The age-mass relation for chromospherically active binaries. 1. The evolutionary status

Abstract

In this paper we present a study of the evolutionary status of a sample of chromospherically active binary systems for which accurate determinations of their stellar parameters are available. Stellar ages have been obtained by using evolutionary tracks. The agreement between the estimates of ages for the two components of a given system is very good, which proves the reliability of our method. It has been possible to separate the chromospherically active binaries in three groups, according to the mass of the primary component: evolved stars with masses in the range 2.5-5 M_⨀ , evolved stars (subgiants) with M≃1.4 M_⨀ and main-sequence stars with M≃1.1 M_⨀. We have found a relationship between stellar masses and ages of the form Log Age = 9.883(±0.022) - 2.965(±0.122) Log (Mass/M_⨀). This relationship is very close to that for stars on the TAMS. The relation can be understood, in the framework of the evolution of the components and orbital elements of binary systems, as an effect of the increase of the stellar radius as the components evolve off the main sequence, and the decrease of the rotation period due to tidal effects which leads to enhanced chromospheric emission levels, several times higher than that of the Sun. The relationship has a dependence on rotation, due to the fact that for a given range of masses, younger stars rotate faster. We have also found that the more evolved stars are the more active, for a given interval of rotation periods.