Lithium-rich giants in the Galactic thick disk

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 12 C/ 13 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 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 12 C/ 13 C isotopic ratio, close to the cool bottom processing predictions.

Searching for Li-rich giants in a sample of 12 open clusters

“…This last suggestion is supported by the asteroseismic analysis of a red giant burning He in its core (Silva Aguirre et al 2014) and by Li-rich red clump stars discovered by Monaco et al (2014) and Strassmeier et al (2015). Finally, other recent works report Li-rich stars all along the RGB challenging the idea that Li production is only possible at certain evolutionary phases (Gonzalez et al 2009;Monaco et al 2011;Lebzelter et al 2012).…”

“…We note that using NLTE abundances would produce the same trend, although the slope would be slightly less steep. A similar behaviour is found in Gonzalez et al (2009) and Monaco et al (2011). However, to disentangle the possible causes for Li depletion we have to compare stars in the same evolutionary stage (see discussion below).…”

A super lithium-rich red-clump star in the open cluster Trumpler 5

“…These initial parameters were then refined following the procedure outlined in Monaco et al (2011). Elemental abundances were then derived from line equivalent widths or by spectrosynthesis as required, and are reported in Table A.2 (see Monaco et al 2011;Villanova & Geisler 2011). Carbon, nitrogen, and oxygen abundances as well as the 12 C/ 13 C carbon isotopic ratio, were determined from the MIKE spectrum for Star #3416 alone.…”

“…The stellar surface gravity (log g) was derived by comparison with theoretical isochrones from the Girardi et al (2002) collection with a Z = 0.008 metallicity (Piatti et al 2004). These initial parameters were then refined following the procedure outlined in Monaco et al (2011). Elemental abundances were then derived from line equivalent widths or by spectrosynthesis as required, and are reported in Table A.2 (see Monaco et al 2011;Villanova & Geisler 2011).…”

“…Early indications supported the red giant branch luminosity bump for low-mass stars and the asymptotic giant branch clump for intermediate-mass stars, as the preferred evolutionary stages for lithium production Article published by EDP Sciences L6, page 1 of 5 (Charbonnel & Balachandran 2000). There are suggestions of possible clustering of Li-rich objects among central He-burning stars (Kumar et al 2011), while indications that there are lithium-rich stars all along the red giant branch (RGB) sequence have also emerged (Monaco et al 2011). Most of these studies were, however, limited by the lack of a proper evolutionary status determination.…”