How stellar flares of M dwarfs impact the composition and spectra of gaseous exoplanets

Stellar flares of active M dwarfs can affect the atmospheric composition of close-orbiting gas giants and can result in time-dependent transmission spectra, which in turn can bias retrieval analyses. Existing studies that have researched this do not account for possible effects of climate dynamics and have not explored a large diversity of stellar flares.

We present here the study of the effects of stellar flares on the composition of a tidally-locked, close-orbiting gaseous planet of 800 K and consider the impact on the transmission spectra on both limbs. We use a two-dimensional thermo-and photochemical kinetics model that takes into account horizontal transport due to equatorial superrotation, the latter dominating the circulation patterns of such planets. We explore a variety of flares differing in energy, duration, wavelength coverage and occurrence frequency.

After a high-energy flare, we find that the dayside and evening limb is depleted in methane and ammonia by over 3 orders of magnitude, which lowers transit depths by 200 ppm during the first (2.3) days after the flare event. After advection to the cooler nightside, photolysis products recombine and enhance the molar fractions of hydrogen cyanide and acetylene. The latter molecule causes a strong increase in transit depth (< 600 ppm) around 14 micron for several days after the flare, although this is strongly coupled to the horizontal wind speed. Furthermore, we identify the peak flux as the most important factor in determining the chemical response of the atmosphere. Additionally, we find that rare high-energy flares do not have a permanent effect on the composition and hypothesize that more frequent low-energy flares are not strong enough to maintain a perturbed atmospheric state that differs from the pre-flare distribution.