The nature of magnetic activity in lower main sequence stars.

Abstract

The chromospheric-coronal emission of lower main sequence single and binary stars is correlated with an activity parameter of the form R = g(B-V)P^-1, where P is the rotation period and g(B-V) is a function of the colour resembling the convective turnover time. A more general form of R, including also gravity, is suggested. The functional dependence on R becomes steeper when the emitting plasma temperature increases, and a saturation is clearly seen at R ≈ 3. The results are interpreted in the framework of a hypothetical two-component stellar surface (quiet and active regions) with static coronal loops, heated by the twisting of magnetic flux tubes. The loop pressure, magnetic field and twisting velocity depend more weakly on R. Contact binaries have smaller twisting velocities by a factor of 2 - 5 than other stars with the same R. The results suggest that the dynamo action operates deep in the convective shell and is not much influenced by binary effects.