Some Manifestations of Nonstationarity in the Spectra of Early-Type Supergiants
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SOME MANIFESTATIONS OF NONSTATIONARITY IN THE SPECTRA OF EARLY-TYPE SUPERGIANTS
A. G. Nikoghossian
This is a theoretical study of changes in several characteristics of the spectra of early-type supergiants owing to scattering of continuum radiation. Thomson scattering on free electrons is treated as the scattering mechanism, although the approach used here is quite general. The reason for changes in the spectral class of a star with constant bolometric luminosity and surface temperature is elucidated and the necessary conditions for their realization are introduced. The effect of scattering in the continuum on the magnitude of the intensity jumps in the hydrogen spectrum depending on the degree of ionization is studied. The conditions under which the Schuster mechanism for formation of emission lines sets in are clarified both for an isothermal atmosphere and for an atmosphere with a temperature gradient. Keywords: supergiant stars: intensity discontinuities: emission lines: Thomson scattering
1. Introduction
Stars of spectral class A and adjacent subclasses of types B and F occupy an important place among the supergiants of early types. These stars are visually the brightest, not only in our galaxy but in other galaxies as well. There is indisputable proof of the efflux of matter and the existence of expanding shells for all these objects. As part of the bright blue variable (LBV) stars, they are characterized by a relatively quiescent state that is occasionally
V. A. Ambartsumyan Byurakan Astrophysical Observatory, Armenia; e-mail: [email protected]
Original article submitted June 12, 2020. Translated from Astrofizika, Vol. 63, No. 4, pp. 611-629 (November 2020) 534
0571-7256/19/6304-0534 ©2020 Springer Science+Business Media, LLC
interrupted by the ejection of stellar matter. The shells are accelerated by radiative pressure and form more or less opaque winds with a relatively low finite velocity [1,2]. It is well known that scattering of radiation in the continuum plays an important role in the formation of these stars. Of the range of possible scattering mechanisms, Thomson scattering on free electrons is usually considered. The high temperature, rarefied plasma with highly ionized hydrogen and, partially, helium make this a basic mechanism [3]. An understanding of the importance of accounting for continuum scattering goes back to the work of Schuster [4]. The mechanism of Thomson scattering on free electrons as applied to the atmospheres of stars of early types was discussed by Ambartsumyan [5]. He concluded that the color temperatures of these stars might differ from their effective temperatures, so that replacement of the latter by the color temperatures leads to a reduction in the estimated radii of stars of this kind. For temperatures on the order of 105 K he determined an upper limit on the gas density below which the mechanism of Thomson scattering becomes important. One of the effects of continuum scattering is its influence on the line spectrum. In particular, the appearance of lines in abs
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