Coronal Heating in dMe and dM Stars: Clues from the X-Ray Surface Fluxes

Astronomy and Astrophysics – Astronomy

Scientific paper

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Stars: Emission-Line, Be, Stars: Coronae, Stars: Late-Type, X-Rays: Stars

Scientific paper

The mean X-ray surface fluxes Fx from a nearly complete sample of M dwarfs within 6-7 pc of the Sun fall into two groups: most dMe stars have Fx above a critical value Fc, while most dM stars have Fx below the same Fc. The formal value of Fc, where the distribution functions of dMe and dM stars are maximally different, is ≍ 105.3-105.4 ergs cm-2 s-1. The value of Fc is close to an upper limit computed by Mullan & Cheng for a model of acoustically heated coronae in an M dwarf (Fac ≍ 105.1 cgs). These results provide quantitative support for the hypothesis that the coronal heating process in dMe stars cannot be predominantly acoustic in nature.
A Monte Carlo analysis of the X-ray data suggests that X- active regions on dMe stars occupy at least 45%-50% of the surface, while, on dM stars the X- active regions occupy no more than 10% of the surface. This analysis leads to the conclusion that quiet coronal regions in both dM and dMe stars may be heated acoustically.

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