Browsing by Subject "galactic disk"

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The APOGEE Red-Clump Catalog: Precise Distances, Velocities, And High-Resolution Elemental Abundances Over A Large Area Of The Milky Way's Disk
(2014-08) Bovy, Jo; Nidever, David L.; Rix, Hans-Walter; Girardi, Leo; Zasowski, Gail; Chojnowski, S. Drew; Holtzman, Jon; Epstein, Courtney; Frinchaboy, Peter M.; Hayden, Michael R.; Rodrigues, Thaise S.; Majewski, Steven R.; Johnson, Jennifer A.; Pinsonneault, Marc H.; Stello, Dennis; Prieto, Carlos Allende; Andrews, Brett; Basu, Sarbani; Beers, Timothy C.; Bizyaev, Dmitry; Burton, Adam; Chaplin, William J.; Cunha, Katia; Elsworth, Yvonne; Garcia, Rafael A.; Garcia-Hernandez, Domingo A.; Garcia Perez, Ana E.; Hearty, Fred R.; Hekker, Saskia; Kallinger, Thomas; Kinemuchi, Karen; Koesterke, Lars; Meszaros, Szabolcs; Mosser, Benoit; O'Connell, Robert W.; Oravetz, Daniel; Pan, Kaike; Robin, Annie C.; Schiavon, Ricardo P.; Schneider, Donald P.; Schultheis, Mathias; Serenelli, Aldo; Shetrone, Matthew; Aguirre, Victor Silva; Simmons, Audrey; Skrutskie, Michael; Smith, Verne V.; Stassun, Keivan; Weinberg, David H.; Wilson, John C.; Zamora, Olga; Shetrone, Matthew
The Sloan Digital Sky Survey III's Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a high-resolution near-infrared spectroscopic survey covering all of the major components of the Galaxy, including the dust-obscured regions of the inner Milky Way disk and bulge. Here we present a sample of 10,341 likely red-clump stars (RC) from the first two years of APOGEE operations, selected based on their position in color-metallicity-surface-gravity-effective-temperature space using a new method calibrated using stellar evolution models and high-quality asteroseismology data. The narrowness of the RC locus in color-metallicity-luminosity space allows us to assign distances to the stars with an accuracy of 5%-10%. The sample extends to typical distances of about 3 kpc from the Sun, with some stars out to 8 kpc, and spans a volume of approximately 100 kpc(3) over 5 kpc less than or similar to R less than or similar to 14 kpc, vertical bar Z vertical bar less than or similar to 2 kpc, and -15 degrees less than or similar to Galactocentric azimuth less than or similar to 30 degrees. The APOGEE red-clump (APOGEE-RC) catalog contains photometry from the Two Micron All Sky Survey, reddening estimates, distances, line-of-sight velocities, stellar parameters and elemental abundances determined from the high-resolution APOGEE spectra, and matches to major proper motion catalogs. We determine the survey selection function for this data set and discuss how the RC selection samples the underlying stellar populations. We use this sample to limit any azimuthal variations in the median metallicity within the approximate to 45 degrees azimuthal region covered by the current sample to be <= 0.02 dex, which is more than an order of magnitude smaller than the radial metallicity gradient. This result constrains coherent non-axisymmetric flows within a few kiloparsecs from the Sun.
Chemical Cartography With Apogee: Large-Scale Mean Metallicity Maps Of The Milky Way Disk
(2014-05) Michael R. Hayden; Jon A. Holtzman; Jo Bovy; Steven R. Majewski; Jennifer A. Johnson; Carlos Allende Prieto; Timothy C. Beers; Katia Cunha; Peter M. Frinchaboy; Ana E. García Pérez; Léo Girardi; Fred R. Hearty; Young Sun Lee; David Nidever; Ricardo P. Schiavon; Katharine J. Schlesinger; Donald P. Schneider; Mathias Schultheis; Matthew Shetrone; Verne V. Smith; Gail Zasowski; Dmitry Bizyaev; Diane Feuillet; Sten Hasselquist; Karen Kinemuchi; Elena Malanushenko; Viktor Malanushenko; Robert O'Connell; Kaike Pan; Keivan Stassun; Shetrone, Matthew D.
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 < R < 15 kpc) at a range of heights above the plane (0 < vertical bar z vertical bar < 3 kpc), are derived from a sample of nearly 20,000 giant stars with unprecedented coverage, including stars in the Galactic mid-plane at large distances. We also split the sample into subsamples of stars with low- and high-[alpha/M] abundance ratios. We assess possible biases in deriving the mean abundances, and find that they are likely to be small except in the inner regions of the Galaxy. A negative radial metallicity gradient exists over much of the Galaxy; however, the gradient appears to flatten for R < 6 kpc, in particular near the Galactic mid-plane and for low-[alpha/M] stars. At R > 6 kpc, the gradient flattens as one moves off the plane, and is flatter at all heights for high-[alpha/M] stars than for low-[alpha/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-[alpha/M] populations. Stars with higher [alpha/M] appear to have a flatter radial gradient than stars with lower [alpha/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.
A Detailed Model Atmosphere Analysis of Cool White Dwarfs in the Sloan DIGITal Sky Survey
(2010-09) Kilic, Mukremin; Leggett, S. K.; Tremblay, P. E.; von Hippel, Ted; Bergeron, P.; Harris, Hugh C.; Munn, Jeffrey A.; Williams, Kurtis A.; Gates, Evalyn; Farihi, J.; Williams, Kurtis A.
We present optical spectroscopy and near-infrared photometry of 126 cool white dwarfs (WDs) in the Sloan Digital Sky Survey (SDSS). Our sample includes high proper motion targets selected using the SDSS and USNOB astrometry and a dozen previously known ultracool WD candidates. Our optical spectroscopic observations demonstrate that a clean selection of large samples of cool WDs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic Survey Telescope data sets) is possible using a reduced proper motion diagram and a tangential velocity cut-off (depending on the proper motion accuracy) of 30 km s(-1). Our near-infrared observations reveal eight new stars with significant absorption. We use the optical and near-infrared photometry to perform a detailed model atmosphere analysis. More than 80% of the stars in our sample are consistent with either pure hydrogen or pure helium atmospheres. However, the eight stars with significant infrared absorption and the majority of the previously known ultracool WD candidates are best explained with mixed hydrogen and helium atmosphere models. The age distribution of our sample is consistent with a Galactic disk age of 8 Gyr. A few ultracool WDs may be as old as 12-13 Gyr, but our models have problems matching the spectral energy distributions of these objects. There are only two halo WD candidates in our sample. However, trigonometric parallax observations are required for accurate mass and age determinations and to confirm their membership in the halo.
The Initial-Final Mass Relationship From White Dwarfs In Common Proper Motion Pairs
(2008-01) Catalan, S.; Iserns, J.; Garcia-Berros, E.; Ribas, I.; Prieto, C. A.; Bonanoss, A. Z.; Prieto, C. A.
Context. The initial-final mass relationship of white dwarfs, which is poorly constrained, is of paramount importance for different aspects of modern astrophysics. From an observational perspective, most of the studies up to now have been done using white dwarfs in open clusters. Aims. In order to improve the initial-final mass relationship, we explore the possibility of deriving a semi-empirical relation studying white dwarf's in common proper motion pairs. If these systems are comprised of a white dwarf and a FGK star, the total age and the metallicity of the progenitor of the white dwarf can be inferred from the detailed analysis of the companion. Methods. We have performed an exhaustive search for common proper motion pairs containing a DA white dwarf and a FGK star using the available literature and crossing the SIMBAD database with the Villanova White Dwarf Catalog. We have acquired long-slit spectra of the white dwarf members of the selected common proper motion pairs, as well as high resolution spectra of their companions. From these observations, a full analysis of the two members of each common proper motion pair leads to the initial and final masses of the white dwarfs. Results. These observations have allowed us to provide updated information for the white dwarfs, since some of them were misclassified. In the case of the DA white dwarfs, their atmospheric parameters, masses, and cooling times have been derived using appropriate white dwarf models and cooling sequences. From a detailed analysis of the FGK star spectra we have inferred the metallicity. Then, using either isochrones or X-ray luminosities we have obtained the main-sequence lifetime of the progenitors, and subsequently their initial masses. Conclusions. This work is the first one using common proper motion pairs to improve the initial-final mass relationship, and has also allowed us to cover the poorly explored low-mass domain. As in the case of studies based on white dwarfs in open clusters, the distribution of the semi-empirical data presents a large scatter, which is higher than the expected uncertainties in the derived values. This suggests that the initial-final mass relationship may not be a single-valued function.
Rubidium In The Interstellar Medium
(2009-11) Walker, Kyle M.; Federman, S. R.; Knauth, David C.; Lambert, David L.; Lambert, David L.
We present observations of interstellar rubidium toward o Per, zeta Per, AE Aur, HD 147889, chi Oph, zeta Oph, and 20 Aql. Theory suggests that stable (85)Rb and long-lived (87)Rb are produced predominantly by high-mass stars, through a combination of the weak s- and r-processes. The (85)Rb/(87)Rb ratio was determined from measurements of the Rb I line at 7800 angstrom and was compared to the solar system meteoritic ratio of 2.59. Within 1 sigma uncertainties, all directions except HD 147889 have Rb isotope ratios consistent with the solar system value. The ratio toward HD 147889 is much lower than themeteoritic value and similar to that toward rho Oph A; both lines of sight probe the Rho Ophiuchus Molecular Cloud. The earlier result was attributed to a deficit of r-processed (85)Rb. Our larger sample suggests instead that (87)Rb is enhanced in these two lines of sight. When the total elemental abundance of Rb is compared to the K elemental abundance, the interstellar Rb/K ratio is significantly lower than the meteoritic ratio for all the sight lines in this study. Available interstellar samples for other s- and r-process elements are used to help interpret these results.
The SEGUE Stellar Parameter Pipeline. III. Comparison With High-Resolution Spectroscopy Of SDSS/SEGUE Field Stars
(2008-11) Prieto, Carlos Allende; Sivarani, Thirupathi; Beers, Timoth C.; Lee, Young Sun; Koesterke, Lars; Shetrone, Matthew; Sneden, Christopher; Lambert, David L.; Wilhelm, Ronald; Rockosi, Constance M.; Lai, David K.; Yanny, Brian; Ivans, Inese I.; Johnson, Jennifer A.; Aoki, Wako; Bailer-Jones, Coryn A. L.; Fiorentin, Paola Re; Koesterke, Lars; Shetrone, Matthew D.; Sneden, Christopher; Lambert, David L.
We report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which we compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R similar to 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/Ns), we empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km s(-1), 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. We estimate random errors for lower S/N based on numerical simulations.
The Spectral Evolution of Convective Mixing White Dwarfs, the Non-DA Gap, and White Dwarf Cosmochronology
(2012-07) Chen, E. Y.; Hansen, B. M. S.; Chen, E. Y.
The spectral distribution of field white dwarfs shows a feature called the "non-DA gap." As defined by Bergeron et al., this is a temperature range (5100-6100 K) where relatively few non-DA stars are found, even though such stars are abundant on either side of the gap. It is usually viewed as an indication that a significant fraction of white dwarfs switch their atmospheric compositions back and forth between hydrogen-rich and helium-rich as they cool. In this Letter, we present a Monte Carlo model of the Galactic disk white dwarf population, based on the spectral evolution model of Chen and Hansen. We find that the non-DA gap emerges naturally, even though our model only allows white dwarf atmospheres to evolve monotonically from hydrogen-rich to helium-rich through convective mixing. We conclude by discussing the effects of convective mixing on the white dwarf luminosity function and the use thereof for Cosmochronology.