Mapping the Instability Domains of GW Vir Stars in the Effective Temperature-Surface Gravity Diagram
We present the results of a detailed stability study of models of GW Vir pulsators. These are extremely hot, evolved, and compact stars that are on their way to the white dwarf cooling phase and that show multiperiodic luminosity variations due to gravity-mode pulsational instabilities. Our nonadiabatic survey confirms that cyclic ionization of carbon and oxygen is at the origin of the GW Vir phenomenon. We find that the extent of the instability domain in the logg-Teff plane is a strong function of the C and O content in the envelopes of these stars. Given that GW Vir stars show important variations in atmospheric composition from one object to another, this implies that the notion of a blue edge for the GW Vir instability domain is necessarily a ``fuzzy'' concept. We investigate the effects of varying the total mass, of adding hydrogen, and of changing the metallicity. We summarize our results in the form of extensive stability maps in the logg-Teff diagram and produce tables and graphs showing the expected ranges of excited periods under various conditions.