The High-energy Radiation Environment around a 10 Gyr M Dwarf: Habitable at Last?
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.
- Publication:
-
The Astronomical Journal
- Pub Date:
- November 2020
- DOI:
- 10.3847/1538-3881/abb465
- arXiv:
- arXiv:2009.01259
- Bibcode:
- 2020AJ....160..237F
- Keywords:
-
- Solar extreme ultraviolet emission;
- Exoplanet atmospheres;
- Stellar activity;
- Stellar flares;
- Habitable zone;
- Hubble Space Telescope;
- 1493;
- 487;
- 1580;
- 1603;
- 696;
- 761;
- Astrophysics - Earth and Planetary Astrophysics;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted to AJ