New evolutionary calculations for the born again scenario

Abstract

We present evolutionary calculations to describe the born-again scenario for post-AGB remnant stars of 0.5842 and 0.5885 M . Results are based on a detailed treatment of the physical processes responsible for the chemical abundance changes. We considered two theories of convection: the standard mixing length theory (MLT) and the double-diffusive GNA convection. The latter accounts for the effect of the chemical gradient (∇µ) in the mixing processes and in the transport of energy. We also explore the dependence of born-again evolution on some physical hypotheses, such as the effect of the existence of non-zero chemical gradients, the prescription for the velocity of the convective elements and the size of the overshooting zones. Attention is paid to the behavior of the born-again times and to the chemical evolution during the ingestion of protons. We find that in our calculations born again times are dependent on time resolution. In particular when the minimum allowed time step is below 5 × 10 −5 yr we obtain, with the standard mixing length theory, born again times of 5-10 yr. This is true without altering the prescription for the efficiency of convective mixing during the proton ingestion. On the other hand we find that the inclusion of chemical gradients in the calculation of the mixing velocity tends to increase the born again times by about a factor of two. In addition we find that proton ingestion can be altered if the occurrence of overshooting is modified by the ∇µ-barrier at the H-He interface, significantly changing born again times.

Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars

“…PIEs can lead to a splitting of the convective region, for example, as energy released by H burning creates a temperature inversion inside the convection zone. The amount of protons ingested in our model is small (up to 6 × 10 −6 M ), but in some PIE models this has been found to be enough to start the splitting of the convective zone (Miller Bertolami et al 2006;Herwig et al 2011). However, this effect is model dependent: the 3D PIE models of Stancliffe et al (2011) did not find the split to occur even for larger amounts of protons, in contrast to the 3D PIE models of Woodward et al (2015).…”

Imaging ejecta from the final flash star V605 Aquilae

“…Lawlor & MacDonald (2003) predicted that V605 Aql will cool back towards the AGB within the next 50−70 years and then resemble FG Sge. Similar two loop models are presented by Miller Bertolami et al (2006). However, FG Sge is more often identified as a LTP object as the surface abundances and time scale are not predicted correctly by a VLTP (Jeffrey & Schönberner 2006).…”

Full evolutionary models for PG 1159 stars. Implications for the helium-rich O(He) stars

“…As can be noted, our numerical simulation covers all the evolutionary phases from the ZAMS to the domain of the early-WD including the stages corresponding to the helium thermal pulses at the tip of the AGB and the born again episode. This last stage is particularly fast and evolution proceeds on a scale of years (Herwig 2001;Miller Bertolami et al 2006). During this stage the star is ballooned back to the giant region both by the helium flash and by the violent H-burning that occurs at the VLTP.…”