In his classical work, Skumanich (1972) showed that chromospheric activity, rotational velocity and lithium abundance in solar-type stars decreases with age. Subsequent work (e.g. Pallavicini et al. 1981, Noyes et al. 1984) demonstrated that the relationship between surface activity and age is indirect. Activity depends on stellar rotational velocity, and the relationship with age reflects the evolution of stellar rotation during main sequence lifetime: due to magnetic spin down, young single stars rotate faster than old ones and thus have a higher level of activity.
The dependence of Li abundance upon age is more intriguing. Since Li is easily destroyed in stellar interiors at temperatures above K, depletion during main-sequence lifetime (and for the coolest stars also during pre-main sequence evolution) is expected for those stars with convective zones deep enough to mix surface material down the point where Li is burned. However the observations are in disagreement with the predictions of standard models even for the best studied and "well behaved" open cluster, the Hyades, unless ad hoc assumptions are made (Swenson et al. 1994).
Li observations of Pop I solar-type stars in the field gave more puzzling results: Duncan (1981), using Ca II emission as a proxy for age, pointed out the presence of stars with low chromospheric activity but high Li abundance. Pallavicini et al. (1987), using a smaller sample but higher quality data, confirmed Duncan's results, finding in addition that, whereas a general trend exists between chromospheric activity and stellar rotation, the same is not true for Li abundances, indicating that the problem is indeed with Li. In a detailed analysis of the Li-rich low-activity subgiant Hyi, Dravins et al. (1993) suggested that lithium could be stored during main sequence evolution just below the convective zone, rather than destroyed, and that this Li-rich material could be brought back to the surface when a star evolves off the main sequence. In this interpretation, however, high Li abundances in old solar-type stars should be found only among evolved stars.
Pasquini et al. (1994), analyzing high quality data for a volume limited sample of nearby G dwarfs, showed that a large dispersion in Li abundances is present among stars otherwise similar to the Sun, and that this spread does not correlate with any of the measurable stellar parameters like absolute magnitude, effective temperature, and metallicity. This study also pointed out that old Li-rich stars are quite common and not limited to a few, exceptional cases.
To conclude, the question arises whether the Sun, which had an initial abundance N(Li) = 3.31 (in the usual logarithmic scale where N(H) = 12.0) and a present abundance N(Li) = 1.16 (i.e., a factor 100 lower, Grevesse et al. 1996) should be regarded as the 'standard' for stars of its mass, age and metallicity or rather if additional depletion mechanims, not account for in the standard models, need to be considered.
The results outlined above are based on field stars, for which distances, magnitudes and ages are poorly known; clearly, observations of an open cluster with solar age would allow determining whether a spread in Li abundance really exists among solar-type stars, with interesting consequences for our understanding of the relationship between Li abundance and age.
M 67 is an ideal target for this study, because the cluster has been extensively observed for proper motions (Sanders 1977, Girard et al. 1989) and thus a reliable membership list exists; moreover, accurate CCD photometry has been obtained (Montgomery et al. 1993), and surveys for detecting the presence of binaries have been carried out (Latham et al. 1993). Finally, the cluster age (about 4.7 Gyrs) and the solar metallicity (Fe/H 0) (Janes and Phelps 1994) offer the possibility of making a direct comparison with the Sun. A few observations of Li in this cluster have been carried out (Hobbs and Pilachowski 1986, Spite et al. 1987, Garcia Lopez et al. 1988), suggesting that a spread in Li abundances may be present among solar-type stars belonging to the cluster. We present here new observations of G-type stars in the cluster which extend those carried out in the past and allow addressing the question of the Li spread on a firmer statistical basis.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998