Recombination of Hot Ionized Nebulae: The Old Planetary Nebula around V4334 Sgr (Sakurai’s Star)

After becoming ionized, low-density astrophysical plasmas will begin a process of slow recombination. Models for this still have significant uncertainties. Recombination cannot normally be observed in isolation, because the ionization follows the evolutionary timescale of the ionizing source. Laboratory experiments are unable to reach the appropriate conditions because of the very long required timescales. The extended nebula around the very late helium pulse (VLTP) star V4334 Sgr provides a unique laboratory for this kind of study. The sudden loss of the ionizing UV radiation after the VLTP event has allowed the nebula to recombine free from other influences. More than 290 long-slit spectra taken with FORS1/2 at ESO’s Very Large Telescope between 2007 and 2022 are used to follow the time evolution of the lines of H, He, N, S, O, and Ar. Hydrogen and helium lines, representing most of the ionized mass, do not show significant changes. A small increase is seen in [N ii ] (+2.8% yr −1 ; 2.7 σ significance), while we see a decrease in [O iii ] (−1.96% yr −1 ; 2.0 σ significance). The [S ii ] lines show a change of +3.0% yr −1 (1.6 σ significance). The lines of [S iii ] and of [Ar iii ] show no significant changes. For [S iii ], the measurement differs from the predicted decrease by 4.5 σ . A possible explanation is that the fractions of S 3+ and higher are larger than expected. Such an effect could provide a potential solution for the sulfur anomaly in planetary nebulae.