Non-LTE analysis of four PG1159 stars.

Abstract

A spectral analysis of four members of the PG1159 class is performed. These stars are believed to be very hot pre-White Dwarf stars. It is shown that the photospheres of PG1159 stars are dominated by helium and carbon and that a significant amount of oxygen is present. It is also found that the nonradial pulsations observed in some PG1159 stars are driven by the cyclic ionization of carbon and oxygen.