Model Planetaries with rapidly evolving central stars: A case for FG Sge

Extract

Ever since the pioneering work of Schönberner (1979, A&A, 79, 108) and Iben (1984, ApJ, 277, 333) who showed that the evolution of post-AGB remnants might be affected by late thermal pulses of the helium-burning shell, resulting in a temporary growth of these objects to red giant dimensions, many attempts were made to explain a number of puzzling objects, among them the well-known variable central star FG Sge as well as the R CrB and PG 1159 stars, by this so-called “born-again AGB” scenario (Iben et al. 1983, ApJ, 264, 605; Iben & MacDonald 1995, in: White Dwarfs, Springer, p. 48). However, it is still not clear if the frequency of occurrence of such events is high enough as to be consistent with the number of born-again candidates. This is due to the very short evolutionary timescales during the pulse and the character of the post-pulse evolution which resembles the first post-AGB phase and makes it difficult for an observer to distinguish such objects from “normal” central stars.