The High-Energy Radiation Environment Around a 10 Gyr M Dwarf: Habitable at Last?


(left) A comparison between the ultraviolet spectrum of the quiet Sun (black, from Woods et al. 2009) and the quiescent flux from Barnard’s Star (GJ 699, shown in red binned to the 2 ˚A resolution of the solar data). Prominent emission lines are labeled. The reconstructed EUV flux (Section 4.3) is enhanced relative to the quiet Sun, whereas the FUV flux (mainly chromospheric and transition region emission lines; Section 4) is comparable and the NUV flux is below the Sun by 2 – 3 orders of magnitude owing to the cooler effective temperature of the M3 star. (right) Lyα is the brightest line in the UV spectrum of M dwarfs, we show the observed (black histogram), reconstructed (blue dashed line; Section 4), and model fit (pink solid line) Lyα emission line, employing the reconstruction technique of Youngblood et al. (2016). The reconstruction parameters find an intrinsic line flux, F(Lyα) = 1.02 × 10−12 erg cm−2 s−1 with an interstellar column density of log10N(HI) = 17.60. Note that the large negative radial velocity of GJ 699 moves the star mostly out of the interstellar H I line core.

High levels of X-ray and UV activity on young M dwarfs may drive rapid atmospheric escape on temperate, terrestrial planets orbiting within the liquid water habitable zone.

However, secondary atmospheres on planets orbiting older, less active M dwarfs may be stable and present more promising candidates for biomarker searches. We present new HST and Chandra 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 ≈ 5000s; E130 ≈ 1029.5 erg each) and one X-ray (EX ≈ 1029.2 erg) flares, and 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 5 A - 10 μm SED 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 non-thermal 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, respectively. 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.

Kevin France, Girish Duvvuri, Hilary Egan, Tommi Koskinen, David J. Wilson, Allison Youngblood, Cynthia S. Froning, Alexander Brown, Julian D. Alvarado-Gomez, Zachory K. Berta-Thompson, Jeremy J. Drake, Cecilia Garraffo, Lisa Kaltenegger, Adam F. Kowalski, Jeffrey L. Linsky, R.O. Parke Loyd, Pablo J. D. Mauas, Yamila Miguel, J. Sebastian Pineda, Sarah Rugheimer, P. Christian Schneider, Feng Tian, Mariela Vieytes

Comments: Accepted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2009.01259 [astro-ph.EP] (or arXiv:2009.01259v1 [astro-ph.EP] for this version)
Submission history
From: Kevin France
[v1] Wed, 2 Sep 2020 18:00:24 UTC (349 KB)
https://arxiv.org/abs/2009.01259
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