Molecular Hydrogen Controls The Temperatures Of Flares On TRAPPIST-1

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 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 (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.

Alexander I. Shapiro Nadiia Kostogryz Sara Seager Veronika Witzke Julien de Wit Valeriy Vasilyev Astrid M. Veronig Robert Cameron Hardi Peter Sami K. Solanki

Comments: Accepted for publication in ApJL
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2601.00386 [astro-ph.SR] (or arXiv:2601.00386v1 [astro-ph.SR] for this version)
https://doi.org/10.48550/arXiv.2601.00386
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Submission history
From: Alexander Shapiro
[v1] Thu, 1 Jan 2026 16:40:35 UTC (1,118 KB)
https://arxiv.org/abs/2601.00386
Astrobiology, exoplanet,