Lithium in M67: From the main sequence to the red giant branch
G. Pace, M. Castro, J. Meléndez, S. Théado, J. D. do Nascimento Jr.
Context. Lithium abundances in open clusters are a very effective probe of mixing processes, and their study can help us to understand the large depletion of lithium that occurs in the Sun. Owing to its age and metallicity, the open cluster M67 is especially interesting on this respect. Many studies of lithium abundances in M67 have been performed, but a homogeneous global analysis of lithium in stars from subsolar masses and extending to the most massive members, has yet to be accomplished for a large sample based on high-quality spectra.
Aims. We test our non-standard models, which were calibrated using the Sun with observational data.
Methods. We collect literature data to analyze, for the first time in a homogeneous way, the non-local thermal equilibrium lithium abundances of all observed single stars in M67 more massive than ~ 0.9 M☉. Our grid of evolutionary models is computed assuming a non-standard mixing at metallicity [Fe/H] = 0.01, using the Toulouse-Geneva evolution code. Our analysis starts from the entrance in the zero-age main-sequence.
Results. Lithium in M67 is a tight function of mass for stars more massive than the Sun, apart from a few outliers. A plateau in lithium abundances is observed for turn-off stars. Both less massive (M ≤ 1.10 M☉) and more massive (M ≥ 1.28 M☉) stars are more depleted than those in the plateau. There is a significant scatter in lithium abundances for any given mass M ≤ 1.1 M☉.
Conclusions. Our models qualitatively reproduce most of the features described above, although the predicted depletion of lithium is 0.45 dex smaller than observed for masses in the plateau region, i.e. between 1.1 and 1.28 solar masses. More work is clearly needed to accurately reproduce the observations. Despite hints that chromospheric activity and rotation play a role in lithium depletion, no firm conclusion can be drawn with the presently available data.
Astronomy and Astrophysics (Accepted)