Lithium-rich giants in the Galactic thick disk Article - 2011

Lorenzo Monaco, S. Villanova, C. Moni Bidin, G. Carraro, D. Geisler, Piercarlo Bonifacio, O. A. Gonzalez, M. Zoccali, L. Jilkova

Lorenzo Monaco, S. Villanova, C. Moni Bidin, G. Carraro, D. Geisler, Piercarlo Bonifacio, O. A. Gonzalez, M. Zoccali, L. Jilkova, « Lithium-rich giants in the Galactic thick disk  », Astronomy and Astrophysics - A&A, 2011, p. 90. ISSN 0004-6361

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 <SUP>12</SUP>C/<SUP>13</SUP>C 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<SUB>&sun ;</SUB> 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 <SUP>12</SUP>C/<SUP>13</SUP>C isotopic ratio, close to the cool bottom processing predictions.

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