Kavli Affiliate: Sara Seager
| Summary:
Early JWST observations of TRAPPIST-1 have revealed an unexpected puzzle: energetic white-light flares ($rmE > 10^30$ 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 $sim4000$,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.
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