Hydrogen recombination continua in stellar flares

Abstract

An increasing interest in stellar flares stimulated various modelling approaches in order to analyse the observed flare fluxes. A particular interest was focused on photometric data obtained from Kepler and TESS satellites which detected thousands of flares on cool dwarf stars, including extremely energetic superflares. Radiation-hydrodynamical simulations, together with a rather rare broad-band spectroscopy, indicate much larger densities in the superflare chromospheres as compared to solar flares. Formation of hydrogen recombination continua under such different densities ranging from 1013 to 1015 cm−3 or more is governed by physics of optically thin to largely thick plasmas, the continuum optical thickness being within the range of four orders of magnitude. Various authors presented simple approximate methods to analyse the photometric data from Kepler or TESS under such diverse regimes of physical conditions. In this letter, we summarize the general physical approach and compute the hydrogen recombination spectra under the above range of electron densities. We show the theoretical contrast with respect to quiet-star continuum for two characteristic stars of G and dMe type. Based on that we distinguish three regimes of the continuum formation and discuss the applicability of various simple approaches.