On the Origin of Optical Radiation during the Impulsive Phase of Flares on dMe Stars. I. Discussion of Gas Dynamic Models

On the Origin of Optical Radiation During the Impulsive Phase of Flares on dMe Stars. II. Continuum and Line Radiation

“…(c) the HI emission lines are mainly localized in the chromospheric condensation and zones of thermal relaxation. Here the populations of the atomic levels and the degree of ionization of the plasma are: (a) close to the equilibrium values in a chromospheric condensation with sufficient geometric thickness 5 (Kowalski and Allred [20], Morchenko et al [1,10]) and in the region where the blue continuum is formed (at the brightness maximum of powerful flares) [3,19]; (b) differ sharply [20] from the equilibrium values in the thermal relaxation zones behind the front of the non-stationary chromospheric shock wave (according to the general rule the deviations for the populations of the atomic levels ν k decrease with increasing principal quantum number k). The gas radiating behind the front of the non-stationary chromospheric shock wave is stable with respect to radiative cooling (this conclusion follows from the calculations of Ref.…”

“…We emphasize that, because of the rapid radiative cooling of the dense gas behind the shock front (e.g., al. [1,10] showed (section 5.3 in [10]; introduction to [1]) that the H α profile in the observations [16] is approximately described by a model with a "Doppler core" and "Stark-broadened wings. "…”

“…al. [1,10] showed (section 5.3 in [10]; introduction to [1]) that the H α profile in the observations [16] is approximately described by a model with a "Doppler core" and "Stark-broadened wings. "…”

“…4) these layers are identified with the photosphere of a red dwarf in a quiescent state, and the photosphere radiation field is assume to be Planckian. 2 In addition, recent 1 Here (as well as in [1]), the "blue" continuum is considered as part of the flare continuum with a sufficiently large optical depth beyond the Balmer jump. This opacity is a necessary condition [3] for explaining the blue color of powerful stellar flares at the brightness maximum.…”

“…For high velocities v sh of the non-stationary chromospheric shock wave propagating in a partially ionized plasma in the direction of the photosphere, this increase in the electron temperature makes it possible to create conditions for ionization of helium atoms (HeI) and excitation of their discrete levels by electron impact (radiative cooling is substantially nonstationary [2], so the degree of ionization and the excitation state of the atoms are determined not only by the current value of T e ); this is a known effect [13] in the theory of stationary shock waves with emission. In this case the subsequent de-excitation by allowed spontaneous transitions can ensure [1] the appearance and enhancement of HeI lines in the spectra of powerful solar and stellar flares (e.g., lines with λ = 10830 Å [14]). We recall, however (see Ref.…”