Morphology of Circumstellar Environment and Some Characterictics of Circumstellar Shells of Stars with the R Coronae Borealis Variability

Abstract

The well-known light minima of stars with the R Coronae Borealis variability are caused by the formation of an additional circumstellar dust shell, the screening shell, inside the permanent shell. Under the assumption of uniform distribution of matter in the circumstellar environment we estimated the optical thickness of the permanent gas-and-dust shell at 0.2-0.7, and its geometrical thickness is no less than 0.4 of its own radius. The wavelength dependence of extinction is close to neutral. From spectral observations of R CrB itself in the 1985 minimum we traced the transformation of the stellar linear and molecular absorption spectrum to the emission spectrum and established that the fast variation of the U-B colour index by -0.6 in the light decline was caused purely by a change of the spectrum type. The spectrum transformation causes an increase of star brightness in the U, B, and V bands by about 1.4, 0.75, and 0.75 mags, correspondingly. It is suggested that a high-velocity (>200 km/s) matter stream through the circumstellar environment is the cause of the excitation of the emissions observed during light minima when the photospheric flux is weakening.