Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...288..829z&link_type=abstract
Astronomy and Astrophysics 288, 829-841 (1994)
Astronomy and Astrophysics
Astrophysics
71
Sun: Abundances, Rotation, Stars: Abundances, Close Binaries, Evolution, Rotation
Scientific paper
Late-type stars possess two firmly established properties: both their rotation speed and their lithium abundance decrease with age. Quite naturally, it has been suggested that the lithium depletion is related with the loss of angular momentum. Such a causal link may just be postulated in the evolutionary calculations, and the free parameters calibrated with the observations (Endal & Sofia 1978; Pinsonneault et al. 1989). But it can also be justified on theoretical reasons, with the meridian circulation playing the major role in the transport of matter and angular momentum (Zahn 1992a). In the present paper, we seek an observational 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 experience 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 they should retain more of their original lithium. Based on the current tidal 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 occur in systems which were tidally locked on the ZAMS, i.e. for orbital periods below =~8 days for solar-type stars of Population I, and 6 days for halo stars. Our theoretical predictions are in good agreement with the latest available data (Soderblom et al. 1993; Thorburn 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 lithium 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 models of the primordial nucleosynthesis. Another implication is that magnetic torquing has played little role in the radiative interior of late-type stars, and that the core of these stars is probably rotating faster than their surface.
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