Geoffrey Clayton, Department of Physics & Astronomy,
Louisiana State University, Baton Rouge, LA 70808, USA
The R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich supergiants which undergo spectacular declines in brightness of up to 8 magnitudes at irregular intervals as dust forms along the line of sight. Most RCB stars have G-K star temperatures. Only about 30 of these stars are known. Their rarity may stem from the fact that they are in an extremely rapid phase of the evolution toward white dwarfs. Understanding the RCB stars is a key test for any theory which aims to explain hydrogen deficiency in post-Asymptotic Giant Branch (AGB) stars.
There are two major evolutionary models for the origin of RCB stars: the Double Degenerate and the Final Helium Shell Flash. The former involves the merger of two white dwarfs, and in the latter a white dwarf/evolved Planetary Nebula (PN) central star is blown up to supergiant size by a final helium flash. A star descending the white dwarf cooling track can undergo a final thermal pulse when the helium shell ignites causing the star to expand to giant dimensions. In the final flash model, there is a close relationship between RCB stars and PN. The connection between RCB stars and PN has recently become stronger, since the central stars of three old PNe (Sakurai's Object, V605 Aql and FG Sge) have had observed outbursts that transformed them from hot evolved central stars into cool giants with the spectral properties of an RCB star.