R Coronae Borealis variable

From Wikipedia, the free encyclopedia

A R Coronae Borealis variable (abbreviated RCB) is an eruptive variable star that varies in luminosity in two modes, one low amplitude pulsation, and one irregularly occurring sudden fading by 1 to 9 magnitudes. The prototype star R Coronae Borealis was discovered by the English amateur astronomer Edward Pigott in 1797, who first observed the enigmatic fadings of the star. Since then, about 30 RCB variables have been identified, making this class being a very rare kind of stars.

R Coronae Borealis variables are supergiant stars in the spectral classes F and G (by convention called "yellow"), with typical C₂ and CN molecular bands, characteristic for yellow supergiants. RCB star atmospheres do however lack hydrogen down to an abundance of 1 part per 1,000 down to 1 part per 1,000,000 compared to helium and other chemical elements, while the universal abundance of hydrogen is about 3 to 1. RCB stars are thus probably helium to carbon nuclear fusors.

The fadings are caused by condensation of carbon to soot, making the star fade in visible light, while measurements in infrared light exhibit no real luminosity decrease. The exact mechanisms how the carbon is condensed, in what stellar atmosphere layer (or outside the star), how it is transported there, and how it is dispersed, are unknown. Various of theories about the mechanisms have been formulated, but not been conclusively confirmed by observations, so the cause of the fadings, and the low hydrogen content, must be regarded as unsolved. However, there seems to be some analogies with Wolf-Rayet stars and hydrogen deficient carbon stars (HdC). There are a few R Coronae Borealis like stars that have an "earlier" (bluer) spectral type than the RCB:s proper.