Lithium in population I subgiants

We present a lithium survey for a sample of 91 Pop. I stars. JHKL photometr y was also obtained for 61 stars in the sample. Besides Li abundances, [Fe/ H] values were derived. Thanks to Hipparcos parallaxes, we could infer abso lute V magnitudes for our sample stars and were able to place them on the c olor-magnitude diagram, which allowed us to constrain their evolutionary st atus. Masses and ages were derived for most of the stars by comparison with evolutionary tracks. The sample was originally selected so to include clas s IV stars later than spectral-type F0, but, based on the location on the c olor-magnitude diagram, we found a posteriori that a fraction of the stars (about 20%) are either main sequence stars or evolved giants. As it is the case for dwarfs and giants, a large spread in lithium abundanc e is present among the subgiants in our sample. As expected, the average li thium decreases as the stars evolve along the subgiant branch; however, the re is not a one-to-one relationship between the position on the color-magni tude diagram and lithium abundance, and the observed dispersion is only par tially explainable as due to a dispersion in mass, metallicity, and age. In particular; a dispersion in lithium is seen among slightly evolved subgian ts with masses close to solar but in the same evolutionary stage as the G2 IV star beta Hyi. The comparison of the beta Hyi-like sample with a sample of non evolved solar-like stars indeed suggests that beta Hyi has most like ly evolved from a main sequence Li-rich star, rather than from a Li-poor st ar (like the Sun) that has dredged-up previously stored lithium. Our sample includes several stars that have completed the first-dredge up l ithium dilution, but that have not yet evolved to thr evolutionary point wh ere extra-mixing in the giant phase is thought to occur. A large number of them have Li abundances considerably below the theoretical predictions of f irst dredge-up dilution. We confirm that this is due to the fact that the p rogenitors of these stars are most likely stars that have depleted lithium while on the main sequence; the fraction of post-dredge up Li rich/poor sta rs, in fact, is consistent with the observed distribution of Li abundances among stars that have just left the main sequence. The signature of the second mixing (or RGB extra-mixing) episode is evident in the log n(Li) vs. B-V and log n(Li) vs. M-bol distributions of the star s in the sample; it seems however that the extra-mixing occurs at luminosit ies lower than predicted by the models of Charbonnel (1994). Finally, a few evolved giants are found that should have passed the second mixing episode, but that do not show signs of it. At least half of them are spectroscopic binaries.