A high-resolution VLT/FLAMES study of individual stars in the centre of the Fornax dwarf spheroidal galaxy^⋆,⋆⋆

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700AV Groningen, The Netherlands
² California Institute of Technology, 1200E. California Blvd, MC105-24, Pasadena, CA 91125, USA
³ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
⁴ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 92125 Meudon Cedex, France
⁵ Université de Nice Sophia-Antipolis, CNRS, Observatoire de Côte d’Azur, Laboratoire Cassiopée, 06304 Nice Cedex 4, France
e-mail: vanessa.hill@oca.eu
⁶ Observatoire de Genève, University of Geneva, 1290 Sauverny, Switzerland
⁷ McDonald Observatory, University of Texas, Fort Davis, TX 79734, USA
⁸ Dept. of Physics & Astronomy, University of Victoria, 3800 Finerty Road, Victoria, BC V8P 1A1, Canada
⁹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁰ European Southern Observatory, Karl-Schwarzschild-str. 2, 85748 Garching bei München, Germany
¹¹ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
¹² National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹³ Astronomical Institute, Osaka Kyoiku University, Asahigaoka, Kashiwara, Osaka 582-8582, Japan

Received: 6 October 2009
Accepted: 6 July 2010

Abstract

For the first time we show the detailed, late-stage, chemical evolution history of a small nearby dwarf spheroidal galaxy in the Local Group. We present the results of a high-resolution (R ~ 20000, λ = 5340–5620; 6120–6701) FLAMES/GIRAFFE abundance study at ESO/VLT of 81 photometrically selected, red giant branch stars in the central 25′ of the Fornax dwarf spheroidal galaxy. We also carried out a detailed comparison of the effects of recent developments in abundance analysis (e.g., spherical models vs. plane-parallel) and the automation that is required to efficiently deal with such large data sets. We present abundances of α-elements (Mg, Si, Ca, and Ti), iron-peak elements (Fe, Ni, and Cr), and heavy elements (Y, Ba, La, Nd, and Eu). Our sample was randomly selected and is clearly dominated by the younger and more metal-rich component of Fornax, which represents the major fraction of stars in the central region. This means that the majority of our stars are 1−4 Gyr old, and thus represent the end phase of chemical evolution in this system. Our sample of stars has unusually low [α/Fe], [Ni/Fe], and [Na/Fe] compared to the Milky Way stellar populations at the same [Fe/H]. The particularly important role of stellar winds from low-metallicity AGB stars in the creation of s-process elements is clearly seen from the high [Ba/Y]. Furthermore, we present evidence of an s-processorigin of Eu.