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Astron. Astrophys. 317, 701-706 (1997)

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1. Introduction

It has long been known that in late-type giant stars both their rotational velocity and their Lithium abundance decrease with age. In fact, the abundance of Lithium in late-type giants is known to depend strongly on the degree of convective dilution as the star evolves to the red giant branch. Herbig (1965) has suggested that Lithium depletion occurs as a consequence of the convective transportation of Lithium-rich surface material to hotter stellar interiors where Lithium is rapidly destroyed by the reaction 7 Li(p, [FORMULA])2 He. Iben (1967a, b) was able to predict the amount of Lithium depletion and dilution in low-mass stars from the stellar evolution theory by using evolutionary models of 1.0-1.5 [FORMULA] main sequence, subgiant and giant stars. Nevertheless, the physical mechanism responsible for Lithium depletion is not yet well established.

If Lithium abundance is related to age, one should expect some correlation between such parameter and rotational velocity at least for stars of similar spectral type (Skumanich 1972). In this sense, Zahn (1992) and Pinsonneault et al. (1989, 1990) have postulated, following different approaches, that the depletion of Lithium in single late-type stars is directly related to the loss of angular momentum. Concerning specifically the subgiant stars, different authors have reported a gradual decrease of Lithium abundances toward later spectral types (e.g.: Duncan 1981; Pallavicini et al. 1987). Such behavior seems to reflect the Lithium depletion process due to the increase of the convective envelope depth, which produces a more efficient convective transport of Lithium-rich material from the surface to the hotter stellar interior. Further, such class of luminosity presents a broad range of Lithium abundances for stars with similar effective temperatures. Such feature is up to now, not completely understood. Several authors consider that this spread of Lithium abundances is related to a spread of ages (Herbig, 1965; Duncan 1981; Soderblom 1983), whereas other authors (Pallavicini et al. 1987; Spite & Spite 1982; Randich et al. 1994) ascribe this spread to other parameters such as metallicity, activity and mass. In addition to the convective mixing, other nonstandard effects have been proposed to affect the Lithium abundances in the subgiant branch. Among such effects one can quote rotational-induced mixing (e.g.: Pinsonneault et al. 1989; Charbonnel & Vauclair 1992), convective overshoot (e.g.: D'Antona & Mazzitelli 1984), mass loss (e.g.: Hobbs et al. 1989; Boothroyd et al. 1991) and gravitational settling (e.g.: Michaud 1986).

The link between rotation and Lithium abundance for subgiant stars is not yet well established. In general, the dearth of rotational velocities, in particular for the generally slowly rotating G and K subgiants has prevented a solid analysis on the relationship between rotation and Lithium abundance in this class of luminosity. Moreover, what has not been determined so far is whether Lithium abundance follows the behavior of the rotation in the spectral region of the rotational discontinuity near the spectral type F8IV (Gray & Nagar 1985; De Medeiros & Mayor 1989; De Medeiros 1990). A cut-off in the distribution of the rotational velocity for subgiant star is very well defined at (B-V) near 0.55, which corresponds to the spectral type F8IV. Essentially, if one puts aside the synchronized binary systems, all subgiants located to the right of the rotational discontinuity are slow rotators. On the left side of the discontinuity there is a large spread of Vsini values, with rotational velocities scattered over a range of about 2.0 km.s-1 to about 150.0 km.s-1. Whereas the most plausible explanation for the rotational discontinuity seems to be a strong magnetic braking (Gray & Nagar 1985; Rutten & Pylyser 1988; De Medeiros & Mayor 1989; De Medeiros 1990), the bahavior of the rotational velocity to the right of such discontinuity seems to be controlled by the increasing of the moment of inertia, as result of the increasing in stellar radius and change in the internal radial distribution of mass, associated with angular momentum loss via a magnetized wind as stars cross the subgiant branch (Pinsonneault et al. 1989; De Medeiros 1990). More recently, Lebre et al. (1995) have claimed a discontinuity in the abundances of Lithium of single subgiant stars near the spectral type F8IV. A study on the effects of enhanced rotation on the process of convective mixing of Lithium in RSCVn binaries and related chromospherically active stars was addressed in a remarkable series of papers by Pallavicini et al. (1992, 1993) and Randich et al. (1993, 1994). These authors have shown that the Lithium abundance is poorly correlated with rotation and chromospheric emission, suggesting that Lithium abundance among chromospherically active evolved stars may not be directly related to chromospheric activity. In these later studies one finds 22 subgiants, essentially binary stars.

In the present work, we address specifically the question of the link between rotation and Lithium abundance in single and binary subgiant stars by using a sample of stars larger than those studied previously, and for which we have now precise rotational velocity. In particular, we look for the link between the rotational discontinuity and the decline in Lithium abundance. In Sect. 2, we present the observational data. In Sect. 3, we present and discuss our results and finally, a summary is given in Sect. 4.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998