On the frequency of planetary nebula nuclei powered by helium burning and on the frequency of white dwarfs with hydrogen-deficient atmospheres

Abstract

The evolutionary behavior of a model central star of a planetary nebula is examined as a function of the phase in its nuclear burning cycle when its progenitor leaves the asymptotic giant branch for the first time. Models may be assigned to one of six distinct groups, and estimates may be made of the probability that a real star will fall into a particular group. In particular, it is suggested that approximately 25% of all central stars of planetary nebulae will fall into groups whose members are expected to become hydrogen-deficient white dwarfs. This suggestion is not in disagreement with the observations. It is argued that winds of at least two types are responsible for this result: a low-velocity or slow wind (or ordinary or super strength) which operates when, and if, the central star returns to the asymptotic giant branch after experiencing a final helium shell flash; and a high-velocity or fast wind which operates when the central star is at high enough surface temperatures that emitted photons will excite nebular emission. It is also inferred that roughly half of all bright central stars of planetary nebulae are burning helium rather than hydrogen. Finally, an attempt is made tomore » assess the relevance of extant models for understanding exotic planetary nebulae such as A30 and A78, helium-rich stars of the R Coronae Borealis variety, and the chameleon star FG Sagittae.« less