WN] central stars of planetary nebulae

Abstract

Hydrogen‐deficient central stars are commonly considered as the progenitors of H‐deficient white dwarfs. Spectroscopically, many H‐deficient central stars resemble massive Wolf‐Rayet stars of the carbon sequence and are therefore classified as [WC] stars. The massive WR stars of the nitrogen sequence (WN), however, have no spectroscopic counterpart among the central stars. With PB 8 we found for the first time a central star with a WR‐type emission line spectrum that resembles the nitrogen sequence with only a slight enhancement of carbon lines, and therefore we classified this star as [WN/C]. Our analysis reveals that its atmosphere consists mainly of helium, with some hydrogen and only traces of carbon, nitrogen, and oxygen. This is very different from any other Wolf‐Rayet type central stars. The results of our analyses, especially the chemical composition, strongly constrains possible scenarios for the formation of PB 8. For the time being, we don’t know any path of single‐star evolution that could explain this enigmatic central star. In this context, we will also discuss the status of the central star of PMR 5, which is another candidate for a [WN] spectral type.