Thomas Lebzelter, Stefan Uttenthaler, Maurizio Busso, Mathias Schultheis, Bernhard Aringer
Context: A small number of K-type giants on the red giant branch (RGB) is
known to be very rich in lithium (Li). This fact is not accounted for by
standard stellar evolution theory. The exact phase and mechanism of Li
enrichment is still a matter of debate. Aims: Our goal is to probe the
abundance of Li along the RGB, from its base to the tip, to confine Li-rich
phases that are supposed to occur on the RGB. Methods: For this end, we
obtained medium-resolution spectra with the FLAMES spectrograph at the VLT in
GIRAFFE mode for a large sample of 401 low-mass RGB stars located in the
Galactic bulge. The Li abundance was measured in the stars with a detectable Li
670.8 nm line by means of spectral synthesis with COMARCS model atmospheres.
Results: Thirty-one stars with a detectable Li line were identified, three of
which are Li-rich according to the usual criterion ($\log\epsilon({\rm
Li})>1.5$). The stars are distributed all along the RGB, not concentrated in
any particular phase of the red giant evolution (e.g. the luminosity bump or
the red clump). The three Li-rich stars are clearly brighter than the
luminosity bump and red clump, and do not show any signs of enhanced mass loss.
Conclusions: We conclude that the Li enrichment mechanism cannot be restricted
to a clearly defined phase of the RGB evolution of low-mass stars
($M\sim1M_{\sun}$), contrary to earlier suggestions from disk field stars.
View original:
http://arxiv.org/abs/1111.3572
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