The Evolution of Oxygen and Magnesium in the Bulge and Disk of the Milky Way

Abstract

We show that the Galactic bulge and disk share a similar, strong, decline in [O/Mg] ratio with [Mg/H]. The similarity of the [O/Mg] trend in these two, markedly different, populations suggests a metallicity-dependent modulation of the stellar yields from massive stars, by mass loss from winds, and related to the Wolf-Rayet phenomenon, as proposed by McWilliam & Rich (2004). We have modified existing models for the chemical evolution of the Galactic bulge and the solar neighborhood with the inclusion of metallicity-dependent oxygen yields from theoretical predictions for massive stars that include mass loss by stellar winds. Our results significantly improve the agreement between predicted and observed [O/Mg] ratios in the bulge and disk above solar metallicity; however, a small zero-point normalization problem remains to be resolved. The zero-point shift indicates that either the semi-empirical yields of Francois et al. (2004) need adjustment, or that the bulge IMF is not quite as flat as found by Ballero et al. (2007); the former explanation is preferred. Our result removes a previous inconsistency between the interpretation of [O/Fe] and [Mg/Fe] ratios in the bulge, and confirms the conclusion that the bulge formed more rapidly than the disk, based on the over-abundances of elements produced by massive stars. We also provide an explanation for the long-standing difference between [Mg/Fe] and [O/Fe] trends among disk stars more metal-rich than the sun.

22 pages including 5 figures. Submitted to the Astronomical Journal

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• Galactic chemical evolution: Carbon through Zinc
• alpha-, r-, and s-process element trends in the Galactic thin and thick disks
• Oxygen, Sodium, Magnesium and Aluminium as tracers of the Galactic Bulge Formation
• Chemical Evolution of the Galactic Bulge as Derived from High-Resolution Infrared Spectroscopy of K and M Red Giants
• Formation & evolution of the Galactic bulge: constraints from stellar abundances
• Explosive Nucleosynthesis from GRB and Hypernova Progenitors: Direct Collapse versus Fallback
• Testing the universal stellar IMF on the metallicity distribution in the bulges of the Milky Way and M31

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