Determination of Mass-Loss Rates of PG 1159 Stars from Far-Ultraviolet Spectroscopy (original) (raw)

Abstract

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 λλ1032, 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(Ṁ/Msolar yr-1)=-8,...,-7, as compared to the [WCE] stars that have mass-loss rates of about log(Ṁ/Msolar yr-1)=-5.5,...,-6.5. By comparing with theory, we conclude that the wind of PG 1159 stars is driven by radiation pressure.

Based on the development and utilization of ORFEUS (Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometers), a collaboration of the Institute for Astronomy and Astrophysics at the University of Tübingen, the Space Astrophysics Group of the University of California at Berkeley, and the Landessternwarte Heidelberg.