Determination of mass-loss rates of PG 1159 stars from far-ultraviolet spectroscopy

We determine the mass-loss rates of four hot, low-gravity PG 1159 stars that are regarded as immediate descendants of Wolf-Rayet central stars of planetary nebulae (i.e,, early spectral type [WCE]). The sample consists of classical hydrogen-deficient PG 1159 stars (K1-16, NGC 246, and RX J2117.1+3412,) as well as one object of the very rare "hybrid" subtype, which also exhibits hydrogen lines (NGC 7094). The sample is complemented by the famous [WC]-PG 1159 transition object Abell 78. Our analysis is based on the O VI lambda lambda 1032, 1038 resonance line, which is the strongest wind feature in these objects. Far-UV observations were performed with the Berkeley spectrograph during the ORFEUS-SPAS II mission. One spectrum is taken from archive data of the ORFEUS-SPAS I mission, and another one was obtained with the Hopkins Ultraviolet Telescope during the Astro-2 mission. We find mass-loss rates in the range log (M/M. yr(-1)) = -8, ... , -7, as compared to the [WCE] stars that have mass-loss rates of about log(M/M. yr(-1)) = -5.5, ... , -6.5. By comparing with theory, we conclude that the wind of PG 1159 stars is driven by radiation pressure.