TOPoS

I. Survey design and analysis of the first sample^⋆

¹ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
e-mail: Elisabetta.Caffau@obspm.fr
² GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, place Jules Janssen, 92190 Meudon, France
³ UPJV, Université de Picardie Jules Verne, 33 rue St-Leu, 80080 Amiens, France
⁴ European Southern Observatory, 19001 Casilla, Santiago, Chile
⁵ Laboratoire Univers et Particules de Montpellier, LUPM, Université Montpellier 2, CNRS, 34095 Montpellier Cedex 5, France
⁶ Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Stra β e 2, 69120 Heidelberg, Germany
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), an der Sternwarte 16, 14482 Potsdam, Germany
⁸ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 14 August 2013
Accepted: 10 October 2013

Abstract

Context. The metal-weak tail of the metallicity distribution function (MDF) of the Galactic Halo stars contains crucial information on the formation mode of the first generation of stars. To determine this observationally, it is necessary to observe large numbers of extremely metal-poor stars.

Aims. We present here the Turn-Off Primordial Stars survey (TOPoS) that is conducted as an ESO Large Programme at the VLT. This project has four main goals: (i) to understand the formation of low-mass stars in a low-metallicity gas: determine the metal-weak tail of the halo MDF below [M/H] = −3.5; in particular, we aim at determining the critical metallicity, that is the lowest metallicity sufficient for the formation of low-mass stars; (ii) to determine in extremely metal-poor stars the relative abundances of the elements that are the signature of the massive first stars; (iii) to determine the trend of the lithium abundance at the time when the Galaxy formed; and (iv) to derive the fraction of C-enhanced extremely metal-poor stars with respect to normal extremely metal-poor stars. The large number of stars observed in the SDSS provides a good sample of candidate stars at extremely low metallicity.

Methods. Candidates with turn-off colours down to magnitude g = 20 were selected from the low-resolution spectra of SDSS by means of an automated procedure. X-Shooter has the potential of performing the necessary follow-up spectroscopy, providing accurate metallicities and abundance ratios for several key elements for these stars.

Results. We present here the stellar parameters of the first set of stars. The nineteen stars range in iron abundance between –4.1 and −2.9 dex relative to the Sun. Two stars have a high radial velocity and, according to our estimate of their kinematics, appear to be marginally bound to the Galaxy and are possibly accreted from another galaxy.