2. Observational overview
Recently, more accurate measurements of in F, G and K dwarfs in young open clusters became available. Among others, Soderblom et al. (1990, 1993a,b) Balachandran et al. (1988, 1996), Boesgaard (1987) and Boesgaard & Budge (1988), García Lopez et al. (1994), in a series of papers compared Li depletion in pre-MS low mass stars in Per, Hyades, Praesepe and Pleiades. These clusters have nearly solar metallicity (Hyades and Praesepe have somewhat larger Z, and the Pleiades perhaps slightly lower -Boesgaard & Friel 1990) and ages ranging from Myr up to Myr (Henry et al. 1977, Boesgard & Budge 1988, Cayrel de Strobel 1990, Meynet et al. 1993). The results of these observations can be summarized as follows:
- stars with do not experience any pre-MS Li-depletion. They reach the MS with , which is generally regarded as the initial Li-content for pop I compositions. Stars in the range reach the MS with little or no Li-depletion.
- stars with show evidence of Li-depletion during pre-MS. It is possible to fix a reasonably tight MS vs. relation, with a small spread around the mean values.
- the picture completely changes for , where the observed spread increases up to one order of magnitude or more. Only the Hyades preserve a tighter correlation for the MS - abundance.
- the situation is even more dramatic for masses , where the spread can grow up to two orders of magnitude. Of course, one can not exclude that at least a fraction of this spread is of "observational" origin (namely it depends on observational scatter and/or on the interpretation of the observed equivalent widths in terms of abundances). Also note that, for , all the theoretical models foresee a complete Li-depletion in pre-MS, at variance with observations.
It is relevant to stress that the observed correlations vs. for both Per ( Myr) and Pleiades ( Myr) are very similar to one another. Since these two clusters have different ages, we can hypothesize that depletion for these stars is largely determined by their pre-MS history, otherwise an age effect should be present too.
Also, given a value of (or, equivalently, of MS mass), an inspection of the run of versus the rotational velocity (, being i the unknown inclination of the rotation axis) shows that in most cases stars with lower rotational velocities have larger Li-depletion (Soderblom et al. 1993a). Indeed, Butler et al. (1987) first noticed that Li-rich stars within the Pleiades appeared to be rapid rotators, with rotational rates up to 100 times the solar one.
Insight on the problem could finally benefit also from results for locked binaries, whose low mass components generally present larger Li-abundances than single stars of same spectral type (and probably mass): see, e.g. Ryan & Deliyannis (1995), Barrado y Navascués & Stauffer (1996).
© European Southern Observatory (ESO) 1998
Online publication: March 3, 1998