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Published online by Cambridge University Press: 02 December 2009
We present abundance results for 53 bulge giant stars using highresolution spectra obtained with FLAMES/UVES at the ESO/VLT for various regions of the Bulge (−12 < b < −4). The trend of the four light elements O, Na, Mg and Al indicates a chemical enrichment of the bulge dominated by massive stars at all metallicities. For [Fe/H] > −0.5, [O/Fe], [Na/Fe], [Mg/Fe] and [Al/Fe] are enhanced relative to both the thin- and the thick-disc trend. This suggests that the bulge formed on a shorter timescale than did the Galactic discs.
Using Mg as a proxy for metallicity (instead of Fe) we further show the following (i) The [O/Mg] ratio for bulge stars follows and extends to higher metallicities the decreasing trend of [O/Mg] found in the galactic discs. (ii) The [Na/Mg] ratio trend with increasing [Mg/H] is found to increase in three distinct sequences in the thin disc, the thick disc, and the bulge. The bulge trend is well represented by the predicted metallicity- dependent yields of massive stars, whereas the galactic discs have Na/Mg ratios that are too high at low metallicities, indicating an additional source of Na from AGB stars. (iii) In contrast to the case with Na, there appears to be no systematic difference in the [Al/Mg] ratio between bulge and disc stars, and the theoretical yields for massive stars agree with the observed ratios, leaving no space for an AGB contribution to Al.
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