Lithium abundances and extra mixing processes in evolved stars of M 67^⋆,⋆⋆

¹ Universidade Federal do Rio Grande do NorteDepartamento de Física, 59072-970 Natal, RN, Brazil
e-mail: brunocanto@dfte.ufrn.br
² Université Montpellier II, CNRS, UMR 5024, Groupe de Recherche en Astronomie et Astrophysique du Languedoc, Place E. Bataillon, 34095 Montpellier, France
³ Université Nice Sophia Antipolis, CNRS, UMR 6202, Observatoire de la Côte d’Azur, Laboratoire Cassiopée, BP 4229, 06304 Nice, France
⁴ European Southern Observatory, Garching, Germany

Received: 18 May 2010
Accepted: 17 December 2010

Abstract

Aims. We present a spectroscopic analysis of a sample of evolved stars in M 67 (turn-off, subgiant and giant stars) in order to bring observational constraints to evolutionary models taking into account non-standard transport processes.

Methods. We determined the stellar parameters (T_eff, log g, [Fe/H]), microturbulent and rotational velocities and, lithium abundances (A_Li) for 27 evolved stars of M 67 with the spectral synthesis method based on MARCS model atmospheres. We also computed non-standard stellar evolution models, taking into account atomic diffusion and rotation-induced transport of angular momentum and chemicals that were compared with this set of homogeneous data.

Results. The lithium abundances that we derive for the 27 stars in our sample follow a clear evolutionary pattern ranging from the turn-off to the Red Giant Branch. Our abundance determination confirms the well known decrease of lithium content for evolved stars. For the first time, we provide a consistent interpretation of both the surface rotation velocity and of the lithium abundance patterns observed in an homogeneous sample of TO and evolved stars of M 67. We show that the lithium evolution is determined by the evolution of the angular momentum through rotation-induced mixing in low-mass stars, in particular for those with initial masses larger than 1.30 M_⊙ when at solar metallicity.