Late main-sequence evolution of lithium and beryllium

¹ Institut d'Astronomie et d'Astrophysique, ULB, CP226, 1050 Brussels, Belgium e-mail: piau@astro.ulb.ac.be
² INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: randich@arcetri.astro.it;palla@arcetri.astro.it

Received: 26 December 2002
Accepted: 4 June 2003

Abstract

We examine the effects of the tachocline diffusion process on the surface abundances of light elements in solar-like stars (). Acting during main sequence evolution, the tachocline diffusion can account for the gradual decrease in lithium surface abundance while preserving beryllium, in agreement with the most recent observational data for open cluster stars older than the Hyades (~600 Myr). We show that helioseismology and observations of surface rotation demand a nearly solid-body rotation of solar analogs after ≈1 Gyr. By then, these stars have become slow rotators and lost most of their initial angular momentum. We argue that mixing due to angular momentum loss does not appear to be a viable mechanism to account for the observed abundances.
Using the results of helioseismology and of the observations of solar lithium abundance, we calibrate the two parameters of the tachocline mixing, the Brunt-Väisälä frequency and the tachocline thickness. We then evolve stellar models starting at the Hyades age and compare the results of ⁷Li and ⁹Be abundances to the observations of M 67 (~4.5 Gyr). Finally, we investigate the effects of slight differences in specific metal abundance ratios on the tachocline diffusion. Because of opacity effects, we find differences in the amount of lithium depletion of ~0.5 dex for solar effective temperature and iron abundance at the age of M 67 that can partly explain the observed lithium spread in this cluster.