Chromospherically young, kinematically old stars

¹ Instituto Astronômico e Geofísico (USP), Av. Miguel Stefano 4200, 04301-904 São Paulo SP, Brazil e-mail: maciel@iagusp.usp.br
² Depart. of Astronomy, University of Virginia, Charlottesville, VA 22903, USA e-mail: helio@virginia.edu
³ Laboratório Nacional de Astrofísica, CP 21, 37500-000 Itajubá MG, Brazil e-mail: bruno@lna.br

Corresponding author: H. J. Rocha-Pinto, helio@virginia.edu

Received: 3 July 2001
Accepted: 18 December 2001

Abstract

We have investigated a group of stars known to have low chromospheric ages, but high kinematical ages. Isochrone, chemical and lithium ages are estimated for them. The majority of stars in this group show lithium abundances much smaller than expected for their chromospheric ages, which is interpreted as an indication of their old age. Radial velocity measurements in the literature also show that they are not close binaries. The results suggest that they can be formed from the coalescence of short-period binaries. Coalescence rates, calculated taking into account several observational data and a maximum theoretical time scale for contact, in a short-period pair, predict a number of coalesced stars similar to what we have found in the solar neighbourhood.