Pulsating White Dwarf - GW Vir Stars

GW Vir Stars

The third known class of pulsating variable white dwarfs is the GW Vir stars, sometimes subdivided into DOV and PNNV stars. Their prototype is PG 1159-035., §1.1. This star (also the prototype for the class of PG 1159 stars) was observed to vary in 1979, and was given the variable star designation GW Vir in 1985, giving its name to the class. These stars are not, strictly speaking, white dwarfs; rather, they are stars which are in a position on the Hertzsprung-Russell diagram between the asymptotic giant branch and the white dwarf region. They may be called pre-white dwarfs., § 1.1; They are hot, with surface temperatures between 75,000 K and 200,000 K, and have atmospheres dominated by helium, carbon, and oxygen. They may have relatively low surface gravities (log g ≤ 6.5.), Table 1 It is believed that these stars will eventually cool and become DO white dwarfs., § 1.1.

The periods of the vibrational modes of GW Vir stars range from about 300 to about 5,000 seconds., Table 1 How pulsations are excited in GW Vir stars was first studied in the 1980s but remained puzzling for almost twenty years. From the beginning, the excitation mechanism was thought to be caused by the so-called κ-mechanism associated with ionized carbon and oxygen in the envelope below the photosphere, but it was thought this mechanism would not function if helium was present in the envelope. However, it now appears that instability can exist even in the presence of helium., §1.