Hydrogen Recombination Continuum As the Radiative Model for Stellar Optical Flares

The study of stellar flares has increased with new observations from CoRoT,Kepler, and TESS satellites, revealing the broadband visible emission fromthese events. Typically, stellar flares have been modelled as 10^4 Kblackbody plasma to obtain estimates of their total energy. In the Sun, whitelight flares (WLFs) are much fainter than their stellar counterparts, andnormally can only be detected via spatially resolved observations. Identifyingthe radiation mechanism for the formation of the visible spectrum from solarand stellar flares is crucial to understand the energy transfer processesduring these events, but spectral data for WLFs are relatively rare, andinsufficient to remove the ambiguity of their origin: photospheric blackbodyradiation and/or Paschen continuum from hydrogen recombination in thechromosphere. We employed an analytical solution for the recombinationcontinuum of hydrogen instead of the typically assumed 10^4 K blackbodyspectrum to study the energy of stellar flares and infer their fractional areacoverage. We investigated 37 events from Kepler-411 and 5 events fromKepler-396, using both radiation mechanisms. We find that estimates for thetotal flare energy from the H recombination spectrum are about an order ofmagnitude lower than the values obtained from the blackbody radiation. Giventhe known energy transfer processes in flares, we argue that the former is aphysically more plausible model than the latter to explain the origin of thebroadband optical emission from flares.