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TOPIC: Extremely Metal Poor Star

Blobrana

L

Posts: 131433

Date: Jan 13 09:40 2013

RE: Extremely Metal Poor Star	Permalink

Title: Very Metal-poor Stars in the Outer Galactic Bulge Found by the Apogee Survey Authors: Ana E. García Pérez, Katia Cunha, Matthew Shetrone, Steven R. Majewski, Jennifer A. Johnson, Verne V. Smith, Ricardo P. Schiavon, Jon Holtzman, David Nidever, Gail Zasowski, Carlos Allende Prieto, Timothy C. Beers, Dmitry Bizyaev, Garrett Ebelke, Daniel J. Eisenstein, Peter M. Frinchaboy, Léo Girardi, Fred R. Hearty, Elena Malanushenko, Viktor Malanushenko, Szabolcs Meszaros, Robert W. O'Connel, Daniel Oravetz, Kaike Pan, Annie C. Robin, Donald P. Schneider, Mathias Schultheis, Michael F. Skrutskie, Audrey Simmonsand, John C. Wilson Despite 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, multifibre, high-resolution spectroscopic survey within Sloan Digital Sky Survey III (SDSS-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 2,403 giant stars in twelve fields in the bulge obtained during APOGEE commissioning yielded five stars with low metallicity([Fe/H] \le-1.7), including two that are very metal-poor [Fe/H] ~ -2.1 by bulge standards. Luminosity-based distance estimates place the five stars within the outer bulge, where other 1,246 of the analysed 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. Read more (64kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Mar 3 18:40 2010

S1020549	Permalink

A newly discovered star outside the Milky Way has yielded important clues about the evolution of our galaxy. Located in the dwarf galaxy Sculptor some 280,000 light-years away, the star has a chemical make-up similar to the Milky Way's oldest stars, supporting theories that our galaxy grew by absorbing dwarf galaxies and other galactic building blocks. Some recent studies had questioned the link between dwarf galaxies and the Milky Way, citing differences between the chemistry of their stars. But the differences may not be so big after all, according to new research published in Nature. Read more __________________

Blobrana

L

Posts: 131433

Date: May 31 17:49 2009

Extremely-Metal Poor Stars	Permalink

Title: Extremely-Metal Poor Stars in the Milky Way: A Second Generation Formed after Reionisation Authors: Michele Trenti (1), Michael Shull (1), ((1) U Colorado) Cosmological simulations of Population III star formation suggest an initial mass function (IMF) biased toward very massive stars (M>100Msun) formed in minihalos at redshift z>20, when the cooling is driven by molecular hydrogen. However, this result conflicts with observations of extremely-metal poor (EMP) stars in the Milky Way halo, whose r-process elemental abundances appear to be incompatible with those expected from very massive Population III progenitors. We propose that the IMF of second-generation stars formed at z>10, before reionisation, is deficient in sub-solar mass stars, owing to the high cosmic microwave background temperature floor. The observed EMP stars are formed preferentially at z<10 in pockets of gas enriched to metallicity Z>10^{-3.5} Zsun by winds from Population II stars. Our cosmological simulations of structure formation show that current samples of EMP stars can only constrain the IMF of late-time Population III stars, formed at z<13 in halos with virial temperature Tvir 10^4 K. This suggests that pair instability supernovae were not produced primarily by this population. To begin probing the IMF of Population III stars formed at higher redshift will require a large survey, with at least 500 and probably several thousand EMP stars of metallicities Z~10^{-3.5} Zsun. Read more (256kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Aug 15 23:26 2008

Extremely Metal-Poor Stars	Permalink

Title: Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultra-Faint Dwarf Spheroidal Satellite Galaxies Authors: Evan N. Kirby (1), Joshua D. Simon (2), Marla Geha (3), Puragra Guhathakurta (1), Anna Frebel (4) ((1) University of California Santa Cruz/Lick Observatory, (2) California Institute of Technology, (3) Yale University, (4) University of Texas Austin/McDonald Observatory) (Version v3) 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 [Fe/H] < -3.0 of stars in a dwarf galaxy, with individual stellar metallicities as low as [Fe/H] = -3.3. Because 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 LCDM galaxy assembly paradigm wherein dwarf galaxies dissolve to form the stellar halo of the MW. Read more (33kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Mar 15 13:58 2007

Extremely Metal Poor Star	Permalink

Title: A New Type of Extremely Metal Poor Star Authors: Judith G. Cohen, Andrew McWilliam, Norbert Christlieb, Stephen Shectman, Ian Thompson, Jorge Melendez, Lutz Wisotzki, Dieter Reimers We present an abundance analysis for the extremely metal poor star HE1424-0241 based on high dispersion spectra from HIRES at Keck. This star is a giant on the lower red giant branch with [Fe/H] ~ -4.0 dex. Relative to Fe, HE1424-0241 has normal Mg, but it shows a very large deficiency of Si, with epsilon(Si)/epsilon(Fe) ~ 1/10 and epsilon(Si)/epsilon(Mg) ~ 1/25 that of all previously known extremely metal poor giants or dwarfs. It also has a moderately large deficiency of Ca and a smaller deficit of Ti, combined with enhanced Mn and Co and normal or low C. We suggest that in HE1424-0241 we see the effect of a very small number of contributing supernovae, and that the SNII contributing to the chemical inventory of HE1424-0241 were biased in progenitor mass or in explosion characteristics so as to reproduce its abnormal extremely low Si/Mg ratio. HE1424-0241 shows a deficiency of the explosive alpha-burning elements Si, Ca and Ti coupled with a ratio [Mg/Fe] normal for EMP stars; Mg is produced via hydrostatic alpha-burning. The latest models of nucleosynthesis in SNII fail to reproduce the abundance ratios seen in HE1424-0241 for any combination of the parameter space of core-collapse explosions they explore. Read more (15kb, PDF) __________________

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