Browsing by Subject "hipparcos parallaxes"
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Item The Chemical Compositions of Variable Field Horizontal-Branch Stars: RR Lyrae Stars(2011-12) For, Bi-Qing; Sneden, Christopher; Preston, George W.; For, Bi-Qing; Sneden, ChristopherWe present a detailed abundance study of 11 RR Lyrae ab-type variables: AS Vir, BS Aps, CD Vel, DT Hya, RV Oct, TY Gru, UV Oct, V1645 Sgr, WY Ant, XZ Aps, and Z Mic. High-resolution and high signal-to-noise ratio echelle spectra of these variables were obtained with the 2.5 m du Pont telescope at the Las Campanas Observatory. We obtained more than 2300 spectra, roughly 200 spectra per star, distributed more or less uniformly throughout the pulsational cycles. A new method has been developed to obtain the initial effective temperatures of our sample stars at a specific pulsational phase. We find that the abundance ratios are generally consistent with those of similar metallicity field stars in different evolutionary states and throughout the pulsational cycles for RR Lyrae stars. TY Gru remains the only n-capture enriched star among the RRab in our sample. A new relation is found between microturbulence and effective temperature among stars of the horizontal-branch population. In addition, the variation of microturbulence as a function of phase is empirically shown to be similar to the theoretical variation. Finally, we conclude that the derived T-eff and log g values of our sample stars follow the general trend of a single mass evolutionary track.Item Testing the Asteroseismic Mass Scale Using Metal-Poor Stars Characterized With APOGEE and Kepler(2014-04) Epstein, Courtney R.; Elsworth, Yvonne P.; Johnson, Jennifer A.; Shetrone, Matthew; Mosser, Benoit; Hekker, Saskia; Tayar, Jamie; Harding, Paul; Pinsonneault, Marc; Aguirre, Victor Silva; Basu, Sarbani; Beers, Timothy C.; Bizyaev, Dmitry; Bedding, Timothy R.; Chaplin, William J.; Frinchaboy, Peter M.; Garcia, Rafael A.; Perez, Ana E. Garcia; Hearty, Fred R.; Huber, Daniel; Ivans, Inese I.; Majewski, Steven R.; Mathur, Savita; Nidever, David; Serenelli, Aldo; Schiavon, Ricardo P.; Schneider, Donald P.; Schoenrichi, Ralph; Sobeck, Jennifer S.; Stassun, Keivan G.; Stello, Dennis; Zasowski, Gail; Shetrone, MatthewFundamental stellar properties, such as mass, radius, and age, can be inferred using asteroseismology. Cool stars with convective envelopes have turbulent motions that can stochastically drive and damp pulsations. The properties of the oscillation frequency power spectrum can be tied to mass and radius through solar-scaled asteroseismic relations. Stellar properties derived using these scaling relations need verification over a range of metallicities. Because the age and mass of halo stars are well-constrained by astrophysical priors, they provide an independent, empirical check on asteroseismic mass estimates in the low-metallicity regime. We identify nine metal-poor red giants (including six stars that are kinematically associated with the halo) from a sample observed by both the Kepler space telescope and the Sloan Digital Sky Survey-III APOGEE spectroscopic survey. We compare masses inferred using asteroseismology to those expected for halo and thick-disk stars. Although our sample is small, standard scaling relations, combined with asteroseismic parameters from the APOKASC Catalog, produce masses that are systematically higher ((Delta M) = 0.17 +/- 0.05 M-circle dot) than astrophysical expectations. The magnitude of the mass discrepancy is reduced by known theoretical corrections to the measured large frequency separation scaling relationship. Using alternative methods for measuring asteroseismic parameters induces systematic shifts at the 0.04 M-circle dot level. We also compare published asteroseismic analyses with scaling relationship masses to examine the impact of using the frequency of maximum power as a constraint. Upcoming APOKASC observations will provide a larger sample of similar to 100 metal-poor stars, important for detailed asteroseismic characterization of Galactic stellar populations.