Effect of Photospheric Radiation on the Upper Layers of a Star’s Atmosphere

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

“…Radiation from near-photospheric layers. Belova and Bychkov [2] assume that "black body radiation that appears at times during flares" . .…”

“…. "comes from a photosphere heated by a flux of suprathermal particles;" "a hot spot with a temperature of 10 4 ÷ 2 • 10 4 K may be formed in it" [2].…”

“…In these assertions the authors [2] do not take the following into account. By definition, the visibility of flares indicates the existence of unperturbed layers of a star's atmosphere.…”

“…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.…”

“…Neutral helium lines. 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]).…”