Table of contents

Since Bekenstein's creation of his tensor-vector-scalar theory (TeVeS), the modified Newtonian dynamics (MOND) paradigm has been redeemed from the embarrassment of lacking a relativistic version. One primary success of TeVeS is that it provides an enhancement of gravitational lensing, which could not be achieved by other MOND theories. Following Bekenstein's work, we investigate the phenomena of gravitational lensing including deflection angles, lens equations, and time delay. We find that the deflection angle maintains its value, while the distance of closest approach varies in the MOND regime. We also use the deflection angle law to derive magnifications and investigate microlensing light curves. We find that the difference in the magnification of the two images in the point-mass model is not a constant, as in general relativity (GR). Besides, microlensing light curves could deviate significantly from GR in the deep MOND regime. Furthermore, the scalar field, which is introduced to enhance the deflection angle in TeVeS, contributes a negative effect on the potential time delay. Unfortunately, this phenomenon is unmeasurable in lensing systems, where we can only observe the time delay between two images for a given source. However, this measurable time delay offers another constraint on the mass ratio of the dark matter and MOND scenarios, which in general differs from that given by the deflection angle. In other words, for a lensing system, if two masses, m_gN and m_gM, are mutual alternatives for the deflection angles in their own paradigm, regarding the time delay they are in general in an exclusive relation.

Gravitational lensing by massive galaxy clusters is a powerful tool for the discovery and study of high-redshift galaxies, including those at z ≥ 6 likely responsible for cosmic reionization. Pelló et al. recently used this technique to discover a candidate gravitationally magnified galaxy at z = 10 behind the massive cluster lens Abell 1835 (z = 0.25). We present new Keck and Spitzer Space Telescope observations of the z = 10 candidate (hereafter #1916, following Pelló et al.'s nomenclature) together with a reanalysis of archival optical and near-infrared imaging from the Hubble Space Telescope and Very Large Telescope, respectively. Our analysis extends from the atmospheric cutoff at λ_obs ≃ 0.35 μm out to λ_obs ≃ 5 μm. The z = 10 galaxy is not detected in any of these data, including an independent reduction of Pelló et al.'s discovery H- and K-band imaging. We conclude that there is no statistically reliable evidence for the existence of #1916. We also assess the implications of our results for ground-based near-infrared searches for gravitationally magnified galaxies at z ≳ 7. The broad conclusion is that such experiments remain feasible, assuming that space-based optical and mid-infrared imaging are available to break the degeneracy with low-redshift interlopers (e.g., z ~ 2-3) when fitting spectral templates to the photometric data.

We investigate the properties and evolution of a sample of galaxies selected to have prominent emission lines in low-resolution grism spectra of the Hubble Ultra Deep Field (HUDF). These objects, eGRAPES, are late-type blue galaxies characterized by small proper sizes (R₅₀ ≤ 2 kpc) in the 4350 Å rest frame, low masses (5 × 10⁹M_☉), and a wide range of luminosities and surface brightnesses. The masses, sizes, and volume densities of these objects appear to change very little up to a redshift of z = 1.5. On the other hand, their surface brightness decreases significantly from z = 1.5 to 0, while their mass-to-light ratio increases twofold. This could be a sign that most low-redshift eGRAPES have a stellar population older than high-redshift eGRAPES and hence that most eGRAPES formed at higher redshifts. The average volume density of eGRAPES is (1.8 ± 0.3) × 10^-3h Mpc^-3 at 0.3 < z ≤ 1.5. Many eGRAPES would have been formally classified as luminous compact blue galaxies (LCBGs) if these had been selected based on small physical size, blue intrinsic color, and high surface brightness, while the remainder of the sample discussed in this paper forms an extension of LCBGs toward fainter luminosities.

We apply a new approach to quantifying galaxy morphology and identifying galaxy mergers to the rest-frame far-ultraviolet images of 82 z ~ 4 Lyman break galaxies (LBGs) and 55 1.2 < z < 1.8 emission-line galaxies in the GOODS and Ultra Deep Field survey. We compare the distributions of the Gini coefficient (G), the second-order moment of the brightest 20% of galaxy light (M₂₀), and concentration (C) for high- and low-redshift galaxies with average signal-to-noise ratios per pixel >2.5 and Petrosian radii >03. Ten of the 82 LBGs have M₂₀ ≥ -1.1 and possess bright double or multiple nuclei, implying a major-merger fraction of star-forming galaxies ~10%-25% at M_FUV < -20, depending on our incompleteness corrections. Galaxies with bulge-like morphologies (G ≥ 0.55, M₂₀ < -1.6) make up ~30% of the z ~ 4 LBG sample, while the remaining ~50% have G- and M₂₀-values higher than expected for smooth bulges and disks and may be star-forming disks, minor mergers, or postmergers. The star-forming z ~ 1.5 galaxy sample has a morphological distribution that is similar to the UDF z ~ 4 LBGs, with an identical fraction of major-merger candidates but fewer spheroids. The observed morphological distributions are roughly consistent with current hierarchical model predictions for the major-merger rates and minor-merger-induced starbursts at z ~ 1.5 and ~4. We also examine the rest-frame FUV - NUV and FUV - B colors as a function of morphology and find no strong correlations at either epoch.

We present results of our Hubble Space Telescope Cycle 11 survey for low-redshift (z < 1.65) damped Lyα (DLA) systems in the UV spectra of quasars selected from the Sloan Digital Sky Survey (SDSS) Early Data Release. These quasars have strong intervening Mg II-Fe II systems that are known signatures of high column density neutral gas. In total, including our previous surveys, UV observations of Lyα absorption in 197 Mg II systems with z < 1.65 and rest equivalent width (REW) W ≥ 0.3 Å have now been obtained. The main results are as follows: (1) The success rate of identifying DLAs in a Mg II sample with W ≥ 0.5 Å and Fe II W ≥ 0.5 Å is 36% ± 6% and increases to 42% ± 7% for systems with W/W < 2 and Mg I W > 0.1 Å. (2) The mean H I column density of Mg II systems with 0.3 Å ≤ W < 0.6 Å is = (9.7 ± 2.2) × 10¹⁸ cm^-2. For the larger REW systems in our sample, = (3.5 ± 0.7) × 10²⁰ cm^-2. (3) The DLA incidence per unit redshift for 0 < z < 5 is n_DLA(z) = n₀(1 + z)^γ, where n₀ = 0.044 ± 0.005 and γ = 1.27 ± 0.11. This parameterization is consistent with no evolution for z ≲ 2 (Ω_Λ = 0.7, Ω_M = 0.3) but exhibits significant evolution for z ≳ 2. (4) The cosmological neutral gas mass density due to DLAs is constant in the redshift interval 0.5 < z < 5.0 to within the uncertainties, Ω_DLA ≈ 1 × 10^-3. This is larger than Ω_gas(z = 0) by a factor of ≈2. (5) The slope of the H I column density distribution does not change significantly with redshift. However, the low-redshift distribution is marginally flatter due to the higher fraction of high column density systems in our sample. (6) Finally, using the precision of Mg II survey statistics, we show that under the assumption of constant DLA fraction and H I column density suggested by our current sample, there may be evidence of a decreasing Ω_DLA from z = 0.5 to 0.

We present results for Lyα forest and metal absorbers from ~7 km s^-1 resolution Space Telescope Imaging Spectrograph data for QSO PKS 0405-123 (z = 0.574). We analyze strong and weak samples of Lyα forest lines, containing 60 absorbers with column density log N ≥ 13.3 over 0.002 < z < 0.423 and 44 with log N ≥ 13.1 over 0.020 < z < 0.234, respectively. Seven absorbers contain metals, all with associated O VI and often offset in velocity from Lyα. The strong and weak Doppler parameter distributions have ⟨b⟩ = 47 ± 22 and 44 ± 21 km s^-1, respectively. Simulated spectra suggest that line blending and signal-to-noise ratio effects inflate b. For absorbers with 13.1 < log N < 14.0, we find a redshift overdensity of ~0.2-0.3 dex at 0.127 < z < 0.234, which we attribute to cosmic variance. There is a void in the strong sample at 0.0320 < z < 0.0814 with probability of random occurrence P < 0.0004. We detect Lyα-Lyα clustering in our sample on a scale of Δv ≤ 250 km s^-1 for log N ≥ 13.3, consistent with a numerical model of structure evolution. We detect velocity correlations of up to 250 km s^-1 between Lyα absorbers and 39 galaxies at z < 0.43 out to a transverse distance of 1.6 h local frame Mpc. The Lyα-galaxy two-point correlation function is significant out to Δv < 250 km s^-1 and grows with minimum H I column density. The strongest signal occurs for log N ≳ 13.5-14.0 absorbers and is similar to the galaxy-galaxy correlation, implying that such Lyα absorbers have masses log(M/M_☉) = 11.3. We find a correlation between local galaxy counts and local summed H I column density, with peak significance on scales of 4000-6000 km s^-1 and probability of random occurrence P = 0.0009. Finally, we present column densities for a number of Galactic species.

We have analyzed Hubble Space Telescope spectroscopy of 24 nearby active galactic nuclei (AGNs) to investigate spatially resolved gas kinematics in the narrow-line region (NLR). These observations effectively isolate the nuclear line profiles on less than 100 pc scales and are used to investigate the origin of the substantial scatter between the widths of strong NLR lines and the stellar velocity dispersion σ_* of the host galaxy, a quantity that relates with substantially less scatter to the mass of the central, supermassive black hole and more generally characterize variations in the NLR velocity field with radius. We find that line widths measured with STIS at a range of spatial scales systematically underestimate both σ_* and the line width measured from ground-based observations, although they do have comparably large scatter to the relation between ground-based NLR line width and σ_*. There are no obvious trends in the residuals when compared with a range of host galaxy and nuclear properties. The widths and asymmetries of [O III] λ5007 and [S II] λλ6716, 6731 as a function of radius exhibit a wide range of behavior. Some of the most common phenomena are substantial width increases from the STIS to the large-scale, ground-based aperture and almost no change in line profile between the unresolved nuclear spectrum and ground-based measurements. We identify asymmetries in a surprisingly large fraction of low-ionization [S II] line profiles and several examples of substantial red asymmetries in both [O III] and [S II]. These results underscore the complexity of the circumnuclear material that constitutes the NLR and suggest that the scatter in the NLR width and σ_* correlation cannot be substantially reduced with a simple set of empirical relations.

We analyze the X-ray spectrum of the quasar PG 1211+143 observed with the CCD and grating spectrometers on board XMM-Newton. Using an ion-by-ion fitting model, we find an outflow component of about 3000 km s^-1, which includes absorption lines of K and L shell ions of the astrophysically abundant elements. We also identify and include in our model broad (FWHM = 6000 km s^-1) emission lines from H-like ions of C, N, O, and Ne, and He-like ions of O, Ne, and Mg. The outflow velocity we find is an alternative interpretation of the data and is in contrast with the ultra high velocity of ~24,000 km s^-1 reported previously for this object. Nevertheless, we cannot completely rule out the presence of a high-velocity component due to the poor signal-to-noise ratio of the data.

Data taken in stereo mode by the High Resolution Fly's Eye (HiRes) air fluorescence experiment are analyzed to search for correlations between the arrival directions of ultrahigh-energy cosmic rays with the positions of BL Lacertae objects. Several previous claims of significant correlations between BL Lac objects and cosmic rays observed by other experiments are tested. These claims are not supported by the HiRes data. However, we verify a recent analysis of correlations between HiRes events and a subset of confirmed BL Lac objects from the 10th Veron Catalog, and we study this correlation in detail. Due to the a posteriori nature of the search, the significance level cannot be reliably estimated and the correlation must be tested independently before any claim can be made. We identify the precise hypotheses that will be tested with statistically independent data.

The central 2 × 1 kpc of the starburst galaxy NGC 253 have been imaged using the Submillimeter Array at a 60 pc resolution in the J = 2-1 transitions of ¹²CO, ¹³CO, and C¹⁸O, as well as in the 1.3 mm continuum. Molecular gas and dust are distributed mainly in a circumnuclear disk of ~500 pc radius, with warm (~40 K) and high area filling factor gas in its central part. Two gas shells or cavities have been discovered in the circumnuclear disk. They have ~100 pc diameters and have large velocity widths of 80-100 km s^-1, suggestive of expansion at ~50 km s^-1. Modeled as an expanding bubble, each shell has an age of ~0.5 Myr and needed kinetic energy of ~1 × 10⁴⁶ J, as well as mean mechanical luminosity of ~1 × 10³³ W, for its formation. The large energy allows each to be called a superbubble. A ~10⁶M_☉ super star cluster can provide the luminosity and could be a building block of the nuclear starburst in NGC 253. Alternatively, a hypernova can also be the main source of energy for each superbubble, not only because it can provide the mechanical energy and luminosity but also because the estimated rate of superbubble formation and that of hypernova explosions are comparable. Our observations indicate that the circumnuclear molecular disk harboring the starburst is highly disturbed on 100 pc or smaller scales, presumably by individual young clusters and stellar explosions, in addition to being globally disturbed in the form of the well-known superwind.

We investigated the dynamical structure of 53 elliptical galaxies using the Chandra archival X-ray data. In X-ray-luminous galaxies, temperature increases with radius and gas density is systematically higher at the optical outskirts, indicating the presence of a significant amount of the group-scale hot gas. In contrast, X-ray-dim galaxies show a flat or declining temperature profile against radius and the gas density is relatively lower at the optical outskirts. Thus, it is found that X-ray-bright and faint elliptical galaxies are clearly distinguished by the temperature and gas density profile. The mass profile is well scaled by a virial radius r₂₀₀ rather than an optical half-radius r_e, is quite similar at (0.001-0.03)r₂₀₀ between X-ray-luminous and dim galaxies, and smoothly connects to those profiles of clusters of galaxies. At the inner region of (0.001-0.01)r₂₀₀ or (0.1-1)r_e, the mass profile well traces a stellar mass with a constant mass-to-light ratio of M/L_B = 3-10 M_☉/L_☉. The M/L_B ratio of X-ray-bright galaxies rises up steeply beyond 0.01r₂₀₀ and thus requires a presence of massive dark matter halo. From the deprojection analysis combined with the XMM-Newton data, we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 also have a high M/L_B ratio of 20-30 at 20 kpc, comparable to that of X-ray-luminous galaxies. Therefore, dark matter is indicated to be common in elliptical galaxies; their dark matter distribution, as well as that of galaxy clusters, almost follows the NFW profile.

Star formation in galaxies is triggered by a combination of processes, including gravitational instabilities, spiral wave shocks, stellar compression, and turbulence compression. Some of these persist in the far outer regions, where the column density is far below the threshold for instabilities, making the outer disk cutoff somewhat gradual. We show that in a galaxy with a single exponential gas profile the star formation rate can have a double exponential, with a shallow one in the inner part and a steep one in the outer part. Such double exponentials have been observed recently in the broadband intensity profiles of spiral and dwarf irregular galaxies. The break radius in our model occurs slightly outside the threshold for instabilities, provided the Mach number for compressive motions remains of order unity to large radii. The ratio of the break radius to the inner exponential scale length increases for higher surface brightness disks because the unstable part extends farther out. This is also in agreement with observations. Galaxies with extended outer gas disks that fall more slowly than a single exponential, such as 1/R, can have their star formation rate scale approximately as a single exponential with radius, even out to 10 disk scale lengths. Hα profiles should drop much faster than the star formation rate as a result of the rapidly decreasing ambient density.

We apply the modified acceleration law obtained from Einstein gravity coupled to a massive skew-symmetric field F_μνλ to the problem of explaining galaxy rotation curves without exotic dark matter. Our sample of galaxies includes low surface brightness (LSB) and high surface brightness (HSB) galaxies and an elliptical galaxy. In those cases for which photometric data are available, a best fit via the single parameter (M/L)_stars to the luminosity of the gaseous (H I plus He) and luminous stellar disks is obtained. In addition, a best fit to the rotation curves of galaxies is obtained in terms of a parametric mass distribution (independent of luminosity observations)—a two-parameter fit to the total galactic mass (or mass-to-light ratio M/L) and a core radius associated with a model of the galaxy cores—using a nonlinear least-squares fitting routine including estimated errors. The fits are compared to those obtained using Milgrom's phenomenological MOND model and to the predictions of the Newtonian/Kepler acceleration law.

We present mid-infrared Spitzer Space Telescope observations of a complete sample of star-forming dwarf galaxies selected from the KPNO International Spectroscopic Survey. The galaxies span a wide range in mid-infrared properties. Contrary to expectations, some of the galaxies emit strongly at 8 μm, indicating the presence of hot dust and/or PAHs. The ratio of this mid-infrared dust emission to the stellar emission is compared with the galaxies' luminosity, star formation rate, metallicity, and optical reddening. We find that the ratio of the 8.0 μm dust emission to the stellar emission is more strongly correlated with the star formation rate than it is with the metallicity or the optical reddening in these systems. Nonetheless, there is a correlation between the 8.0 μm luminosity and metallicity. The slope of this luminosity-metallicity correlation is shallower than corresponding ones in the B band and 3.6 μm. The precise nature of the 8.0 μm emission seen in these galaxies (i.e., PAH vs. hot dust or some combination of the two) will require future study, including deep mid-IR spectroscopy.

We explore the abundances and depletion levels of elements important to dust composition in four Small Magellanic Cloud (SMC) sight lines observed with the Hubble Space Telescope. The apparent optical depth method is used to interpret the high-resolution ultraviolet absorption data toward AzV 18, AzV 456, Sk 108, and Sk 155; the latter two sight lines have been previously explored using component fitting. In contrast to previous studies, we find evidence that silicon is depleted in the SMC's interstellar medium (ISM). This makes it improbable that SMC-like extinction, i.e., no 2175 Å bump and a steep far-ultraviolet rise, results from a lack of silicate grains in the SMC. Notable differences between our sight lines that have SMC-like and Milky Way-like extinction, AzV 18 and AzV 456, respectively, are that the former has a substantially larger gas-to-dust ratio and smaller fractional H₂ abundance. Iron abundances and depletions in the SMC's ISM often diverge from the pattern shown by Si and Mg. This is evidence that Fe is not tied to the same grains as silicon, and therefore that most silicate grains are likely magnesium based. The presence of iron depletion in the SMC then suggests that this element is probably incorporated into grain types such as metals or oxides.

In this paper we report the second soft gamma-ray source catalog obtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRAL satellite. The scientific data set is based on more than 10 Ms of high-quality observations performed during the first 2 years of Core Program and public IBIS/ISGRI observations, and covers ~50% of the whole sky. The main aim of the first survey was to scan systematically, for the first time at energies above 20 keV, the whole Galactic plane to achieve a limiting sensitivity of ~1 mcrab in the central radian. The target of the second year of the INTEGRAL mission lifetime was to expand as much as possible our knowledge of the soft gamma-ray sky, with the same limiting sensitivity, to at least 50% of the whole sky, mainly by including a substantial coverage of extragalactic fields. This catalog comprises more than 200 high-energy sources detected in the energy range 20-100 keV, including new transients not active during the first year of operation, faint persistent objects revealed with longer exposure time, and several Galactic and extragalactic sources in sky regions not observed in the first survey. The mean position error for all the sources detected with significance above 10 σ is ~40'', enough to identify most of them with a known X-ray counterpart and to unveil the nature of most of the strongly absorbed ones, even though they are very difficult to detect in X-rays.

We report on a survey of the inner part of the Galactic plane in very high energy gamma rays with the H.E.S.S. Cerenkov telescope system. The Galactic plane between ±30° in longitude and ±3° in latitude relative to the Galactic center was observed in 500 pointings for a total of 230 hr, reaching an average flux sensitivity of 2% of the Crab Nebula at energies above 200 GeV. Fourteen previously unknown sources were detected at a significance level greater than 4 σ after accounting for all trials involved in the search. Initial results on the eight most significant of these sources were already reported elsewhere (Aharonian and coworkers). Here we present detailed spectral and morphological information for all the new sources, along with a discussion on possible counterparts in other wavelength bands. The distribution in Galactic latitude of the detected sources appears to be consistent with a scale height in the Galactic disk for the parent population smaller than 100 pc, consistent with expectations for supernova remnants and/or pulsar wind nebulae.

Recent near-IR (NIR) and X-ray observations of Sagittarius A*'s spectrum have yielded several strong constraints on the transient energizing mechanism, justifying a reexamination of the stochastic acceleration model proposed previously for these events. We here demonstrate that the new results are fully consistent with the acceleration of electrons via the transit-time damping process. But more importantly, these new NIR and X-ray flares now can constrain the source size, the gas density, the magnetic field, and the wave energy density in the turbulent plasma. Future simultaneous multiwavelength observations with good spectral information will, in addition, allow us to study their temporal evolution, which will eventually lead to an accurate determination of the behavior of the plasma just minutes prior to its absorption by the black hole.

We perform an analysis of spectra and photometry for 22,770 stars included in the third data release (DR3) of the Sloan Digital Sky Survey (SDSS). We derive atmospheric parameters and distances. Our analysis procedures are throughly checked using three recently published spectroscopic libraries of nearby stars and alternative methods. The SDSS sample covers a range in stellar brightness of 14 < V < 22, primarily at intermediate Galactic latitudes, and comprises large numbers of F- and G-type stars from the thick-disk and halo populations, therefore including some of the oldest stars in the Milky Way. We find that halo stars exhibit a broad range of iron abundances, with a peak at [Fe/H] ≃ -1.4. This population exhibits essentially no Galactic rotation. Thick-disk G-dwarf stars at distances from the Galactic plane in the range 1 < |z| < 3 kpc show a much more compact metallicity distribution, with a maximum at [Fe/H] ≃ -0.7, and a median Galactic rotation lagging the local standard of rest by 63 km s^-1. A comparison of color indices and metal abundances with isochrones indicates that no significant star formation has taken place in the halo in the last ~11 Gyr, but there are thick-disk stars that are at least 2 Gyr younger. We find the metallicities of thick-disk stars to be nearly independent of Galactocentric distance between 5 and 14 kpc from the Galactic center, in contrast with the marked gradients found in the literature for the thin disk. No vertical metallicity gradient is apparent for the thick disk, but we detect a gradient in its rotational velocity of -16 ± 4 km s^-1 kpc^-1 between 1 and 3 kpc from the plane. We estimate that among the stars in our sample there are over 2000 with an iron abundance [Fe/H] < -2, and over 150 stars with an iron abundance [Fe/H] < -3.

We present the first results of a new abundance survey of the Milky Way bulge based on Keck HIRES spectra of 27 K giants in the Baade's Window (l = 1°, b = -4°) field. The spectral data used in this study are of much higher resolution and signal-to-noise ratio than previous optical studies of Galactic bulge stars. The [Fe/H] values of our stars, which range between -1.29 and +0.51, were used to recalibrate large low-resolution surveys of bulge stars. Our best value for the mean [Fe/H] of the bulge is -0.10 ± 0.04. This mean value is similar to the mean metallicity of the local disk and indicates that there cannot be a strong metallicity gradient inside the solar circle. The metallicity distribution of stars confirms that the bulge does not suffer from the so-called G dwarf problem. This paper also details the new abundance techniques necessary to analyze very metal-rich K giants, including a new Fe line list and regions of low blanketing for continuum identification.

Recent spectroscopic studies have revealed the presence of numerous carbon-enhanced, metal-poor stars with [Fe/H] < -2.0 that exhibit strong enhancements of s-process elements. These stars are believed to be the result of a binary mass transfer episode from a former asymptotic giant branch (AGB) companion that underwent s-process nucleosynthesis. However, several such stars exhibit significantly lower Ba/Eu ratios than solar s-process values. This might be explained if there were an additional contribution from the r-process, thereby diluting the Ba/Eu ratio by extra production of Eu. We propose a model in which the double enhancements of r-process and s-process elements originate from a former 8-10 M_☉ companion in a wide binary system, which may undergo s-processing during an AGB phase, followed by r-processing during its subsequent supernova explosion. The mass of Eu (as representative of r-process elements) captured by the secondary through the wind from the supernova is estimated and is assumed to be proportional to the geometric fraction of the secondary (low-mass, main-sequence) star with respect to the primary (exploding) star. We find that the estimated mass is in good agreement with a constraint on the Eu yield per supernova event obtained from a Galactic chemical evolution study, when the initial orbital separation is taken to be ~1 yr. If one assumes an orbital period on the order of 5 yr, the efficiency of wind pollution from the supernova must be enhanced by a factor of ~10. This may, in fact, be realized if the expansion velocity of the supernova's innermost ejecta, in which the r-process has taken place, is significantly slow, resulting in an enhancement of accretion efficiency by gravitational focusing.

We set new near-infrared and optical magnitude limits for the central X-ray point source (XPS) in the Cassiopeia A supernova remnant based on HST images. Near-infrared images of the center of Cas A taken with the NICMOS 2 camera in combination with the F110W and F160W filters (~J and H bands) have magnitude limits ≥26.2 and ≥24.6, respectively. These images reveal no sources within a 12 radius (corresponding to a 99% confidence limit) of the Chandra XPS position. The NICMOS data, taken together with broadband optical magnitude limits (R ~ 28 mag) obtained from a deep STIS CCD exposure taken with a clear filter (50CCD), indicate that the XPS luminosities are very low in the optical/NIR bands (e.g., L_H < 3 × 10²⁹ ergs s^-1) with no optical, J-, or H-band counterpart to the XPS easily detectable by HST. The closest detected object lies 18 from the XPS's nominal coordinates, with magnitudes R = 25.7, m_F110W = 21.9, and m_F160W = 20.6, and is a foreground, late-type star as suggested by Kaplan, Kulkarni, and Murray. We discuss the nature of the Cas A central compact object on the basis of these near-infrared and optical flux limits.

Analysis of broadband HST ACS and WFPC2 images of the young Galactic supernova remnant Cassiopeia A reveals a far larger population of outlying, high-velocity knots of ejecta with a broader range of chemical properties than previously suspected. In this paper, we concentrate on a ≃1.5 arcmin² region located along the eastern limb of the remnant where we identify three main classes of outer ejecta: (1) knots dominated by [N II] λλ6548, 6583 emission, (2) knots dominated by oxygen emission lines, especially [O II] λλ7319, 7330, and (3) knots with emission-line strengths similar to the [S II]-strong fast-moving knot (FMK) ejecta commonly seen in the main emission shell. Mean transverse velocities derived from observed proper motion for N-rich, O-rich, and FMK-like knots identified in this region were found to be 8100, 7900, and 7600 km s^-1, respectively. The discovery of a significant population of O-rich ejecta situated between the suspected N-rich outer photospheric layer and S-rich FMK-like ejecta suggests that the progenitor's chemical layers were not completely disrupted by the supernova explosion outside of the remnant's northeast and southwest high-velocity "jet" regions. In addition, we find the majority of O-rich outer ejecta at projected locations out beyond the remnant's fastest moving Fe-rich X-ray emission material seen in Chandra and XMM-Newton data along the eastern limb, suggesting that penetration of Fe-rich material up through the S- and Si-rich mantle did not extend past the progenitor's N- or O-rich outer layers for this section of the remnant.

Two Chandra observations have been used to search for thermal X-ray emission from within and around the Crab Nebula. Dead time was minimized by excluding the brightest part of the nebula from the field of view. A dust-scattered halo comprising 5% of the strength of the Crab is clearly detected, with surface brightness measured out to a radial distance of 18'. Coverage is 100% at 4', 50% at 12', and 25% at 18'. The observed halo is compared with predictions based on three different interstellar grain models, and one can be adjusted to fit the observation. This dust halo and mirror scattering form a high background region that has been searched for emission from shock-heated material in an outer shell. We find no evidence for such emission. We can set upper limits a factor of 10-1000 less than the surface brightness observed from outer shells around similar remnants. The upper limit for X-ray luminosity of an outer shell is ≈10³⁴ ergs s^-1. Although it is possible to reconcile our observation with an 8-13 M_☉ progenitor, we argue that this is unlikely.

The effect of a newly born star cluster inside a giant molecular cloud (GMC) is to produce a hot bubble and a thin, dense shell of interstellar gas and dust swept up by the H II expansion, strong stellar winds, and repeated supernova explosions. Lying at the inner side of the shell is the photodissociation region (PDR), the origin of much of the far-infrared/submillimeter/millimeter (FIR/submm/mm) radiation from the interstellar medium (ISM). We present a model for the expanding shell at different stages of its expansion that predict mm/submm and far-IR emission line intensities from a series of key molecular and atomic constituents in the shell. The kinematic properties of the swept-up shell predicted by our model are in very good agreement with the measurements of the supershell detected in the nearby starburst galaxy M82. We compare the modeling results with the ratio-ratio plots of the FIR/submm/mm line emission in the central 1.0 kpc region to investigate the mechanism of star-forming activity in M82. Our model has yielded appropriate gas densities, temperatures, and structure scales compared to those measured in M82, and the total H₂ content is compatible with the observations. This implies that the neutral ISM of the central star-forming region is a product of fragments of the evolving shells.

We report results from a Far Ultraviolet Spectroscopic Explorer (FUSE) survey of interstellar molecular hydrogen (H₂) along 45 sight lines to AGNs at high Galactic latitudes ( > 20°). Most (39 out of 45) of the sight lines show detectable Galactic H₂ absorption from Lyman and Werner bands between 1000 and 1126 Å, with column densities ranging from N = 10^14.17 to 10^19.82 cm^-2. In the northern Galactic hemisphere, we identify many regions of low N (≤10¹⁵ cm^-2) between l = 60° and 180° and at b > 54°. These "H₂ holes" provide valuable, uncontaminated sight lines for extragalactic UV spectroscopy, and a few may be related to the "Northern Chimney" (low Na I absorption) and the "Lockman Hole" (low N). A comparison of high-latitude H₂ with 139 OB star sight lines surveyed in the Galactic disk suggests that high-latitude and disk H₂ clouds may have different rates of heating, cooling, and UV excitation. For rotational states J = 0 and 1, the mean excitation temperature at high latitude, ⟨T⟩ = 124 ± 8 K, is somewhat higher than that in the Galactic disk, ⟨T⟩ = 86 ± 20 K. For J ≥ 2, the mean ⟨T_exc⟩ = 498 ± 28 K, and the column-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate a comparable degree of UV excitation in the disk and low halo for sight lines with N ≥ 10¹⁸ cm^-2. The distribution of molecular fractions at high latitude shows a transition at lower total hydrogen column density (log N ≈ 20.38 ± 0.13) than in the Galactic disk (log N ≈ 20.7). If the UV radiation fields are similar in disk and low halo, this suggests an enhanced H₂ (dust-catalyzed) formation rate in higher density, compressed clouds, which could be detectable as high-latitude, sheetlike infrared cirrus.

We combine data from our recent FUSE survey of interstellar molecular hydrogen absorption toward 50 high-latitude AGNs with COBE-corrected IRAS 100 μm emission maps to study the correlation of infrared cirrus with H₂. A plot of the H₂ column density versus IR cirrus intensity shows the same transition in molecular fraction, f, as seen with total hydrogen column density, N_H. This transition is usually attributed to H₂ "self-shielding," and it suggests that many diffuse cirrus clouds contain H₂ in significant fractions, f ≈ 1%-30%. These clouds cover ~50% of the northern sky at b > 30°, at temperature-corrected 100 μm intensities D ≥ 1.5 MJy sr^-1. The sheetlike cirrus clouds, with hydrogen densities n_H ≥ 30 cm^-3, may be compressed by dynamical processes at the disk-halo interface, and they are conducive to H₂ formation on grain surfaces. Exploiting the correlation between N and 100 μm intensity, we estimate that cirrus clouds at b > 30° contain ~3000 M_☉ in H₂. Extrapolated over the inner Milky Way, the cirrus may contain 10⁷M_☉ of H₂ and 10⁸M_☉ in total gas mass. If elevated to 100 pc, their gravitational potential energy is ~10⁵³ ergs.

We report the first detection of the N = 1₁₁ → 0₀₀ and 1₁₀ → 0₀₀ground state rotational lines of o-ND₂H at 335.5 and 388.7 GHz, obtained in the Lynds 1689N, Barnard 1, and Lynds 1544 molecular clouds using the Caltech Submillimeter Observatory (CSO). The submillimeter ND₂H lines have moderate opacities and simple hyperfine patterns, which allow accurate determination of the excitation temperature, H₂ volume density, and molecular column density. Both transitions have high critical densities. The 389 GHz line, in particular, traces molecular material with densities above a few × 10⁶ cm^-3. The strong 389 GHz ND₂H emission in LDN 1689N implies a high fraction of dense gas in this source, ~30%, as compared to ~15% in B1 and LDN 1544. All these regions are sites of strong molecular depletion and heavy deuteration. Nonaccreting molecules, H and its isotopologues, are difficult to study, but in the sources studied here it appears that ammonia and its isotopologues are not completely frozen out, even in the high density gas. In the well-studied case of LDN 1544, the volume probed by the ND₂H emission has densities of ~10⁶-10⁷ cm^-3, within the range where the "complete freezeout" has been predicted to occur. The critical density of the 389 GHz ND₂H line is close to that of the 309 GHz ND₃ line. Observations of these two transitions thus provide an accurate measure of the [ND₃]/[ND₂H] fractionation ratio in the very dense gas. The [ND₃]/[ND₂H] ratio in LDN 1689N (~3%) appears lower than the values measured in B1 and LDN 1544 (~7%-10%), indicating that different chemical processes may be at work in these environments. The submillimeter lines of deuteroammonia are relatively strong and detectable from good sites, such as Mauna Kea or Chajnantor. Interferometric observations of these lines with the Submillimeter Array (SMA), and subsequently the Atacama Large Millimeter Array (ALMA), will provide new opportunities to study the physics and chemistry of cold, dense ISM, where most molecules are depleted onto dust grains.

The results for the calculation of the CH⁺ formation rate by radiative association are presented. The calculation is performed on the lowest potential energy curves, which are coupled by the spin-orbit and rotational interaction. The association rate coefficient is calculated at temperatures up to 1000 K. The energy-dependent radiative association rate coefficient is marked by the presence of many resonances, which are responsible for significant contributions to the rate. The calculated rate coefficient is lower than that predicted by a previous quantum calculation. An analytical expression for the temperature-dependent association rate coefficient is given for the interval of temperatures 5-1000 K.

In order to elucidate the unusually high [HNC]/[HCN] abundance ratio observed in interstellar space, we have carried out wave packet simulations for the dissociative recombination reaction HCNH⁺ + e^- → HNC/HCN + H on the two-dimensional potential energy surfaces (PESs) of two dissociative electronic states of HCNH, 1 ²Σ⁺ and 2 ²Σ⁺. Two adiabatic PESs of 1 ²Σ⁺ and 2 ²Σ⁺ have been determined as functions of NH and CH bond lengths for a linear geometry of HCNH with a fixed CN bond length, by the multireference single and double configuration interaction plus Davidson's quadruple excitation correction [MR-SDCI(+Q)] method. The wave packets, generated from the low-lying vibrational eigenstates of HCNH⁺, have been put initially on the 2 ²Σ⁺ adiabatic PES. The resulting branching ratios of [HNC]/[HCN] vary from 0.77 to 1.32, depending on the initial vibrational quantum numbers, which support that HCNH⁺ is the precursor of both HNC and HCN in interstellar space.

We present a high spatial resolution, 10-20 μm survey of 65 T Tauri binary stars in Taurus, Ophiuchus, and Corona Australis using the Keck 10 m telescopes. Designed to probe the inner ~1 AU region of the circumstellar disks around the individual stellar components in these binary systems, this study increases the number of binaries with spatially resolved measurements at 10 μm by a factor of ~5. Combined with resolved near-infrared photometry and spectroscopic accretion diagnostics, we find that ~10% of stars with a mid-infrared excess do not appear to be accreting. In contrast to an actively accreting disk system, these passive disks have significantly lower near-infrared colors that are, in most cases, consistent with photospheric emission, suggesting the presence of an inner disk hole. In addition, there appears to be a spectral type/mass dependence associated with the presence of a passive disk, with all passive disks occurring around M-type stars. The presence of a passive disk does not appear to be related to the fact that these objects are in visual binary systems; the passive disk systems span the entire range of binary separations present in the sample, and a similar fraction of passive disks is observed in a sample of single stars. The possibility that the passive disks are caused by the presence of an as yet undetected companion at a small separation (0.3-3 AU) is possible for any individual system; however, it cannot account for the spectral type dependence of the passive disk sample as a whole. We propose that these passive disks represent a subset of T Tauri stars that are undergoing significant disk evolution. The fraction of observed passive disks and the observed spectral type dependence can both be explained by models of disk evolution that include disk photoevaporation from the central star.

We present interferometric observations of N₂H⁺(1-0) in the starless, dense core L694-2 and compare them to previously published maps of L1544. Both cores are starless, centrally condensed, and show spectral signatures of rotation and collapse. We fit radially averaged spectra using a two-layer infall model and measure the variation of opacity and infall speed in each core. Both functions increase toward the center of each core, but the radial gradients are shallower, and the central values lower, in L694-2. This general behavior is predicted in models of gravitational collapse with thermal plus magnetic support, and the lower values in L694-2 may be due to its lower mass or a slightly earlier evolutionary state. In either case, it appears that both cores will form stars within a few 10⁴ yr.

We present light curves of the afterglow of GRB 050502A, including very early data at t - t_GRB < 60 s. The light curve is composed of unfiltered ROTSE-IIIb optical observations from 44 s to 6 hr postburst, R-band MDM observations from 1.6 to 8.4 hr postburst, and PAIRITEL JHK_s observations from 0.6 to 2.6 hr postburst. The optical light curve is fit by a broken power law, where t^α steepens from α = -1.13 ± 0.02 to -1.44 ± 0.02 at ~5700 s. This steepening is consistent with the evolution expected for the passage of the cooling frequency ν_c through the optical band. Even in our earliest observation at 44 s postburst, there is no evidence that the optical flux is brighter than a backward extrapolation of the later power law would suggest. The observed decay indices and spectral index are consistent with either an ISM or a wind fireball model, but slightly favor the ISM interpretation. The expected spectral index in the ISM interpretation is consistent within 1 σ with the observed spectral index β = -0.8 ± 0.1; the wind interpretation would imply a spectral index slightly (~2 σ) shallower than observed. A small amount of dust extinction at the source redshift could steepen an intrinsic spectrum sufficiently to account for the observed value of β. In this picture, the early optical decay, with the peak at or below 4.7 × 10¹⁴ Hz at 44 s, requires very small electron and magnetic energy partitions from the fireball.

GRB 031203 was a very low apparent luminosity γ-ray burst (GRB). Coincidentally, it was also the first GRB with a dust-scattered X-ray halo. The observation of the halo allowed us to infer the presence of a large soft X-ray fluence in the total burst output. It has also been claimed, however, that GRB 031203 was intrinsically subenergetic, representative of a class of spectrally hard, low-energy bursts quite different from other GRBs. A careful reanalysis of the available data confirms our original finding that GRB 031203 had a very large soft X-ray component, the time of which can be constrained to within a few minutes after the burst, strongly suggesting that while GRB 031203 did indeed have a very low apparent luminosity, it was also very soft. Notions propagated in the literature regarding the uncertainties in the determination of the soft X-ray fluence from the halo data and on the available constraints from the hard X-ray data are addressed: the properties of the scattering dust along the line of sight (grain sizes, precise location, and geometry) are determined directly from the high-quality X-ray data so that there is little uncertainty about the scatterer; constraints on the X-ray light curve from the INTEGRAL spacecraft at the time of the soft X-ray blast are not complete because of a slew in the spacecraft pointing shortly after the burst. Claims that GRB 031203 was intrinsically underenergetic and that it represents a deviation from the luminosity-peak-energy relation do not appear to be substantiated by the data, regardless of whether the soft X-ray component is (arbitrarily) declared part of the prompt emission or the afterglow. We conclude that the difference between the soft and hard X-ray spectra from XMM-Newton and INTEGRAL indicate that a second soft pulse probably occurred in this burst, as has been observed in other GRBs, notably GRB 050502B.

We report the results of observations of the black hole binaries XTE J1550-564 and H1743-322 in their quiescent state using the Chandra X-Ray Observatory. Both sources are detected at their faintest level of X-ray emission ever observed with a 0.5-10 keV unabsorbed luminosity of 2 × 10³² (d/5 kpc)² ergs s^-1 for XTE J1550-564 and 9 × 10³¹ (d/8 kpc)² ergs s^-1 for H1743-322. These luminosities are in the upper range compared to the faintest levels observed in other black hole systems, possibly related to residual accretion for these sources with frequent outbursts. For XTE J1550-564, the Chandra observations also constrain the X-ray spectrum, as a fit with an absorbed power-law model yields a photon index of 2.25 ± 0.08, clearly indicating a softening of the X-ray spectrum at lower luminosities compared to the standard hard state. Similar softening at low luminosity is seen for several black hole transients with orbital periods less than 60 hr. Most of the current models of accreting black holes are able to reproduce such softening in quiescence. In contrast, we find that systems with orbital periods longer than 60 hr appear to have hard spectra in quiescence, and their behavior may be consistent with hardening in quiescence.

Observations of Galactic and extragalactic globular clusters have shown that, on average, metal-rich clusters are ~3 times more likely to contain a bright X-ray source than their metal-poor counterparts. We propose that this can be explained by taking into account the difference in the stellar structure of main-sequence donors with masses between ~0.85 and ~1.25 M_☉ at different metallicities. Metal-poor main-sequence stars in this mass range do not have an outer convective zone, while metal-rich stars do. The absence of this zone turns off magnetic braking, a powerful mechanism of orbital shrinkage, leading to the failure of dynamically formed main-sequence-neutron star binaries to start mass transfer or appear as bright, low-mass X-ray binaries.

We propose that the shape of the upper-end X-ray luminosity function (XLF) observed in elliptical galaxies for point sources carries valuable information about the black hole (BH) mass spectrum among old X-ray transients formed in the galaxies. Here we present the line of arguments and analysis that support this connection and the methodology for deriving the BH mass spectrum slope from the observed XLF slope. We show that this underlying BH mass spectrum is modified by a weighting factor that is related to the transient duty cycle, and it generally depends on the host galaxy age, the BH mass, and the X-ray binary (XRB) donor type (main-sequence, red giant, or white dwarf donors). We find that the observed XLF is dominated by transient BH systems in outburst (a prediction possibly testable by future observations) but that the assumption of a constant duty cycle for all systems leads to results inconsistent with current observations. We also find that the derived BH mass slope depends on the strength of the angular momentum loss due to magnetic braking for main-sequence donors. More specifically, we find that for "standard" magnetic braking, BH XRBs with red giant donors dominate the upper-end XLF but that for weaker magnetic braking prescriptions, main-sequence donors are found to be dominant. The methodology presented here can be used in the future as our understanding of the transient duty and its dependence on binary and mass transfer properties improves. Under certain assumptions for this dependence, we derive a differential BH mass spectrum slope of ≃2.5; an upper BH mass cutoff at ≃20 M_☉ is needed to understand the very brightest of the BH XRBs in elliptical galaxies. We also show that our quantitative results are robust against expected variations by factors of a few of the outburst peak X-ray luminosities. We expect that our analysis will eventually help to constrain binary population synthesis models and the adopted relations between black holes and the masses of their progenitors.

We derive an analytic model for nonlinear "photon bubble" wave trains driven by buoyancy forces in magnetized, radiation pressure-dominated atmospheres. Continuous, periodic wave solutions exist when radiative diffusion is slow compared to the dynamical timescale of the atmosphere. We identify these waves with the saturation of a linear instability discovered by Arons; therefore, these wave trains should develop spontaneously. The buoyancy-driven waves are physically distinct from a second family of photon bubbles discovered by Gammie, which evolve into trains of gas pressure-dominated shocks as they become nonlinear. Like the gas pressure-driven shock trains, buoyancy-driven photon bubbles can exhibit very large density contrasts, which greatly enhance the flow of radiation through the atmosphere. However, steady state solutions for buoyancy-driven photon bubbles exist only when an extra source of radiation is added to the energy equation, in the form of a flux divergence. We argue that this term is required to compensate for the radiation flux lost via the bubbles, which increases with height. We speculate that an atmosphere subject to buoyancy-driven photon bubbles, but lacking this compensating energy source, would lose pressure support and collapse on a timescale much shorter than the radiative diffusion time in the equivalent homogeneous atmosphere.

We describe observational evidence for a new kind of interacting binary star outburst that involves both an accretion instability and an increase in thermonuclear shell burning on the surface of an accreting white dwarf. We refer to this new type of eruption as a combination nova. In late 2000, the prototypical symbiotic star Z Andromedae brightened by roughly 2 mag in the optical. We observed the outburst in the radio with the VLA and MERLIN, in the optical both photometrically and spectroscopically, in the far-ultraviolet with FUSE, and in the X-rays with both Chandra and XMM-Newton. The 2 year long event had three distinct stages. During the first stage, the optical rise closely resembled an earlier, small outburst that was caused by an accretion disk instability. In the second stage, the hot component ejected an optically thick shell of material. In the third stage, the shell cleared to reveal a white dwarf whose luminosity remained on the order of 10⁴L_☉ for approximately 1 yr. The eruption was thus too energetic to have been powered by accretion alone. We propose that the initial burst of accretion was large enough to trigger enhanced nuclear burning on the surface of the white dwarf and the ejection of an optically thick shell of material. This outburst therefore combined elements of both a dwarf nova and a classical nova. Our results have implications for the long-standing problem of producing shell flashes with short recurrence times on low-mass white dwarfs in symbiotic stars.

κ Pegasi is a well-known, nearby triple star system. It consists of a "wide" pair with semimajor axis = 235 mas, one component of which is a single-line spectroscopic binary (semimajor axis = 2.5 mas). Using high-precision differential astrometry and radial velocity observations, the masses for all three components are determined and the relative inclination between the wide and narrow pairs' orbits is found to be 438 ± 30, just over the threshold for the three-body Kozai resonance. The system distance is determined to be 34.60 ± 0.21 pc and is consistent with trigonometric parallax measurements.

We present new Spitzer IRS observations of HD 164270 (WC9, WR103). A quantitative analysis of the UV, optical, near-, and mid-IR spectra of HD 164270 is presented, allowing for line blanketing and wind clumping, revealing T_* ~ 48 kK, log L/L_☉ ~ 4.9, and ~ 10^-5M_☉ yr^-1 for a volume filling factor of f ~ 0.1. Our models predict that He is partially recombined in the outer stellar wind, such that recent radio-derived mass-loss rates of WC9 stars have been underestimated. We obtain C/He ~ 0.2 and O/He ~ 0.01 by number from optical diagnostics. Mid-IR fine-structure lines of [Ne II] 12.8 μm and [S III] 18.7 μm are observed, with [Ne III] 15.5 μm and [S IV] 10.5 μm absent. From these we obtain Ne/He ~ Ne⁺/He = 2.2 × 10^-3 by number, 7 times higher than the solar value (as recently derived by Asplund et al.), and S/He ~ S²⁺/He = 5.1 × 10^-5 by number. From a comparison with similar results for other WC subtypes we conclude that WC9 stars are as chemically advanced as earlier subtypes. We consider why late WC stars are exclusively observed in high-metallicity environments. In addition, we compare the UV/optical/mid-IR spectroscopic morphology of HD 164270 with the planetary nebula central star BD +30 3639 ([WC9]). Their UV and optical signatures are remarkably similar, such that our quantitative comparisons confirm similarities in stellar temperature, wind densities, and chemistry first proposed by Smith & Aller, in spite of completely different evolutionary histories, with HD 164270 presently a factor of 10 more massive than BD +30 3639. At mid-IR wavelengths, the dust from the dense young nebula of BD +30 3639 completely dominates its appearance, in contrast with HD 164270.

We report the discovery of He II λλ4850 and 6545 Raman-scattered by atomic hydrogen in the high-resolution spectrum of the very compact and young planetary nebula IC 5117 obtained with the ESPaDOnS (Echelle Spectropolarimetric Device for the Observation of Stars) installed on the 3.6 m Canada-France-Hawaii Telescope. By adopting case B recombination atomic data and considering all the fine-structure emission components, we calculate the weighted averages for the line centers of the Raman-scattered features, as well as those for the emission lines He II λλ4859 and 6560 to determine redward velocity shifts of Δv₄₈₅₀ = +29 km s^-1 and Δv₆₅₄₅ = +24 km s^-1. We compute the Raman conversion efficiency by a Monte Carlo technique adopting a simple scattering geometry, in which the hot central star is surrounded by a hollow cylindrical H I region characterized by thickness T that is in turn parameterized by the H I column density. It is proposed that the central far-UV emission region is covered significantly by the neutral scattering region characterized by the H I column density N ≃ 4 × 10²¹ cm^-2. The H I column density is higher by an order of magnitude than the value proposed from 21 cm radio observations. We briefly discuss the importance of Raman scattering as a probe of neutral material around a newly formed hot white dwarf and the mass-loss process occurring in the late stage of stellar evolution.

The hypothesis that CAK-type line driving is responsible for the large observed Wolf-Rayet (W-R) mass-loss rates has been called into question in recent theoretical studies. The purpose of this paper is to reconsider the plausibility of line driving of W-R winds within the standard approach using the Sobolev approximation while advancing the conceptual understanding of this topic. Due to the multiple scattering required in this context, of particular importance is the role of photon frequency redistribution into spectral gaps, which in the extreme limit yields the statistical Sobolev-Rosseland (SSR) mean approximation. Interesting limits to constrain are the extremes of no frequency redistribution, in which the small radii and corresponding high W-R surface temperature induces up to twice the mass-loss rate relative to cooler stars, and the SSR limit, in which the reduced efficiency of the driving drops the mass flux by as much as an order of magnitude whenever there exist significant gaps in the spectral line distribution. To see how this efficiency drop might be sufficiently avoided to permit high W-R mass loss, we explore the suggestion that ionization stratification may serve to fill the gaps globally over the wind. We find that global ionization changes can only fill the gaps sufficiently to cause about a 25% increase in the mass-loss rate over the local SSR limit. Higher temperatures and more ionization states (especially of iron) may be needed to achieve optically thick W-R winds, unless strong clumping corrections eliminate the need for such winds.

Mid-infrared observations of IK Tau have been made at 11.15 μm with the three-telescope Infrared Spatial Interferometer on Mount Wilson and also using individual segments of the Keck telescope for multiple-aperture interferometry on the Keck telescope at 10.7 μm. Both experiments provided closure phase and show temporal variations and asymmetries in the surrounding dust, with a difference of about 15% in intensity between two sides of the star. Asymmetries have been previously observed in the distribution of SiO masers closely surrounding the star. Comparison with earlier interferometric measurements shows substantial reduction in dust surrounding the star over the last decade. Several asymmetric dust models are investigated and simple images constructed.

The space telescope MOST is now providing us with extremely accurate low-frequency p-mode oscillation data for the star η Boo. We demonstrate in this paper that these data, when combined with ground-based measurements of the high-frequency p-mode spectrum, can be reproduced with stellar models that include the effects of turbulence in their outer layers. Without turbulence, the l = 0 modes of our models deviate from either the ground-based or the space data by about 1.5-4 μHz. This discrepancy can be completely removed by including turbulence in the models, and we can exactly match 12 out of 13 MOST frequencies that we identified as l = 0 modes, in addition to 13 out of 21 ground-based frequencies within their observational 2 σ tolerances. The better agreement between model frequencies and observed frequencies depends for the most part on the turbulent kinetic energy that was taken from a three-dimensional convection simulation for the Sun.

We report the successful fitting of a Roche model, with a surface temperature gradient following the von Zeipel gravity darkening law, to observations of Altair made with the Navy Prototype Optical Interferometer. We confirm the claim by Ohishi et al. that Altair displays an asymmetric intensity distribution due to rotation, the first such detection in an isolated star. Instrumental effects due to the high visible flux of this first magnitude star appear to be the limiting factor in the accuracy of this fit, which nevertheless indicates that Altair is rotating at 0.90 ± 0.02 of its breakup (angular) velocity. Our results are consistent with the apparent oblateness found by van Belle et al. and show that the true oblateness is significantly larger owing to an inclination of the rotational axis of ~64° to the line of sight. Of particular interest, we conclude that instead of being substantially evolved as indicated by its classification, A7 IV-V, Altair is only barely off the zero-age main sequence and represents a good example of the difficulties rotation can introduce in the interpretation of this part of the HR diagram.

We have searched for infrared excesses around a well-defined sample of 69 FGK main-sequence field stars. These stars were selected without regard to their age, metallicity, or any previous detection of IR excess; they have a median age of ~4 Gyr. We have detected 70 μm excesses around seven stars at the 3 σ confidence level. This extra emission is produced by cool material (<100 K) located beyond 10 AU, well outside the "habitable zones" of these systems and consistent with the presence of Kuiper Belt analogs with ~100 times more emitting surface area than in our own planetary system. Only one star, HD 69830, shows excess emission at 24 μm, corresponding to dust with temperatures ≳300 K located inside of 1 AU. While debris disks with L_dust/L_⋆ ≥ 10^-3 are rare around old FGK stars, we find that the disk frequency increases from 2% ± 2% for L_dust/L_⋆ ≥ 10^-4 to 12% ± 5% for L_dust/L_⋆ ≥ 10^-5. This trend in the disk luminosity distribution is consistent with the estimated dust in our solar system being within an order of magnitude greater or less than the typical level around similar nearby stars. Although there is no correlation of IR excess with metallicity or spectral type, there is a weak correlation with stellar age, with stars younger than a gigayear more likely to have excess emission.

Solid particles with the composition of interstellar dust and power-law size distribution dn/da ∝ a^-p (for a ≤ a_max with a_max ≳ 3λ and 3 < p < 4) will have submillimeter opacity spectral index β(λ) ≡ d ln κ/d ln ν ≈ (p - 3)β_ISM, where β_ISM ≈ 1.7 is the opacity spectral index of interstellar dust material in the Rayleigh limit. For the power-law index p ≈ 3.5, which characterizes interstellar dust and may apply for particles growing by agglomeration in protoplanetary disks, grain growth to sizes a ≳ 3 mm will result in β(1 mm) ≲ 1. Grain growth can naturally account for β ≈ 1 observed for protoplanetary disks, provided that a_max ≳ 3λ.

We explore the effect of magnetorotational turbulence on the dynamics and concentrations of boulders in local box simulations of a sub-Keplerian protoplanetary disk. The solids are treated as particles, each with an independent space coordinate and velocity. We find that the turbulence has two effects on the solids. (1) Meter and decameter bodies are strongly concentrated, locally up to a factor of 100 times the average dust density, whereas decimeter bodies only experience a moderate density increase. The concentrations are located in large-scale radial gas density enhancements that arise from a combination of turbulence and shear. (2) For meter-sized boulders, the concentrations cause the average radial drift speed to be reduced by 40%. We find that the densest clumps of solids are gravitationally unstable under physically reasonable values for the gas column density and for the dust-to-gas ratio due to sedimentation. We speculate that planetesimals can form in a dust layer that is not in itself dense enough to undergo gravitational fragmentation, and that fragmentation happens in turbulent density fluctuations in this sublayer.

During the period when the Sun was intensely active in 2003 October-November, two remarkable solar neutron events were observed by the ground-based neutron monitors. On 2003 October 28, in association with an X17.2 large flare, solar neutrons were detected with high statistical significance (6.4 σ) by the neutron monitor at Tsumeb, Namibia. On 2003 November 4, in association with an X28-class flare, relativistic solar neutrons were observed by the neutron monitors at Haleakala in Hawaii and Mexico City and by the solar neutron telescope at Mauna Kea in Hawaii simultaneously. Clear excesses were observed at the same time by these detectors, with the significance calculated as 7.5 σ for Haleakala and 5.2 σ for Mexico City. The detector on board the INTEGRAL satellite observed a high flux of hard X-rays and γ-rays at the same time in these events. By using the time profiles of the γ-ray lines, we can explain the time profile of the neutron monitor. It appears that neutrons were produced at the same time as the γ-ray emission.

We study the temporal evolution of energetic particle beams in an intermittently turbulent solar wind environment using the propagating source method developed by Zank and coworkers, which is based on the separation of the total particle distribution function into a main beam component (unscattered part) and a secondary component (produced by the scattered beam particles). We show here that intermittent changes in the turbulence responsible for scattering particles in the radial direction of the solar wind medium can contribute to the generation of fine-scale structure in the intensity profiles of impulsive events. Our aim in this paper is to simplify the problem to the extent that it allows us to demonstrate the suggested process. Implications of a more complicated transport equation are also discussed. We further address the issue of "dropouts" observed by Mazur and coworkers on the basis of transport in an intermittently turbulent medium.

The MHD virial theorem may be used to estimate the magnetic energy of active regions on the basis of vector magnetic fields measured at the photosphere or chromosphere. However, the virial estimate depends on the measured vector magnetic field being force-free. Departure from the force-free condition leads to an unknown systematic error in the virial energy estimate and an origin dependence of the result. We present a method for estimating the systematic error by assuming that magnetic forces are confined to a thin layer near the photosphere. If vector magnetic field measurements are available at two levels in the low atmosphere (e.g., the photosphere and the chromosphere), the systematic error may be directly calculated using the observed horizontal and vertical field gradients, resulting in an energy estimate that is independent of the choice of origin. If (as is generally the case) measurements are available at only one level, the systematic error may be approximated using the observed horizontal field gradients together with a simple linear force-free model for the vertical field gradients. The resulting "improved" virial energy estimate is independent of the choice of origin but depends on the choice of the model for the vertical field gradients, i.e., the value of the linear force-free parameter α. This procedure is demonstrated for five vector magnetograms, including a chromospheric magnetogram.

The spatio-spectral maximum entropy method (SSMEM) has been developed by Komm and coworkers in 1997 for use with solar multifrequency interferometric observation. In this paper we further improve the formulation of the SSMEM to establish it as a tool for astronomical imaging spectroscopy. We maintain their original idea that spectral smoothness at neighboring frequencies can be used as an additional a priori assumption in astrophysical problems and that this can be implemented by adding a spectral entropy term to the usual maximum entropy method (MEM) formulation. We, however, address major technical difficulties in introducing the spectral entropy into the imaging problem that are not encountered in the conventional MEM. These include calculation of the spectral entropy in a generally frequency-dependent map grid, simultaneous adjustment of the temperature variables and Lagrangian multipliers in the spatial and spectral domain, and matching the solutions to the observational constraints at a large number of frequencies. We test the performance of the SSMEM in comparison with the conventional MEM.

We present extensive configuration interaction calculations of oscillator strengths and transition rates of certain near-infrared emission lines of Fe II observed in the BD Weigelt blobs of η Carinae. Comparison is made between our work and results available in existing databases. We offer arguments in favor of our results being the most accurate, where differences occur.

A new redshift mechanism—the electric redshift—is proposed, in accord with the five-dimensional Kaluza-Klein theory, which unifies Einsteinian general relativity and Maxwellian electromagnetic theory. It is shown that a dense, massive, and charged object can significantly shift a light ray that is emitted from the object's surface toward the red as compared with the gravitational redshift. A compact, electrically charged object with density and mass comparable to those of a neutron star can impart a redshift as great as quasars have. Therefore, if quasars are dense, massive, and electrically charged objects, their large redshifts do not imply that all quasars are extremely distant; thus, the luminosity of quasars and their association with low-redshift galaxies can be understood. This interpretation does not conflict with big bang cosmology, because the electric redshifts are negligible for normal stars, galaxies, and large-scale matter, which are not dense and electrically charged.

Analysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL) Core Programme and public open-time observations performed up to 2005 April provides a sample of 62 active galactic nuclei in the 20-100 keV band above a flux limit of ~1.5 × 10^-11 ergs cm^-2 s^-1. Most (42) of the sources in the sample are Seyfert galaxies, almost equally divided between type 1 and type 2 objects; six are blazars, and 14 are still unclassified. Excluding the blazars, the average redshift of our sample is 0.021, while the mean luminosity is log L = 43.45. We find that absorption is present in 65% of the objects, with 14% of the total sample due to Compton-thick active galaxies. In agreement with both Swift BAT team results and 2-10 keV studies, the fraction of absorbed objects decreases with the 20-100 keV luminosity. All Seyfert 2's in our sample are absorbed, as are 33% of Seyfert 1's. The present data highlight the capability of INTEGRAL to probe the extragalactic gamma-ray sky and to find new and/or absorbed active galaxies.

GRB 050904 is very interesting, since it is by far the most distant gamma-ray burst event known to date (z = 6.29). It was reported that during the prompt high-energy emission phase, a very bright optical flare was detected that was temporally coincident with an X-ray flare. Here we use two models to explain the optical flare. One is the "late internal shock model," in which the optical flare is produced by the synchrotron radiation of the electrons accelerated by the late internal shock and the X-ray flare is produced by the synchrotron self-Compton mechanism. The other is the external forward-reverse shock model, in which the optical flare is from the reverse-shock emission and the X-ray flare is attributed to the activity of the central engine. We show that with the proper parameters, a bright optical flare can appear in either model. We think that the late internal shock model is more favored, since in this model the optical flash and the X-ray flare have the same origin, which provides a natural explanation of their temporal coincidence. In the forward-reverse shock scenario, fits to the optical flare and the late afterglow suggest that the physical parameters of the reverse shock are much different from those of the forward shock, as found in previous modeling of the optical flash of GRB 990123.

We report Swift Burst Alert Telescope (BAT) observations of the X-ray flash (XRF) XRF 050416A. The fluence ratio between the 15-25 and 25-50 keV energy bands of this event is 1.1, thus making it the softest gamma-ray burst (GRB) observed by BAT so far. The spectrum is well fitted by a Band function with E of 15.0 keV. Assuming the redshift of the host galaxy (z = 0.6535), the isotropic equivalent radiated energy E_iso and the peak energy at the GRB rest frame (E) of XRF 050416A are not only consistent with the correlation found by Amati et al. and extended to XRFs by Sakamoto et al. but also fill in the gap of this relation around the 30-80 keV range of E. This result tightens the validity of the E-E_iso relation from XRFs to GRBs. We also find that the jet break time estimated using the empirical relation between E and the collimation corrected energy E_γ is inconsistent with the afterglow observation by the Swift X-Ray Telescope. This could be due to the extra external shock emission overlaid around the jet break time or to the nonexistence of a jet break feature for XRFs, which might be a further challenge for GRB jet emission models and XRF/GRB unification scenarios.

We present the results of a study of the evolution of the parameters that characterize the structure and dynamics of the relaxed elliptical-like objects (ELOs) identified at redshifts z = 0, z = 1, and z = 1.5 in a set of hydrodynamic, self-consistent simulations operating in the context of a concordance cosmological model. The values of the stellar mass M, the stellar half-mass radius r, and the mean square velocity for stars σ have been measured in each ELO and found to populate, at any z, a flattened ellipsoid close to a plane (the dynamical plane, DP). Our simulations indicate that at the intermediate z's considered, individual ELOs evolve, increasing their M, r, and σ parameters as a consequence of ongoing mass assembly, but nevertheless, their DP is roughly preserved within its scatter, in agreement with observations of the fundamental plane at different z's. We briefly discuss how this lack of significant dynamical and structural evolution in ELO samples arises, in terms of the two different phases operating in the mass aggregation history of their dark matter halos. According to our simulations, most dissipation involved in ELO formation takes place at the early violent phase, causing the M, r, and σ parameters to settle down to the DP and, moreover, the transformation of most of the available gas into stars. In the subsequent slow phase, ELO stellar mass growth preferentially occurs through nondissipative processes, so that the DP is preserved and the ELO star formation rate considerably decreases. These results hint, for the first time, at a possible way of explaining, in the context of cosmological simulations, different and apparently paradoxical observational results for elliptical galaxies.

The luminosity dependence of kinematic and isophotal properties of elliptical galaxies is investigated using numerical simulations of galaxy merging, combined with semianalytical models of hierarchical structure formation. Mergers of spiral galaxies as the only formation mechanism for elliptical galaxies can reproduce neither the kinematic and photometric properties of very massive elliptical galaxies nor the change from rotationally flattened, disky systems to anisotropic, boxy systems with increasing luminosity. We present numerical simulations showing that binary mergers of early-type galaxies open an additional channel for the formation of anisotropic, slowly rotating, boxy elliptical galaxies. Including this channel in a semianalytical model, we can successfully reproduce the observed trend that more luminous giant elliptical galaxies are more boxy and less flattened by rotation. This trend can be more strongly reproduced by suppressing residual gas infall and star formation for galaxies with stellar bulge masses M_* ≥ 3 × 10¹⁰M_☉. Hence we propose that mergers of early-type galaxies play an important role in the assembly of massive elliptical galaxies.

Star counts obtained from a 2° × 2° area centered on NGC 6822 have revealed an optical image of this galaxy composed of two components: in addition to the well-known H I disk with its young stellar component, there is a spheroidal stellar structure as extensive as its H I disk, but with its major axis at roughly right angles to it, that we traced to at least 36'. Radial velocities of over 100 intermediate-age carbon stars found within this structure display kinematics contrasting strongly with those of the H I disk. These C stars belong to the spheroid. Although devoid of gas, the spheroid rotation is consistent with the I-band Tully-Fisher relation. The orientation of the rotation axis that minimizes the stellar velocity dispersion coincides with the minor axis of the stellar population ellipsoid, lying very nearly in the plane of the H I disk. We conclude that the H I disk is a polar ring and that the spheroidal component is an erstwhile disk, a fossil remainder of a past close encounter episode.

We present a color-magnitude diagram (CMD) for a field in the giant tidal stream of the Andromeda galaxy (M31). These observations, taken with the Advanced Camera for Surveys on the Hubble Space Telescope, are 50% complete at V ≈ 30 mag, reaching 1 mag below the oldest main-sequence turnoff. Striking similarities between the stream and a previous spheroid CMD imply that they have very similar age and metallicity distributions, but present something of an enigma; we speculate on possible interpretations of this result, but we note that none are without problems. Distinct multiple turnoffs, as might be expected from pulses of star formation caused by interaction with Andromeda, are not apparent in the stream CMD. Star formation in both fields lasted about 6 billion years, building up to relatively high metallicities and being largely complete 6 billion years ago. The close similarity of the spheroid and stream suggests that both may have derived from the same event; it would be worth exploring to what extent stars in these structures are the remnants of a disk galaxy that interacted with M31 or even were disrupted from the M31 disk itself by the interaction.

The cluster 037-B327 is of interest because it is both the most luminous and the most highly reddened cluster known in M31. Deep observations with the Advanced Camera for Surveys on the Hubble Space Telescope provide photometric data in the F606W band and also show that this cluster is crossed by a dust lane. We have determined the structural parameters of 037-B327 by fitting the observed surface brightness distribution to a King model with r_c = 072 (=2.69 pc), r_t = 587 (=21.93 pc), and a concentration index c = log (r_t/r_c) = 0.91. The surface brightness profile appears to be essentially flat within 025 of the center and shows no signs of core collapse. Although the dust lane affects the photometry, the King model fits the surface brightness profile well except for the regions badly affected by the dust lane. We also calculate the half-light radius, r_h = 111 (=4.15 pc). Combined with previous photometry, we find that this object falls in the same region of the M_V versus log R_h diagram as do ω Centauri, M54, and NGC 2419 in the Milky Way and the massive cluster G1 in M31. All four of these objects have been claimed to be the stripped cores of former dwarf galaxies. This suggests that 037-B327 may also be the stripped core of a former dwarf companion to M31.

Eighteen RR Lyrae variables (RRLs) that lie in the "124 clump" identified by the Quasar Equatorial Survey Team (QUEST) have been observed spectroscopically to measure their radial velocities and metal abundances. Ten blue horizontal branch (BHB) stars identified by the Sloan Digital Sky Survey (SDSS) were added to this sample. Six of the nine stars in the densest region of the clump have a mean radial velocity in the Galactic rest frame (V_gsr) of 99.8 and σ = 17.3 km s^-1, which is slightly smaller than the average error of the measurements. The whole sample contains eight RRLs and five BHB stars that have values of V_gsr suggesting membership in this stream. For seven of these RRLs, the measurements of [Fe/H], which have an internal precision of 0.08 dex, yield ⟨[Fe/H]⟩ = -1.86 and σ = 0.40. These values suggest that the stream is a tidally disrupted dwarf spheroidal galaxy of low luminosity. Photometry from the database of the SDSS indicates that this stream covers at least 106 deg² of the sky in the constellation Virgo. The name Virgo stellar stream is suggested.

We discuss the possibility of mapping interstellar clouds at unprecedentedly high spatial resolution by means of near-IR imaging of their scattered light. We calculate the scattering of the interstellar radiation field by a cloud model obtained from the simulation of a supersonic turbulent flow. Synthetic maps of scattered light are computed in the J, H, and K bands and allow us to make an accurate estimate of column density, in the range of visual extinction between 1 and 20 mag. We provide a formalism to convert the intensity of scattered light at these near-IR bands into a total gas column density. We also show that this new method of mapping interstellar clouds is within the capability of existing near-IR facilities, which can achieve a subarcsecond spatial resolution. This opens up new perspectives in the study of interstellar dust and gas structure on very small scales. The validity of the method has been recently demonstrated by the extraordinary images of the Perseus region that were obtained by Foster & Goodman and that motivated this investigation.

We present new deep near-infrared images of dark clouds in the Perseus molecular complex. These images show beautiful extended emission that we model as scattered ambient starlight and name "cloudshine." The brightness and color variation of cloudshine complicates the production of extinction maps, the best tracer of column density in clouds. However, since the profile of reflected light is essentially a function of mass distribution, cloudshine provides a new way to study the structure of dark clouds. Previous work has used optical scattered light to study the density profile of tenuous clouds; extending this technique into the infrared provides a high-resolution view into the interiors of very dense clouds, bypassing the complexities of using thermal dust emission, which is biased by grain temperature, or molecular tracers, which have complicated depletion patterns. As new wide-field infrared cameras are used to study star-forming regions at greater depth, cloudshine will be widely observed and should be seen as a new high-resolution tool, rather than an inconvenience.

The inferred black hole in the Galactic center spans the largest angle on the sky among all known black holes. Forthcoming observational programs plan to localize or potentially resolve the image of Sgr A* to an exquisite precision, comparable to the scale of the black hole horizon. Here we show that the location of the image centroid of Sgr A* should depend on the observing frequency because of relativistic and radiative transfer effects. The same effects introduce a generic dependence of the source polarization on frequency. Future detection of the predicted centroid shift and the polarization dependence on frequency can be used to determine the unknown black hole spin and verify the validity of general relativity.

We fit X-ray spectral data in the thermal-dominant, or high-soft, state of two dynamically confirmed black holes, GRO J1655-40 and 4U 1543-47, and estimate the dimensionless spin parameters a_* ≡ a/M of the two holes. For GRO J1655-40, using a spectral hardening factor computed for a non-LTE relativistic accretion disk, we estimate a_* ~ 0.75 and a_* ~ 0.65-0.75, respectively, from ASCA and RXTE data. For 4U 1543-47, we estimate a_* ~ 0.75-0.85 from RXTE data. Thus, neither black hole has a spin approaching the theoretical maximum a_* = 1.

The discovery of the first gravitationally redshifted spectral line from a neutron star (NS) by Cottam et al. has triggered theoretical studies of the physics of atomic line formation in NS atmospheres. Chang et al. showed that the hydrogenic Fe Hα line formed above the photosphere of a bursting NS is intrinsically broad. We now include rotational broadening within general relativity and compare the resulting profile to that observed during type I bursts from EXO 0748-676. We show that the fine-structure splitting of the line precludes a meaningful constraint on the radius. Our fitting of the data show that the line-forming Fe column is log(N_{Fe, n=2}/cm^-2) = 17.9 and gravitational redshift 1 + z = 1.345 with 95% confidence. We calculate the detectability of this spectral feature for a large range of spins and inclinations, assuming that the emission comes from the entire surface. We find that at 300 and 600 Hz only 10%-20% and 5%-10% of NSs would have spectral features as deep as those seen in EXO 0748-676.

We analyze Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array (PCA) data of a double-peaked burst from the low-mass X-ray binary (LMXB) 4U 1636-536 that shows no evidence of photospheric radius expansion (PRE). We find that the X-ray-emitting area on the star increases with time as the burst progresses, even though the photosphere does not expand. We argue that this is a strong indication of thermonuclear flame spreading on the stellar surface during such bursts. We propose a model for such double-peaked bursts, based on thermonuclear flame spreading, that can qualitatively explain their essential features as well as the rarity of these bursts.

We report the results of a synthetic spectral analysis of HST STIS spectra of the AM CVn-type cataclysmic variable CP Eri obtained when the system was in quiescence. The FUV spectrum is best fitted by a helium-dominated, hybrid composition (DBAZ) white dwarf with T_eff ~ 17,000 ± 1000 K, log g ~ 8, He/H abundance ratio by number ~1000, metallicity Z ~ 0.05 × solar, and V sin i ~ 400 ± 100 km s^-1. This is the first directly detected primary white dwarf in any AM CVn system, and the surface abundance and rotation rate for the white dwarf primary are the first to be reported for AM CVn systems. The model-predicted distance is ~1000 pc. The spectral fits using pure He photospheres or He-rich accretion disks were significantly less successful. Based on the analysis of our FUV spectra, CP Eri appears to contain a hybrid composition DBAZ white dwarf with a metallicity that sets it apart from the other two AM CVn stars that have been observed in quiescence and are metal-poor. The implications of this analysis for evolutionary channels leading to AM CVn systems are discussed.

We report the discovery of an SX Phoenicis type pulsating component in the Algol-type semidetached eclipsing binary QU Sge, in the metal-rich globular cluster M71. QU Sge is only about 80'' from the center of M71 and is located in the blue straggler region in the color-magnitude diagram of M71. It is considered to be a probable member of M71, with a membership probability greater than 60% deduced from a proper-motion study in the literature. From time-series CCD photometry, we find that QU Sge has an orbital period of 3.790818 days and a primary minimum depth of ΔV = 1.333 mag. The eclipsing light curve solution shows that QU Sge has a semidetached binary configuration with the secondary component fully filling its Roche lobe. After subtracting the eclipses from the light curve, we discover an SX Phoenicis type pulsation feature. It is found to have a short period of about 0.03 days and a small amplitude of about 0.024 mag. This is the first eclipsing binary system in a globular cluster to exhibit a pulsating feature. This result supports the model in which the origin of some blue stragglers in globular clusters is mass transfer between two components in the primordial binary systems.

We report on the serendipitous discovery of ~500 low-mass candidate pre-main-sequence (PMS) stars in the vicinity of the stellar association LH 52 in the Large Magellanic Cloud. We present evidence that the red faint sequence of these stars seen in the CMD of LH 52 from Hubble Space Telescope (HST) WFPC2 observations belongs only to the association and follows almost perfectly isochrone models for PMS stars of masses down to ~0.3 M_☉. We find that this feature has a Galactic counterpart and that the mass spectrum of the candidate PMS stars in LH 52 seems to correspond to a Salpeter initial mass function with a slope Γ ≃ -1.26 in the mass range 0.8-1.4 M_☉.

We observed the HH 211 jet in the submillimeter continuum and the CO (3-2) and SiO (8-7) transitions with the Submillimeter Array. The continuum source detected at the center of the outflow shows an elongated morphology, perpendicular to the direction of the outflow axis. The high-velocity emission of both molecules shows a knotty and highly collimated structure. The SiO (8-7) emission at the base of the outflow, close to the driving source, spans a wide range of velocities, from -20 up to 40 km s^-1. This suggests that a wide-angle wind may be the driving mechanism of the HH 211 outflow. For distances ≥5'' (~1500 AU) from the driving source, emission from both transitions follows a Hubble-law behavior, with SiO (8-7) reaching higher velocities than CO (3-2) and being located upstream of the CO (3-2) knots. This indicates that the SiO (8-7) emission is likely tracing entrained gas very close to the primary jet, while the CO (3-2) is tracing less dense entrained gas. From the SiO (5-4) data of Hirano et al., we find that the SiO (8-7)/SiO (5-4) brightness temperature ratio along the jet decreases for knots far from the driving source. This is consistent with the density decreasing along the jet, from (3-10) × 10⁶ cm^-3 at 500 AU to (0.8-4) × 10⁶ cm^-3 at 5000 AU from the driving source.

We have mapped the SiO J = 5-4 line at 217 GHz from the HH 211 molecular outflow with the Submillimeter Array (SMA). The high-resolution map (16 × 09) shows that the SiO J = 5-4 emission comes from the central narrow jet along the outflow axis with a width of ~08 (~250 AU) FWHM. The SiO jet consists of a chain of knots separated by 3''-4'' (~1000 AU), and most of the SiO knots have counterparts in shocked H₂ emission seen in a new, deep VLT near-infrared image of the outflow. A new, innermost pair of knots have been discovered at just ±2'' from the central star. The line ratio between the SiO J = 5-4 data and the upper limits from the SiO J = 1-0 data of Chandler & Richer suggests that these knots have a temperature in excess of 300-500 K and a density of (0.5-1) × 10⁷ cm^-3. The radial velocity measured for these knots is ~30 km s^-1, comparable to the maximum velocity seen in the entire jet. The high temperature, high density, and velocity structure observed in this pair of SiO knots suggest that they are closely related to the primary jet launched close to the protostar.

Spitzer Space Telescope spectra of the low-mass young stellar object (YSO) IRS 46 (L_bol ≈ 0.6 L_☉) in Ophiuchus reveal strong vibration-rotation absorption bands of gaseous C₂H₂, HCN, and CO₂. This is the only source out of a sample of ~100 YSOs that shows these features, and this is the first time that they are seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined with Keck L- and M-band spectra reveals excitation temperatures of ≳350 K and abundances of 10^-6 to 10^-5 with respect to H₂, orders of magnitude higher than those found in cold clouds. In spite of this high abundance, the HCN J = 4-3 line is barely detected with the James Clerk Maxwell Telescope (JCMT), indicating a source diameter less than 13 AU. The (sub)millimeter continuum emission and the absence of scattered light in near-infrared images limit the mass and temperature of any remnant collapsing envelope to less than 0.01 M_☉ and 100 K, respectively. This excludes a hot-core-type region as found in high-mass YSOs. The most plausible origin of this hot gas rich in organic molecules is in the inner (<6 AU radius) region of the disk around IRS 46, either the disk itself or a disk wind. A nearly edge-on two-dimensional disk model fits the spectral energy distribution (SED) and gives a column of dense warm gas along the line of sight that is consistent with the absorption data. These data illustrate the unique potential of high-resolution infrared spectroscopy to probe the organic chemistry, gas temperatures, and gas kinematics in the planet-forming zones close to a young star.

Observational studies show that the probability of finding gas giant planets around a star increases with the star's metallicity. Our latest simulations of disks undergoing gravitational instabilities (GIs) with realistic radiative cooling indicate that protoplanetary disks with lower metallicity generally cool faster and thus show stronger overall GI activity. More importantly, the global cooling times in our simulations are too long for disk fragmentation to occur, and the disks do not fragment into dense protoplanetary clumps. Our results suggest that direct gas giant planet formation via disk instabilities is unlikely to be the mechanism that produced most observed planets. Nevertheless, GIs may still play an important role in a hybrid scenario, compatible with the observed metallicity trend, where structure created by GIs accelerates planet formation by core accretion.

We discovered a unique morphology in a disk around the Herbig Ae star HD 142527 by near-infrared (H and K bands) adaptive optics imaging observations. The almost face-on disk consists of two bright arcs facing one another along the east-west direction (banana-split structure) and one spiral arm extending to the north from the western arc. The eastern arc is located at ~100-400 AU in radius from the star, and the western one is detected at ~150-490 AU. The stellar position is displaced from the center of the disk by about 20 AU to the north, and also from the center of the inner hole. The two arcs show an asymmetry in their size and brightness; the larger western arc is brighter than the east one by about 2 mag. The morphology of the disk, consisting of a banana-split structure and a spiral arm, most likely suggests the presence of an unseen eccentric binary and a recent stellar encounter.

We present the first images of the 691.473 GHz CO J = 6-5 line in a protoplanetary disk, obtained along with the 690 GHz dust continuum, toward the classical T Tauri star TW Hya, using the Submillimeter Array. Imaging in the CO J = 6-5 line reveals a rotating disk, consistent with previous observations of CO J = 3-2 and 2-1 lines. Using an irradiated accretion disk model and two-dimensional Monte Carlo radiative transfer, we find that additional surface heating is needed to simultaneously fit the absolute and relative intensities of the CO J = 6-5, 3-2, and 2-1 lines. In particular, the vertical gas temperature gradient in the disk must be steeper than that of the dust, mostly likely because the CO emission lines probe nearer to the surface of the disk. We have used an idealized X-ray heating model to fit the line profiles of CO J = 2-1 and 3-2 with χ² analysis, and the prediction of this model yields CO J = 6-5 emission consistent with the observations.

We discuss the physical reasons that lead to the deflection of the interstellar neutral hydrogen flow from the direction of propagation of neutral helium in the inner heliosheath. On the basis of numerical simulations, the possibilities are investigated for deriving the orientation of the interstellar magnetic field as a function of the deflection angle.

The temperature at the base of the solar corona is one of the important factors in determining the solar coronal structure. In this Letter, we performed the time-dependent magnetohydrodynamic (MHD) simulation for the solar corona utilizing the temperature map derived from the multiwavelength observation by the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) and the magnetic field map from the Michelson Doppler Imager (MDI) on SOHO. We analyzed the difference in three-dimensional magnetic field topology obtained when the uniform base temperature adopted in standard simulations is replaced by the observation-based, nonuniform temperature distribution. The differences in the magnetic field topology obtained as the response of the solar corona to the changes of the temperature at the coronal base depict the role of the plasma conditions at the coronal base in the dynamics of the global solar corona. This work is our first effort to utilize the data of the solar coronal plasma as the boundary condition to enhance the MHD simulations of a solar corona.

In this Letter, we report a direct SOHO LASCO C1 observation of low coronal "magnetic breakout" that occurred during the coronal mass ejection (CME) on 1998 March 23. The LASCO C1 images show that a slowly expanding, small coronal loop on the northeastern limb erupted, becoming a CME with the typical three-part structure (core, void, and front). Just after the CME front went out of the C1 field of view (about 2 solar radii), a wedgelike open structure is clearly formed. From this observation, together with Yohkoh SXT and SOHO MDI images, we inferred the change of the coronal magnetic field configuration during the eruption, which shows a morphological consistency with the breakout CME model. However, our observation shows that the initial acceleration (~100 m s^-2) of the CME front began about 1 hr before the apparent field opening. This observation disagrees with the CME initiation mechanism of the breakout model. We note that the observed eruption progressed in four distinct phases: a slow rise of loop structures, the initial acceleration of the CME, the magnetic breakout and second acceleration, and the CME propagation at almost-constant speed.

In this Letter, we present the results from a high-cadence (~40 ms) Hα blue-wing observation of an M1.1-class solar flare, which occurred in NOAA AR 10687 on 2004 November 1. In collaboration with RHESSI, the observation was made with the Hα Fine Structure Telescope at the GanYu Solar Station of the Purple Mountain Observatory. For this flare, a pair of conjugate Hα kernels shows a kind of converging motion during the impulsive phase. After the impulsive phase, there appears a normal separation motion. The motion of one Hα kernel is perpendicular to the magnetic neutral line, while another kernel's converging shows both perpendicular and parallel components. Nevertheless, the shear angle decreases during the converging motion, clearly showing the relaxation of a sheared magnetic field. All of the above features are confirmed with hard X-ray (HXR) footpoints observed by RHESSI. We also obtained the time profiles of the rate of change of the shear angle and the relative velocity of the two kernels with Hα observations. Both of these time profiles show a good correlation with RHESSI HXR light curves in the higher energy range (≳50 keV). This indicates that, during the peak times of the flare, the relaxation process may have occurred rapidly. This event was also observed by the Nobeyama Radio Heliograph (NoRH), showing a single microwave source. Using NoRH maps at 17 GHz with 1 s cadence, we obtained the time profile of the radio source's velocity using the same method that we used with Hα images. The velocity-time curve of the microwave source shows a good correlation with that obtained from the two Hα kernels.