Volume 543, July 2012
|Number of page(s)||10|
|Section||Galactic structure, stellar clusters and populations|
|Published online||22 June 2012|
Two distinct halo populations in the solar neighborhood
1 Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
2 Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México, Apartado Postal 877, C.P. 22800 Ensenada, B.C., México
Received: 4 April 2012
Accepted: 13 May 2012
Context. A previous study of F and G main-sequence stars in the solar neighborhood has revealed the existence of two distinct halo populations with a clear separation in [α/Fe] for the metallicity range −1.4 < [Fe/H] < −0.7. Taking into account the kinematics and ages of the stars, some Galactic formation models suggest that the “high-alpha” halo stars were formed in situ, whereas the “low-alpha” stars have been accreted from satellite galaxies.
Aims. In this paper we investigate if there is a systematic difference in the lithium abundances of stars belonging to the high- and low-alpha halo populations.
Methods. Equivalent widths of the Li i 6707.8 Å resonance line are measured from high resolution VLT/UVES and NOT/FIES spectra and used to derive Li abundances on the basis of MARCS model atmospheres. Furthermore, masses of the stars are determined from the log Teff – log g diagram by interpolating between evolutionary tracks based on Yonsei-Yale models.
Results. There is no significant systematic difference in the lithium abundances of high- and low-alpha stars. For the large majority of stars with masses 0.7 < M/M⊙ < 0.9 and heavy-element mass fractions 0.001 ≲ Z < 0.006, the lithium abundance is well fitted by a relation A(Li) = a0 + a1 M + a2 Z + a3 M Z, where a0, a1, a2, and a3 are constants. Extrapolating this relation to Z = 0 leads to a lithium abundance close to the primordial value predicted from standard Big Bang nucleosynthesis calculations and the WMAP baryon density. The relation, however, does not apply to stars with metallicities below [Fe/H] ≃ − 1.5.
Conclusions. We suggest that metal-rich halo stars were formed with a lithium abundance close to the primordial value, and that lithium in their atmospheres has been depleted in time with an approximately linear dependence on stellar mass and Z. The lack of a systematic difference in the Li abundances of high- and low-alpha stars indicates that an environmental effect is not important for the destruction of lithium.
Key words: stars: abundances / stars: atmospheres / stars: interiors / Galaxy: halo / early Universe
Based on observations made with the Nordic Optical Telescope on La Palma, and on data from the European Southern Observatory ESO/ST-ECF Science Archive Facility (programs 65.L-0507, 67.D-0086, 67.D-0439, 68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585, 76.B-0133 and 77.B-0507).
Table 1 is available in electronic form at http://www.aanda.org
© ESO, 2012
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