Mark Walker

Winds, disruptions and accretion in R Coronae Borealis

Luminous stars can drive fast dust-winds from the surfaces of hydrogen snow clouds, leading to deep eclipses lasting for several months when a cloud’s orbit takes it within a few AU of the star. Such close interactions involve tidal disruption of the cloud, with the star accreting helium as a result. This scenario provides a simple account of the dimming events of R Coronae Borealis stars, and simultaneously explains why these events occur specifically in hydrogen-deficient, carbon-rich supergiants: as fast as it accretes, the helium is burnt on the surface of a degenerate core. By way of contrast we will show eclipse light-curves for systems where the luminosity is moderate or low, as would be expected for a non-degenerate disruptor/accretor. We also identify the source of the observed chromospheric emission lines and near-/mid-IR excess flux, in the context of this disruption/accretion model of RCBs.