[WN] central stars of planetary nebulae

TL;DR

This paper discusses the spectral characteristics and evolutionary origins of hydrogen-deficient central stars of planetary nebulae, focusing on newly identified [WN] type stars that resemble massive Wolf-Rayet stars of the nitrogen sequence.

Contribution

It provides spectral analyses of [WN] type central stars and explores their evolutionary status, expanding understanding of hydrogen-deficient stellar evolution.

Findings

Identification of [WN] type central stars with nitrogen-rich spectra

Spectral analysis supports their similarity to massive Wolf-Rayet stars

Discussion of potential evolutionary pathways for [WN] stars

Abstract

While most of the low-mass stars stay hydrogen-rich on their surface throughout their evolution, a considerable fraction of white dwarfs as well as central stars of planetary nebulae have a hydrogen-deficient surface composition. The majority of these H-deficient central stars exhibit spectra very similar to massive Wolf-Rayet stars of the carbon sequence, i.e. with broad emission lines of carbon, helium, and oxygen. In analogy to the massive Wolf-Rayet stars, they are classified as [WC] stars. Their formation, which is relatively well understood, is thought to be the result of a (very) late thermal pulse of the helium burning shell. It is therefore surprising that some H-deficient central stars which have been found recently, e.g. IC 4663 and Abell 48, exhibit spectra that resemble those of the massive Wolf-Rayet stars of the nitrogen sequence, i.e. with strong emission lines of nitrogen instead of carbon. This new type of central stars is therefore labelled [WN]. We present spectral analyses of these objects and discuss the status of further candidates as well as the evolutionary status and origin of the [WN] stars.