Browsing by Subject "metallicity distribution"
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Item High-Resolution Spectroscopy Of Extremely Metal-Poor Stars In The Least Evolved Galaxies: Ursa Major II And Coma Berenices(2010-01) Frebel, Anna; Simon, Joshua D.; Geha, Marla; Willman, Beth; Frebel, AnnaWe present spectra of six metal-poor stars in two of the ultra-faint dwarf galaxies orbiting the Milky Way (MW), Ursa Major II, and Coma Berenices obtained with the Keck/High Resolution Echelle Spectrometer (HIRES). These observations include the first high-resolution spectroscopic observations of extremely metal-poor ([Fe/H] < -3.0) stars not belonging to the MW halo field star population. We obtain abundance measurements and upper limits for 26 elements between carbon and europium. The entire sample of stars spans a range of -3.2 < [Fe/H] < -2.3, and we confirm that each galaxy contains a large intrinsic spread of Fe abundances. A comparison with MW halo stars of similar metallicities reveals substantial agreement between the abundance patterns of the ultra-faint dwarf galaxies and the MW halo for the light, alpha, and iron-peak elements (C to Zn). This agreement contrasts with the results of earlier studies of more metal-rich stars (-2.5 less than or similar to [Fe/H] less than or similar to -1.0) in more luminous dwarf spheroidal galaxies, which found significant abundance discrepancies with respect to the MW halo data. The abundances of neutron-capture elements (Sr to Eu) in the ultra-faint dwarf galaxies are extremely low, consistent with the most metal-poor halo stars, but not with the typical halo abundance pattern at [Fe/H] greater than or similar to -3.0. Not only are our results broadly consistent with a galaxy formation model that predicts that massive dwarf galaxies are the source of the metal-rich component ([Fe/H] > -2.5) of the MW halo, but they also suggest that the faintest known dwarfs may be the primary contributors to the metal-poor end of the MW halo metallicity distribution.Item Multi-Element Abundance Measurements From Medium-Resolution Spectra. I. The Sculptor Dwarf Spheroidal Galaxy(2009-11) Kirby, Evan N.; Guhathakurta, Puragra; Bolte, Michael; Sneden, Christopher; Geha, Marla C.; Sneden, ChristopherWe present measurements of Fe, Mg, Si, Ca, and Ti abundances for 388 radial velocity member stars in the Sculptor dwarf spheroidal galaxy (dSph), a satellite of the Milky Way (MW). This is the largest sample of individual a element (Mg, Si, Ca, and Ti) abundance measurements in any single dSph. The measurements are made from Keck/Deep Imaging Multi-Object Spectrometer medium-resolution spectra (6400-9000 angstrom, R similar to 6500). Based on comparisons to published high-resolution (R greater than or similar to 20,000) spectroscopic measurements, our measurements have uncertainties of sigma[Fe/H] = 0.14 and sigma[alpha/Fe] = 0.13. The Sculptor [Fe/H] distribution has a mean <[Fe/H]> = -1.58 and is asymmetric with a long, metal-poor tail, indicative of a history of extended star formation. Sculptor has a larger fraction of stars with [Fe/H] < -2 than the MW halo. We have discovered one star with [Fe/H] = -3.80 +/- 0.28, which is the most metal-poor star known anywhere except the MW halo, but high-resolution spectroscopy is needed to measure this star's detailed abundances. As has been previously reported based on high-resolution spectroscopy, [alpha/Fe] in Sculptor falls as [Fe/H] increases. The metal-rich stars ([Fe/H] similar to -1.5) have lower [alpha/Fe] than Galactic halo field stars of comparable metallicity. This indicates that star formation proceeded more gradually in Sculptor than in the Galactic halo. We also observe radial abundance gradients of -0.030 +/- 0.003 dex arcmin(-1) in [Fe/H] and +0.013 +/- 0.003 dex arcmin(-1) in [alpha/Fe] out to 11 arcmin (275 pc). Together, these measurements cast Sculptor and possibly other surviving dSphs as representative of the dwarf galaxies from which the metal-poor tail of the Galactic halo formed.Item Very Metal-Poor Stars in the Outer Galactic Bulge Found By the APOGEE Survey(2013-04) Garcia Perez, Ana E.; Cunha, Katia; Shetrone, Matthew; Majewski, Steven R.; Johnson, Jennifer A.; Smith, Verne V.; Schiavon, Ricardo P.; Holtzman, Jon; Nidever, David; Zasowski, Gail; Allende Prieto, Carlos; Beers, Timothy C.; Bizyaev, Dmitry; Ebelke, Garrett; Eisenstein, Daniel J.; Frinchaboy, Peter M.; Girardi, Leo; Hearty, Fred R.; Malanushenko, Elena; Malanushenko, Viktor; Meszaros, Szabolcs; O'Connell, Robert W.; Oravetz, Daniel; Pan, Kaike; Robin, Annie C.; Schneider, Donald P.; Schultheis, Mathias; Skrutskie, Michael F.; Simmonsand, Audrey; Wilson, John C.; Shetrone, MatthewDespite its importance for understanding the nature of early stellar generations and for constraining Galactic bulge Formation models, at present little is known about the metal-poor stellar content of the central Milky Way. This is a consequence of the great distances involved and intervening dust obscuration, which challenge optical studies. However, the Apache Point Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber, high-resolution spectroscopic survey within Sloan Digital Sky Survey III, is exploring the chemistry of all Galactic stellar populations at infrared wavelengths, with particular emphasis on the disk and the bulge. An automated spectral analysis of data on 2403 giant stars in 12 fields in the bulge obtained during APOGEE commissioning yielded five stars with low metallicity ([Fe/H] <= -1.7), including two that are very metal-poor [Fe/H] similar to -2.1 by bulge standards. Luminosity-based distance estimates place the 5 stars within the outer bulge, where 1246 of the other analyzed stars may reside. A manual reanalysis of the spectra verifies the low metallicities, and finds these stars to be enhanced in the alpha-elements O, Mg, and Si without significant alpha-pattern differences with other local halo or metal-weak thick-disk stars of similar metallicity, or even with other more metal-rich bulge stars. While neither the kinematics nor chemistry of these stars can yet definitively determine which, if any, are truly bulge members, rather than denizens of other populations co-located with the bulge, the newly identified stars reveal that the chemistry of metal-poor stars in the central Galaxy resembles that of metal-weak thick-disk stars at similar metallicity.Item VLT/FLAMES Spectroscopy Of Red Giant Branch Stars In The Carina Dwarf Spheroidal Galaxy(2012-02) Lemasle, B.; Hill, V.; Tolstoy, E.; Venn, K. A.; Shetrone, M. D.; Irwin, M. J.; de Boer, T. J. L.; Starkenburg, E.; Salvadori, S.; Shetrone, Matthew D.Context. The ages of individual red giant branch stars can range from 1 Gyr old to the age of the Universe, and it is believed that the abundances of most chemical elements in their photospheres remain unchanged with time (those that are not affected by the first dredge-up). This means that they trace the interstellar medium in the galaxy at the time the star formed, and hence the chemical enrichment history of the galaxy. Aims. Colour-magnitude diagram analysis has shown the Carina dwarf spheroidal to have had an unusually episodic star formation history and this is expected to be reflected in the abundances of different chemical elements. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode (R approximate to 20 000) to measure the abundances of several chemical elements, including Fe, Mg, Ca and Ba, in a sample of 35 individual Red Giant Branch stars in the Carina dwarf spheroidal galaxy. We also combine these abundances with photometry to derive age estimates for these stars. This allows us to determine which of two distinct star formation episodes the stars in our sample belong to, and thus to define the relationship between star formation and chemical enrichment during these two episodes. Results. As is expected from the star formation history, Carina contains two distinct populations of Red Giant Branch stars: one old (greater than or similar to 10 Gyr), which we have found to be metal-poor ([Fe/H] < -1.5), and alpha-rich ([Mg/Fe] > 0); the other intermediate age (approximate to 2-6 Gyr), which we have found to have a metallicity range (-1.8 < [Fe/H] < -1.2) with a large spread in [alpha/Fe] abundance, going from extremely low values ([Mg/Fe] < -0.3) to the same mean values as the older population (<[Mg/Fe]> similar to 0.3). Conclusions. We show that the chemical enrichment history of the Carina dwarf spheroidal was different for each star formation episode. The earliest was short (similar to 2-3 Gyr) and resulted in the rapid chemical enrichment of the whole galaxy to [Fe/H] similar to -1.5 with both SNe II and SNe Ia contributions. The subsequent episode occured after a gap of similar to 3-4 Gyr, forming similar to 70% of the stars in the Carina dSph, but it appears to have resulted in relatively little evolution in either [Fe/H] or [alpha/Fe].