The exotic chemical composition of the Sagittarius dwarf spheroidal galaxy

¹ CIFIST Marie Curie Excellence Team, France
² Observatoire de Paris, GEPI, 5 place Jules Janssen, 92195 Meudon, France e-mail: luca.sbordone@obspm.fr
³ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Rome, Italy
⁴ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
⁵ Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Rome, Italy
⁶ ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
⁷ ESO – European Southern Observatory, Alonso de Cordova, 3107 Santiago, Chile
⁸ INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padua, Italy

Received: 12 September 2006
Accepted: 29 November 2006

Abstract

Context.The Sagittarius dwarf spheroidal galaxy is the nearest neighbor of the Milky Way. Moving along a short period quasi-polar orbit within the Halo, it is being destroyed by the tidal interaction with our Galaxy, losing its stellar content along a huge stellar stream.

Aims.We study the detailed chemical composition of 12 giant stars in the Sagittarius dwarf Spheroidal main body, together with 5 more in the associated globular cluster Terzan 7, by means of high resolution VLT-UVES spectra.

Methods.Abundances are derived for up to 21 elements from O to Nd, by fitting lines EW or line profiles against ATLAS 9 model atmospheres and SYNTHE spectral syntheses calculated ad-hoc. Temperatures are derived from or colors and gravities from – ionization equilibrium.

Results.The metallicity of the observed stars is between [Fe/H] = -0.9 and 0. We detected a highly peculiar “chemical signature”, with undersolar α elements, Na, Al, Sc, V, Co, Ni, Cu, and Zn, among others, and overabundant La, Ce, and Nd. Many of these abundance ratios (in particular light-odd elements and iron peak ones) are strongly at odds with what is observed within the Milky Way, so they may be a very useful tool for recognizing populations originating within the Sagittarius dwarf. This can be clearly seen in the case of the globular Palomar 12, which is believed to have been stripped from Sagittarius: the cluster shows precisely the same chemical “oddities”, thus finally confirming its extragalactic origin.