Authors: Kai Yang (Institute for Astronomy, University of Hawaii at Manoa) Xudong Sun (Institute for Astronomy, University of Hawaii at Manoa) Graham Kerr (Department of Physics, Catholic University of America)

M-dwarf flares observed by the Transiting Exoplanet Survey Satellite (TESS) sometimes exhibit a “peak-bump” light-curve morphology, characterized by a secondary, gradual peak well after the main, impulsive peak. A similar “late phase” is frequently detected in solar flares observed in the extreme-ultraviolet from long, hot coronal loops. White-light emission has also been observed in off-limb flare loops. Here we perform a suite of one-dimensional hydrodynamic loop simulations for M-dwarf flares inspired by these solar examples. Our results suggest that coronal plasma condensation following impulsive flare heating can yield high electron number densities in the loop, allowing it to contribute significantly to the optical light curves via bound-free and free-free emission mechanisms. Our simulation results qualitatively agree with the observations: the longer evolutionary time scale of coronal loops produces a distinct, secondary emission peak; its intensity increases with the input flare energy density. We argue that coronal plasma condensation is a possible mechanism for the TESS late-phase flares.