ROTATION AND LITHIUM DEPLETION IN LATE-TYPE BINARIES

Late-type stars possess two firmly established properties: both their rotation speed and their lithium abundance decrease with age. Quite na turally, it has been suggested that the lithium depletion is related w ith the loss of angular momentum. Such a causal link may just be postu lated in the evolutionary calculations, and the free parameters calibr ated with the observations (Endal & Sofia 1978; Pinsonneault et al. 19 89). But it can also be justified on theoretical reasons, with the mer idian circulation playing the major role in the transport of matter an d angular momentum (Zahn 1992a). In the present paper, we seek an obse rvational confirmation of this link in the behavior of close binaries. In addition to the torque exerted on them by the stellar wind, which is responsible for the spin-down of single stars, binary stars experie nce a tidal torque, which tends to synchronize their rotation with the orbital motion. As a result, they exchange lesser amounts of angular momentum than single stars of the same mass and age, and therefore the y should retain more of their original lithium. Based on the current t idal theory (Zahn 1989), we calculate the dynamical evolution of close binaries, and infer from it the differences in lithium abundance one should expect between them and single stars. Underdepletion should occ ur in systems which were tidally locked on the ZAMS, i.e. for orbital periods below almost-equal-to 8 days for solar-type stars of Populatio n I, and 6 days for halo stars. Our theoretical predictions are in goo d agreement with the latest available data (Soderblom et al. 1993; Tho rburn et al. 1993; Spite et al. 1994), and we take this as a proof for the existence of a physical link between the depletion of lithium and the loss of angular momentum. We conclude that the abundance of lithi um measured in old disk and halo stars by Spite & Spite (1982) is less than the original one, and cannot be taken as such to constrain the m odels of the primordial nucleosynthesis. Another implication is that m agnetic torquing has played little role in the radiative interior of l ate-type stars, and that the core of these stars is probably rotating faster than their surface.