All talk and no action: coronal mass ejections from M dwarfs

M dwarfs are favoured exoplanet hosts because their small masses and low surface temperatures ensure that their habitable zones are close to the star where any exoplanets may be easily detected. The slow spin-down rates of these stars, however, mean that they remain magnetically active for a long time. Their frequent and powerful flares produce high-energy X-ray and EUV photons capable of ionising the upper reaches of an exoplanet atmosphere. More damaging, however, might be the stellar equivalent of Solar Energetic Particles. On the Sun, we see these accelerated ahead of flare-driven solar Coronal Mass Ejections that typically carry 200 times the X-ray energy of the flare. Do these exist on M dwarfs and if so, how frequent and how powerful are they? Existing sparse observations suggest that M dwarf coronal mass ejections are much less powerful than the solar analogy might predict. We present a study of ~ 150 stars for which we have mapped the stars’ surface magnetic fields using the spectropolarimetric technique of Zeeman-Doppler imaging. We show that fully convective M dwarfs have fundamentally different magnetic fields from higher mass stars such as the Sun. These fields are energetic (and hence capable of powering the very large observed flares) but have relatively low helicity density (believed to be the trigger for solar coronal mass ejections). It seems that M dwarfs, despite their powerful flares, may be less dangerous for exoplanet atmospheres than we feared.