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

Bensby, Thomas LU ; Adén, Daniel LU ; Melendez, J.; Gould, A.; Feltzing, Sofia LU ; Asplund, M.; Johnson, J. A.; Lucatello, S.; Yee, J. C. and Ramirez, I., et al. (2011) In Astronomy & Astrophysics 533.
Abstract
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 Li-7 line at 670.8 nm. The results from the 26 microlensed dwarf and subgiant stars show that the bulge metallicity distribution... (More)
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 Li-7 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. (Less)
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publication status
published
subject
keywords
gravitational lensing: micro, Galaxy: bulge, Galaxy: formation, Galaxy:, evolution, stars: abundances
in
Astronomy & Astrophysics
volume
533
publisher
EDP Sciences
external identifiers
  • wos:000295168100134
  • scopus:80052855851
ISSN
0004-6361
DOI
10.1051/0004-6361/201117059
language
English
LU publication?
yes
id
6594d581-2a47-483f-a353-201112aa399a (old id 2179693)
date added to LUP
2011-10-25 11:52:35
date last changed
2017-10-01 03:59:52
@article{6594d581-2a47-483f-a353-201112aa399a,
  abstract     = {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 Li-7 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.},
  author       = {Bensby, Thomas and Adén, Daniel and Melendez, J. and Gould, A. and Feltzing, Sofia and Asplund, M. and Johnson, J. A. and Lucatello, S. and Yee, J. C. and Ramirez, I. and Cohen, J. G. and Thompson, I. and Bond, I. A. and Gal-Yam, A. and Han, C. and Sumi, T. and Suzuki, D. and Wada, K. and Miyake, N. and Furusawa, K. and Ohmori, K. and Saito, To. and Tristram, P. and Bennett, D.},
  issn         = {0004-6361},
  keyword      = {gravitational lensing: micro,Galaxy: bulge,Galaxy: formation,Galaxy:,evolution,stars: abundances},
  language     = {eng},
  publisher    = {EDP Sciences},
  series       = {Astronomy & Astrophysics},
  title        = {Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars IV. Two bulge populations},
  url          = {http://dx.doi.org/10.1051/0004-6361/201117059},
  volume       = {533},
  year         = {2011},
}