Observatoire de Paris-Meudon, GEPI and CNRS UMR 8111, 5 place J. Janssen, 92125 Meudon Cedex, France e-mail: [Aurelie.Lecureur;Vanessa.Hill;Ana.Gomez]@obspm.fr
2 P. Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 306, Santiago 22, Chile e-mail: [mzoccali;dante]@astro.puc.cl
3 Universidade de São Paulo, IAG, Rua do Matão 1226, São Paulo 05508-900, Brazil e-mail: email@example.com
4 Universidi Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: firstname.lastname@example.org
5 Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: email@example.com
Accepted: 2 October 2006
Aims.This paper investigates the peculiar behaviour of the light even (alpha-elements) and odd atomic number elements in red giants in the galactic bulge, both in terms of the chemical evolution of the bulge, and in terms of possible deep-mixing mechanisms in these evolved stars.
Methods.Abundances of the four light elements O, Na, Mg, and Al are measured in 13 core He-burning giant stars (red clump stars) and 40 red giant branch stars in four 25′ fields spanning the bulge from -3 to -12° galactic latitude. Special care was taken in the abundance analysis, performing a differential analysis with respect to the metal-rich solar-neighbourhood giant μLeo, which best resembles our bulge sample stars. This approach minimises systematic effects that can arise in the analysis of cool metal-rich stars due to continuum definition issues and blending by molecular lines (CN) and, cancels out possible model atmosphere deficiencies.
Results.We show that the resulting abundance patterns point towards a chemical enrichment dominated by massive stars at all metallicities. Oxygen, magnesium, and aluminium ratios with respect to iron are overabundant with respect to both galactic disks (thin and thick) for [Fe/H] > -0.5. A formation timescale for the galactic bulge shorter than for both the thin and thick disks is therefore inferred. To isolate the massive-star contribution to the abundances of O, Mg, Al, and Na, we use Mg as a proxy for metallicity (instead of Fe), and further show that: (i) the bulge stars [O/Mg] ratio follows and extends the decreasing trend of [O/Mg] found in the galactic disks to higher metallicities. This is a challenge for predictions of O and Mg yields in massive stars, which so far predicted no metallicity dependence in this ratio; (ii) the [Na/Mg] ratio trend with increasing [Mg/H] is found to increase in three distinct sequences in the thin disk, the thick disk, and the bulge. The bulge trend is well represented by the predicted metallicity-dependent yields of massive stars, whereas the galactic disks have Na/Mg ratios that are too high at low metallicities, pointing to an additional source of Na from AGB stars; (iii) contrary to the case of the [Na/Mg] ratio, there appears to be no systematic difference in the [Al/Mg] ratio between bulge and disk stars, and the theoretical yields by massive stars agree with the observed ratios, leaving no space for AGB contribution to Al.
Key words: Galaxy: bulge / Galaxy: formation / Galaxy: abundances / stars: abundances / stars: atmospheres
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