Vulnerability of Atmospheres of M Dwarf Planets

Two interesting papers regarding the effect of a red dwarf on the atmospheres of its (close in) planets.

Atmospheric mass loss by stellar wind from planets around main sequence M stars
http://arxiv.org/abs/1006.0021

Abstract wrote:

We present an analytic model for the interaction between planetary atmospheres and stellar winds from main sequence M stars, with the purpose of obtaining a quick test-model that estimates the timescale for total atmospheric mass loss due to this interaction. Planets in the habitable zone of M dwarfs may be tidally locked and may have weak magnetic fields, because of this we consider the extreme case of planets with no magnetic field. The model gives the planetary atmosphere mass loss rate as a function of the stellar wind and planetary properties (mass, atmospheric pressure and orbital distance) and an entrainment efficiency coefficient $\alpha$. We use a mixing layer model to explore two different cases: a time-independent stellar mass loss and a stellar mass loss rate that decreases with time. For both cases we consider planetary masses within the range of $1\to10$ M$_{\oplus}$ and atmospheric pressures with values of 1, 5 and 10 atm. For the time dependent case, planets without magnetic field in the habitable zone of M dwarfs with initial stellar mass losses of $\leq \dot{M}_{w} < 10^{-11}$ M$_{\odot}$ yr$^{-1}$, may retain their atmospheres for at least 1 Gyr. This case may be applied to early spectral type M dwarfs (earlier than M5). Studies have shown that late type M dwarfs (later than M5) may be active for long periods of time ($\geq 4$Gyr), and because of that our model with constant stellar mass loss rate may be more accurate. For these stars most planets may have lost their atmospheres in 1 Gyr or less because most of the late type M dwarfs are expected to be active. We emphasize that our model only considers planets without magnetic fields. Clearly we must expect a higher resistance to atmospheric erosion if we include the presence of a magnetic field.

The Effect of a Strong Stellar Flare on the Atmospheric Chemistry of an Earth-like Planet Orbiting an M dwarf
http://arxiv.org/abs/1006.0022

Abstract wrote:

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.