Model for R Coronae Borealis stars
Abstract
We propose an evolutionary model in which R Coronae Borealis stars are in a second common-envelope phase, with a low mass companion orbiting well inside the envelope. The high mass loss rate in the first common-envelope phase turns the massive star into a hydrogen-deficient star. In the second common-envelope phase, as a result of shear induced by the inner envelope rotation and possibly by excited gravity-wave induced turbulence, carbon and oxygen are being dredged up from the core into the envelope. This evolutionary scenario smoothly connects R CrB stars to the double-mass-loss-episodes evolutionary scenario of hydrogen-deficient binaries suggested by Plavec in 1973. This model may have consequences for the nature of mass loss in R Coronae Borealis stars: The orbiting companion breaks spherical symmetry allowing for nonradial pulsations and asymmetric mass loss. The optical polarization and IR observations may point to axisymmetric mass loss with the same axis of symmetry for the different mass loss episodes.