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Session 66 - Galaxies.
Display session, Thursday, June 11
The Sagittarius dwarf spheroidal galaxy (dSph) is slowly disintegrating and merging with the Milky Way. A powerful way of determining if many such mergers have occurred in the past is to compare the ratios of chemical abundances found in Galactic stars to those found in dSph stars. Most Galactic halo stars have chemical abundance ratios that are nearly identical to the average yield of Type II supernovae. However, the complex star-formation rates that have recently been deduced for most dSphs in the Local Group will generate stars that contain a mixture of chemicals created in both Type Ia and Type II supernovae. We have obtained spectra with the HIRES spectrograph on the Keck I Telescope for seven stars in the Sagittarius dSph and two stars in the Sculptor dSph. We present our results on Sagittarius stars in this paper. Analysis of our high dispersion and high signal-to-noise ratio spectra show that the metal-poor stars in Sagittarius, [Fe/H]\simeq -1.5, have abundances that are similar Galactic halo stars. Thus the initial mass function for massive stars in the Sagittarius dSph was similar to that in Galaxy. One star has depleted oxygen and enhanced sodium that indicates internal chemical processing via proton burning similar to what is seen in some Galactic globular clusters. Two stars are surprisingly metal rich for such a dwarf galaxy; the stars have solar metallicity and solar element ratios ([Fe/H]=+0.11, [0/Fe]=-0.41, and [Ca/Fe]=-0.24). Thus the chemical enrichment history of the Sagittarius dSph was complex. Specifically, the subsolar element ratios hint that the galaxy may have undergone quiescent periods in which enrichment continued from Type Ia supernovae while the star-formation rate and the enrichment from Type II supernovae declined. The potential for forming the Galactic halo or bulge from merged dSphs that are similar to the Sagittarius dSph is severly limited.
Program listing for Thursday