Volume 465, Number 3, April III 2007
|Page(s)||799 - 814|
|Section||Galactic structure, stellar clusters, and populations|
|Published online||02 January 2007|
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
Full Tables 2 to 6 are only available in electronic form at the CDS via anonymous ftp to cdsarc-ustrasbg.fr (188.8.131.52) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/799
© ESO, 2007
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.