The Composition of the Sagittarius Dwarf Spheroidal Galaxy and Implications for Nucleosynthesis and Chemical Evolution

Abstract

We have measured the chemical composition of 14 stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) using high S/N Keck HIRES echelle spectra. For the Sgr dSph stars with [Fe/H]≥−1 the abundances are highly unusual, showing a striking enhancement in heavy s-process elements, increasing with [Fe/H], deficiencies of the α-elements (O, Si, Ca, and Ti), deficiencies of Al and Na, and deficiencies of the odd-numbered iron-peak elements Mn and Cu. Our abundances suggest that the composition of the metal-rich Sgr dSph stars is dominated by the ejecta of an old, metal-poor population, including products of AGB stars and type Ia supernovae (SN). The simplest scenario to explain our abundance results is that chemical enrichment of the Sgr dSph occurred over long timescales (several Gyr), during which time this galaxy experienced significant gas loss. This led to the situation where injection of newly synthesized material was dominated by low-metallicity, low-mass, AGB stars and type Ia SN. Since mass-loss from low-mass galaxies is generally expected to have occurred, on the basis of their low mean metallicity, we anticipate that our unusual abundance patterns may be quite common among low-mass galaxies. Published abundance results for the Fornax dSph and the Galactic globular cluster ω Cen are similar to Sgr dSph, which suggests similarities in the chemical enrichment histories of all three systems. Filled circles represent our Sgr dSph results; others are solar neighborhood. Fig. 1. Left: Enhancements up to 1 dex in [La/Fe]. Right: Solid and dashed lines show the predicted evolution with halo mix plus pure s-process material; the dot-dash line indicates 95% s-process. Our data indicate s-process dominated after [Fe/H]∼−0.6. Composition of the Sagittarius Dwarf Galaxy 237 Fig. 2. Enhancements of [La/Y], by ∼0.5 dex, implicate metal-poor AGB s-process. Fig. 3. Depletions of O, Al and Na suggest enhanced type Ia supernova contribution. Fig. 4. Mn and Cu depletions indicate enhancement of low-Z SNIa synthesis products.