Hydrogen-deficient central stars of planetary nebulae
H. Todt, A. Y. Kniazev, V. V. Gvaramadze, W.-R. Hamann, M. Pena, G. Gräfener, D. Buckley , L. Crause, S. M. Crawford, A. A. S. Gulbis , C. Hettlage, E. Hooper, T.-O. Husser, P. Kotze, N. Loaring, K. H. Nordsieck, D. O'Donoghue, T. Pickering, S. Potter, E. Romero-Colmenero, P. Vaisanen, T. Williams, M. Wolf
A significant number of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient and are considered as the progenitors of H-deficient white dwarfs. Almost all of these H-deficient CSPNe show a chemical composition of helium, carbon, and oxygen. Most of them exhibit Wolf-Rayet-like emission line spectra and are therefore classified as of spectral type [WC]. In the last years, CSPNe of other Wolf-Rayet spectral subtypes have been identified, namely PB 8 (spectral type [WN/WC]), IC 4663 and Abell 48 (spectral type [WN]). We performed spectral analyses for a number of Wolf-Rayet type central stars of different evolutionary stages with the help of our Potsdam Wolf-Rayet (PoWR) model code for expanding atmospheres to determine relevant stellar parameters. The results of our recent analyses will be presented in the context of stellar evolution and white dwarf formation. Especially the problems of a uniform evolutionary channel for [WC] stars as well as constraints to the formation of [WN] or [WN/WC] subtype stars will be addressed.