Increased chromospheric activity in subgiants related to evolutionary effects
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.
KeywordsRotation Period Convection Zone Main Sequence Rossby Number Dynamo Activity
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