Detailed chemical evolution of Carina and Sagittarius dwarf spheroidal galaxies

¹ IAG-USP, R. do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brazil e-mail: glanfran@usp.br
² Dipartimento di Astronomia-Universitá di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
³ I.N.A.F. Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy

Received: 1 December 2005
Accepted: 9 March 2006

Abstract

Aims. In order to verify the effects of the most recent data on the evolution of Carina and Sagittarius Dwarf Spheroidal Galaxies (dSph) and to set tight constraints on the main parameters of chemical evolution models, we study in detail the chemical evolution of these galaxies through comparisons between the new data and the predictions of a model, already tested to reproduce the main observational constraints in dSphs.

Methods. Several abundance ratios, such as [ α/Fe] , [Ba/Fe] and [Eu/Fe], in Sagittarius and Carina and the metallicity distribution of stars in Carina are compared to the predictions of our models adopting the observationally derived star formation histories in these galaxies.

Results. These new comparisons confirm our previously suggested scenario for the evolution of these galaxies, and allow us to better fix the star formation and wind parameters. In particular, for Carina the comparison between the new observed metallicity distribution of stars and our predictions indicates that the best efficiency of star formation for this galaxy is  Gyr^-1, that the best wind efficiency parameter is w_i = 5 (the wind rate is five times stronger than the star formation rate), and that the star formation history, which produces the best fit to the observed metallicity distribution of stars is characterized by several episodes of activity. In the case of Sagittarius there are now much more data on abundances and our results suggest that  Gyr^-1 and w_i=9, again in agreement with our previous work. Finally, we show new predictions for [N/Fe] and [C/Fe] ratios for the two galaxies suggesting a scenario for Sagittarius very similar to the one of the solar vicinity in the Milky Way, except for a slight decrease of [N/Fe] ratio at high metallicities due to the galactic wind which is not present in the Milky Way. For Carina we predict a larger [N/Fe] ratio at low metallicities, reflecting the lower star formation efficiency of this galaxy relative to Sagittarius and the Milky Way.