[vsnet-chat 2997] Re: R CrB variability

["A.E. Rosenbush" <mijush@mao.kiev.ua>]

>It seems to be at least two main kinds of RCB behaviour: the RCB that fall
>from grace and the one-time performers. V366 Lac faded deeply but there is
>another star I can't recall now that also "dissapeared" from the sky. And we
>have V4334 Sgr, Sakurai's Object, that appeared suddenly and vanished
>showing RCB behaviour but with too great amplitude. Or we have FG Sge, that
>has changed its spectrum from B to K. Are we observing examples of real time
>evolution? May a star go through this evolutinary phase that fast? And if
>they are really examples of RCB, why do other stars spend more time behaving
>like that?


In any case the evolutionary stage with the R CrB type variability is 
very short-term. FG Sge quickly changes some of its variability parameters: 
the last deep minimum was once and lasted one pulsation period! It "grazes" 
this stage?
UV Cas before our eyes finished this variability in the beginning of 
the XX century. 
The mass of similar stars is low, therefore the mass layers with one 
abundance are very thin. 
And V4334 Sgr (Sakurai's Object) is the unique example, 
and I think very slow nova. 
(Or it is the bridge between the novae and the planetary nebulae.) 
It is quite possible there is very small group (one population) 
of classical R CrB (R CrB itself, RY Sgr, SU Tau and one-two more) stars.  
Only now we accumulate knowledge on these unique objects 
from visual (in the first turn), infrared, UV, spectral, polarimetry, 
theoretic.. 
Previously one identificated the R CrB stars on their deep minima. 
and now it can make on rather shallow minima but very typical shape. 

My best wishes, 
clear sky to all, 
Alexander Rosenbush

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