  Main sequence M stars pose an interesting problem for astrobiology: their
abundance in our galaxy makes them likely targets in the hunt for habitable
planets, but their strong chromospheric activity produces high energy radiation
and charged particles that may be detrimental to life. We studied the impact of
the 1985 April 12 flare from the M dwarf, AD Leonis (AD Leo), simulating the
effects from both UV radiation and protons on the atmospheric chemistry of a
hypothetical, Earth-like planet located within its habitable zone. Based on
observations of solar proton events and the Neupert effect we estimated a
proton flux associated with the flare of $5.9\times 10^{8}$ protons cm$^{-2}$
sr$^{-1}$ s$^{-1}$ for particles with energies >10 MeV. Then we calculated the
abundance of nitrogen oxides produced by the flare by scaling the production of
these compounds during a large solar proton event called the "Carrington
event". The simulations were performed using a 1-D photochemical model coupled
to a 1-D radiative/convective model. Our results indicate that the ultraviolet
radiation emitted during the flare does not produce a significant change in the
ozone column depth of the planet. When the action of protons is included, the
ozone depletion reached a maximum of 94% two years after the flare for a planet
with no magnetic field. At the peak of the flare, the calculated UV fluxes that
reach the surface, in the wavelength ranges that are damaging for life, exceed
those received on Earth during less than 100 s. Flares may therefore not
present a direct hazard for life on the surface of an orbiting habitable
planet. Given that AD Leo is one of the most magnetically-active M dwarfs
known, this conclusion should apply to planets around other M dwarfs with lower
levels of chromospheric activity.
