The evolution of lithium in intermediate mass stars is a powerful probe of the convective envelope conditions. In particular, during the Asymptotic Giant Branch (AGB) phase in stars of mass large enough for the Hot Bottom Burning (HBB) process to occur, the lithium abundance has a complex behaviour, whose knowledge can shed light on the efficiency of convection and on the occurrence of convective overshooting in the preceding phases. The most suitable targets for such a study are the relatively massive and thermally pulsating (TP) AGB stars in (young) clusters, whose mass can, in principle, be derived by fitting the cluster color-magnitude diagram. The need for a relatively well populated AGB naturally favors the choice of young clusters in the Large Magellanic Cloud.
We carried out, therefore, an observing program to explore the lithium
abundance in the TP-AGB phase of stars belonging to four Large Magellanic
Cloud open clusters and whose initial mass was determined by fitting the
morphology of the color-magnitude diagram (CMD), turnoff included. We
obtained mid-resolution spectra of several stars in NGC 1866 and NGC 2031
(which have ages 
150 Myr), of one star in the younger (100 Myr old) cluster NGC 2214 and of one star in the older NGC 2107 (250 Myr) (e.g. Corsi et al. 1994; Girardi et al. 1995). The most
luminous (three) stars in NGC 1866 and (two) in NGC 2031, and the stars in the
other clusters, were selected from the list of Frogel et al. (1990, hereafter FMB90).
Being these also the latest spectral type clusters' stars, they
are good TP-AGBs candidates. Additional stars were selected in NGC 1866 and
NGC 2031, as good candidates for the "early-AGB'' phase of evolution on the
base of our own near IR photometry.
We looked for and derived the strength of the lithium line at
Å, and explored its dependence on the AGB luminosity and on the
cluster age. The observed spectra were compared with synthetic ones, for
evaluation of the lithium abundance. Though a higher dispersion is
required for a precise abundance determination, our mid-resolution spectra
already provide interesting results.
In the next sections we present the theoretical background of our project (Sect. 2), the criteria for cluster and target selection (Sect. 3), the observations and data reduction (Sect. 4), the analysis of the lithium abundance (Sect. 5) and its theoretical implications (Sect. 6), the analysis of the evolutionary stage represented by the selected stars (Sect. 7), and the final discussion and conclusions (Sect. 8).
Copyright ESO 2002