The extremely low-metallicity tail of the Sculptor dwarf spheroidal galaxy^⋆,⋆⋆

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV, Groningen, The Netherlands
² Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8P 1A1, Canada
e-mail: else@uvic.ca
³ CIfAR Junior Fellow and CITA National Fellow  
⁴ Laboratoire Lagrange, UMR 7293, Universit de Nice Sophia-Antipolis, CNRS, Observatoire de la Cte d’Azur, 06300 Nice, France
⁵ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁶ UPJV, Université de Picardie Jules Verne, 33 rue St. Leu, 80080 Amiens, France
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB03 0HA, UK
⁸ Laboratoire d’astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
⁹ INAF Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁰ Department of Astrophysics, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹¹ Sterrenkundig Instituut “Anton Pannekoek”, University of Amsterdam, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands

Received: 7 September 2012
Accepted: 4 November 2012

Abstract

We present abundances for seven stars in the (extremely) low-metallicity tail of the Sculptor dwarf spheroidal galaxy, from spectra taken with X-shooter on the ESO VLT. Targets were selected from the Ca II triplet (CaT) survey of the dwarf abundances and radial velocities team (DART) using the latest calibration. Of the seven extremely metal-poor candidates, five stars are confirmed to be extremely metal-poor (i.e., [Fe/H]  < −3 dex), with [Fe/H] = –3.47  ± 0.07 for our most metal-poor star. All have [Fe/H]  ≤ −2.5 dex from the measurement of individual Fe lines. These values are in agreement with the CaT predictions to within error bars. None of the seven stars is found to be carbon-rich. We estimate a 2–13% possibility of this being a pure chance effect, which could indicate a lower fraction of carbon-rich extremely metal-poor stars in Sculptor compared to the Milky Way halo. The [α/Fe] ratios show a range from +0.5 to –0.5, a larger variation than seen in Galactic samples although typically consistent within 1–2σ. One star seems mildly iron-enhanced. Our program stars show no deviations from the Galactic abundance trends in chromium and the heavy elements barium and strontium. Sodium abundances are, however, below the Galactic values for several stars. Overall, we conclude that the CaT lines are a successful metallicity indicator down to the extremely metal-poor regime and that the extremely metal-poor stars in the Sculptor dwarf galaxy are chemically more similar to their Milky Way halo equivalents than the more metal-rich population of stars.