Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans^⋆

¹ Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland
e-mail: pascale.jablonka@unige.ch
² GEPI, Observatoire de Paris, CNRS UMR 8111, Université Paris Diderot, 92125 Meudon Cedex, France
³ Department Cassiopée, University of Nice Sophia-Antipolis, Observatoire de Côte d’Azur, CNRS, 06304 Nice Cedex 4, France
⁴ McDonald Observatory, University of Texas, Fort Davis, TX 79734, USA
⁵ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700AV Groningen, The Netherlands
⁶ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁷ European Southern Observatory, Karl-Schwarzschild-str. 2, 85748, Garching bei München, Germany
⁸ Dept. of Physics & Astronomy, University of Victoria, 3800 Finerty Road, Victoria, BC V8P 1A1, Canada
⁹ Kavli Institute for Particle-Astrophysics and Cosmology, Stanford University, SLAC National Accelerator Laboratory, Menlo Park 94025, USA
¹⁰ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile

Received: 4 April 2010
Accepted: 19 August 2010

Abstract

We present the results of a dedicated search for extremely metal-poor stars in the Fornax, Sculptor, and Sextans dSphs. Five stars were selected from two earlier VLT/Giraffe and HET/HRS surveys and subsequently followed up at high spectroscopic resolution with VLT/UVES. All of them turned out to have [Fe/H]  ≲ −3 and three stars are below [Fe/H]  ~ −3.5. This constitutes the first evidence that the classical dSphs Fornax and Sextans join Sculptor in containing extremely metal-poor stars and suggests that all of the classical dSphs contain extremely metal-poor stars. One giant in Sculptor at [Fe/H]  = −3.96 ± 0.06 is the most metal-poor star ever observed in an external galaxy. We carried out a detailed analysis of the chemical abundances of the α, iron peak, and the heavy elements, and we performed a comparison with the Milky Way halo and the ultra faint dwarf stellar populations. Carbon, barium, and strontium show distinct features characterized by the early stages of galaxy formation and can constrain the origin of their nucleosynthesis.