Evolution of Hen\,3-1357, the Stingray Nebula

TL;DR

This paper documents the spectroscopic evolution of the Stingray Nebula over three decades, showing its recombination phase and changes in the central star's temperature, with implications for stellar evolution models.

Contribution

It provides a detailed analysis of the nebula's changing physical conditions and chemical abundances, suggesting the star experienced a late thermal pulse and is returning to the AGB phase.

Findings

Decreasing intensity of highly-ionized emission lines over time.

Increasing emission of low-ionization lines, indicating recombination.

Central star's temperature has decreased from 60,000 K to about 40,000 K.

Abstract

The spectroscopic evolution of Hen\,3-1357, the Stingray Nebula, is presented by analysing data from 1990 to 2021. High resolution data obtained in 2021 with South African Large Telescope High Resolution Spectrograph and in 2009 with European Southern Observatory-Very Large Telescope UVES spectrograph are used to determine physical conditions and chemical abundances in the nebula. From comparison of these data with data from different epochs it is found that the intensity of highly-ionized emission lines has been decreasing with time, while the emission of low-ionization lines has been increasing, confirming that the nebula is recombining, lowering its excitation class, as a consequence of the changes in the central star which in 2002 had an effective temperature of 60,000 K and from then it has been getting colder. The present effective temperature of the central star is about 40,000 K. It has been suggested that the central star has suffered a late thermal pulse and it is returning to the AGB phase. The nebular chemistry of Hen\,3-1357 indicates that all the elements, except He and Ne, present sub solar abundances. The comparison of the nebular abundances with the values predicted by stellar nucleosynthesis models at the end of the AGB phase, shows that the central star had an initial mass lower than 1.5 M$_{\odot}$. We estimated the ADF(O$^{+2}$) to be between 2.6 and 3.5.