On the evolution of those nuclei of planetary nebulae that experience a final helium shell flash

The authors suggest that some of the central stars of planetary nebulae experience a final thermal pulse after having achieved a white dwarf configuration and begun their descent along a cooling white dwarf sequence of nearly constant radius. Theoretical calculations show that there are two possible paths for the evolution of a PN nucleus past the final thermal pulse. In the first case, the pulse occurs well up on the white dwarf cooling track while the star is surrounded by a visible planetary nebula. The subsequent giant phase is of too short a duration to be observable, and the star next appears on the high-luminosity portion of the quiescent helium-burning track, where it may be identified with the nuclei of planetaries like Abell 30 and Abell 78. In the second case, the pulse occurs farther down on the track, when the surrounding nebulae would no longer be visible on existing surveys. The star remains longer in its giant state, where it is identified with the R CrB stars. Eventually, both of these fade to become non-DA white dwarfs.