Letter to the Editor

Low-mass lithium-rich AGB stars in the Galactic bulge: evidence for cool bottom processing?^*

¹ Department of Astronomy, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria e-mail: [uttenthaler;lebzelter;aringer;lederer]@astro.univie.ac.at
² Department of Physics, University of Perugia, via A. Pascoli 1, 06123 Perugia, Italy e-mail: [palmerini;busso]@fisica.unipg.it

Received: 14 May 2007
Accepted: 26 June 2007

Abstract

Context.The stellar production of the light element lithium is still a matter of debate.

Aims.We report the detection of low-mass, Li-rich Asymptotic Giant Branch (AGB) stars located in the Galactic bulge.

Methods.A homogeneous and well-selected sample of low mass, oxygen-rich AGB stars in the Galactic bulge has been searched for the absorption lines of Li. Using spectral synthesis techniques, we determine from high resolution UVES/VLT spectra the Li abundance in four out of 27 sample stars, and an upper limit for the remaining stars.

Results.Two stars in our sample have a solar Li abundance or above; these stars seem to be a novelty, since they do not show any s-element enhancement. Two more stars have a Li abundance slightly below solar; these stars do show s-element enhancement in their spectra. Different scenarios which lead to an increased Li surface abundance in AGB stars are discussed.

Conclusions.Of the different enrichment scenarios presented, Cool Bottom Processing (CBP) is the most likely one for the Li-rich objects identified here. Self-enrichment by Hot Bottom Burning (HBB) seems very unlikely as all Li-rich stars are below the HBB mass limit. Also, the ingestion of a low mass companion into the stars' envelope is unlikely because the associated additional effects are lacking. Mass transfer from a former massive binary companion is a possible scenario, if the companion produced little s-process elements. A simple theoretical estimation for the Li abundance due to CBP is presented and compared to the observed values.