Li-rich red giant branch stars in the Galactic bulge

Abstract

Aims. We present lithium abundance determination for a sample of K giant stars in the Galactic bulge. The stars presented here are the only 13 stars with a detectable lithium line (6767.18 Å) among ∼400 stars for which we have spectra in this wavelength range, half of them in Baade's Window (b = −4 • ) and half in a field at b = −6 • . Methods. The stars were observed with the GIRAFFE spectrograph of FLAMES mounted on VLT, with a spectral resolution of R ∼ 20 000. Abundances were derived from spectral synthesis and the results are compared with those of stars with similar parameters, but no detectable Li line. Results. We find 13 stars with a detectable Li line, among which 2 have abundances A(Li) > 2.7. No clear correlations were found between the Li abundance and those of other elements. With the exception of the two most Li rich stars, the others follow a fairly tight A(Li) − T eff correlation. Conclusions. There is strong indication of a Li production phase during the red giant branch (RGB), acting either on a very short timescale, or selectively only in some stars. That the proposed Li production phase is associated with the RGB bump cannot be excluded, although our targets are significantly brighter than the predicted RGB bump magnitude for a population at 8 kpc.

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

“…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).…”

“…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).…”

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

“…The only other detections of Li in the Galactic bulge are from observations of RGB and AGB stars (e.g., Gonzalez et al 2009) in which the atmospheric Li abundance have been altered. Dwarf A134, page 14 of 20 and subgiant stars with effective temperatures greater than about 5900 K are the only reliable tracers of Li (e.g., Weymann & Sears 1965;Boesgaard & Tripicco 1986).…”

Li-rich giants in the Galactic Bulge. Is Li linked only to evolutionary status?

“…However, in a very short stage prior to its destruction, 7 Li could be produced by Cameron-Fowler mechanism (Cameron & Fowler 1971). In complete contrast, the number of Li-rich stars not fulfilling the RGB bump connection has been recently increased (Monaco et al 2007;Uttenthaler et al 2007;Gonzalez et al 2009) requiring stellar models to explain a Li production phase almost at any instance along the giant evolutionary sequence.…”

“…In our detailed study of chemical abundances in the Galactic bulge (see Zoccali et al 2008 for a description of the entire project) we have measured Li abundances by fitting synthetic spectra to the 7 Li (6707.18Å) line for ∼ 400 giants in Baade's Window and a field at b = -6 (Gonzalez et al 2009). We have found 13 stars showing strong 7 Li lines in complete contrast to the rest of the sample for which only upper limits could be obtained.…”