Shock heating in inactive M dwarf atmospheres

Astronomy and Astrophysics – Astronomy

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Scientific paper

Surface fluxes of MgII and Ly-alpha have recently been reported for a sample of M dwarfs. We have developed a model to determine if the minimum (basal) fluxes in stars with effective temperatures 3000-4000 K can be understood in terms of shock dissipation. Using a prescribed input of mechanical energy at the lower boundary, the hydrodynamic equations, including radiative and conductive losses, are evolved to a (nearly) stationary state. Radiative losses and/or gains in the photosphere are handled using opacities tabulated by Kurucz. In the chromosphere, radiative losses are treated by solving the transfer equation for the two strongest lines emitted by the chromosphere: MgII and Ly-alpha . We find that the models are successful in reproducing the lower envelope of the observed Ly-alpha versus MgII fluxes. In M dwarfs, the transmission of acoustic power upwards into the corona is expected to be more efficient than in the Sun: using plausible choices for the coronal transmission parameters, we have explored the possibility that the X-ray fluxes observed in the least active M dwarfs might also be due largely to acoustic heating.

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