A primordial star in the heart of the Lion^⋆

¹ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
² GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, Place Jules Janssen, 92190 Meudon, France
e-mail: Elisabetta.Caffau@obspm.fr
³ European Southern Observatory, 19001 Casilla, Santiago, Chile
⁴ UPJV, Université de Picardie Jules Verne, 33 rue St Leu, 80080 Amiens, France
⁵ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Institute of Astronomy, Russian Academy of Sciences, 119017 Moscow, Russia
⁸ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁹ Laboratoire Univers et Particules de Montpellier, LUPM, Université Montpellier 2, CNRS, 34095 Montpellier Cedex 5, France
¹⁰ Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, Laboratoire Cassiopée, BP 4229, 06304 Nice Cedex 4, France
¹¹ Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 26 December 2011
Accepted: 2 March 2012

Abstract

Context. The discovery and chemical analysis of extremely metal-poor stars permit a better understanding of the star formation of the first generation of stars and of the Universe emerging from the Big Bang.

Aims. We report the study of a primordial star situated in the centre of the constellation Leo (SDSS J102915+172927).

Methods. The star, selected from the low-resolution spectrum of the Sloan Digital Sky Survey, was observed at intermediate (with X-Shooter at VLT) and at high spectral resolution (with UVES at VLT). The stellar parameters were derived from the photometry. The standard spectroscopic analysis based on 1D ATLAS models was completed by applying 3D and non-LTE corrections.

Results. An iron abundance of  [Fe/H ]  =  −4.89 makes SDSS J102915+172927 one of the lowest [Fe/H] stars known. However, the absence of measurable C and N enhancements indicates that it has the lowest metallicity, Z ≤ 7.40 × 10^-7 (metal-mass fraction), ever detected. No oxygen measurement was possible.

Conclusions. The discovery of SDSS J102915+172927 highlights that low-mass star formation occurred at metallicities lower than previously assumed. Even lower metallicity stars may yet be discovered, with a chemical composition closer to the composition of the primordial gas and of the first supernovae.