Formation and evolution of the Galactic bulge: constraints from stellar abundances

¹ Dipartimento di Astronomia, Universitá di Trieste, via G. B.Tiepolo 11, 34131 Trieste, Italy e-mail: ballero@oats.inaf.it
² INAF – Osservatorio Astronomico di Trieste, via G. B.Tiepolo 11, 34131 Trieste, Italy
³ INAF – Osservatorio Astronomico di Bologna, via G. Ranzani 1, 40127 Bologna, Italy
⁴ Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA

Received: 18 October 2006
Accepted: 29 January 2007

Abstract

Aims.We compute the chemical evolution of the Galactic bulge in the context of an inside-out model for the formation of the Milky Way. The model contains updated stellar yields from massive stars. The main purpose of the paper is to compare the predictions of this model with new observations of chemical abundance ratios and metallicity distributions in order to put constraints on the formation and evolution of the bulge.

Methods.We computed the evolution of several α-elements and Fe and performed several tests by varying different parameters such as star formation efficiency, slope of the initial mass function and infall timescale. We also tested the effect of adopting a primary nitrogen contribution from massive stars.

Results.The [ α/Fe] abundance ratios in the Bulge are predicted to be supersolar for a very large range in [Fe/H], each element having a different slope. These predictions are in very good agreement with most recent accurate abundance determinations. We also find a good fit of the most recent Bulge stellar metallicity distributions.

Conclusions.We conclude that the Bulge formed on a very short timescale (even though timescales much shorter than ~0.1 Gyr are excluded) with a quite high star formation efficiency of 20 Gyr^-1 and with an initial mass function more skewed toward high masses (i.e. 0.95) than the solar neighbourhood and rest of the disk. The results obtained here are more robust than previous ones since they are based on very accurate abundance measurements.