A&A 384, 912–924 (2002) Astronomy DOI: 10.1051/0004-6361:20011815 & c ESO 2002 Astrophysics Chromospherically young, kinematically old stars H. J. Rocha-Pinto1,3,B.V.Castilho2, and W. J. Maciel1 1 Instituto Astronˆomico e Geof´ısico (USP), Av. Miguel Stefano 4200, 04301-904 S˜ao Paulo SP, Brazil e-mail:
[email protected] 2 Laborat´orio Nacional de Astrof´ısica, CP 21, 37500-000 Itajub´a MG, Brazil e-mail:
[email protected] 3 Depart. of Astronomy, University of Virginia, Charlottesville, VA 22903, USA e-mail:
[email protected] 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. Key words. stars: late-type – stars: chromospheres – Galaxy: evolution 1. Introduction investigate the chromospheric activity in single stars. Due to this, the division of these surveys into two classes, of The chromospheric activity of a late-type star is frequently active and inactive stars, corresponds closely to an age interpreted as a sign of its youth.