S1242: a lithium-rich subgiant star in the open cluster M 67

¹ Groupe de Recherche en Astronomie et Astrophysique du Languedoc-UMR 5024/ISTEEM (CNRS), Université Montpellier II, Place E. Bataillon, 34095 Montpellier, France e-mail: lebre@graal.univ-montp2.fr
² Departamento de Física, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, RN, Brazil
³ Observatoire de la Côte d'Azur, Département Cassiopée, UMR 6202 (CNRS), BP 4229, 06304 Nice, France
⁴ European Southern Observatory, Santiago, Chile
⁵ Departamento de Astronomìa, Universidad de Chile, Casilla Postal 36D, Santiago de Chile, Chile

Received: 28 April 2005
Accepted: 29 December 2005

Abstract

Aims.We report the serendipitous discovery of a lithium-rich subgiant star, S1242. This object is member of a large eccentricity spectroscopic binary system in the solar-age open cluster M 67.

Methods.Using VLT/FLAMES-UVES observations and synthetic spectrum analysis, we derived for this star stellar parameters (temperature, gravity, metallicity and rotational velocity) and a surface Li abundance () larger than the higher abundance of M 67 stars near the turn-off.

Results.The high Li abundance we found in S1242 points to an excess of lithium at the subgiant stage. We discuss two possible explanations for this unusually high Li content in this binary system: a preservation of the Li at the turn-off stage due to tidal effects, or an efficient dredge-up of Li, hidden below the convective zone by atomic diffusion occuring in the post turn-off stage.