On the Origin of Optical Radiation During the Impulsive Phase of Flares on dMe Stars. II. Continuum and Line Radiation
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ON THE ORIGIN OF OPTICAL RADIATION DURING THE IMPULSIVE PHASE OF FLARES ON dMe STARS. II. CONTINUUM AND LINE RADIATION
E. S. Morchenko
It is argued that, not only the blue (at the brightness maximum) but also the red (in the slow decay phase) components of the optical continuum of powerful flares on dMe stars are formed near the photosphere. The possible appearance of HeI lines in the relaxation zones for the plasma to a state of thermal equilibrium (as a result of a rise in electron temperature owing to elastic collisions of electrons with atoms and ions) is noted (for sufficiently high speeds of a non-stationary chromospheric shock wave propagating toward the photosphere of the Sun and stars). A scheme is proposed for the positioning of the “layers” of plasma responsible for the generation of radiation in the white light continuum during the impulsive phase of powerful stellar flares. Keywords: red dwarf stars: flares: models of flares: optical radiation
1. Introduction
This article completes the discussion begun by the author in Ref. 1.
Physics Faculty, M. V. Lomonosov Moscow State University, Moscow, Russia; e-mail: [email protected]
Original article submitted December 16, 2019; accepted for publication June 24, 2020. Translated from Astrofizika, Vol. 63, No. 3, pp. 501-509 (August 2020) 440
0571-7256/19/6303-0440 ©2020 Springer Science+Business Media, LLC
2. Radiation from near-photospheric layers
Belova and Bychkov [2] assume that “black body radiation that appears from time to time during flares” ... “comes from a photosphere heated by a flux of suprathermal particles;” “a hot spot with a temperature of 10 4 y 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 ondicates the existence of unperturbed layers of a star’s atmosphere. In the method of Ref. 3 for estimating the area s of the source of a quasi-black-body blue continuum of powerful flares on dMe stars (the brightness maximum) in the approximation of an absolute black body (used, for example, in Ref. 4) these layers are identified with the photosphere of a red dwarf in a quiescent state, and its radiation field is assume to be Planckian.1 In addition, recent observations [5] of the flare activity of Proxima Cen (dM5.5e) indicate that the molecular absorption spectra of the atmosphere of the red dwarf (the reversing layer) “are essentially independent of the flare events.” In turn, Honda et al. [6] did not detect changes in the molecular bands during the time of a flare in EV Lac (dM4.5e). The quasi-black-body shape of the blue continuum of powerful flares on dMe stars at the brightness maximum is indicated by the existence of continuum optical radiation on the long-wavelength (red) side of the Hb line that does not correspond to a Planck function with Tbb ~ 104 K (see Fig. 8a in the article by Kowalski et al. [7]). This radiation originates in deeper, less heated near-photospheric layers. Here an important role in generating the quasi
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