The effect of a strong stellar flare on the atmospheric chemistry of an earth-like planet orbiting an M dwarf

Abstract

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 × 10⁸ 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 with a 1-D photochemical model coupled to a 1-D radiative/convective model. Our results indicate that the UV 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 reaches 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. Therefore, flares may 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.

FIG. 1.

Model results for present Earth around the Sun (dotted lines), compared with vertical profiles from the 1976 U.S. Standard Atmosphere (solid lines) and the profiles for the AD Leo planet (dashed lines): (a) temperature, (b) H₂O mixing ratio, and (c) O₃ number density. The profiles of the AD Leo planet are the ones calculated for steady state and used as starting point in the simulations.

FIG. 1.

Model results for present Earth around the Sun (dotted lines), compared with vertical profiles from the 1976 U.S. Standard Atmosphere (solid lines) and the profiles for the AD Leo planet (dashed lines): (a) temperature, (b) H₂O mixing ratio, and (c) O₃ number density. The profiles of the AD Leo planet are the ones calculated for steady state and used as starting point in the simulations.

FIG. 2.

Flux received at the top of the atmosphere of a planet on the AD Leo habitable zone (0.16 AU). The dotted line is the solar flux received by Earth. Times listed on the right lower corner of each panel correspond to the flare fluxes plotted on that panel. AD Leo spectrum during quiescence is always shown in a black continuous line to be used as a reference. Color images available online at www.liebertonline.com/ast.

FIG. 3.

AD Leo flux in quiescent state (black line) and during the flare as observed from Earth. Vertical dotted lines show the wavelength extent of the various spectrograph gratings: the IUE short wavelength camera (SWP) grating, from 1150 to 2000 Å, the IUE long wavelength camera (LWP) grating from 1850 to 3100 Å, and the blue edge of the optical spectrum at 3560 Å, obtained with the McDonald Observatory Electronic Spectrograph No. 2. IUE spectra were taken in low-resolution mode, providing a resolution of 6 Å, while the optical spectra have a resolution of 3.5 Å. As the bulk flux distribution, rather than the detailed line profiles, were the primary concern in this work, the spectra were rebinned prior to being input to the photochemical/climate model. Color images available online at www.liebertonline.com/ast.

FIG. 4.

Effect of incident UV radiation from a flare on the temperature profile. Left: Temperature profile for an Earth-like planet around AD Leo before, during, and after a big UV flare event. Right: Difference between the initial steady-state temperature (T_qsc) and the temperature calculated for the AD Leo planet during and after the big UV flare event. Color images available online at www.liebertonline.com/ast.

FIG. 5.

Water profile for an Earth-like planet around AD Leo before, during, and after a big UV flare event. Color images available online at www.liebertonline.com/ast.

FIG. 6.

Ozone number density for an Earth-like planet around AD Leo, before, during, and after a big UV flare event. The ozone concentration calculated for Earth is presented for comparison (dotted line). Color images available online at www.liebertonline.com/ast.

FIG. 7.

Time evolution of the ozone column depth compared to the initial steady state before, during, and after a big UV flare event. The lines show simulations made with different time steps after the flare ended. Times used for each run are listed in the figure.

FIG. 8.

Time evolution before, during, and after the flare for the atmospheric nitric oxide abundance profile. These results show the combined influence of the flare's incident UV radiation and a proton event at the peak of the flare. Color images available online at www.liebertonline.com/ast.

FIG. 9.

Time evolution before, during, and after the flare for the atmospheric ozone abundance profile. These results show the combined influence of the flare's incident UV radiation and a proton event at the peak of the flare. The ozone concentration calculated for Earth is presented for comparison (dotted line). Color images available online at www.liebertonline.com/ast.

FIG. 10.

Time evolution of the ozone column depth compared to the initial steady state. These results show the combined influence of the flare's incident UV radiation and a proton event at the peak of the flare. Line with diamonds: O₃ fraction change for a simultaneous UV and proton flux peak. Line with crosses: O₃ fraction change for a proton event with a maximum delayed by 889 s with respect to the UV flare peak. Vertical dotted lines indicate the time for the peak of the UV flare and the end of the UV flare.

FIG. 11.

Surface UV flux before, during, and after the UV flare (solid lines). The top of the atmosphere UV flux at quiescence (dashed black line) and at the peak of the flare (dashed blue line) are drawn for comparison. The dotted line is the surface UV radiation calculated for Earth. Vertical solid lines show the boundaries between the regions known as UVA (3150–4000 Å), UVB (2800–3150 Å), and UVC (<2800 Å). Color images available online at www.liebertonline.com/ast.

FIG. 12.

Surface UV flux before, during a UV flare plus proton event, and after the flare (solid lines). The top of the atmosphere UV flux at quiescence (dashed black line) and at the peak of the flare (dashed blue line) are drawn for comparison. The dotted line is the surface UV radiation calculated for Earth. Vertical solid lines show the boundaries between the regions known as UVA (3150–4000 Å), UVB (2800–3150 Å), and UVC (<2800 Å). The first 800 s of the flare are omitted here because fluxes are identical to those shown in Fig. 11. Color images available online at www.liebertonline.com/ast.