Global trends in winds of M dwarf stars

Abstract: M dwarf stars are currently the main targets in searches for potentially habitable planets. However, their winds have been suggested to be harmful to planetary atmospheres. Here, in order to better understand the winds of M dwarfs and also infer their physical properties, we perform a one-dimensional magnetohydrodynamic parametric study of winds of M dwarfs that are heated by dissipation of Alfv\'en waves. These waves are triggered by sub-surface convective motions and propagate along magnetic field lines. Here, we vary the magnetic field strength and density at the wind base (chromosphere), while keeping the same relative wave amplitude ($0.1 B_0$) and dissipation lenghtscale. We find that our winds very quickly reach isothermal temperatures with mass-loss rates proportional to base density square. We compare our results with Parker wind models and find that, in the high-beta regime, both models agree. However, in the low-beta regime, the Parker wind underestimates the terminal velocity by around one order of magnitude and mass-loss rate by several orders of magnitude. We also find that M dwarfs could have chromospheres extending to 18% to 180% of the stellar radius. We apply our model to the planet-hosting star GJ 436 and find, from X-ray observational constraints, $\dot{M}<7.6\times 10^{-15}\,M_{\odot}~\text{yr}^{-1}$. This is in agreement with values derived from the Lyman-alpha transit of GJ 436b, indicating that spectroscopic planetary transits could be used as a way to study stellar wind properties.