Modeling Repeated M Dwarf Flaring at an Earth-like Planet in the Habitable Zone: Atmospheric Effects for an Unmagnetized Planet

Abstract

Understanding the impact of active M dwarf stars on the atmospheric equilibrium and surface conditions of a habitable zone Earth-like planet is key to assessing M dwarf planet habitability. Previous modeling of the impact of electromagnetic (EM) radiation and protons from a single large flare on an Earth-like atmosphere indicated that significant and long-term reductions in ozone were possible, but the atmosphere recovered. However, these stars more realistically exhibit frequent flaring with a distribution of different total energies and cadences. Here, we use a coupled 1D photochemical and radiative-convective model to investigate the effects of repeated flaring on the photochemistry and surface UV of an Earth-like planet unprotected by an intrinsic magnetic field. As input, we use time-resolved flare spectra obtained for the dM3 star AD Leonis, combined with flare occurrence frequencies and total energies (typically 10^30.5 to 10³⁴ erg) from the 4-year Kepler light curve for the dM4 flare star GJ1243, with varied proton event impact frequency. Our model results show that repeated EM-only flares have little effect on the ozone column depth but that multiple proton events can rapidly destroy the ozone column. Combining the realistic flare and proton event frequencies with nominal CME/SEP geometries, we find the ozone column for an Earth-like planet can be depleted by 94% in 10 years, with a downward trend that makes recovery unlikely and suggests further destruction. For more extreme stellar inputs, O₃ depletion allows a constant ∼0.1-1 W m^-2 of UVC at the planet's surface, which is likely detrimental to organic complexity. Our results suggest that active M dwarf hosts may comprehensively destroy ozone shields and subject the surface of magnetically unprotected Earth-like planets to long-term radiation that can damage complex organic structures. However, this does not preclude habitability, as a safe haven for life could still exist below an ocean surface.

Keywords: Flares; M dwarf; Magnetic field.; Planetary atmospheres; Stellar activity; aHbitable zone.

FIG. 1.

The FFD and amplitudes observed from GJ1243 used to generate flare distributions in the present work.

FIG. 2.

Top: Six months, flare distribution (1277 events) generated from the GJ1243 FFD. The inset identifies an example complex flare near ∼9 days, produced by flare flux stacking. Bottom: One-year flare distribution (2555 events) from the GJ1243 FFD.

FIG. 3.

Comparison of light curve evolution from Segura et al. (2010) vs. the method adopted in the present work, based on the empirical modeling results of Davenport et al. (2014).

FIG. 4.

The relationship of proton fluence (left y axis) and proton flux units (pfu, right y axis) for the flares simulated in the present work, as function of relative flux increase (bottom x axis) and total flare energy (top x axis).

FIG. 5.

Full UV-visible spectrum for the impulsive phase of a characteristic AD Leo great flare–sized (10³⁴ erg) event. The spectra to the right of the vertical dashed red line have been added in the present work.

FIG. 6.

O₃ evolution for single-flare energy parameter comparison with Segura et al. (2010) for EM-only (a) and EM+protons (b) 10^30.5 to 10³⁴ erg flares.

FIG. 7.

The effects on the O₃ column of EM-only, 10³⁴ erg flares with varying interflare separations. Separations of 1 day, 1 week, 1 month, and 1 year included simulations of 10³ flares, where the 2 h separation included 10⁴ flares to obtain extended effects for long-term prediction of O₃ column (dash-dotted red line).

FIG. 8.

O₃ evolution driven by repeated proton events, 100 flare events for all cases. Top: 10^30.5 erg flares+protons. The 1 d⁻¹ period is likely to occur for a planet orbiting GJ1243; the dash-dotted blue line extrapolates the predicted O₃ loss rate. Middle: Carrington equivalent proton events at 10^31.9 erg. The dash-dotted red line predicts O₃ evolution for the most likely frequency experienced at a GJ1243-orbiting planet. Bottom: AD Leo equivalent proton events with 10³⁴ erg.

FIG. 9.

O₃ evolution for EM-only flare events generated from the GJ1243 FFD for periods of 1 month, 6 months, 1 year, and 15 years. The dash-dotted black line predicts continued effects of flaring beyond 15 years.

FIG. 10.

Distribution of proton events by flare event energy and proton fluence for the 10-year simulations. CME probability P = 0.25 (0.08) is shown in magenta (green).

FIG. 11.

O₃ column depth response to multiple proton events, generated by the GJ1243 FFD and taking into account CME geometries. Vertical lines represent the 1-year and 10-year time steps. The average (black line) and standard deviation for flaring period (shaded violet) and recovery period (shaded green) for 1-year simulations, and one 10-year simulation (red line) are shown. Top: Events with more conservative CME with per-event probability for impact of P = 0.083. Bottom: Events with more-permissive geometry with per-event impact probability of 0.25.

FIG. 12.

UV flux for (top) GJ1243 FFD–generated flares and (bottom) extreme O₃ loss. Steady-state O₃ column at the TOA (dotted black line) and planetary surface (dash-dotted black line); conditions with depleted O₃ column at surface (green dash-dotted line); at top (bottom) conditions at the peak of a 10^31.9 (10^30.5) erg flare at TOA (blue dotted line) and surface (blue dash-dotted line); conditions at the peak of a 10³⁴ erg flare at TOA (red dotted line) and surface (red dash-dotted line). Integrated UVC flux values are given in Table 3.

FIG. 13.

Atmospheric mixing ratio (f_V) and temperature profiles for the three parameter studies of 100 flares of log(E) 30.5 at frequency of 1 per day (top row), 31.9 with frequency of 1 per month (middle row), and 34.0 with frequency 1 per year (bottom row). In all cases, the dashed line represents initial steady state, the dash-dotted line signifies the state at the end of the flaring (before recovery), and the solid line represents the state at peak ozone loss during recovery.

FIG. 14.

Atmospheric mixing ratio (f_V) and temperature for two 10-year periods of flare activity, generated from the GJ1243 FFD. The top row is with a conservative CME impact probability of 0.08, and the bottom row is the more permissive probability of 0.25. In all cases, the dashed line represents initial steady state, the dash-dotted line signifies the state at the end of the flaring (before recovery), and the solid line represents the state at peak ozone loss during recovery.