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Astron. Astrophys. 357, 931-937 (2000)

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1. Introduction

This paper is the second in a series about the study of lithium and rotation in evolved stars based on both new high resolution spectroscopic observations and precise rotational velocities obtained with the CORAVEL spectrometer. In Lèbre et al. (1999, hereafter Paper I) we derived lithium abundances by spectral synthesis analysis and presented the observational data for a large and homogeneous sample of F, G, and K-type Population I subgiant stars. On the basis of this uniform data set, we could confirm the occurrence of the rotational discontinuity near the spectral type F8IV (Gray & Nagar 1985; De Medeiros & Mayor 1989, De Medeiros 1990), and localize a lithium drop-off around the spectral type G2IV. No clear correlation appeared between the lithium abundance and the rotational velocity (see also De Medeiros et al. 1997).

In the present work we investigate the physical processes that underline the lithium and rotational discontinuities along the subgiant branch. For this purpose we first determine the evolutionary status and individual masses of our sample stars by using the HIPPARCOS parallaxes and by comparing the observational Hertzsprung-Russell diagram with evolutionary tracks computed with the Toulouse-Geneva code (Sect. 2). This allows us to study very precisely the behaviour of the lithium abundance and of the rotational velocity as a function of effective temperature, stellar mass, metallicity and evolutionary stage. The lithium main features are discussed in Sect. 3 where we compare the observations with theoretical predictions related to the dilution mechanism (Iben 1967a,b). Finally, the connection between the observed rotation behaviour and the magnetic braking due to the deepening of the convective envelope (Gray 1981; Gray & Nagar 1985) is quantified in Sect. 4. This study, based on a close examination of the stellar parameters, sheds new light on the question of the link between rotation and lithium discontinuities in subgiant stars.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000