The cooling-down central star of the planetary nebula SwSt\,1: a late thermal pulse in a massive post-AGB star?

TL;DR

This study investigates the evolution of the central star of planetary nebula SwSt 1, providing evidence for a late thermal pulse in a massive post-AGB star that caused temperature changes without significant expansion.

Contribution

It presents new spectral analyses showing the star's temperature decrease and loop in the H-R diagram, supporting the occurrence of a late thermal pulse in a massive post-AGB star.

Findings

Star's temperature dropped from 42 kK to 40.5 kK between 1976 and 1993.

Spectral changes indicate a loop in the H-R diagram.

Late thermal pulse can remove the H envelope without significant stellar expansion.

Abstract

SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing a late [WC]-type central star. Previous studies suggested that the nebular and stellar lines are slowly changing with time. We studied new and archival optical and ultraviolet spectra of the object. The [OIII] 4959 and 5007 A to $\mathrm{H}\beta$ line flux ratios decreased between about 1976 and 1997/2015. The stellar spectrum also shows changes between these epochs. We modeled the stellar and nebular spectra observed at different epochs. The analyses indicate a drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and 1993. We do not detect significant changes between 1993 and 2015. The observations show that the star performed a loop in the H-R diagram. This is possible when a shell source is activated during its post-AGB evolution. We infer that a late thermal pulse (LTP) experienced by a massive post-AGB star can explain the evolution of the central star. Such a star does not expand significantly as the result of the LTP and does not became a born-again red giant. However, the released energy can remove the tiny H envelope of the star.