Chemical abundances of RGB-Tip stars in the Sagittarius dwarf Spheroidal galaxy

We present preliminary iron abundances and α element (Ca, Mg) abundance ratios for a sample of 22 Red Giant Branch (RGB) Stars in the Sagittarius galaxy (Sgr), selected near the RGB-Tip. The sample is representative of the Sgr dominant population. The mean iron abundance is [Fe/H]=-0.49. The α element abundance ratios are slightly subsolar, in agreement with the results recently prensented by [2]. 1 Target Selection and abundance analysis The Sagittarius dwarf Spheroidal galaxy (Sgr dSph) is currently disrupting under the strain of the Milky Way (MW) tidal field. The study of the Sgr chemical composition allows us to study at the same time the star formation history of a dwarf galaxy and the relevance of the hierarchical merging process for the formation of large galaxies such as the MW. In May 2003, we obtained spectra for 24 Sgr stars using the high resolution spectrograph FLAMES-UVES@VLT. The target selection has been performed using the 2Mass infrared photometry (see, e.g., [6]) where the Sgr Red Giant Branch stands out very clearly from the contaminating MW field. In this way we selected stars belonging to the Sgr dominant population near the RGB-Tip. Such a selection turned out to be very efficient: 23 out of 24 stars are Sgr radial velocity members. Up to date we derived the iron abundance for 22 stars and alpha element abundance ratios (Mg, Ca) for 20 stars of the sample. Temperatures have been derived from the (V-I) color using the calibration of [1] and E(B-V)=0.14 [4]. Gravities were determined superposing theoretical isochrones [3] to the observed optical color magnitude diagram while microturbolent velocities have been derived minimizing the dependence of the derived abundance from the measured equivalent width. A model atmosphere has been computed for each star using the ATLAS 9 code. Equivalent widths have been measured by using the standard IRAF task SPLOT and, finally, abundances were determined using the WIDTH code. As can be seen from the metallicity distribution (see Fig. 1, upper panel), the mean metallicity of the Sgr dominant population is <[Fe/H]>=-0.49±0.19, in excellent agreement with the results derived in [5] from the magnitude of the RGB-bump and the shape of the Red Giant Branch. A component extending towards lower metallicities is also present. The α element abundance ratio (see Fig. 1, bottom panel) is slightly subsolar, <[α/Fe]>=-0.20±0.10, in agreement with [2] and, apart from a small offset, also with [7].