Molecular hydrogen controls the temperatures of flares on TRAPPIST-1

Abstract

Early JWST observations of TRAPPIST-1 have revealed an unexpected puzzle: energetic white-light flares ($\mathrm{E}>10^{30}$ erg) reach temperatures of only $\sim$3500--4000\,K, nearly three times cooler than typical solar flares, which peak around 9000--10000\,K. Here we explain this difference by identifying the physical mechanism that regulates flare temperatures on late M-dwarfs. The key factor is that in the cool, dense atmosphere of TRAPPIST-1, magnetic heating is strongly moderated by the dissociation of molecular hydrogen (H${}_2$) into atomic hydrogen. This "H${}_2$ dissociation thermostat" acts as an efficient energy sink, preventing flare regions from heating above $\sim 4000$\,K. Our chemical equilibrium and heat capacity calculations show that this effect depends sensitively on stellar atmospheric pressure and the local abundance of H${}_2$. In hotter stars, from early M dwarfs to solar-type stars, the scarcity of molecular hydrogen renders this mechanism ineffective; instead, atomic hydrogen ionization limits flare temperatures near $\sim$9000\,K.

Accepted for publication in ApJL