Past and Future Evolution of Sakurai's Object

Abstract

Abstract. We present a stellar evolution model sequence of the past and future evolution of the post-AGB born again star Sakurai's object (V 4334 Sgr). In order to match the short evolutionary time scale we have to assume that the convective ingestion of hydrogen-rich envelope material into the He-flash convection zone proceeds slower than predicted by the mixing length theory. For the future we predict a swift second evolution through the central star region before a second born-again evolution occurs.The fast evolving born again asymptotic giant branch (AGB) star V 4334 Sgr provides a unique opportunity to test stellar evolution and nucleosynthesis models of the very late thermal pulse (VLTP) which occurs in about 10% of all central stars of planetary nebulae (CSPN) (Then & McDonald, 1995). V 4334 Sgr displayed an extraordinary fast evolution from the pre-white dwarf (WD) stage to the AGB in only three years (Duerbeck et al., 2000). In addition a peculiar realtime abundance evolution has been documented for this star by Asplund et al. (1999). We address both of these properties by exploring theoretical models. For example, Herwig & Langer (2001) proposed a new nucleosynthesis mechanism for the lithium overabundance of V 4334 Sgr, because the well known hot-bottom burning mechanism which operates in massive AGB stars had to be excluded here. However, the latest VLTP stellar evolution model by Herwig et al. (1999) could not match the observed fast evolution of V 4334 Sgr although considerable effort had been put into a consistent numerical method for simultaneous mixing and nucleosynthesis. Herwig (2001) assumed that convective mixing of envelope material (cold fuel) into the He-flash convection zone is retarded with respect to the mixing-length theory (MLT) velocity. In that case the p-capture nuclear energy is released closer to the surface which leads to a faster inflation of the outermost layers. With this model the observed born again evolution time scale of V 4334 Sgr can be matched (Fig.

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

“…Furthermore V605 Aql should have stayed at least several decades at the AGB before reheating to the temperature seen nowadays in the CIV lines (Kimeswenger 2003, Lechner & Kimeswenger 2003. This gives somewhat longer timescales than those predicted by Lawlor & MacDonald (2003) or by Herwig (2003).…”

“…Whilst Iben et al (1983) do not show such a loop at all, the transition time for the model of Lawlor & MacDonald (2003) gives timescales of only a few tens of years for the heating process after the first "Nova event" for the transition E-F. The timescale in Herwig (2003) is even shorter. Then it stays near F for a long time at high luminosity and high temperature.…”

“…The timescale for this transition is very controversial. While Lawlor & MacDonald have transition timescales of up to several thousand years, Iben et al (1983) and Iben & MacDonald (1995) give a few tens of years, Herwig et al (1999) and Herwig (2001Herwig ( , 2003 give a timescale of less than one year for a 0.604 M CSPN.…”

New evolutionary calculations for the born again scenario

“…These comings and goings in T eff caused by H and He-burning driven expansions produce the typical double loop in the HR diagram (see Fig. 6; and also Lawlor & MacDonald 2003;and Herwig 2003).…”

Element Abundance Determination in Hot Evolved Stars

“…The high effective temperatures require non-LTE modeling of the atmospheres. Such models for H-deficient compositions have only become Evolutionary tracks are from [14] and [1] (dashed lines), [23] and [6] (dot-dashed line) (labels: mass in M⊙). The latter 0.604 M⊙ track is the final CSPN track following a VLTP evolution and therefore has a H-deficient composition.…”