of M31 interacting with M32 and NGC 205, we find that, qualitatively, such interactions can produce an offset, split ring like that seen in the MIPS images.

We present spectroscopy and discuss the photometric history of a previously obscure star in M31. The spectrum of the star is an extremely close match to that of P Cygni, one of the archetypes of luminous blue variables (LBVs). The star has not shown much variability over the past 40 years (<0.2 mag), although small-scale (0.05 mag) variations over a year appear to be real. Nevertheless, the presence of a subarcsecond extension around the star is indicative of a past outburst, and from the nebula's size (0.5 pc diameter) we estimate that the outburst took place roughly 2000 years ago. P Cygni itself exhibits a similar photometric behavior and has a similar nebula (0.2 pc diameter). We argue that this may be more typical behavior for LBVs than commonly assumed. The star's location in the H-R diagram offers substantial support for stellar evolutionary models that include the effects of rotation, as the star is just at a juncture in the evolutionary track of a 85 M_☉ star. The star is likely in a transition from an O star to a late-type WN Wolf-Rayet star.

We have detected absorption lines from the high-velocity cloud (HVC) complex WB in the spectrum of the star HE 1048+0231. This detection sets an upper limit on the distance to the cloud of 8.8 kpc. Nondetection (at greater than 3 σ confidence) in the star HE 1138-1303 at 7.7 ± 0.2 kpc sets a probable lower limit. The equivalent width of the Ca II K line due to the HVC [W_λ(Ca II K) = 114.6 ± 4.4 mÅ] corresponds to a column density of N(Ca II) = (1.32 ± 0.05) × 10¹² cm^-2. Using an H I spectrum from the Leiden/Argentine/Bonn survey, we calculate N(Ca II)/N(H I) = (81 ± 16) × 10^-9. These distance limits imply an H I mass limit of 3.8 × 10⁵M_☉ < M_{H I} < 4.9 × 10⁵M_☉. The upper distance limit imposed by these observations shows that this HVC complex has a probable Galactic or circumgalactic origin. Future metallicity measurements will be able to confirm or refute this interpretation.

Recently, the Galactic center has been reported to be a source of very high energy (VHE) γ-rays by the CANGAROO, VERITAS, and HESS experiments. The energy spectra as measured by these experiments show substantial differences. In this Letter we present MAGIC observations of the Galactic center, resulting in the detection of a differential γ-ray flux consistent with a steady, hard-slope power law, described as dN_γ/(dAdtdE) = (2.9 ± 0.6) × 10^-12(E/TeV)^-2.2±0.2 cm^-2 s^-1 TeV^-1. The γ-ray source is centered at (R.A., decl.) = (17^h45^m20^s, -29°2'). This result confirms the previous measurements by the HESS experiment and indicates a steady source of TeV γ-rays. We briefly describe the observational technique used and the procedure implemented for the data analysis, and we discuss the results in the perspective of different models proposed for the acceleration of the VHE γ-rays.

Planar polycyclic aromatic hydrocarbons (PAHs) are candidates for the diffuse interstellar band (DIB) carriers. Nanometer-size Fe metallic particles, expected to be present during astrophysical graphite growth, are catalytic for formation of a related species—large tubular PAH molecules. We propose that tubular PAH molecules are a component of the interstellar medium. Electronic structure calculations, on a specific family of tubular PAH molecules derived from elongated C₆₀, reveal intense electronic transitions in the visible and near-IR, which vary systematically with length. We analyze these molecules as DIB carriers within the known constraints.

Renewed interest in the first stars that were formed in the universe has led to the discovery of extremely iron-poor stars. Since several competing scenarios exist, our understanding of the mass range that determines the observed elemental abundances remains unclear. In this study, we consider three well-studied metal-poor stars in terms of the theoretical supernova (SN) model. Our results suggest that the observed abundance patterns in the metal-poor star BD +80 245 and the pair of stars HD 134439/40 agree strongly with the theoretical possibility that these stars inherited their heavy-element abundance patterns from SNe initiated by thermonuclear runaways in the degenerate carbon-oxygen cores of primordial asymptotic giant branch stars with masses of ~3.5-5 M_☉. Recent theoretical calculations have predicted that such SNe could be originated from metal-free stars in the intermediate-mass range. On the other hand, intermediate-mass stars containing some metals would end their lives as white dwarfs after expelling their envelopes in the wind due to intense momentum transport from outgoing photons to heavy elements. This new pathway for the formation of SNe requires that stars be formed from the primordial gas. Thus, we suggest that stars of a few solar masses were formed from the primordial gas and that some of them caused thermonuclear explosions when the mass of their degenerate carbon-oxygen cores increased to the Chandrasekhar limit without experiencing efficient mass loss.

An asymptotic theory is presented to describe the possible mechanism for the excitation and maintenance of torsional oscillations in rotating convective stars and planets. It is demonstrated analytically that the torsional oscillations can be excited and sustained by global thermal convection.

Simultaneous observations of explosive chromospheric evaporation are presented using data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory. For the first time, cospatial imaging and spectroscopy have been used to observe explosive evaporation within a hard X-ray emitting region. RHESSI X-ray images and spectra were used to determine the flux of nonthermal electrons accelerated during the impulsive phase of an M2.2 flare. When we assumed a thick-target model, the injected electron spectrum was found to have a spectral index of ~7.3, a low-energy cutoff of ~20 keV, and a resulting flux of ≥4 × 10¹⁰ ergs cm^-2 s^-1. The dynamic response of the atmosphere was determined using CDS spectra; we found a mean upflow velocity of 230 ± 38 km s^-1 in Fe XIX (592.23 Å) and associated downflows of 36 ± 16 and 43 ± 22 km s^-1 at chromospheric and transition region temperatures, respectively, relative to an averaged quiet-Sun spectra. The errors represent a 1 σ dispersion. The properties of the accelerated electron spectrum and the corresponding evaporative velocities were found to be consistent with the predictions of theory.

We demonstrate that the inclusion of configuration interaction (CI) results in significant values for the K-shell fluorescence yields of Li-like ions, which are zero in a single-configuration approach. Modeling codes for simulating supernova remnants under nonequilibrium ionization conditions or photoionized plasmas such as active galactic nuclei or X-ray binaries need to be updated accordingly. A two-parameter fitting formula for the fluorescence yields has been developed. The generality of important CI effects on atomic calculations is pointed out.