SIMBAD references

Early JWST observations of TRAPPIST-1 have revealed an unexpected puzzle: energetic white-light flares (E > 10³⁰ 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 (H₂) into atomic hydrogen. This “H₂ 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 H₂. 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.