The progenitor of the “born-again” core V605 Aql and the relation to its younger twin V4334 Sgr

Abstract

We derived the properties of V605 Aql before the final helium flash pulse by studying its surrounding PN A58. Photoionizing models of our spectral data together with a new distance estimate and a closer look at the recombination timescales lead to a consistent model. Comparing our findings with the only hydrogen-poor twin, namely Sakurai's Object, we conclude that these born-again objects have normal PNe core masses. We are able to prove V605 Aql indeed to be, similar to V4332 Sgr, a very late thermal pulse object and to put constraints on the evolutionary time scales for the transition back to the AGB.