The High-energy Radiation Environment around a 10 Gyr M Dwarf: Habitable at Last?

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; VIEYTES, MARIELA

The Astronomical Journal

IOP

Año: 2020 vol. 160

High levels of X-ray and UV activity on young M dwarfs may drive rapidatmospheric escape on temperate, terrestrial planets orbiting within the liquidwater habitable zone. However, secondary atmospheres on planets orbiting older,less active M dwarfs may be stable and present more promising candidates forbiomarker searches. We present new HST and Chandra observations of Barnard´sStar (GJ 699), a 10 Gyr old M3.5 dwarf, acquired as part of the Mega-MUSCLESprogram. Despite the old age and long rotation period of Barnard´s star, weobserve 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 ofthe time the star is in a flare state) of ∼ 25%. A 5 A - 10 μm SED ofGJ 699 is created and used to evaluate the atmospheric stability of ahypothetical, unmagnetized terrestrial planet in the habitable zone (rHZ∼ 0.1 AU). Both thermal and non-thermal escape modeling indicate (1) thequiescent stellar XUV flux does not lead to strong atmospheric escape:atmospheric heating rates are comparable to periods of high solar activity onmodern Earth, and (2) the flare environment could drive the atmosphere into ahydrodynamic loss regime at the observed flare duty cycle: sustained exposureto the flare environment of GJ 699 results in the loss of ≈ 87 Earthatmospheres Gyr−1 through thermal processes and ≈ 3 Earthatmospheres Gyr−1 through ion loss processes, respectively. These resultssuggest that if rocky planet atmospheres can survive the initial ∼ 5 Gyrof high stellar activity, or if a second generation atmosphere can be formed oracquired, the flare duty cycle may be the controlling stellar parameter for thestability of Earth-like atmospheres around old M stars.