Astronomy and Astrophysics, volume 465, 799-814 (2007/4-3)
Oxygen, sodium, magnesium, and aluminium as tracers of the Galactic Bulge formation.
LECUREUR A., HILL V., ZOCCALI M., BARBUY B., GOMEZ A., MINNITI D., ORTOLANI S. and RENZINI A.
Abstract (from CDS):
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. 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. 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.
Galaxy: bulge - Galaxy: formation - Galaxy: abundances - stars: abundances - stars: atmospheres
VizieR on-line data:
<Available at CDS (J/A+A/465/799): table1.dat ew.dat abund.dat>
Tables 1, 11, 12: [LHZ2007] BWc-NN (Nos 1-13), [LHZ2007] BW-aN N=5+6, [LHZ2007] BN-bN N=7+5, [LHZ2007] BN-fN N=6+7, [LHZ2007] BL-N N=7.
Table 1 Objects BW-NN not created in Simbad because the authors do not give coordinates.
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