Lithium abundances in a flux-limited sample of galactic carbon stars.

Lithium abundances, or upper limits, have been derived for 161 galactic carbon stars, selected from the Two Micron Sky Survey, using high resolution, high signal to noise ratio spectra and synthetic spectrum fitting. Fourteen of these C-stars present surface lithium abundances [log ɛ(Li) ≥ 1.0] higher than theoretically expected during the AGB phase, but only three are super-rich lithium stars [log ɛ(Li) >4], which confirms the scarce number of galactic C-stars (∼2%) of this type. The average masses of the sample stars suggests the plume mixing, instead of the hot bottom burning mechanism, as the nucleosynthetic scenario for the production of lithium in these stars. However, the expected theoretical correlations among lithium abundances and ¹²C/¹³C, C/O ratios do not show up. The lithium abundance distribution of the remaining stars shows a pronounced peak at log ɛ(Li) ≈ -0.5 suggesting, when compared to those in M and SC stars, that no important lithium depletion occurs during the transition from M to C-star.