The High-energy Radiation Environment around a 10 Gyr M Dwarf: Habitable at Last?
Loyd, R. O. Parke; Kowalski, Adam F.; Brown, Alexander; Drake, Jeremy J.; France, Kevin; Youngblood, Allison; Pineda, J. Sebastian; Linsky, Jeffrey L.; Mauas, Pablo J. D.; Duvvuri, Girish; Miguel, Yamila; Kaltenegger, Lisa; Schneider, P. Christian; Froning, Cynthia S.; Berta-Thompson, Zachory K.; Alvarado-Gómez, Julián D.; Wilson, David J.; Koskinen, Tommi; Rugheimer, Sarah; Vieytes, Mariela; Tian, Feng; Garraffo, Cecilia; Egan, Hilary
United States, Germany, Argentina, Netherlands, United Kingdom, Macao SAR
Abstract
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 (δ130 ≈ 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.