Increased chromospheric activity in subgiants related to evolutionary effects

Abstract

A scenario based on detailed stellar evolution computations is presented which predicts development of more intense dynamo activity with evolution of early F stars off the main sequence. On the main sequence a typical early F star rotates rapidly but has a very short convective turnover time scale; the resulting lack of rotational influence on the convection cannot support dynamo activity. The main sequence lifetime is passed in a state of inactivity with little or no rotational braking. With evolution to the subgiant domain, the convection zone deepens, the convective turnover time scale lengthens with a consequent increase of rotational influence on convection, and significant dynamo activity is expected to result. The above scenario is supported by the observations of HD 124850, an active subgiant with a surprisingly short rotation period. Indeed the mere existence of an evolved star with Prot = 7.6 days (Noyes et al. 1984) and a moderate activity level fairly well demands that such a scenario be true, since magnetic braking over less evolution has slowed many (less massive) stars of similar activity levels to much longer rotation periods.