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Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars IV. Two bulge populations

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Bensby, T. [1, 2] ; Aden, D. [1] ; Melendez, J. [3] ; Gould, A. [4] ; Feltzing, S. [1] ; Asplund, M. [5] ; Johnson, J. A. [4] ; Lucatello, S. [6] ; Yee, J. C. [4] ; Ramirez, I. [7] ; Cohen, J. G. [8] ; Thompson, I. [7] ; Bond, I. A. [9] ; Gal-Yam, A. [10] ; Han, C. [11] ; Sumi, T. [12] ; Suzuki, D. [12] ; Wada, K. [12] ; Miyake, N. [13] ; Furusawa, K. [13] ; Ohmori, K. [13] ; Saito, To. [14] ; Tristram, P. [15] ; Bennett, D. [16]
Número total de Autores: 24
Afiliação do(s) autor(es):
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[1] Lund Observ, Dept Astron & Theoret Phys, S-22100 Lund - Sweden
[2] European So Observ, Santiago 19001 19 - Chile
[3] Univ Sao Paulo, Dept Astron, IAG, BR-05508900 Sao Paulo - Brazil
[4] Ohio State Univ, Dept Astron, Columbus, OH 43210 - USA
[5] Max Planck Inst Astrophys, D-85741 Garching - Germany
[6] INAF Astron Observ Padova, I-35122 Padua - Italy
[7] Carnegie Observ, Pasadena, CA 91101 - USA
[8] CALTECH, Palomar Observ, Pasadena, CA 91125 - USA
[9] Massey Univ, Inst Informat & Math Sci, N Shore Ctr, Auckland - New Zealand
[10] Weizmann Inst Sci, Benoziyo Ctr Astrophys, IL-76100 Rehovot - Israel
[11] Chungbuk Natl Univ, Dept Phys, Cheongju 361763 - South Korea
[12] Osaka Univ, Dept Earth & Space Sci, Osaka 5600043 - Japan
[13] Nagoya Univ, Solar Terr Enivironment Lab, Chikusa Ku, Nagoya, Aichi 4648601 - Japan
[14] Tokyo Metropolitan Coll Ind Technol, Tokyo 1168523 - Japan
[15] Univ Auckland, Dept Phys, Auckland 1142 - New Zealand
[16] Univ Notre Dame, Dept Phys, Notre Dame, IN 46556 - USA
Número total de Afiliações: 16
Tipo de documento: Artigo Científico
Fonte: Astronomy & Astrophysics; v. 533, SEP 2011.
Citações Web of Science: 125

Based on high-resolution (R approximate to 42 000 to 48 000) and high signal-to-noise (S/N approximate to 50 to 150) spectra obtained with UVES/VLT, we present detailed elemental abundances (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, and Ba) and stellar ages for 12 new microlensed dwarf and subgiant stars in the Galactic bulge. Including previous microlensing events, the sample of homogeneously analysed bulge dwarfs has now grown to 26. The analysis is based on equivalent width measurements and standard 1-D LTE MARCS model stellar atmospheres. We also present NLTE Li abundances based on line synthesis of the (7)Li line at 670.8 nm. The results from the 26 microlensed dwarf and subgiant stars show that the bulge metallicity distribution (MDF) is double-peaked; one peak at {[}Fe/H] approximate to -0.6 and one at {[}Fe/H] approximate to +0.3, and with a dearth of stars around solar metallicity. This is in contrast to the MDF derived from red giants in Baade's window, which peaks at this exact value. A simple significance test shows that it is extremely unlikely to have such a gap in the microlensed dwarf star MDF if the dwarf stars are drawn from the giant star MDF. To resolve this issue we discuss several possibilities, but we can not settle on a conclusive solution for the observed differences. We further find that the metal-poor bulge dwarf stars are predominantly old with ages greater than 10 Gyr, while the metal-rich bulge dwarf stars show a wide range of ages. The metal-poor bulge sample is very similar to the Galactic thick disk in terms of average metallicity, elemental abundance trends, and stellar ages. Speculatively, the metal-rich bulge population might be the manifestation of the inner thin disk. If so, the two bulge populations could support the recent findings, based on kinematics, that there are no signatures of a classical bulge and that the Milky Way is a pure-disk galaxy. Also, recent claims of a flat IMF in the bulge based on the MDF of giant stars may have to be revised based on the MDF and abundance trends probed by our microlensed dwarf stars. (AU)

Processo FAPESP: 10/50930-6 - Evolução química e populações estelares galácticas e extragalácticas, por espectroscopia e imageamento
Beneficiário:Beatriz Leonor Silveira Barbuy
Linha de fomento: Auxílio à Pesquisa - Temático