Chemical composition of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy ^*,**

¹ National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan e-mail: [aoki.wako;arimoto.n]@nao.ac.jp
² Department of Astronomical Science, Graduate University of Advanced Studies, Mitaka, Tokyo 181-8588, Japan
³ Astronomical Institute, Osaka Kyoiku University, Asahigaoka, Kashiwara, Osaka 582-8582, Japan e-mail: sadakane@cc.osaka-kyoiku.ac.jp
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: [etolstoy;ahelmi]@astro.rug.nl
⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: [gbattagl;fprimas]@eso.org
⁶ Observatoire de Genéve, Laboratoire d'Astrophysique de l'École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland e-mail: Pascale.Jablonka@obs.unige.ch; martin.tafelmeyer@epfl.ch
⁷ University of Texas, McDonald Observatory, HC75 Box 1337-McD, Fort Davis, TX 79734, USA e-mail: shetrone@astro.as.utexas.edu
⁸ California Institute of Technology, Pasadena, CA 91125, USA e-mail: bruno@astro.caltech.edu
⁹ Institute of Astronomy, Madingley Road, Cambridge CB03 0HA, UK e-mail: mike@ast.cam.ac.uk
¹⁰ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France e-mail: [Vanessa.Hill;carine.babusiaux]@obspm.fr
¹¹ Observatoire de Paris-Meudon, GEPI, 61 avenue de l'Observatoire, 75014 Paris, France e-mail: patrick.Francois@obspm.fr
¹² Department of Physics and Astronomy, University of Victoria, Elliott Building, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada e-mail: kvenn@uvic.ca
¹³ European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile e-mail: [akaufer;tszeifer]@eso.org

Received: 27 February 2009
Accepted: 22 April 2009

Abstract

Context. Individual stars in dwarf spheroidal galaxies around the Milky Way Galaxy have been studied both photometrically and spectroscopically. Extremely metal-poor stars among them are very valuable because they should record the early enrichment in the Local Group. However, our understanding of these stars is very limited because detailed chemical abundance measurements are needed from high resolution spectroscopy.

Aims. To constrain the formation and chemical evolution of dwarf galaxies, metallicity and chemical composition of extremely metal-poor stars are investigated.

Methods. Chemical abundances of six extremely metal-poor ([Fe/H] < -2.5) stars in the Sextans dwarf spheroidal galaxy are determined based on high resolution spectroscopy () with the Subaru Telescope High Dispersion Spectrograph.

Results. (1) The Fe abundances derived from the high resolution spectra are in good agreement with the metallicity estimated from the Ca triplet lines in low resolution spectra. The lack of stars with [Fe/H]  -3 in Sextans, found by previous estimates from the Ca triplet, is confirmed by our measurements, although we note that high resolution spectroscopy for a larger sample of stars will be necessary to estimate the true fraction of stars with such low metallicity. (2) While one object shows an overabundance of Mg (similar to Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar to the solar value. This is the first time that low Mg/Fe ratios at such low metallicities have been found in a dwarf spheroidal galaxy. No evidence for over-abundances of Ca and Ti are found in these five stars, though the measurements for these elements are less certain. Possible mechanisms to produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are discussed. (3) Ba is under-abundant in four objects, while the remaining two stars exhibit large and moderate excesses of this element. The abundance distribution of Ba in this galaxy is similar to that in the Galactic halo, indicating that the enrichment of heavy elements, probably by the r-process, started at metallicities [Fe/H] ≤ -2.5, as found in the Galactic halo.