1. Shell-model parameters of a star with the R Coronae Borealis type variability Alexander E. Rosenbush Main Astronomical Observatory of the National Academy of Sciences of the Ukraine, Zabolotnoho str. 27, 03680 Kyiv, Ukraine e-mail: aeros@mao.kiev.ua

2. INTRODUCTION Against the spherical shell model of visual light minimum two basic arguments are put forward: 1 - absence of any correlation between variations in infra-red excess and visual light variation during a minimum (Forrest et al. 1972) ; 2 - decrease of color indexes of a star during a light minimum (Tatarnikov & Yudin 1998) ;

3. It is proposed to return to the model of homogeneous circumstellar shell with one important addition: the visual light minimum is caused by formation of one more shell, internal in relation to the permanent shell.

4. The circumstellar environment of a star with the R Coronae Borealis type variability out of a light minimum.

5. The circumstellar environment of a star with the R Coronae Borealis type variability in a light minimum.

6. A light curve of R CrB in 70-th years of XX century. The IR observations in 1976 according to Shenavrin & Khruzina (1979). (Some stars by data Feast et al. 1997.) Δ t ~ 60-100 d

7. Dependence of a polarization degree of radiation in the photometric V band at R CrB versus the light decline during light minima. Polarization during the light recovery is always less than by the decline. I shell II shell

8. Light V and color B-V and U-B indexes curves of R CrB in 1985 minimum. Da ta of Goncharova (1990) and Efimov (1988)). Horizontal dashed lines – an normal level of the V and, the B-V and the U-B. Vertical solid lines – the moments of key changes in the stellar line spectrum.

9. The scheme of the formation of a blue-shifted profile of a broad emission line in the permanent shell. The screening shell eclipses leaving parts of the permanent shell. Profile of broad emission is superposition: 1 – broad emission with the screening red wing; 2 – sharp emission; 3 – photospheric absorption; 4 – high-speed circumstellar absorption The broad emissions are traced up to 1000 a.u. from a star as, for example, C II 133.5 nm in V854 Cen (Clayton & Ayres 2001)

10. Profiles of cores of the IR triplet Са II line λ 854.2 nm in spectrum of R CrB in the 1998 minimum normalized to the continuum in a light maximum. Sharp emission lines are formed between the screening and permanent shells.

11. Very essential argument in favour of our model we consider consecutive development of the RCB phenomenon in FG Sge: The increase of the IR excess in 1992 was a consequence of the formation of the screening shell. FG Sge has allowed to define directly the optical thickness of a permanent shell after comparison of its brightness before and after 1992: τ ~ 0.7.

12. Stellar environment Out of a light minimum In a light minimum

13. Some physical parameters of a star with the R Coronae Borealis phenomenon *) 25 km/s is the velocity of matter on the internal bound of the screening and permanent shells, i.e., before a dust condensation. The velocity of matter after the dust condensation is increased rapidly up to 200 km/s and more (previously it was present). ~ 25*  200

14. Observed light curve in 1998 – 2003 (the VSNET) and its approximation

15. Observed light curve in 2007 and its approximation, as of September 8 (data by the VSNET) First shell, τ = 6.3 Second shell, τ = 5.7 If it will be not form the third shell shell formation Max optical thickness

16. Comparison of visual V and 38.6-days pulsations UV( 240 nm) brightness of visual (squares) of RY Sgr. and UV (plus) light of RY Sgr

17. CONCLUSION At a result of my researches of stars with the R Coronae Borealis type variability I came to understanding that it is necessary to investigate the phenomenon of R Coronae Borealis widespread among stars on final phases of evolution, novae, for example. It is possible to give such definition of the R Coronae Borealis type variability or the R Coronae Borealis phenomenon. The R Coronae Borealis phenomenon is the phenomenon, which meets in stars on the late stages of the stellar evolution possessing both the high mass-loss rate by sub-Eddington luminosity, the overabundance of carbon and the high enough abundance of hydrogen. Enough of hydrogen in the atmosphere of a star is the necessary condition of processes which leads to light minima and the full exhaustion of hydrogen means disappearance of the RCB phenomenon in the star (Jurcsik 1996: the II Conference).

18. One of two fundamental questions Answers: by Rao et al. (1999): what are the physical processes that trigger and control development of the unpredictable minima? <─ Pulsations of a star <─ Maximum (Pugach 1977, ….) and <─ Minimum (RY Sgr: Feast 1996, FG Sge: Arkhipova 1996) of pulsation (Göres, Woitke et al., 1996…) <─ Activity of star controls by the 4284-days cycle (visual - Rosenbush 1997, 2001; confirmed in the infrared - Yudin et al. 2002 = 4342 days). =>

19. Synchronization of the 4284-days activity cycle of R CrB itself and the H-def Conferences Number of cycle Duration of Cycle Month year V4435August 1983 VI4291October 1995 VII429130 June 2007 VIII4284: (±140) 20 March: 2019 Number of Conference City, country Month Year IMaysor India November 1985 IIBamberg Germany August 1995 IIITübingen Germany September 2007 IV ? October  2019

20. This is all! Thank you very much for your attention!