Central stars of planetary nebulae: The white dwarf connection

TL;DR

This review explores the transition of hydrogen-deficient central stars of planetary nebulae into white dwarfs, highlighting new spectral discoveries, evolutionary sequences, and chemical compositions in this phase.

Contribution

It presents new spectral line identifications, evidence for multiple evolutionary pathways, and insights into the chemical evolution of hydrogen-deficient post-AGB stars.

Findings

Discovery of Ne VII and Ne VIII lines in PG1159 stars

Evidence for two distinct post-AGB evolutionary sequences

Identification of a hydrogen-deficient post-super AGB evolution sequence

Abstract

This paper is focused on the transition phase between central stars and white dwarfs, i.e. objects in the effective temperature range 100,000 - 200,000 K. We confine our review to hydrogen-deficient stars because the common H-rich objects are subject of the paper by Ziegler et al. in these proceedings. We address the claimed iron-deficiency in PG1159 stars and [WC] central stars. The discovery of new Ne VII and Ne VIII lines in PG1159 stars suggests that the identification of O VII and O VIII lines that are used for spectral classification of [WCE] stars is wrong. We then present evidence for two distinct post-AGB evolutionary sequences for H-deficient stars based on abundance analyses of the He-dominated O(He) stars and the hot DO white dwarf KPD0005+5106. Finally, we report on evidence for an H-deficient post-super AGB evolution sequence represented by the hottest known, carbon/oxygen-atmosphere white dwarf H1504+65 and the recently discovered carbon-atmosphere "hot DQ" white dwarfs.