Observations and three-dimensional ionization structure of the planetary nebula SuWt 2

TL;DR

This study combines optical spectroscopy and 3D photoionization modeling to investigate the ionization structure and evolutionary status of the unusual planetary nebula SuWt 2, revealing a likely hydrogen-deficient, PG 1159-type central star that experienced a very late thermal pulse.

Contribution

It provides the first detailed 3D ionization analysis of SuWt 2 and constrains the nature and evolutionary stage of its central star, suggesting a very late thermal pulse and a PG 1159-type star.

Findings

Central star is likely hydrogen-deficient PG 1159-type.

The nebula's age is about 25,000 years, consistent with the star's evolutionary stage.

The star's mass is approximately 0.64 solar masses, originating from a 3 solar mass progenitor.

Abstract

The planetary nebula SuWt 2 (PN G311.0+02.4), is an unusual object with a prominent, inclined central emission ellipse and faint bipolar extensions. It has two A-type stars in a proven binary system at the centre. However, the radiation from these two central stars is too soft to ionize the surrounding material leading to a so far fruitless search for the responsible ionizing source. Such a source is clearly required and has already been inferred to exist via an observed temporal variation of the centre-of-mass velocity of the A-type stars. Moreover, the ejected nebula is nitrogen-rich which raises question about the mass-loss process from a likely intermediate-mass progenitor. We use optical integral-field spectroscopy to study the emission lines of the inner nebula ring. This has enabled us to perform an empirical analysis of the optical collisionally excited lines, together with a fully three-dimensional photoionization modelling. Our empirical results are used to constrain the photoionization models, which determine the evolutionary stage of the responsible ionizing source and its likely progenitor. The time-scale for the evolutionary track of a hydrogen-rich model atmosphere is inconsistent with the dynamical age obtained for the ring. This suggests that the central star has undergone a very late thermal pulse. We conclude that the ionizing star could be hydrogen-deficient and compatible with what is known as a PG 1159-type star. The evolutionary tracks for the very late thermal pulse models imply a central star mass of ~ 0.64M${}_{\odot}$, which originated from a ~ 3M${}_{\odot}$ progenitor. The evolutionary time-scales suggest that the central star left the asymptotic giant branch about 25,000 years ago, which is consistent with the nebula's age.