  Understanding whether M-dwarf stars may host habitable planets with
Earth-like atmospheres and biospheres is a major goal in exoplanet research. If
such planets exist, the question remains as to whether they could be identified
via spectral signatures of biomarkers. Such planets may be exposed to extreme
intensities of cosmic rays that could perturb their atmospheric photochemistry.
Here, we consider stellar activity of M-dwarfs ranging from quiet up to strong
flaring conditions and investigate one particular effect upon biomarkers,
namely, the ability of secondary electrons caused by stellar cosmic rays to
break up atmospheric molecular nitrogen (N2), which leads to production of
nitrogen oxides in the planetary atmosphere, hence affecting biomarkers such as
ozone. We apply a stationary model, that is, without a time-dependence, hence
we are calculating the limiting case where the atmospheric chemistry response
time of the biomarkers is assumed to be slow and remains constant compared with
rapid forcing by the impinging stellar flares. This point should be further
explored in future work with time-dependent models. For the flaring case O3 is
mainly destroyed via direct titration with nitrogen oxides and not via the
familiar catalytic cycle photochemistry, which occurs on Earth. For scenarios
with low O3, Rayleigh scattering by the main atmospheric gases became more
important for shielding the planetary surface from ultra-violet radiation. A
major result of this work is that the biomarker O3 survived all the
stellar-activity scenarios considered except for the strong case, whereas the
biomarker nitrous oxide could survive in the planetary atmosphere under all
conditions of stellar activity considered here, which clearly has important
implications for missions that aim to detect spectroscopic biomarkers.
