First stars

XIV. Sulfur abundances in extremely metal-poor stars^⋆

¹ GEPI Observatoire de Paris, CNRS, Université Paris Diderot, 92195 Meudon Cedex, France
e-mail: monique.spite@obspm.fr
² Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
³ Department of Astronomy and Astronomical Observatory, Odessa National University, and Isaac Newton Institute of Chile, Odessa branch, Shevchenko Park, 65014 Odessa, Ukraine
⁴ Las Cumbres Observatory, Goleta, CA 93117, USA
⁵ Observatoire de la Côte d’Azur, CNRS UMR6202, BP4229, 06304 Nice Cedex 4, France
⁶ GRAAL, Université de Montpellier II, 34095 Montpellier Cedex 05, France
⁷ The Niels Bohr Institute, Astronomy, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁸ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
⁹ Universidade de São Paulo, Departamento de Astronomia, Rua do Matão 1226, 05508-900 São Paulo, Brazil
¹⁰ Department of Physics & Astronomy and JINA: Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI 48824, USA
¹¹ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹² European Southern Observatory (ESO), Karl-Schwarschild-Str. 2, 85748 Garching b. München, Germany

Received: 14 October 2010
Accepted: 9 December 2010

Abstract

Context. Precise S abundances are important in the study of the early chemical evolution of the Galaxy. In particular the site of the formation remains uncertain because, at low metallicity, the trend of this α-element versus [Fe/H] remains unclear. Moreover, although sulfur is not bound significantly in dust grains in the ISM, it seems to behave differently in DLAs and old metal-poor stars.

Aims. We attempt a precise measurement of the S abundance in a sample of extremely metal-poor stars observed with the ESO VLT equipped with UVES, taking into account NLTE and 3D effects.

Methods. The NLTE profiles of the lines of multiplet 1 of S I were computed with a version of the program MULTI, including opacity sources from ATLAS9 and based on a new model atom for S. These profiles were fitted to the observed spectra.

Results. We find that sulfur in EMP stars behaves like the other α-elements, with [S/Fe] remaining approximately constant below [Fe/H] = –3. However, [S/Mg] seems to decrease slightly with increasing [Mg/H]. The overall abundance patterns of O, Na, Mg, Al, S, and K are most closely matched by the SN model yields by Heger & Woosley. The [S/Zn] ratio in EMP stars is solar, as also found in DLAs. We derive an upper limit to the sulfur abundance [S/Fe]  < +0.5 for the ultra metal-poor star CS 22949-037. This, along with a previously reported measurement of zinc, argues against the conjecture that the light-element abundance pattern of this star (and by analogy, the hyper iron-poor stars HE 0107-5240 and HE 1327-2326) would be due to dust depletion.