Showing papers in "The Astronomical Journal in 2014"

A search for stars of very low metal abundance. vi. detailed abundances of 313 metal-poor stars*

Abstract: We present radial velocities, equivalent widths, model atmosphere parameters, and abundances or upper limits for 53 species of 48 elements derived from high resolution optical spectroscopy of 313 metal-poor stars. A majority of these stars were selected from the metal-poor candidates of the HK Survey of Beers, Preston, and Shectman. We derive detailed abundances for 61% of these stars for the first time. Spectra were obtained during a 10 yr observing campaign using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert G. Tull Coude Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory, and the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We perform a standard LTE abundance analysis using MARCS model atmospheres, and we apply line-by-line statistical corrections to minimize systematic abundance differences arising when different sets of lines are available for analysis. We identify several abundance correlations with effective temperature. A comparison with previous abundance analyses reveals significant differences in stellar parameters, which we investigate in detail. Our metallicities are, on average, lower by ≈0.25 dex for red giants and ≈0.04 dex for subgiants. Our sample contains 19 stars with [Fe/H] ≤–3.5, 84 stars with [Fe/H]more » ≤–3.0, and 210 stars with [Fe/H] ≤–2.5. Detailed abundances are presented here or elsewhere for 91% of the 209 stars with [Fe/H] ≤–2.5 as estimated from medium resolution spectroscopy by Beers, Preston, and Shectman. We will discuss the interpretation of these abundances in subsequent papers.« less

CSI 2264: SIMULTANEOUS OPTICAL AND INFRARED LIGHT CURVES OF YOUNG DISK-BEARING STARS IN NGC 2264 WITH CoRoT and SPITZER—EVIDENCE FOR MULTIPLE ORIGINS OF VARIABILITY*

Abstract: We present the Coordinated Synoptic Investigation of NGC 2264, a continuous 30 day multi-wavelength photometric monitoring campaign on more than 1000 young cluster members using 16 telescopes. The unprecedented combination of multi-wavelength, high-precision, high-cadence, and long-duration data opens a new window into the time domain behavior of young stellar objects. Here we provide an overview of the observations, focusing on results from Spitzer and CoRoT. The highlight of this work is detailed analysis of 162 classical T Tauri stars for which we can probe optical and mid-infrared flux variations to 1% amplitudes and sub-hour timescales. We present a morphological variability census and then use metrics of periodicity, stochasticity, and symmetry to statistically separate the light curves into seven distinct classes, which we suggest represent different physical processes and geometric effects. We provide distributions of the characteristic timescales and amplitudes and assess the fractional representation within each class. The largest category (>20%) are optical "dippers" with discrete fading events lasting ~1-5 days. The degree of correlation between the optical and infrared light curves is positive but weak; notably, the independently assigned optical and infrared morphology classes tend to be different for the same object. Assessment of flux variation behavior with respect to (circum)stellar properties reveals correlations of variability parameters with Hα emission and with effective temperature. Overall, our results point to multiple origins of young star variability, including circumstellar obscuration events, hot spots on the star and/or disk, accretion bursts, and rapid structural changes in the inner disk.

From Binaries to Multiples. II. Hierarchical Multiplicity of F and G Dwarfs

Abstract: Statistics of hierarchical multiplicity among solar-type dwarfs are studied using the distance-limited sample of 4847 targets presented in the accompanying Paper I. Known facts about binaries (multiplicity fraction 0.46, log-normal period distribution with median period 100 yr and logarithmic dispersion 2.4, and nearly uniform mass-ratio distribution independent of the period) are confirmed with a high statistical significance. The fraction of hierarchies with three or more components is 0.13 ± 0.01, the fractions of targets with n = 1, 2, 3, . . . components are 54:33:8:4:1. Sub-systems in the secondary components are almost as frequent as in the primary components, but in half of such cases both inner pairs are present. The high frequency of those 2+2 hierarchies (4%) suggests that both inner pairs were formed by a common process. The statistics of hierarchies can be reproduced by simulations, assuming that the field is a mixture coming from binary-rich and binary-poor environments. Periods of the outer and inner binaries are selected recursively from the same log-normal distribution, subject to the stability constraint and accounting for the correlation between inner sub-systems. The simulator can be used to evaluate the frequency of multiple systems with specified parameters. However, it does not reproduce the observed excess of inner periods shorter than 10 d, caused by tidal evolution. Subject headings: stars: binaries; stars: solar-type; stars: statistics

unWISE: Unblurred Coadds of the WISE Imaging

Abstract: The Wide-field Infrared Survey Explorer (WISE) satellite observed the full sky in four mid-infrared bands in the 2.8-28 μm range. The primary mission was completed in 2010. The WISE team has done a superb job of producing a series of high-quality, well-documented, complete data releases in a timely manner. However, the "Atlas Image" coadds that are part of the recent AllWISE and previous data releases were intentionally blurred. Convolving the images by the point-spread function while coadding results in "matched-filtered" images that are close to optimal for detecting isolated point sources. But these matched-filtered images are sub-optimal or inappropriate for other purposes. For example, we are photometering the WISE images at the locations of sources detected in the Sloan Digital Sky Survey through forward modeling, and this blurring decreases the available signal-to-noise by effectively broadening the point-spread function. This paper presents a new set of coadds of the WISE images that have not been blurred. These images retain the intrinsic resolution of the data and are appropriate for photometry preserving the available signal-to-noise. Users should be cautioned, however, that the W3- and W4-band coadds contain artifacts around large, bright structures (large galaxies, dusty nebulae, etc.); eliminating these artifacts is the subject of ongoing work. These new coadds, and the code used to produce them, are publicly available at http://unwise.me.

Color-mass-to-light-ratio relations for disk galaxies

Abstract: We combine Spitzer 3.6µm observations of a sample of disk galaxies spanning over 10 magnitudes in luminosity with optical luminosities and colors to test population synthesis prescriptions for computing stellar mass. Many commonly employed models fail to provide self-consistent results: the stellar mass estimated from the luminosity in one band can differ grossly from that of another band for the same galaxy. Independent models agree closely in the optical (V -band), but diverge at longer wavelengths. This effect is particularly pronounced in recent models with substantial contributions from TP-AGB stars. We provide revised color–mass-to-light ratio relations that yield self-consistent stellar masses when applied to real galaxies. The B − V color is a good indicator of the mass-to-light ratio. Some additional information is provided by V − I, but neither it nor J − Ks are particularly useful for constraining the mass-to-light ratio on their own. In the near-infrared, the mass-to-light ratio depends weakly on color, with typical values of 0.6 M⊙/L⊙ in the Ks-band and 0.47 M⊙/L⊙ at 3.6µm. Subject headings: galaxies: evolution — galaxies: fundamental parameters — galaxies: photometry — galaxies: kinematics and dynamics — galaxies: stellar content

A stellar census of the tucana-horologium moving group

Abstract: We report the selection and spectroscopic confirmation of 129 new late-type (SpT = K3-M6) members of the Tucana-Horologium moving group, a nearby (d ~ 40 pc), young (τ ~ 40 Myr) population of comoving stars. We also report observations for 13 of the 17 known Tuc-Hor members in this spectral type range, and that 62 additional candidates are likely to be unassociated field stars; the confirmation frequency for new candidates is therefore 129/191 = 67%. We have used radial velocities, Hα emission, and Li6708 absorption to distinguish between contaminants and bona fide members. Our expanded census of Tuc-Hor increases the known population by a factor of ~3 in total and by a factor of ~8 for members with SpT ≥ K3, but even so, the K-M dwarf population of Tuc-Hor is still markedly incomplete. Our expanded census allows for a much more detailed study of Tuc-Hor than was previously feasible. The spatial distribution of members appears to trace a two-dimensional sheet, with a broad distribution in X and Y, but a very narrow distribution (±5 pc) in Z. The corresponding velocity distribution is very small, with a scatter of ±1.1 km s–1 about the mean UVW velocity for stars spanning the entire 50 pc extent of Tuc-Hor. We also show that the isochronal age (τ ~ 20-30 Myr) and the lithium depletion boundary age (τ ~ 40 Myr) disagree, following the trend in other pre-main-sequence populations for isochrones to yield systematically younger ages. The Hα emission line strength follows a trend of increasing equivalent width with later spectral type, as is seen for young clusters. We find that moving group members have been depleted of measurable lithium for spectral types of K7.0-M4.5. None of our targets have significant infrared excesses in the WISE W3 band, yielding an upper limit on warm debris disks of F 95% completeness for τ ~ 40 Myr populations with GALEX data available.

Stellar abundances in the solar neighborhood: the hypatia catalog

Abstract: We compile spectroscopic abundance data from 84 literature sources for 50 elements across 3058 stars in the solar neighborhood, within 150 pc of the Sun, to produce the Hypatia Catalog. We evaluate the variability of the spread in abundance measurements reported for the same star by different surveys. We also explore the likely association of the star within the Galactic disk, the corresponding observation and abundance determination methods for all catalogs in Hypatia, the influence of specific catalogs on the overall abundance trends, and the effect of normalizing all abundances to the same solar scale. The resulting stellar abundance determinations in the Hypatia Catalog are analyzed only for thin-disk stars with observations that are consistent between literature sources. As a result of our large data set, we find that the stars in the solar neighborhood may reveal an asymmetric abundance distribution, such that a [Fe/H]-rich group near the midplane is deficient in Mg, Si, S, Ca, Sc II, Cr II, and Ni as compared to stars farther from the plane. The Hypatia Catalog has a wide number of applications, including exoplanet hosts, thick- and thin-disk stars, and stars with different kinematic properties.

Optical spectra of 73 stripped-envelope core-collapse supernovae

Abstract: We present 645 optical spectra of 73 supernovae (SNe) of Types IIb, Ib, Ic, and broad-lined Ic. All of these types are attributed to the core collapse of massive stars, with varying degrees of intact H and He envelopes before explosion. The SNe in our sample have a mean redshift \textless cz \textgreater = 4200 km s(-1). Most of these spectra were gathered at the Harvard-Smithsonian Center for Astrophysics (CfA) between 2004 and 2009. For 53 SNe, these are the first published spectra. The data coverage ranges from mere identification (1-3 spectra) for a few SNe to extensive series of observations (10-30 spectra) that trace the spectral evolution for others, with an average of 9 spectra per SN. For 44 SNe of the 73 SNe presented here, we have well-determined dates of maximum light to determine the phase of each spectrum. Our sample constitutes the most extensive spectral library of stripped-envelope SNe to date. We provide very early coverage (as early as 30 days before V-band max) for photospheric spectra, as well as late-time nebular coverage when the innermost regions of the SN are visible (as late as 2 yr after explosion, while for SN 1993J, we have data as late as 11.6 yr). This data set has homogeneous observations and reductions that allow us to study the spectroscopic diversity of these classes of stripped SNe and to compare these to SNe-gamma-ray bursts. We undertake these matters in follow-up papers.

Probing for Exoplanets Hiding in Dusty Debris Disks: Disk Imaging, Characterization, and Exploration with HST/STIS Multi-roll Coronagraphy

Abstract: Spatially resolved scattered-light images of circumstellar debris in exoplanetary systems constrain the physical properties and orbits of the dust particles in these systems. They also inform on co-orbiting (but unseen) planets, the systemic architectures, and forces perturbing the starlight-scattering circumstellar material. Using Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) broadband optical coronagraphy, we have completed the observational phase of a program to study the spatial distribution of dust in a sample of 10 circumstellar debris systems and 1 'mature' protoplanetrary disk, all with HST pedigree, using point-spread-function-subtracted multi-roll coronagraphy. These observations probe stellocentric distances ≥5 AU for the nearest systems, and simultaneously resolve disk substructures well beyond corresponding to the giant planet and Kuiper Belt regions within our own solar system. They also disclose diffuse very low-surface-brightness dust at larger stellocentric distances. Herein we present new results inclusive of fainter disks such as HD 92945 (F {sub disk}/F {sub star} = 5 × 10{sup –5}), confirming, and better revealing, the existence of a narrow inner debris ring within a larger diffuse dust disk. Other disks with ring-like substructures and significant asymmetries and complex morphologies include HD 181327, for which we posit a spray of ejecta from a recent massivemore » collision in an exo-Kuiper Belt; HD 61005, suggested to be interacting with the local interstellar medium; and HD 15115 and HD 32297, also discussed in the context of putative environmental interactions. These disks and HD 15745 suggest that debris system evolution cannot be treated in isolation. For AU Mic's edge-on disk, we find out-of-plane surface brightness asymmetries at ≥5 AU that may implicate the existence of one or more planetary perturbers. Time-resolved images of the MP Mus protoplanetary disk provide spatially resolved temporal variability in the disk illumination. These and other new images from our HST/STIS GO/12228 program enable direct inter-comparison of the architectures of these exoplanetary debris systems in the context of our own solar system.« less

Overstable librations can account for the paucity of mean motion resonances among exoplanet pairs

Abstract: We assess the multi-planet systems discovered by the Kepler satellite in terms of current ideas about orbital migration and eccentricity damping due to planet-disk interactions. Our primary focus is on first order mean motion resonances, which we investigate analytically to lowest order in eccentricity. Only a few percent of planet pairs are in close proximity to a resonance. However, predicted migration rates (parameterized by ) imply that during convergent migration most planets would have been captured into first order resonances. Eccentricity damping (parameterized by ) offers a plausible resolution. Estimates suggest ? e /? n ~ (h/a)2 ~ 10?2, where h/a is the ratio of disk thickness to radius. Together, eccentricity damping and orbital migration give rise to an equilibrium eccentricity, e eq ~ (? e /? n )1/2. Capture is permanent provided e eq ?1/3, where ? denotes the planet to star mass ratio. But for e eq ?1/3, capture is only temporary because librations around equilibrium are overstable and lead to passage through resonance on timescale ? e . Most Kepler planet pairs have e eq > ?1/3. Since ? n ? e is the timescale for migration between neighboring resonances, only a modest percentage of pairs end up trapped in resonances after the disk disappears. Thus the paucity of resonances among Kepler pairs should not be taken as evidence for in situ planet formation or the disruptive effects of disk turbulence. Planet pairs close to a mean motion resonance typically exhibit period ratios 1%-2% larger than those for exact resonance. The direction of this shift undoubtedly reflects the same asymmetry that requires convergent migration for resonance capture. Permanent resonance capture at these separations from exact resonance would demand ?(? n /? e )1/2 0.01, a value that estimates of ? from transit data and (? e /? n )1/2 from theory are insufficient to match. Plausible alternatives involve eccentricity damping during or after disk dispersal. The overstability referred to above has applications beyond those considered in this investigation. It was discovered numerically by Meyer & Wisdom in their study of the tidal evolution of Saturn's satellites.

Transmission Spectroscopy of the Hot Jupiter WASP-12b from 0.7 to 5 μm

Abstract: Since the first report of a potentially non-solar carbon-to-oxygen ratio (C/O) in its dayside atmosphere, the highly irradiated exoplanet WASP-12b has been under intense scrutiny and the subject of many follow-up observations. Additionally, the recent discovery of stellar binary companions ~1" from WASP-12 has obfuscated interpretation of the observational data. Here we present new ground-based multi-object transmission-spectroscopy observations of WASP-12b that we acquired over two consecutive nights in the red optical with Gemini-N/GMOS. After correcting for the influence of WASP-12's stellar companions, we find that these data rule out a cloud-free H_2 atmosphere with no additional opacity sources. We detect features in the transmission spectrum that may be attributed to metal oxides (such as TiO and VO) for an O-rich atmosphere or to metal hydrides (such as TiH) for a C-rich atmosphere. We also reanalyzed NIR transit-spectroscopy observations of WASP-12b from HST/WFC3 and broadband transit photometry from Warm Spitzer. We attribute the broad spectral features in the WFC3 data to either H_2O or CH_4 and HCN for an O-rich or C-rich atmosphere, respectively. The Spitzer data suggest shallower transit depths than the models predict at infrared wavelengths, albeit at low statistical significance. A multi-instrument, broad-wavelength analysis of WASP-12b suggests that the transmission spectrum is well approximated by a simple Rayleigh scattering model with a planet terminator temperature of 1870 ± 130 K. We conclude that additional high-precision data and isolated spectroscopic measurements of the companion stars are required to place definitive constraints on the composition of WASP-12b's atmosphere.

Clustering of local group distances: publication bias or correlated measurements? i. the large magellanic cloud

Abstract: The distance to the Large Magellanic Cloud (LMC) represents a key local rung of the extragalactic distance ladder yet the galaxy's distance modulus has long been an issue of contention, in particular in view of claims that most newly determined distance moduli cluster tightly—and with a small spread—around the "canonical" distance modulus, (m – M)0 = 18.50 mag. We compiled 233 separate LMC distance determinations published between 1990 and 2013. Our analysis of the individual distance moduli, as well as of their two-year means and standard deviations resulting from this largest data set of LMC distance moduli available to date, focuses specifically on Cepheid and RR Lyrae variable-star tracer populations, as well as on distance estimates based on features in the observational Hertzsprung-Russell diagram. We conclude that strong publication bias is unlikely to have been the main driver of the majority of published LMC distance moduli. However, for a given distance tracer, the body of publications leading to the tightly clustered distances is based on highly non-independent tracer samples and analysis methods, hence leading to significant correlations among the LMC distances reported in subsequent articles. Based on a careful, weighted combination, in a statistical sense, of the main stellar population tracers, we recommend that a slightly adjusted canonical distance modulus of (m – M)0 = 18.49 ± 0.09 mag be used for all practical purposes that require a general distance scale without the need for accuracies of better than a few percent.

The abundance properties of nearby late-type galaxies. i. the data

Abstract: We investigate the oxygen and nitrogen abundance distributions across the optical disks of 130 nearby late-type galaxies using around 3740 published spectra of H II regions. We use these data in order to provide homogeneous abundance determinations for all objects in the sample, including H II regions in which not all of the usual diagnostic lines were measured. Examining the relation between N and O abundances in these galaxies we find that the abundances in their centers and at their isophotal R {sub 25} disk radii follow the same relation. The variation in N/H at a given O/H is around 0.3 dex. We suggest that the observed spread in N/H may be partly caused by the time delay between N and O enrichment and the different star formation histories in galaxies of different morphological types and dimensions. We study the correlations between the abundance properties (central O and N abundances, radial O and N gradients) of a galaxy and its morphological type and dimension.

FROM BINARIES TO MULTIPLES. I. DATA ON F AND G DWARFS WITHIN 67 pc OF THE SUN

Abstract: Data on the multiplicity of F- and G-type dwarf stars within 67 pc of the Sun are presented. This distance-limited sample based on the Hipparcos catalog contains 4847 primary stars (targets) with 0.5 90% complete. There are 2196 known stellar pairs, some of them belong to 361 hierarchical systems from triples to quintuples. Models of companion detection by radial velocity, astrometric acceleration, direct resolution, and common proper motion are developed. They serve to compute completeness for each target, using the information on its coverage collected here. About 80% of companions to the primary stars are detected, but the census of sub-systems in the secondary components is only about 30%. Masses of binary components are estimated from their absolute magnitudes or by other methods, the periods of wide pairs are evaluated from their projected separations. A third of binaries with periods shorter than ∼100 yr are spectroscopic and/or astrometric pairs with yet unknown periods and mass ratios. These data are used in the accompanying Paper II to derive unbiased statistics of hierarchical multiple systems. Subject headings: stars: binaries; stars: solar-type; stars: statistics 1. GOALS AND STRATEGY

Chemical cartography with apogee: large-scale mean metallicity maps of the milky way disk

Abstract: We present Galactic mean metallicity maps derived from the first year of the SDSS-III APOGEE experiment. Mean abundances in different zones of projected Galactocentric radius (0 6 kpc, the gradient flattens as one moves off the plane, and is flatter at all heights for high-[α/M] stars than for low-[α/M] stars. Alternatively, these gradients can be described as vertical gradients that flatten at larger Galactocentric radius; these vertical gradients are similar for both low- and high-[α/M] populations. Stars with higher [α/M] appear to havemore » a flatter radial gradient than stars with lower [α/M]. This could suggest that the metallicity gradient has grown steeper with time or, alternatively, that gradients are washed out over time by migration of stars.« less

DISCOVERY OF EIGHT z ∼ 6 QUASARS FROM Pan-STARRS1

Abstract: High-redshift quasars are currently the only probes of the growth of supermassive black holes and potential tracers of structure evolution at early cosmic time. Here we present our candidate selection criteria from the Panoramic Survey Telescope & Rapid Response System 1 and follow-up strategy to discover quasars in the redshift range 5.7 lsim z lsim 6.2. With this strategy we discovered eight new 5.7 ≤ z ≤ 6.0 quasars, increasing the number of known quasars at z > 5.7 by more than 10%. We additionally recovered 18 previously known quasars. The eight quasars presented here span a large range of luminosities (–27.3 ≤ M 1450 ≤ –25.4; 19.6 ≤ z P1 ≤ 21.2) and are remarkably heterogeneous in their spectral features: half of them show bright emission lines whereas the other half show a weak or no Lyα emission line (25% with rest-frame equivalent width of the Lyα +N V line lower than 15 A). We find a larger fraction of weak-line emission quasars than in lower redshift studies. This may imply that the weak-line quasar population at the highest redshifts could be more abundant than previously thought. However, larger samples of quasars are needed to increase the statistical significance of this finding.

How the geysers, tidal stresses, and thermal emission across the south polar terrain of enceladus are related

Abstract: We present the first comprehensive examination of the geysering, tidal stresses, and anomalous thermal emission across the south pole of Enceladus and discuss the implications for the moon's thermal history and interior structure. A 6.5 yr survey of the moon's south polar terrain (SPT) by the Cassini imaging experiment has located ~100 jets or geysers erupting from four prominent fractures crossing the region. Comparing these results with predictions of diurnally varying tidal stresses and with Cassini low resolution thermal maps shows that all three phenomena are spatially correlated. The coincidence of individual jets with very small (~10 m) hot spots detected in high resolution Cassini VIMS data strongly suggests that the heat accompanying the geysers is not produced by shearing in the upper brittle layer but rather is transported, in the form of latent heat, from a sub-ice-shell sea of liquid water, with vapor condensing on the near-surface walls of the fractures. Normal stresses modulate the geysering activity, as shown in the accompanying paper; we demonstrate here they are capable of opening water-filled cracks all the way down to the sea. If Enceladus' eccentricity and heat production are in steady state today, the currently erupting material and anomalous heat must have been produced in an earlier epoch. If regional tidal heating is occurring today, it may be responsible for some of the erupting water and heat. Future Cassini observations may settle the question.

Light, alpha, and Fe-peak element abundances in the galactic bulge

Abstract: We present radial velocities and chemical abundances of O, Na, Mg, Al, Si, Ca, Cr, Fe, Co, Ni, and Cu for a sample of 156 red giant branch stars in two Galactic bulge fields centered near (l, b) = (+5.25,–3.02) and (0,–12). The (+5.25,–3.02) field also includes observations of the bulge globular cluster NGC 6553. The results are based on high-resolution (R ~ 20,000), high signal-to-noise ration (S/N gsim 70) FLAMES-GIRAFFE spectra obtained through the European Southern Observatory archive. However, we only selected a subset of the original observations that included spectra with both high S/N and that did not show strong TiO absorption bands. This work extends previous analyses of this data set beyond Fe and the α-elements Mg, Si, Ca, and Ti. While we find reasonable agreement with past work, the data presented here indicate that the bulge may exhibit a different chemical composition than the local thick disk, especially at [Fe/H] gsim –0.5. In particular, the bulge [α/Fe] ratios may remain enhanced to a slightly higher [Fe/H] than the thick disk, and the Fe-peak elements Co, Ni, and Cu appear enhanced compared to the disk. There is also some evidence that the [Na/Fe] (but not [Al/Fe]) trends between the bulge and local disk may be different at low and high metallicity. We also find that the velocity dispersion decreases as a function of increasing [Fe/H] for both fields, and do not detect any significant cold, high-velocity populations. A comparison with chemical enrichment models indicates that a significant fraction of hypernovae may be required to explain the bulge abundance trends, and that initial mass functions that are steep, top-heavy (and do not include strong outflow), or truncated to avoid including contributions from stars >40 M ☉ are ruled out, in particular because of disagreement with the Fe-peak abundance data. For most elements, the NGC 6553 stars exhibit abundance trends nearly identical to comparable metallicity bulge field stars. However, the star-to-star scatter and mean [Na/Fe] ratios appear higher in the cluster, perhaps indicating additional self-enrichment.

TYPE Ia SUPERNOVA RATE MEASUREMENTS TO REDSHIFT 2.5 FROM CANDELS: SEARCHING FOR PROMPT EXPLOSIONS IN THE EARLY UNIVERSE

Abstract: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) was a multi-cycle treasury program on the Hubble Space Telescope(HST) that surveyed a total area of approx 025 deg(sup 2) with approx900 HST orbits spread across five fields over three years Within these survey images we discovered 65 supernovae (SNe) of all types, out to z approx 25 We classify approx 24 of these as Type Ia SNe (SNe Ia) based on host galaxy redshifts and SN photometry (supplemented by grism spectroscopy of six SNe) Here we present a measurement of the volumetric SN Ia rate as a function of redshift, reaching for the first time beyond z = 2 and putting new constraints on SN Ia progenitor models Our highest redshift bin includes detections of SNe that exploded when the universe was only approx 3 Gyr old and near the peak of the cosmic star formation history This gives the CANDELS high redshift sample unique leverage for evaluating the fraction of SNe Ia that explode promptly after formation (500 Myr) Combining the CANDELS rates with all available SN Ia rate measurements in the literature we find that this prompt SN Ia fraction isfP053009stat010010sys026, consistent with a delay time distribution that follows a simplet1power law for all timest40 Myr However, mild tension is apparent between ground-based low-z surveys and space-based high-z surveys In both CANDELS and the sister HST program CLASH (Cluster Lensing And Supernova Survey with Hubble), we find a low rate of SNe Ia at z > 1 This could be a hint that prompt progenitors are in fact relatively rare, accounting for only 20 of all SN Ia explosions though further analysis and larger samples will be needed to examine that suggestion

The solar neighborhood. XXXII. The hydrogen burning limit

Abstract: We construct a Hertzsprung-Russell diagram for the stellar/substellar boundary based on a sample of 63 objects ranging in spectral type from M6V to L4. We report newly observed VRI photometry for all 63 objects and new trigonometric parallaxes for 37 objects. The remaining 26 objects have trigonometric parallaxes from the literature. We combine our optical photometry and trigonometric parallaxes with 2MASS and WISE photometry and employ a novel spectral energy distribution fitting algorithm to determine effective temperatures, bolometric luminosities, and radii. Our uncertainties range from ~20?K to ~150?K in temperature, ~0.01 to ~0.06 in log (L/L ?) and ~3% to ~10% in radius. We check our methodology by comparing our calculated radii to radii directly measured via long baseline optical interferometry. We find evidence for the local minimum in the radius-temperature and radius-luminosity trends that signals the end of the stellar main sequence and the start of the brown dwarf sequence at T eff ~ 2075 K, log (L/L ?) ~ ?3.9, and (R/R ?) ~ 0.086. The existence of this local minimum is predicted by evolutionary models, but at temperatures ~400?K cooler. The minimum radius happens near the locus of 2MASS?J0523?1403, an L2.5 dwarf with V ? K = 9.42. We make qualitative arguments as to why the effects of the recent revision in solar abundances accounts for the discrepancy between our findings and the evolutionary models. We also report new color-absolute magnitude relations for optical and infrared colors which are useful for estimating photometric distances. We study the optical variability of all 63 targets and find an overall variability fraction of 36% at a threshold of 15?mmag in the I band, which is in agreement with previous studies.

Revealing a Universal Planet-Metallicity Correlation for Planets of Different Sizes Around Solar-type Stars

Abstract: The metallicity of exoplanet systems serves as a critical diagnostic of planet formation mechanisms. Previous studies have demonstrated the planet–metallicity correlation for large planets (); however, a correlation has not been found for smaller planets. With a sample of 406 Kepler objects of interest whose stellar properties are determined spectroscopically, we reveal a universal planet–metallicity correlation: not only gas-giant planets () but also gas-dwarf () and terrestrial planets () occur more frequently in metal-rich stars. The planet occurrence rates of gas-giant planets, gas-dwarf planets, and terrestrial planets are , , and times higher for metal-rich stars than for metal-poor stars, respectively.

Csi 2264: characterizing accretion-burst dominated light curves for young stars in ngc 2264*

Abstract: Based on more than four weeks of continuous high cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA’s Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high quality, multi-wavelength light curves for young stellar objects (YSOs) whose optical variability is dominated by short duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief – several hour to one day – brightenings at optical and near-infrared (IR) wavelengths with amplitudes generally in the range 5-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a thirty day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u − g vs. g − r color-color diagram with the largest UV excesses. These stars also have large Hɑ equivalent widths, and either centrally peaked, lumpy Hɑ emission profiles or profiles with blue-shifted absorption dips associated with disk or stellar winds. Light curves of this type have been predicted for stars whose accretion is dominated by Rayleigh-Taylor instabilities at the boundary between their magnetosphere and inner circumstellar disk, or where magneto-rotational instabilities modulate the accretion rate from the inner disk. Amongst the stars with the largest UV excesses or largest Hɑ equivalent widths, light curves with this type of variability greatly outnumber light curves with relatively smooth sinusoidal variations associated with long-lived hot spots. We provide quantitative statistics for the average duration and strength of the accretion bursts and for the fraction of the accretion luminosity associated with these bursts.

THE SOLAR NEIGHBORHOOD. XXXIII. PARALLAX RESULTS FROM THE CTIOPI 0.9 m PROGRAM: TRIGONOMETRIC PARALLAXES OF NEARBY LOW-MASS ACTIVE AND YOUNG SYSTEMS

Abstract: We present basic observational data and association membership analysis for 45 young and active low-mass stellar systems from the ongoing Research Consortium On Nearby Stars photometry and astrometry program at the Cerro Tololo Inter-American Observatory. Most of these systems have saturated X-ray emission (log (LX /L bol) > ?3.5) based on X-ray fluxes from the ROSAT All-Sky Survey, and many are significantly more luminous than main-sequence stars of comparable color. We present parallaxes and proper motions, Johnson-Kron-Cousins VRI photometry, and multiplicity observations from the CTIOPI program on the CTIO 0.9?m telescope. To this we add low-resolution optical spectroscopy and line measurements from the CTIO 1.5?m telescope, and interferometric binary measurements from the Hubble Space Telescope Fine Guidance Sensors. We also incorporate data from published sources: JHKS photometry from the Two Micron All Sky Survey point source catalog, X-ray data from the ROSAT All-Sky Survey, and radial velocities from literature sources. Within the sample of 45 systems, we identify 21 candidate low-mass pre-main-sequence members of nearby associations, including members of ? Pictoris, TW Hydrae, Argus, AB Doradus, two ambiguous 30?Myr old systems, and one object that may be a member of the Ursa Major moving group. Of the 21 candidate young systems, 14 are newly identified as a result of this work, and six of those are within 25?pc of the Sun.

MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. XI. Spectral Distributions

Abstract: We have obtained milliarcsecond-scale spectral index distributions for a sample of 190 extragalactic radio jets through the Monitoring of Jets in Active Galactic Nuclei with the VLBA Experiments (MOJAVE) project. The sources were observed in 2006 at 8.1, 8.4, 12.1, and 15.4 GHz, and we have determined spectral index maps between 8.1 and 15.4 GHz to study the four-frequency spectrum in individual jet features. We have performed detailed simulations to study the effects of image alignment and (u, v)-plane coverage on the spectral index maps to verify our results. We use the spectral index maps to study the spectral index evolution along the jet and determine the spectral distributions in different locations of the jets. The core spectral indices are on average flat with a mean value of +0.22 ± 0.03 for the sample, while the jet spectrum is in general steep with a mean index of –1.04 ± 0.03. A simple power-law fit is often inadequate for the core regions, as expected if the cores are partially self-absorbed. The overall jet spectrum steepens at a rate of about –0.001 to –0.004 per deprojected parsec when moving further out from the core with flat spectrum radio quasars having significantly steeper spectra (mean –1.09 ± 0.04) than the BL Lac objects (mean –0.80 ± 0.05). However, the spectrum in both types of objects flattens on average by ~0.2 at the locations of the jet components indicating particle acceleration or density enhancements along the jet. The mean spectral index at the component locations of –0.81 ± 0.02 corresponds to a power-law index of ~2.6 for the electron energy distribution. We find a significant trend that jet components with linear polarization parallel to the jet (magnetic field perpendicular to the jet) have flatter spectra, as expected for transverse shocks. Compared to quasars, BL Lacs have more jet components with perpendicular magnetic field alignment, which may explain their generally flatter spectra. The overall steepening of the spectra with distance can be explained with radiative losses if the jets are collimating or with the evolution of the high-energy cutoff in the electron spectrum if the jets are conical. This interpretation is supported by a significant correlation with the age of the component and the spectral index, with older components having steeper spectra.

Kepler eclipsing binary stars. IV. Precise eclipse times for close binaries and identification of candidate three-body systems

Abstract: We present a catalog of precise eclipse times and analysis of third-body signals among 1279 close binaries in the latest Kepler Eclipsing Binary Catalog. For these short-period binaries, Kepler's 30 minute exposure time causes significant smearing of light curves. In addition, common astrophysical phenomena such as chromospheric activity, as well as imperfections in the light curve detrending process, can create systematic artifacts that may produce fictitious signals in the eclipse timings. We present a method to measure precise eclipse times in the presence of distorted light curves, such as in contact and near-contact binaries which exhibit continuously changing light levels in and out of eclipse. We identify 236 systems for which we find a timing variation signal compatible with the presence of a third body. These are modeled for the light travel time effect and the basic properties of the third body are derived. This study complements J. A. Orosz et al. (in preparation), which focuses on eclipse timing variations of longer period binaries with flat out-of-eclipse regions. Together, these two papers provide comprehensive eclipse timings for all binaries in the Kepler Eclipsing Binary Catalog, as an ongoing resource freely accessible online to the community.

Black holes at the centers of nearby dwarf galaxies

Abstract: Using a distance-limited portion of the Sloan Digital Sky Survey (SDSS) Data Release 7, we have identified 28 active galactic nuclei (AGNs) in nearby ( Mpc) low-mass, low-luminosity dwarf galaxies. The accreting objects at the galaxy centers are expected to be intermediate-mass black holes (IMBHs) with . The AGNs were selected using several optical emission-line diagnostics after careful modeling of the continuum present in the spectra. We have limited our survey to objects with spectral characteristics similar to those of Seyfert nuclei, excluding emission-line galaxies with ambiguous spectra that could be powered by stellar processes. Thus, as a set, the host galaxies in our sample are the least massive objects in the very local universe certain to contain central black holes. Our sample is dominated by narrow-line (type 2) AGNs, and it appears to have a much lower fraction of broad-line objects than that observed for luminous, optically selected Seyfert galaxies. Given our focus on the nearest objects included in the SDSS, our survey is more sensitive to low-luminosity emission than previous optical searches for AGNs in low-mass galaxies. The [O iii] luminosities of the Seyfert nuclei in our sample have a median value of and extend down to . Using published data for broad-line IMBH candidates, we have derived an [O iii] bolometric correction of , which is significantly lower than values obtained for high-luminosity AGNs. Applying this correction to our sample, we obtain minimum black hole mass estimates that fall mainly in the 103 ?104 range, which is roughly where the predicted mass functions for different black hole seed formation scenarios overlap the most. In the stellar mass range that includes the bulk of the AGN host galaxies in our sample, we derive a lower limit on the AGN fraction of a few percent, indicating that active nuclei in dwarf galaxies are not as rare as previously thought.

Hazmat. i. the evolution of far-uv and near-uv emission from early m stars*

Abstract: The spectral energy distribution, variability, and evolution of the high-energy radiation from an M dwarf planet host is crucial in understanding the planet's atmospheric evolution and habitability and in interpreting the planet's spectrum. The star's extreme-UV (EUV), far-UV (FUV), and near-UV (NUV) emission can chemically modify, ionize, and erode the atmosphere over time. This makes determining the lifetime exposure of such planets to stellar UV radiation critical for both the evolution of a planet's atmosphere and our potential to characterize it. Using the early M star members of nearby young moving groups, which sample critical ages in planet formation and evolution, we measure the evolution of the GALEX NUV and FUV flux as a function of age. The median UV flux remains at a 'saturated' level for a few hundred million years, analogous to that observed for X-ray emission. By the age of the Hyades Cluster (650 Myr), we measure a drop in UV flux by a factor of 2-3 followed by a steep drop from old (several Gyrs) field stars. This decline in activity beyond 300 Myr follows roughly t {sup –1}. Despite this clear evolution, there remains a wide range, of 1-2 orders of magnitude, in observed emission more » levels at every age. These UV data supply the much-needed constraints to M dwarf upper-atmosphere models, which will provide empirically motivated EUV predictions and more accurate age-dependent UV spectra as inputs to planetary photochemical models. « less

APASS Landolt-Sloan BVgri Photometry of RAVE Stars. I. Data, Effective Temperatures, and Reddenings

Abstract: We provide AAVSO Photometric All-Sky Survey (APASS) photometry in the Landolt BV and Sloan g'r'i' bands for all 425,743 stars included in the fourth RAVE Data Release. The internal accuracy of the APASS photometry of RAVE stars, expressed as the error of the mean of data obtained and separately calibrated over a median of four distinct observing epochs and distributed between 2009 and 2013, is 0.013, 0.012, 0.012, 0.014, and 0.021 mag for the B, V, g', r', and i' bands, respectively. The equally high external accuracy of APASS photometry has been verified on secondary Landolt and Sloan photometric standard stars not involved in the APASS calibration process and on a large body of literature data on field and cluster stars, confirming the absence of offsets and trends. Compared with the Carlsberg Meridian Catalog (CMC-15), APASS astrometry of RAVE stars is accurate to a median value of 0.098 arcsec. Brightness distribution functions for the RAVE stars have been derived in all bands. APASS photometry of RAVE stars, augmented by 2MASS JHK infrared data, has been χ{sup 2} fitted to a densely populated synthetic photometric library designed to widely explore temperature, surface gravity, metallicity, and reddening. Resulting T {sub eff} andmore » E {sub B–V}, computed over a range of options, are provided and discussed, and will be kept updated in response to future APASS and RAVE data releases. In the process, we find that the reddening caused by a homogeneous slab of dust, extending for 140 pc on either side of the Galactic plane and responsible for E{sub B−V}{sup poles} = 0.036 ± 0.002 at the Galactic poles, is a suitable approximation of the actual reddening encountered at Galactic latitudes |b| ≥ 25°.« less

Stellar diameters and temperatures. iv. predicting stellar angular diameters

Abstract: The number of stellar angular diameter measurements has greatly increased over the past few years due to innovations and developments in the field of long baseline optical interferometry. We use a collection of high-precision angular diameter measurements for nearby, main-sequence stars to develop empirical relations that allow the prediction of stellar angular sizes as a function of observed photometric color. These relations are presented for a combination of 48 broadband color indices. We empirically show for the first time a dependence on metallicity of these relations using Johnson (B – V) and Sloan (g – r) colors. Our relations are capable of predicting diameters with a random error of less than 5% and represent the most robust and empirical determinations of stellar angular sizes to date.

Absolute-Magnitude Distributions of Supernovae

Abstract: The absolute-magnitude distributions of seven supernova (SN) types are presented. The data used here were primarily taken from the Asiago Supernova Catalogue, but were supplemented with additional data. We accounted for both foreground and host-galaxy extinction. A bootstrap method is used to correct the samples for Malmquist bias. Separately, we generate volume-limited samples, restricted to events within 100 Mpc. We find that the superluminous events (M{sub B} –15) make up about 3%. The normal Ia distribution was the brightest with a mean absolute blue magnitude of –19.25. The IIP distribution was the dimmest at –16.75.