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

Matt A Tilley; Antígona Segura; Victoria Meadows; Suzanne Hawley; James Davenport

doi:10.1089/ast.2017.1794

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

Astrobiology. 2019 Jan;19(1):64-86. doi: 10.1089/ast.2017.1794. Epub 2018 Aug 2.

Authors

Matt A Tilley^{1

2

3}, Antígona Segura^{2

4}, Victoria Meadows^{2

3

5}, Suzanne Hawley^{2

3

5}, James Davenport^{2

6}

Affiliations

¹ 1 Department of Earth and Space Sciences, University of Washington , Seattle, Washington, USA.
² 2 NASA Astrobiology Institute-Virtual Planetary Laboratory Lead Team , USA.
³ 3 Astrobiology Program, University of Washington , Seattle, Washington, USA.
⁴ 4 Instituto de Ciencias Nucleares , Universidad Nacional Autónoma de México, México.
⁵ 5 Department of Astronomy, University of Washington , Seattle, Washington, USA.
⁶ 6 Department of Physics and Astronomy, Western Washington University , Bellingham, Washington, USA.

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.

Publication types

Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

Atmosphere*
Electromagnetic Radiation*
Models, Theoretical*
Planets*
Protons
Stars, Celestial*

Substances

Protons