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

Abstract

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; 2.7σ significance), while we see a decrease in [O iii] (−1.96% yr−1; 2.0σ significance). The [S ii] lines show a change of +3.0% yr−1 (1.6σ 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 4.5σ. A possible explanation is that the fractions of S3+ and higher are larger than expected. Such an effect could provide a potential solution for the sulfur anomaly in planetary nebulae.

Whither or Wither the Sulfur Anomaly in Planetary Nebulae?

“…The S/H deficiency seen in Galactic PNs is suggested as due to underestimation of higher ionization stages of sulfur in photoionization models, e.g., Henry et al (2012) and Reichel et al (2022). S/H values from mid-infrared Spitzer spectra in Pottasch & Bernard-Salas (2015, hereafter PBS15) enable observation of the higher ionization stage S 3+ and are systematically higher than our results, with a median difference of 0.17 dex.…”

“…Henry et al (2012) tested the sulfur abundance deficit across a range of nebular properties but found no strong correlations. Recently, by studying the born-again nebula V4334 Sgr and using ESO Very Large Telescope (VLT) spectroscopic observations from 2006 to 2022, Reichel et al (2022) confirmed an [S III] level, contrary to expectation of a decrease, because the S 2+ stage, if it dominates, will rapidly recombine to S + . A possible explanation is recharge from underestimated fractions of higher ionization stages.…”