M Dwarf flares can impact the habitability of exoplanets

About 75% of main-sequence stars are M dwarfs. They show the highest exoplanetary occurrence rate, among rocky planets orbiting within the Habitable Zone (HZ). More than 60% of these M dwarfs are active, displaying very frequent (up to ∼4 per day) and powerful (up to ∼1035 ergs) flares and superflares events. This can be a double-edge sword in regard of exoplanet habitability. On one hand, stellar activity can alter the atmosphere of a planet by photo-dissociating atmospheric species such as O3, SO2 and CH4. On the other hand, flares can promote the abiotic synthesis of organic molecules relevant to the origin of life (nucleobases, sugars, carboxylic acids or aminoacids). Interest in understanding how the Flare Frequency Distribution (FFD) can impact the habitability of an exoplanet is therefore growing. Recent observations have shown that the contrast in flux between flares and the photospheres of cool stars is maximized at UV wavelengths (up to ×100 times stronger than in the optical). UV is also preferable to constrain both the prebiotic abiogenesis and the Ozone depletion. Moreover, flares in the UV band contain emission lines (C IV, He II and Mg II) from transition and chromospheric regions. These will be extremely useful to study short- and long-term variability of active M dwarfs and provide crucial inputs to stellar models. For these reasons, we propose to use a CubeSat to complement current flare surveys operating in the optical. This CubeSat will observe a high number of flaring M dwarfs, performing all-sky photometric optical and NUV surveys, following an all-sky scanning law every two days. The NUV flares detected by this CubeSat can also be used to trigger ground-based follow-up time-resolved spectroscopy. The evolution of continuum and emission lines until reaching quiescent state will deliver precious information of the flare physics scenario. Finally, the study of M dwarfs stellar activity in the NUV band will provide valuable data for larger forthcoming missions that will survey exoplanets (namely PLATO, ARIEL and LUVOIR) as well as for models of planetary NUV photodissociation of biosignatures.