France, Kevin, Duvvuri, Girish, Egan, Hilary, Koskinen, Tommi, Wilson, David J., Youngblood, Allison, Froning, Cynthia S., Brown, Alexander, Alvarado-Gómez, Julián D., Berta-Thompson, Zachory K., Drake, Jeremy J., Garraffo, Cecilia, Kaltenegger, Lisa, Kowalski, Adam F., Linsky, Jeffrey L., Loyd, R. O. Parke, Mauas, Pablo J. D., Miguel, Yamila, Pineda, J. Sebastian, Rugheimer, Sarah, Schneider, P. Christian, Tian, Feng, and Vieytes, Mariela. 2020. "The High-energy Radiation Environment around a 10 Gyr M Dwarf: Habitable at Last?" The Astronomical Journal, 160 237. https://doi.org/10.3847/1538-3881/abb465.
Recent work has demonstrated that high levels of X-ray and UV activity on young M dwarfs may drive rapid atmospheric escape on temperate, terrestrial planets orbiting within the habitable zone. However, secondary atmospheres on planets orbiting older, less active M dwarfs may be stable and present more promising candidates for biomarker searches. In order to evaluate the potential habitability of Earth-like planets around old, inactive M dwarfs, we present new Hubble Space Telescope and Chandra X-ray Observatory observations of Barnard&'s Star (GJ 699), a 10 Gyr old M3.5 dwarf, acquired as part of the Mega-MUSCLES program. Despite the old age and long rotation period of Barnard&'s Star, we observe two FUV (d130 ? 5000 s; E130 ? 1029.5 erg each) and one X-ray (EX ? 1029.2 erg) flares, and we estimate a high-energy flare duty cycle (defined here as the fraction of the time the star is in a flare state) of ˜25%. A publicly available 5 Å to 10 µm spectral energy distribution of GJ 699 is created and used to evaluate the atmospheric stability of a hypothetical, unmagnetized terrestrial planet in the habitable zone (rHZ ˜ 0.1 au). Both thermal and nonthermal escape modeling indicate (1) the quiescent stellar XUV flux does not lead to strong atmospheric escape: atmospheric heating rates are comparable to periods of high solar activity on modern Earth, and (2) the flare environment could drive the atmosphere into a hydrodynamic loss regime at the observed flare duty cycle: sustained exposure to the flare environment of GJ 699 results in the loss of ?87 Earth atmospheres Gyr-1 through thermal processes and ?3 Earth atmospheres Gyr-1 through ion loss processes. These results suggest that if rocky planet atmospheres can survive the initial ˜5 Gyr of high stellar activity, or if a second-generation atmosphere can be formed or acquired, the flare duty cycle may be the controlling stellar parameter for the stability of Earth-like atmospheres around old M stars.
