Abstract
Abstract Stellar post asymptotic giant branch (post-AGB) evolution can be completely altered by a final thermal pulse (FTP) which may occur when the star is still leaving the AGB (AFTP), at the departure from the AGB at still constant luminosity (late TP, LTP) or after the entry to the white-dwarf cooling sequence (very late TP, VLTP). Then convection mixes the He-rich material with the H-rich envelope. According to stellar evolution models the result is a star with a surface composition of $\mathrm{H}\approx \, 20$ per cent by mass (AFTP), ≈ 1 per cent (LTP), or (almost) no H (VLTP). Since FTP stars exhibit intershell material at their surface, spectral analyses establish constraints for AGB nucleosynthesis and stellar evolution. We performed a spectral analysis of the so-called hybrid PG 1159-type central stars (CS) of the planetary nebulae Abell 43 and NGC 7094 by means of non-local thermodynamical equilibrium models. We confirm the previously determined effective temperatures of $\mbox{$T_\mathrm{eff}$}\, = 115\, 000\pm 5\, 000 \, \mathrm{K}$ and determine surface gravities of $\log\,(g\,/\,(\mathrm{cm}\,\mathrm{s}^{-2} )) = 5.6\pm 0.1$ for both. From a comparison with AFTP evolutionary tracks, we derive stellar masses of $0.57^{+0.07}_{-0.04}$ M⊙ and determine the abundances of H, He, and metals up to Xe. Both CS are likely AFTP stars with a surface H mass fraction of 0.25 ± 0.03 and 0.15 ± 0.03, respectively, and an Fe deficiency indicating subsolar initial metallicities. The light metals show typical PG 1159-type abundances and the elemental composition is in good agreement with predictions from AFTP evolutionary models. However, the expansion ages do not agree with evolution time-scales expected from the AFTP scenario and alternatives should be explored.
Highlights
Asymptotic giant branch (AGB) stars are important contributors to the formation of elements heavier than iron
Abundances of H, He, C, N, and Ne are reasonably well reproduced by AFTP models
AFTP models computed from sequences that include convective boundary mixing at the bottom of the pulse driven convective zone display O abundances much larger than those of our program stars
Summary
Asymptotic giant branch (AGB) stars are important contributors to the formation of elements heavier than iron (trans-iron elements, TIEs). It is composed of an inner C/O core, the two burning shells with an He, C, and O rich intershell region in between and an H-rich convective envelope on top. Still on the AGB (AGB final thermal pulse, AFTP), flash-induced mixing of the H-rich envelope (≈ 10−2 M ) with the He-rich intershell layer (≈ 10−2 M ) reduces the H abundance to about 10–20 per cent but H I lines remain detectable. If the nuclear burning is still ‘on’, i.e. the star evolves at constant luminosity, the mixing due to a late thermal pulse (LTP) reduces H below the detection limit (about 10 per cent by mass at the relatively high surface gravity).
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