Precise rotational velocities have been determined for 66 subgiant stars by using the CORAVEL spectrometer. From these data, one sees clearly the rotational discontinuity at nearly (B-V) 0.55, which corresponds to the spectral type F8IV. To the right of this discontinuity, only synchronized binary systems, typically those with orbital period lower than about 20 days and eccentricity lower than about 0.10, show enhanced rotation. These measurements of rotational velocity offer strong support to a detailed analysis of the link between rotation and Lithium abundance in subgiant stars. Based on this sample, we have found a clear dependence of Lithium abundance upon rotation for single subgiants. For the binary systems we have found that Lithium abundance is independent of rotational velocity. There is a very good coincidence between the location of the rotational discontinuity and the drop in Lithium abundance. By considering that magnetic braking plays a relevant role as the root cause of the rotational discontinuity, this fact might indicate that Lithium abundance is also affected by magnetic braking. Binary systems with orbital period lower than 20 days and circularized orbit, located to the right of the drop in Lithium, seem to retain more of their original Lithium than the binary systems with orbital period greater than about 20 days and non circularized orbit, and also more than their single subgiant counterparts.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998