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 Alfvén waves. These waves are triggered by sub-surface convective motions and propagate along magnetic field lines. Here, we vary the magnetic field strength B 0 and density ρ 0 at the wind base (chromosphere), while keeping the same relative wave amplitude (0.1B 0 ) and dissipation lenghtscale. Our simulations thus range from low plasma-β to high plasma-β (0.005 to 3.7). We find that our winds very quickly reach isothermal temperatures with mass-loss rates M ∝ ρ 2 0 . We compare our results with Parker wind models and find that, in the high-β regime, both models agree. However, in the low-β regime, the Parker wind underestimates the terminal velocity by around one order of magnitude and M 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, M < 7.6 × 10 −15 M yr −1 . This is in agreement with values derived from the Lyman-α transit of GJ 436b, indicating that spectroscopic planetary transits could be used as a way to study stellar wind properties.

New Observational Constraints on the Winds of M Dwarf Stars

“…Cranmer & Saar (2011) suggest that M dwarf winds may actually be dominated not by Alfvén wave driving, but by CMEs, a possibility we will return to in Section 5.4. Other Alfvén wave based models infer more substantial M dwarf winds, without resorting to CMEs (Vidotto et al 2014;Alvarado-Gómez et al 2016;Mesquita & Vidotto 2020). These models predict increases in wind strength with stellar activity qualitatively consistent with the observed relation in Figure 10, although quantitative comparison with the models is complicated by the extensive scatter in the data.…”

The large-scale magnetic field of Proxima Centauri near activity maximum

“…At large distance from the star (typically a few stellar radii), the extended magnetic field of Proxima Cen is mainly dipolar. Moreover, Proxima Cen features a corona likely reaching the MK level, which would be enough to entirely ionize the stellar wind (as demonstrated for a few M dwarfs of earlier types; e.g., Vidotto et al 2019;Mesquita & Vidotto 2020). Under these conditions, ud-Doula & Owocki (2002) semi-analytical relations provide us with a first order estimate of the equatorial Aflvèn radius of ∼25 𝑅 S .…”

The evolution of the solar wind

“…For HD189733, the derived local stellar wind speed at the orbit of the planet of 190 km/s and density of 3 Â 10 3 cm À3 (Bourrier and Lecavelier des Etangs 2013) yielded a mass-loss rate of approximately 4 Â 10 À15 M yr À1 . One caveat to keep in mind, though, is that the values derived above are likely not unique, and some degeneracy might exist (Mesquita and Vidotto 2020).…”