Table of contents

We present a list of 552 sources with suspected variability, based on a comparison of mid-infrared photometry from the GLIMPSE I and Midcourse Space Experiment (MSX) surveys, which were carried out nearly a decade apart. We were careful to address issues such as the difference in resolution and sensitivity between the two surveys, as well as the differences in the spectral responses of the instruments. We selected only sources where the IRAC 8.0 μm and MSX 8.28 μm fluxes differ by more than a factor of 2, in order to minimize contamination from sources where the difference in fluxes at 8 μm is due to a strong 10 μm silicate feature. We present a subset of 40 sources for which additional evidence suggests variability, using 2MASS and MIPSGAL data. Based on a comparison with the variability flags in the IRAS and MSX point-source catalogs we estimate that at least a quarter of the 552 sources and at least half of the 40 sources in the subset are truly variable. In addition, we tentatively confirm the variability of one source using multiepoch IRAS LRS spectra. We suggest that most of the sources in our list are likely to be asymptotic giant branch stars.

Radial velocity information has been collected on R Aquarii since 1946. Various stellar radiating systems have been used to determine the velocity centroid of the long-period variable. These velocity values have been combined and analyzed in an attempt to extract the orbital parameters of the binary star system. The analysis produced an orbital period of 34.6 ± 1.2 yr, an eccentricity of 0.52 ± 0.08, and a projected semimajor axis of 3.5 ± 0.4 AU. If accurate, these values would indicate a Roche lobe overflow at periastron and significant effects on the mass transfer of the system. The accuracy of these results is suspect due to inconsistencies with other parameters of the system.

We report early follow-up observations of the error box of the short burst GRB 050813 using the telescopes at Calar Alto and Observatorio Sierra Nevada, followed by deep VLT FORS2 I-band observations obtained under very good seeing conditions 5.7 and 11.7 days after the event. Neither a fading afterglow nor a rising SN component was found, so the potential GRB host galaxy has not been identified based on a comparison of the two VLT images taken at different epochs. We discuss whether any of the galaxies present in the original 10'' XRT error circle could be the host. In any case, the optical afterglow of GRB 050813 was of very low luminosity. We conclude that all these properties are consistent with the binary compact merger hypothesis for the progenitor of GRB 050813.

We discuss the near-infrared (NIR) properties of the nuclei in the 11 merging galaxies of the Toomre sequence, based on high spatial resolution J, H, and K imaging data using NICMOS on board the Hubble Space Telescope (HST). The observations are less affected by dust extinction than our previous HST WFPC2 observations and offer higher spatial resolution than existing ground-based NIR data. Nuclear positions are generally found to be consistent with those reported from data in other wave bands. In NGC 7764A we detect for the first time two nuclei with a separation of about 260 pc, consistent with its placement in the middle of the merging sequence. We see a marginal trend for the nuclei to become bluer with advancing merger stage, which we attribute to a dispersal of dust at late times in the merging process. Our data also indicate a statistically significant trend for the nuclei in the sequence to become more luminous, within an aperture of fixed physical size and after correcting for dust extinction, with advancing merger stage. We derive K-band surface brightness profiles for those nuclei for which the morphology allows a meaningful isophotal analysis, and fit the profiles with a "Nuker" law for comparison with other samples of galaxies observed with HST. The majority of the nuclei have steep profiles that can be characterized as power-law type. In general, the Toomre-sequence galaxies tend to have steeper profiles and higher central luminosity surface densities than E/S0 galaxies. Our findings can be explained if the Toomre-sequence galaxies have newly formed stars that are concentrated toward their centers. We derive V - K color profiles for the nuclei to further address this possibility, but find that the large amounts of dust extinction complicate their interpretation. Overall, our results are consistent with the generic predictions of N-body simulations of spiral galaxy mergers. If left to evolve and fade for several gigayears, it is possible that the properties of the Toomre-sequence nuclei would resemble the properties of the nuclei of normal E/S0 galaxies. Our results therefore support the view that mergers of spiral galaxies can lead to the formation of early-type galaxies.

In this paper we present new high-resolution Very Large Array 1.4 GHz radio continuum observations of five FIR-bright CO-rich early-type galaxies and two dwarf early-type galaxies. The position on the radio-FIR correlation combined with striking agreements in morphology between high-resolution CO and radio maps show that the radio continuum is associated with star formation in at least four of the eight galaxies. The average star formation rate for the sample galaxies detected in radio is ∼2 M_⊙ yr^-1. There is no evidence of a luminous active galactic nucleus in any of our sample galaxies. We estimate Toomre Q-values and find that the gas disks may well be gravitationally unstable, consistent with the above evidence for star formation activity. The radio continuum emission thus corroborates other recent suggestions that star formation in early-type galaxies may not be uncommon.

The collisional family of Kuiper Belt object (KBO) 2003 EL61 opens the possibility for many interesting new studies of processes important in the formation and evolution of the outer solar system. As the first family in the Kuiper Belt, it can be studied using techniques developed for studying asteroid families, although some modifications are necessary. Applying these modified techniques allows for a dynamical study of the 2003 EL61 family. The velocity required to change orbits is used to quantitatively identify objects near the collision. A method for identifying family members that have potentially diffused in resonances (like 2003 EL61) is also developed. Known family members are among the very closest KBOs to the collision and two new likely family members are identified: 2003 UZ117 and 1999 OY3. We also give tables of candidate family members that require future observations to confirm membership. We estimate that a minimum of ∼1 Gyr is needed for resonance diffusion to produce the current position of 2003 EL61, implying that the family is likely primordial. Future refinement of the age estimate is possible once (many) more resonant objects are identified. The ancient nature of the collision contrasts with the seemingly fresh surfaces of known family members, suggesting that our understanding of outer solar system surfaces is incomplete.

We present the results of a survey for star clusters in M33 using HST WFPC2 archive images. We have found 104 star clusters, including 32 new ones, in the images of 24 fields that were not included in previous studies. Combining these with previous data in the literature, we increase the number of M33 star clusters found in the HST images to 242. We have derived BVI integrated photometry of these star clusters from the CCD images taken with the CFH12k mosaic camera at the CFHT. Integrated color-magnitude diagrams of the M33 star clusters are found to be similar in general to those of star clusters in the Large Magellanic Cloud, except that M33 has a much lower fraction of blue star clusters. We find 29 red star clusters with 0.5 ≤ (B - V)₀ ≤ 1.1 and 0.7 ≤ (V - I)₀ ≤ 1.2, making them old globular cluster candidates. We divide the cluster sample into three groups according to their (B - V)₀ color: blue star clusters with (B - V)₀ ≤ 0.3, intermediate-color star clusters with 0.3 < (B - V)₀ < 0.5, and red star clusters with (B - V)₀ ≥ 0.5. Most of the new clusters in M33 are located along a sequence that is consistent with the theoretical evolutionary path for Z = 0.004, Y = 0.24 in the (B - V)₀-(V - I)₀ diagram, while a few of them are on the redder side in the (V - I)₀ color. Relatively more red clusters are found in the outer region of M33 than blue and intermediate-color clusters, and many of the blue stars are located in H II regions. The luminosity function for the blue star clusters shows a peak at M_V ≈ -7.3 mag, while that for the intermediate-color star clusters shows a peak at the fainter magnitude M_V ≈ -6.3 mag. The luminosity function for the red star clusters also shows a peak at M_V ≈ -6.8 mag, although the number of clusters is small.

An alternative model is proposed for the excess radiation emitted by the larger planets, each with a liquid metallic hydrogen annular domain about a central core of ice and rocks. This model is based on the mutual attraction between elements of an aggregate of charged bosons in a charge-neutralizing background, in equilibrium at very high pressure, and the property that spin-1 deuterons are bosons. Assuming valid parameters for Jupiter, it is derived that the deuteron density in Saturn is approximately equal to that in Jupiter and that particles emitted in reactions in the liquid metal domain are thermalized in the liquid hydrogen domain, resulting in infrared radiation, in accord with observed values. With corroborating properties of Neptune, it is proposed that this planet likewise contains a spherical shell of liquid metallic hydrogen outside and close to its rocky core. Whereas data are insufficient to support degenerate fusion, the known magnetic moment of Neptune is found to be consistent with positive charge components rotating in the frame of the liquid metallic hydrogen fluid with current density ≈8.4 × 10^-6 A m^-2. It is proposed that the related coupling between current and magnetic field is supported by a dynamo effect. A brief description is included describing the influence of convective storms in the large planets.

We present the latest results of the Meudon Multicolor Survey. This survey is aimed at characterizing the color properties and trends of Centaurs and trans-Neptunian objects. We report IJHK photometry of objects obtained with CFHT-IR at the 3.6 m Canada-France-Hawaii Telescope (Hawaii), JHK photometry with INGRID at the 4.2 m William Hershel Telescope (La Palma), and BVRI photometry with OIG at the 3.6 m Telescopio Nazionale Galileo Telescope (La Palma). We present visible—near-IR colors for 38 objects. Either these were acquired simultaneously, or the new near-IR photometry was tied to previously published visible measurements using the I magnitude measured in both sets. This large sample allows an extended characterization of the color properties of these primitive objects over the B (0.4 μm) to K (2.2 μm) wavelength range. We performed a detailed statistical analysis of all available IR colors in order to search for significant trends. The most relevant conclusion about visible and near-IR color-color correlations is that, basically, JHK bands alone do not show evidence of correlations, either between them or with BVRIJ bands. Only Centaurs show an anticorrelation between visible colors and H - K. Colors within each dynamical family compare very similarly.

Using the Hubble Space Telescope (HST), we have carried out a survey of candidate preplanetary nebulae (PPNs). We report here our discoveries of objects having well-resolved geometric structures, and we use the large sample of PPNs now imaged with HST (including previously studied objects in this class) to devise a comprehensive morphological classification system for this category of objects. The wide variety of aspherical morphologies which we have found for PPNs are qualitatively similar to those found for young planetary nebulae (PNs) in previous surveys. We also find prominent halos surrounding the central aspherical shapes in many of our objects; these are direct signatures of the undisturbed circumstellar envelopes of the progenitor AGB stars. Although the majority of these have surface brightness distributions consistent with a constant mass-loss rate with a constant expansion velocity, there are also examples of objects with varying mass-loss rates. As in our surveys of young PNs, we find no round PPNs. The similarities in morphologies between our survey objects and young PNs supports the view that the former are the progenitors of aspherical PNs. This suggests that the primary shaping of a PN does not occur during the PN phase via the fast radiative wind of the hot central star, but significantly earlier in its evolution.

The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the extinction of a known source. In this paper we compare two techniques, one based on emission and one on absorption, applied to sections of 14 disk galaxies. The two measurements reflect, respectively, the average and apparent optical depth of a disk section. Hence, they depend differently on the average number and optical depth of ISM structures in the disk. The small-scale geometry of the cold ISM is critical for accurate models of the overall energy budget of spiral disks. ISM geometry, relative contributions of different stellar populations, and dust emissivity are all free parameters in galaxy spectral energy distribution (SED) models; they are also sometimes degenerate, depending on wavelength coverage. Our aim is to constrain the typical ISM geometry. The apparent optical depth measurement comes from the number of distant galaxies seen in Hubble Space Telescope (HST) images through the foreground disk, calibrated with the synthetic field method (SFM). We discuss what can be learned from the SFM measurement alone regarding ISM geometry. We measure the IR flux in images from the Spitzer Infrared Nearby Galaxy Survey in the same section of the disk that was covered by HST. A physical model of the dust is fit to the SED to estimate the dust surface density, mean temperature, and brightness in these disk sections. The surface density is subsequently converted into the average optical depth estimate. The two measurements generally agree, and the SED model finds mostly cold dust (T < 25 K). The ratios between the measured average and apparent optical depths of the disk sections imply optically thin (τ_c = 0.4) clouds in these disks. Optically thick disks are likely to have more than a single cloud along the line of sight.

We quantify the variability of faint unresolved optical sources using a catalog based on multiple SDSS imaging observations. The catalog covers SDSS stripe 82, which lies along the celestial equator in the southern Galactic hemisphere (22^h24^m < α_J2000.0 < 04^h08^m, -1.27° < δ_J2000.0 < +1.27°, ∼290 deg²), and contains 34 million photometric observations in the SDSS ugriz system for 748,084 unresolved sources at high Galactic latitudes (b < -20°) that were observed at least four times in each of the ugri bands (with a median of 10 observations obtained over ∼6 yr). In each photometric bandpass we compute various low-order light-curve statistics, such as rms scatter, χ² per degree of freedom, skewness, and minimum and maximum magnitude, and use them to select and study variable sources. We find that 2% of unresolved optical sources brighter than g = 20.5 appear variable at the 0.05 mag level (rms) simultaneously in the g and r bands (at high Galactic latitudes). The majority (2 out of 3) of these variable sources are low-redshift (<2) quasars, although they represent only 2% of all sources in the adopted flux-limited sample. We find that at least 90% of quasars are variable at the 0.03 mag level (rms) and confirm that variability is as good a method for finding low-redshift quasars as the UV excess color selection (at high Galactic latitudes). We analyze the distribution of light-curve skewness for quasars and find that it is centered on zero. We find that about one-fourth of the variable stars are RR Lyrae stars, and that only 0.5% of stars from the main stellar locus are variable at the 0.05 mag level. The distribution of light-curve skewness in the g - r versus u - g color-color diagram on the main stellar locus is found to be bimodal (with one mode consistent with Algol-like behavior). Using over 600 RR Lyrae stars, we demonstrate rich halo substructure out to distances of 100 kpc. We extrapolate these results to the expected performance by the Large Synoptic Survey Telescope and estimate that it will obtain well-sampled, 2% accurate, multicolor light curves for ∼2 million low-redshift quasars and discover at least 50 million variable stars.

We have analyzed 793 atomic hydrogen emission and absorption spectrum pairs toward continuum background sources in the northern and southern Galactic plane. In this paper we focus on absorption features from cold gas in the outermost arms of the Galaxy. We find H I absorption associated on a global scale with the outer spiral arm (first and second Galactic quadrants). To a lesser extent we also see absorption associated with the most distant spiral arm in the third and fourth Galactic quadrants. In total, 236 spectra contain clear absorption features associated with the outermost spiral arms. Cool-phase gas therefore exists throughout these spiral arms. The mean distances between absorbing clouds are on the order of 90-220 pc. We identify a number of striking H I structures with masses on the order of 10⁵-10⁶M_⊙ containing cool H I gas with temperatures below 100 K. These clouds are only marginally stable against gravitational collapse.

We have carried out a major survey for visual binaries toward the Orion Nebula Cluster, using images obtained with the Advanced Camera for Surveys on the Hubble Space Telescope through an Hα filter. From 1051 stars more than 60'' from θ¹ Ori C, we have selected 781 that fulfill the criteria for membership in the Orion Nebula Cluster. Among these, we find 78 multiple systems (75 binaries and 3 triples), of which 55 are new discoveries, in the range from 0.1'' to 1.5''. We perform a statistical study of the 72 binaries and 3 triples that have separations in the limited range 0.15'' to 1.5'', within which we need no incompleteness correction. An analysis of the stellar density in our images suggests that of these binaries, nine are line-of-sight associations. When corrected for this, we find a binary fraction of 8.8% ± 1.1% within the limited separation range from 67.5 to 675 AU (counting the three triples as six binaries). The field binary fraction in the same range from Duquennoy & Mayor is a factor of 1.5 higher. Within the range 150-675 AU that overlaps the study of binaries in T Tauri associations by Reipurth & Zinnecker, we find that the associations have a factor of 2.2 more binaries than the Orion Nebula Cluster, in approximate agreement with earlier results based on data from the inner Trapezium region with small-number statistics. The binary separation distribution function of the Orion Nebula Cluster shows unusual structure, with a sudden steep decrease in the number of binaries as the separation increases beyond 0.5'', corresponding to 225 AU. We have measured the ratio of binaries wider than 0.5'' to binaries closer than 0.5'' as a function of distance from the Trapezium, and we find that this ratio is significantly depressed in the inner region of the Orion Nebula Cluster. The deficit of binaries with larger separations in the central part of the cluster is likely due to dissolution or orbital change of the wider binaries during their passage through the potential well of the inner cluster region. All of our primaries appear to be T Tauri stars, with the exception of one Herbig Ae/Be star, and there are indications that a substantial number of secondaries could be brown dwarfs.

A release of multicolor broadband (BVRI) photometry for a subsample of 44 isolated spirals drawn from the Catalogue of Isolated Galaxies is presented. Total magnitudes and colors at various circular apertures, as well as some global structural/morphological parameters, are estimated. Morphology is reevaluated through optical and sharp-filtered R-band images, (B - I) color index maps, and archived near-IR JHK images from the Two Micron All Sky Survey. The CAS structural parameters (concentration, asymmetry, and clumpiness) were calculated from the images in each of the bands. The fraction of galaxies with well-identified optical/near-IR bars (SB) is 63%, while another 17% show evidence of weak or suspected bars (SAB). The sample average value of the maximum bar ellipticity is _max ≈ 0.4. Half of the galaxies in the sample show rings. We identify two candidates for isolated galaxies with disturbed morphology. The structural CAS parameters change with the observed band, and the tendencies they follow with morphological type and global color are more evident in the redder bands. In any band, the major difference between our isolated spirals and a sample of interacting spirals is revealed in the A-S plane. A deep and uniformly observed sample of isolated galaxies is intended for various purposes, including (1) comparative studies of environmental effects, (2) comparing model predictions of galaxy evolution, and (3) evaluating the change of galaxy properties with redshift.

We present a multiwavelength analysis of the supernova remnant N49 in the Large Magellanic Cloud. Using high-resolution Hubble Space Telescope WFPC2 images of Hα, [S II], and [O III] emission, we study the morphology of the remnant and calculate the rms electron densities in different regions. We detect an offset of [O III] and Hα emission peaks of about 0.5'' and discuss possible scenarios that could give rise to such high values. The kinematics of the remnant is analyzed by matching individual kinematic features in the echelle spectra obtained at the Cerro Tololo Inter-American Observatory with the morphological features revealed in the WFPC2 images. We detect a narrow Hα emission component and identify it as diffuse preshock recombination radiation, and discrete broad emission features that correspond to the shocked gas in filaments. The overall expansion of the remnant is about 250 km s^-1. The dense clouds are shocked up to line-of-sight velocities of 250 km s^-1, and the less dense gas up to 300 km s^-1. A few cloudlets have even higher radial velocities, reaching up to 350 km s^-1. We confirm the presence of the cavity in the remnant and identify the center of explosion. Using archival Chandra and XMM-Newton data, we observe the same trends in surface brightness distribution for the optical and X-ray images. We carry out a spectral analysis of three regions that represent the most significant optical features.

Chemical abundances in three M supergiants in the Local Group dwarf irregular galaxy IC 1613 have been determined using high-resolution spectra obtained with the UVES spectrograph on the ESO 8.2 m Kueyen telescope. A detailed synthetic-spectrum analysis has been used to determine the atmospheric parameters and abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Fe, Co, Ni, La, and Eu. We find the overall metallicity of the stars to be [Fe/H] = -0.67 ± 0.09 and the age 9-13 Myr, which is in excellent agreement with the present-day values in the age-metallicity relationship model of IC 1613 by Skillman et al. We have found that the three supergiants investigated have a mean [α/Fe] equal to about -0.1, which is lower than seen in Galactic stars at the same metallicity and is in agreement with the results obtained in other dwarf irregular galaxies. The oxygen abundances are in agreement with the upper values of the nebular oxygen determinations in IC 1613. The abundance ratios of s- and r-process elements to iron are enhanced relative to solar by about 0.3 dex. The abundance pattern of the elements studied is similar to that of the Small Magellanic Cloud, except for Co and Ni, which are underabundant in the SMC. The observed elemental abundances are generally in very good agreement with the recent chemical evolution model of Yuk and Lee.

We use high signal-to-noise ratio (∼150-450), high-resolution (R ∼ 45,000) Keck HIRES spectroscopy of 13 candidate post-T Tauri stars (PTTs) to derive basic physical parameters, lithium abundances, and radial velocities. We place our stars in the M_v-T_eff plane for use in determining approximate ages from pre-main-sequence isochrones, and we confirm these using three relative age indicators in our analysis: Li abundances, chromospheric emission, and the kinematic U-V plane. Using the three age criteria we identify five stars (HIP 54529, HIP 62758, HIP 63322, HIP 74045, and HIP 104864) as probable PTTs with ages between 10 and 100 Myr. We confirm HIP 54529 as an SB2 star and HIP 63322 as an SB1 star. We also examine irregular photometric variability of PTTs using the Hipparcos photometry annex. Two of our PTTs exhibit near-IR excesses compared to Kurucz model flux; while recent work suggests classical T Tauri stars evince similar JHK excesses presumably indicative of nonphotospheric (disk) emission, our results may be illusory artifacts of the chosen I-band normalization. The near-IR excesses we see in a literature-based sample of PTTs appear to be artifacts of previous spectral type-based T_eff values. Indeed, comparison of the homology of their observed and model photospheric spectral energy distributions suggests that photometric temperatures are more reliable than temperatures based on spectral standards for the cooler temperature ranges of the stars in this sample. We conclude that our age-oriented analysis is a robust means to select samples of nearby, young, isolated PTTs that otherwise masquerade as normal field stars.

We present the results of a stellar membership survey of the nearby open clusters Praesepe and Coma Berenices. We have combined archival survey data from the SDSS, 2MASS, USNOB1.0, and UCAC-2.0 surveys to compile proper motions and photometry for ∼5 million sources over 300 deg². Of these sources, 1010 stars in Praesepe and 98 stars in Coma Ber are identified as candidate members with probability >80%; 442 and 61 are identified as high-probability candidates for the first time. We estimate that this survey is >90% complete across a wide range of spectral types (F0-M5 in Praesepe, F5-M6 in Coma Ber). We have also investigated the stellar mass dependence of each cluster's mass and radius in order to quantify the role of mass segregation and tidal stripping in shaping the present-day mass function and spatial distribution of stars. Praesepe shows clear evidence of mass segregation across the full stellar mass range; Coma Ber does not show any clear trend, but low number statistics would mask a trend of the same magnitude as in Praesepe. The mass function for Praesepe (τ ∼ 600 Myr; M ∼ 500 M_⊙) follows a power law consistent with that of the field present-day mass function, suggesting that any mass-dependent tidal stripping could have removed only the lowest mass members (<0.15 M_⊙). Coma Ber, which is younger but much less massive (τ ∼ 400 Myr; M ∼ 100 M_⊙), follows a significantly shallower power law. This suggests that some tidal stripping has occurred, but the low-mass stellar population has not been strongly depleted down to the survey completeness limit (∼0.12 M_⊙).

We present results of the Canada-France-Hawaii Telescope adaptive optics (AO) search for companions of a homogeneous group of contact binary stars, as a contribution to our attempts to prove the hypothesis that these binaries require a third star to become as close as observed. In addition to directly discovering companions at separations of ≥1'', we introduced a new method of AO image analysis utilizing distortions of the AO diffraction ring pattern at separations of 0.07''-1''. Very close companions, with separations in the latter range, were discovered in the systems HV Aqr, OO Aql, CK Boo, XY Leo, BE Scl, and RZ Tau. More distant companions were detected in V402 Aur, AO Cam, and V2082 Cyg. Our results provide a contribution to the mounting evidence that the presence of close companions is a very common phenomenon for very close binaries with orbital periods <1 day.

We present the results on millimeter interferometric observations of four luminous infrared galaxies (LIRGs), Arp 220, Mrk 231, IRAS 08572+3915, and VV 114, and one Wolf-Rayet galaxy, He 2-10, using the Nobeyama Millimeter Array (NMA). Both the HCN(1-0) and HCO⁺(1-0) molecular lines were observed simultaneously, and their brightness-temperature ratios were derived. High-quality infrared L-band (2.8-4.1 μm) spectra were also obtained for the four LIRGs to better constrain their energy sources deeply buried in dust and molecular gas. When combined with other LIRGs we have previously observed with NMA, the final sample comprised nine LIRGs (12 LIRG nuclei) with available interferometric HCN(1-0) and HCO⁺(1-0) data, sufficient to investigate the overall trend in comparison with known AGNs and starburst galaxies. We found that LIRGs with luminous buried AGN signatures at other wavelengths tend to show high HCN(1-0)/HCO⁺(1-0) brightness-temperature ratios as seen in AGN-dominated galaxies, while the Wolf-Rayet galaxy He 2-10 displays a small ratio. An enhanced HCN abundance in the interstellar gas surrounding a strongly X-ray-emitting AGN, as predicted by some chemical calculations, and/or infrared radiative pumping, are possible explanations of our results.

A spiral galaxy partially overlapping a more distant elliptical offers a unique opportunity to measure the dust extinction in the foreground spiral. From the Sloan Digital Sky Survey (SDSS) DR4 spectroscopic sample, we selected 83 occulting galaxy pairs and measured disk opacity over the redshift range z = 0.0-0.2 with the goal of determining the recent evolution of disk dust opacity. The enrichment of the ISM changes over the lifetime of a disk, and it is reasonable to expect the dust extinction properties of spiral disks as a whole to change over their lifetime. When they do, the change will affect our measurements of galaxies over the observable universe. From the SDSS pairs we conclude that spiral disks show evidence of extinction to ∼2 effective radii. However, no evidence for recent evolution of disk opacity is evident, due to the limited redshift range and our inability to distinguish other factors on disk opacity such as the presence of spiral arms and Hubble type. Such effects also mask any relation between surface brightness and optical depth that has been found in nearby galaxies. Hence, we conclude that the SDSS spectral catalog is an excellent way to find occulting pairs and construct a uniform local sample. However, a higher resolution than that of the SDSS images is needed to disentangle the effects of spiral arms and Hubble type from evolution since z = 0.2.

The Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey (2MASS) are rich resources for studying stellar astrophysics and the structure and formation history of the Galaxy. As new surveys and instruments adopt similar filter sets, it is increasingly important to understand the properties of the ugrizJHK_s stellar locus, both to inform studies of "normal" main-sequence stars and enable robust searches for point sources with unusual colors. Using a sample of ∼600,000 point sources detected by SDSS and 2MASS, we tabulate the position and width of the ugrizJHK_s stellar locus as a function of g - i color, and provide accurate polynomial fits. We map the Morgan-Keenan spectral type sequence to the median stellar locus by using synthetic photometry of spectral standards and by analyzing 3000 SDSS stellar spectra with a custom spectral typing pipeline, described in the Appendix to this paper. We develop an algorithm to calculate a point source's minimum separation from the stellar locus in a seven-dimensional color space, and use it to robustly identify objects with unusual colors, as well as spurious SDSS/2MASS matches. Analysis of a final catalog of 2117 color outliers identifies 370 white-dwarf/M dwarf (WDMD) pairs, 93 QSOs, and 90 M giant/carbon star candidates, and demonstrates that WDMD pairs and QSOs can be distinguished on the basis of their J - K_s and r - z colors. We also identify a group of objects with correlated offsets in the u - g versus g - r and g - r versus r - i color-color spaces, but subsequent follow-up is required to reveal the nature of these objects. Future applications of this algorithm to a matched SDSS-UKIDSS catalog may well identify additional classes of objects with unusual colors by probing new areas of color-magnitude space.

We have assembled a spectroscopic sample of low-mass dwarfs observed as part of the Sloan Digital Sky Survey along one Galactic sight line, designed to investigate the observable properties of the thin and thick disks. This sample of ∼7400 K and M stars also has measured ugriz photometry, proper motions, and radial velocities. We compute UVW space-motion distributions, and investigate their structure with respect to vertical distance from the Galactic plane. We place constraints on the velocity dispersions of the thin and thick disks, using two-component Gaussian fits. We also compare these kinematic distributions to a leading Galactic model. Finally, we investigate other possible observable differences between the thin and thick disks, such as color, active fraction, and metallicity.

We present a new set of photometric transformations for red stars observed with the Sloan Digital Sky Survey (SDSS) 0.5 m Photometric Telescope (PT) and the SDSS 2.5 m telescope at the Apache Point Observatory in New Mexico. Nightly PT observations of US Naval Observatory standards are used to determine extinction corrections and calibration terms for SDSS 2.5 m photometry. Systematic differences between the PT and native SDSS 2.5 m ugriz photometry require conversions between the two systems which have previously been undefined for the reddest stars. By matching ∼43,000 stars observed with both the PT and SDSS 2.5 m, we extend the present relations to include low-mass stars with colors 0.6 ≤ r - i ≤ 1.7. These corrections will allow us to place photometry of bright, low-mass trigonometric parallax stars previously observed with the PT on the 2.5 m system. We present new transformation equations and discuss applications of these data to future low-mass star studies using the SDSS.

The Canada-France High-z Quasar Survey (CFHQS) is an optical survey designed to locate quasars during the epoch of reionization. In this paper we present the discovery of the first four CFHQS quasars at redshifts greater than 6, including the most distant known quasar, CFHQS J2329-0301 at z = 6.43. We describe the observational method used to identify the quasars and present optical, infrared, and millimeter photometry and optical and near-infrared spectroscopy. We investigate the dust properties of these quasars, finding an unusual dust extinction curve for one quasar and a high far-infrared luminosity due to dust emission for another. The mean millimeter continuum flux for CFHQS quasars is substantially lower than that for SDSS quasars at the same redshift, likely due to a correlation with quasar UV luminosity. For two quasars with sufficiently high signal-to-noise ratio optical spectra, we use the spectra to investigate the ionization state of hydrogen at z > 5. For CFHQS J1509-1749 at z = 6.12 we find significant evolution (beyond a simple extrapolation of lower redshift data) in the Gunn-Peterson optical depth at z > 5.4. The line of sight to this quasar has one of the highest known optical depths at z ≈ 5.8. An analysis of the sizes of the highly ionized near-zones in the spectra of two quasars at z = 6.12 and 6.43 suggest that the intergalactic medium surrounding these quasars was substantially ionized before these quasars turned on. Together, these observations point toward an extended reionization process, but we caution that cosmic variance is still a major limitation in z > 6 quasar observations.

Using 4 years of data from the Mass Time-of-Flight and Proton Monitor, two CELIAS sensors on board SOHO, we report the sulfur abundances in comparison to magnesium and calcium (two low first ionization potential elements) for the slow solar wind, and for the first time we measure the sulfur isotopic abundance ratio. For the period in which the proton velocity was 380 ± 2 km s^-1 we obtain [S]/[Mg] = 0.26 ± 0.03, [S]/[Ca] = 4.7 ± 0.5, and [³⁴S]/[S] = 0.043 ± 0.009. We compare these measurements with the available measurements reported in the literature, and we check the quality of the results by using the magnesium isotopic ratio and the calcium-to-magnesium abundance ratio as a control. Finally, as a further result we also obtain the absolute abundance of the previous elements. In astronomical notation we have A_S = 7.44 ± 0.04, A_Mg = 8.03 ± 0.05, and A_Ca = 6.77 ± 0.04.

We investigate the star formation history of the central regions of four luminous compact blue galaxies (LCBGs) at intermediate redshift using evolutionary population synthesis techniques. LCBGs are blue (B - V ≤ 0.6), compact (μ_B ≤ 21.0 mag arcsec^-2) galaxies with absolute magnitudes M_B brighter than -17.5. The LCBGs analyzed here are located at 0.436 ≤ z ≤ 0.525. They are among the most luminous (M_B < -20.5), blue (B - V ≤ 0.4), and high surface brightness (μ_B ≤ 19.0 mag arcsec^-2) of this population. The observational data used were obtained with the Hubble Space Telescope (HST) STIS, the WFPC2, and the first NICMOS camera. We have disentangled the stellar generations found in the central regions of the observed targets using a very simple model. This is one of the first times this has been done for compact galaxies at this redshift using HST data, and it provides a comparison benchmark for future work on this kind of galaxy using instruments with adaptive optics in 10 m class telescopes. We find evidence for multiple stellar populations. One of them is identified as the ionizing population, and the other corresponds to the underlying stellar generation. The estimated masses of the inferred stellar populations are compatible with the dynamical masses, which are typically (2-10) × 10⁹M_⊙. Our models also indicate that the first episodes of star formation these LCBGs underwent happened between 5 and 7 Gyr ago. We compare the stellar populations found in LCBGs with the stellar populations present in bright, local H II galaxies, nearby spheroidal systems, and blue compact dwarf galaxies. It turns out that the underlying stellar populations of LCBGs are similar to yet bluer than those of local H II galaxies. It is also the case that the passive color evolution of the LCBGs could convert them into local spheroidal galaxies if no further episode of star formation takes place. Our results help to impose constraints on evolutionary scenarios for the population of LCBGs found commonly at intermediate redshifts.

We describe a search for Hα emission-line stars in M31, M33, and seven dwarfs in or near the Local Group (IC 10, NGC 6822, WLM, Sextans B, Sextans A, Pegasus, and the Phoenix dwarf) using interference filter imaging with the KPNO and CTIO 4 m telescopes and Mosaic cameras. The survey is aimed primarily at identifying new luminous blue variables (LBVs) from their spectroscopic similarity to known LBVs, avoiding the bias toward photometric variability, which may require centuries to manifest itself if LBVs go through long quiescent periods. Follow-up spectroscopy with WIYN confirms that our survey detected a wealth of stars whose spectra are similar to the known LBVs. We "classify" the spectra of known LBVs and compare these to the spectra of the new LBV candidates. We demonstrate spectacular spectral variability for several of the new LBV candidates, such as AM2, previously classified as a Wolf-Rayet star (WR), which now shows Fe I, Fe II, and Balmer emission lines but neither the N III λλ4634, 4642 nor the He II λ4686 emission it did in 1982. Profound spectral changes are also noted for other suspected and known LBVs. Several of the LBV candidates also show >0.5 mag changes in V over the past 10-20 years. The number of known or suspected LBVs is now 24 in M31, 37 in M33, 1 in NGC 6822, and 3 in IC 10. We estimate that the total number of LBVs in M31 and M33 may be several hundred, in contrast to the eight known historically through large-scale photometric variability. This has significant implications for the timescale of the LBV phase. We also identify a few new WRs and peculiar emission-line objects.