The early days of the Sculptor dwarf spheroidal galaxy^⋆

¹ Laboratoire d’astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Versoix, Switzerland
e-mail: pascale.jablonka@epfl.ch
² GEPI, Observatoire de Paris, CNRS, Université de Paris Diderot, 92195 Meudon Cedex, France
³ Institute of Astronomy, Russian Academy of Sciences, 119017 Moscow, Russia
⁴ Department Cassiopee, University of Nice Sophia-Antipolis, Observatoire de Côte d’Azur, CNRS, 06304 Nice Cedex 4, France
⁵ McDonald Observatory, University of Texas, Fort Davis, TX 79734, USA
⁶ Department of Physics and Astronomy, University of Victoria, PO Box 3055 STN CSC, Victoria, BC V8W 3P6, Canada
⁷ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700AV Groningen, The Netherlands
¹⁰ Instituto de Astrofisica de Canarias, calle via Lactea s/n, 38205 San Cristobal de La Laguna, Tenerife, Spain
¹¹ Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹² Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8P 1A1, Canada
¹³ European Southern Observatory, Karl-Schwarzschild-str. 2, 85748 Garching bei München, Germany

Received: 14 January 2015
Accepted: 24 June 2015

Abstract

We present the high-resolution spectroscopic study of five −3.9 ≤ [Fe/H] ≤ −2.5 stars in the Local Group dwarf spheroidal, Sculptor, thereby doubling the number of stars with comparable observations in this metallicity range. We carry out a detailed analysis of the chemical abundances of α, iron peak, and light and heavy elements, and draw comparisons with the Milky Way halo and the ultra-faint dwarf stellar populations. We show that the bulk of the Sculptor metal-poor stars follow the same trends in abundance ratios versus metallicity as the Milky Way stars. This suggests similar early conditions of star formation and a high degree of homogeneity of the interstellar medium. We find an outlier to this main regime, which seems to miss the products of the most massive of the Type II supernovae. In addition to its help in refining galaxy formation models, this star provides clues to the production of cobalt and zinc. Two of our sample stars have low odd-to-even barium isotope abundance ratios, suggestive of a fair proportion of s-process. We discuss the implication for the nucleosynthetic origin of the neutron capture elements.