New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

The heating mechanism at high densities during M-dwarf flares is poorly understood. Spectra of M-dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at , and 3) an apparent pseudo-continuum of blended high-order Balmer lines between and . These properties are not reproduced by models that employ a typical solar-type flare heating level of in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological three-component interpretation. We present a new 1D radiative-hydrodynamic model of an M-dwarf flare from precipitating nonthermal electrons with a high energy flux of . The simulation produces bright near-ultraviolet and optical continuum emission from a dense (), hot () chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a blackbody-like continuum component and a low Balmer jump ratio result from optically thick Balmer () and Paschen recombination () radiation, and thus the properties of the flux spectrum are caused by blue ( ) light escaping over a larger physical depth range than by red ( ) and near-ultraviolet ( ) light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau-Zener transitions that result from merged, high-order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.