Browsing by Subject "branch stars"

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Heavy Element Abundances In Giant Stars Of The Globular Clusters M4 And M5
(2008-12) Yong, David; Karakas, Amanda I.; Lambert, David L.; Chieffi, Alessandro; Limongi, Marco; Lambert, David L.
We present a comprehensive abundance analysis of 27 heavy elements in bright giant stars of the globular clusters M4 and M5 based on high-resolution, high signal-to-noise ratio spectra obtained with the Magellan Clay Telescope. We confirm and expand on previous results for these clusters by showing that (1) all elements heavier than, and including, Si have constant abundances within each cluster, (2) the elements from Ca to Ni have indistinguishable compositions in M4 and M5, (3) Si, Cu, Zn, and all s-process elements are approximately 0.3 dex overabundant in M4 relative to M5, and (4) the r-process elements Sm, Eu, Gd, and Th are slightly overabundant in M5 relative to M4. The cluster-to-cluster abundance differences for Cu and Zn are intriguing, especially in light of their uncertain nucleosynthetic origins. We confirm that stars other than Type Ia supernovae must produce significant amounts of Cu and Zn at or below the clusters' metallicities. If intermediate-mass AGB stars or massive stars are responsible for the Cu and Zn enhancements in M4, the similar [Rb/Zr] ratios and (preliminary) Mg isotope ratios in both clusters may be problematic for either scenario. For the elements from Ba to Hf, we assume that the s-and r-process contributions are scaled versions of the solar s-and r-process abundances. We quantify the relative fractions of s-and r-process material for each cluster and show that they provide an excellent fit to the observed abundances.
Rubidium And Lead Abundances In Giant Stars Of The Globular Clusters M4 And M5
(2008-02) Yong, David; Lambert, David L.; Paulson, Diane B.; Carney, Bruce W.; Lambert, David L.
We present measurements of the neutron-capture elements Rb and Pb for bright giants in the globular clusters M4 and M5. The clusters are of similar metallicity ([Fe/ H] similar or equal to -1.2), but M4 is decidedly s-process enriched relative to M5: [Ba/Fe] = +0.6 for M4 but 0.0 for M5. The Rb and Pb abundances were derived by comparing synthetic spectra with high-resolution, high signal-to-noise ratio spectra obtained with MIKE on the Magellan Telescope. Abundances of Y, Zr, La, and Eu were also obtained. In M4, the mean abundances from 12 giants are [Rb/Fe] = 0.39 +/- 0.02 (sigma = 0.07), [Rb/Zr] = 0.17 +/- 0.03 (sigma = 0.08), and [Pb/Fe] = 0.30 +/- 0.02 (sigma = 0.07). In M5, the mean abundances from two giants are [Rb/Fe] = 0.00 +/- 0.05 (sigma = 0.06), [Rb/Zr] = 0.08 +/- 0: 08 (sigma = 0.11), and [Pb/Fe] = -0.35 +/- 0.02 (sigma = 0.04). Within the measurement uncertainties, the abundance ratios [Rb/Fe], [Pb/Fe], and [Rb/X] for X = Y, Zr, and La are constant from star to star in each cluster, and none of these ratios are correlated with O or Na abundances. While M4 has a higher Rb abundance than M5, the ratios [Rb/X] are similar in both clusters, indicating that the nature of the s-products is very similar for each cluster but the gas from which M4's stars formed had a higher concentration of these products.
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, Matthew
Fundamental 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.
Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultrafaint Dwarf Spheroidal Satellite Galaxies
(2008-09) Kirby, Evan N.; Simon, Joshua D.; Geha, Marla; Guhathakurta, Puragra; Frebel, Anna; Frebel, Anna
We present new metallicity measurements for 298 individual red giant branch stars in eight of the least luminous dwarf spheroidal galaxies (dSphs) in the Milky Way ( MW) system. Our technique is based on medium-resolution Keck DEIMOS spectroscopy coupled with spectral synthesis. We present the first spectroscopic metallicities at of stars in a dwarf galaxy, with individual stellar metallicities as low as. Because [Fe/H] < -3.0 [Fe/H] = -3.3 our [Fe/H] measurements are not tied to empirical metallicity calibrators and are sensitive to arbitrarily low metallicities, we are able to probe this extremely metal-poor regime accurately. The metallicity distribution of stars in these dSphs is similar to the MW halo at the metal-poor end. We also demonstrate that the luminosity-metallicity relation previously seen in more luminous dSph galaxies (M(v) = -13.4 to -8.8) extends smoothly down to an absolute magnitude of M(v) = -3.7. The discovery of extremely metal-poor stars in dSphs lends support to the Lambda CDM galaxy assembly paradigm wherein dwarf galaxies dissolve to form the stellar halo of the MW.