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 ( E > 10 30 erg) reach temperatures of only ∼3500–4000 K, nearly 3 times cooler than typical solar flares, which peak around 9000–10,000 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 ∼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 ∼9000 K.