Hydrogen recombination continuum as the radiative model for stellar optical flares

Abstract

The study of stellar flares has increased with new observations from CoRoT, Kepler, and TESS satellites, revealing the broadband visible emission from these events. Typically, stellar flares have been modelled as 104 K blackbody plasma to obtain estimates of their total energy. In the Sun, white light flares (WLFs) are much fainter than their stellar counterparts, and normally can only be detected via spatially resolved observations. Identifying the radiation mechanism for the formation of the visible spectrum from solar and stellar flares is crucial to understand the energy transfer processes during these events, but spectral data for WLFs are relatively rare, and insufficient to remove the ambiguity of their origin: photospheric blackbody radiation and/or Paschen continuum from hydrogen recombination in the chromosphere. We employed an analytical solution for the recombination continuum of hydrogen instead of the typically assumed 104 K blackbody spectrum to study the energy of stellar flares and infer their fractional area coverage. We investigated 37 events from Kepler-411 and 5 events from Kepler-396, using both radiation mechanisms. We find that estimates for the total flare energy from the H recombination spectrum are about an order of magnitude lower than the values obtained from the blackbody radiation. Given the known energy transfer processes in flares, we argue that the former is a physically more plausible model than the latter to explain the origin of the broadband optical emission from flares.

Time-dependent Stellar Flare Models of Deep Atmospheric Heating

“…Stellar flare RHD models have considered smaller electron-beam heating fluxes, but the model Balmer jumps are larger than in all known spectral observations; the photosphere is radiatively backheated by this Balmer continuum radiation, which originates at chromospheric heights (Allred et al 2006). Other models of stellar chromospheric flares include static non-LTE (NLTE) slab calculations (Jevremovic et al 1998;García-Alvarez et al 2002;Morchenko et al 2015), static local thermodynamic equilibrium (LTE) slab calculations (Kunkel 1970;Eason et al 1992;Heinzel & Shibata 2018;Simões et al 2024), semiempirical atmospheric adjustments with NLTE radiative transfer (Houdebine et al 1991;Christian et al 2003;Fuhrmeister et al 2005Fuhrmeister et al , 2010Kowalski et al 2012;Schmidt et al 2012), and static energy equilibrium loop models with NLTE radiative transfer (Hawley & Fisher 1992). The semiempirical model atmospheres of Cram & Woods (1982) explored six variations of temperature and electron density adjustments within the deep chromosphere and photosphere, one of which produced very broad Balmer line wings and a hot optical blackbody color temperature.…”