Lithium-rich giants in the Galactic thick disk^⋆

¹ European Southern Observatory, Casilla 19001, Santiago, Chile
e-mail: lmonaco@eso.org
² Universidad de Concepción, Casilla 160-C, Concepción, Chile
³ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁴ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁵ Departamento Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860 Stgo., Chile
⁶ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic

Received: 8 December 2010
Accepted: 24 February 2011

Abstract

Context. Lithium is a fragile element, which is easily destroyed in the stellar interior. The existence of lithium-rich giants still represents a challenge for stellar evolution models.

Aims. We have collected a large database of high-resolution stellar spectra of 824 candidate thick-disk giants having 2 MASS photometry and proper motions measured by the Southern Proper-Motion Program (SPM). In order to investigate the nature of Li-rich giants, we searched this database for giants presenting a strong Li I resonance line.

Methods. We performed a chemical abundance analysis on the selected stars with the MOOG code along with proper ATLAS-9 model atmospheres. The iron content and atmospheric parameters were fixed by using the equivalent width of a sample of Fe lines. We also derive abundances for C, N, and O and measure or derive lower limits on the ¹²C/¹³C isotopic ratios, which is a sensible diagnostic of the stars evolutionary status.

Results. We detected five stars with a lithium abundance higher than 1.5, i.e. Li-rich according to the current definition. One of them (SPM-313132) has A(Li) > 3.3 and, because of this, belongs to the group of the rare super Li-rich giants. Its kinematics makes it a likely thin-disk member and its atmospheric parameters are compatible with it being a 4 M_⊙ star either on the red giant branch (RGB) or the early asymptotic giant branch. This object is the first super Li-rich giant detected at this phase. The other four are likely low-mass thick-disk stars evolved past the RGB luminosity bump, as determined from their metallicities and atmospheric parameters. The most evolved of them lies close to the RGB-tip. It has A(Li) > 2.7 and a low ¹²C/¹³C isotopic ratio, close to the cool bottom processing predictions.