The discovery of nonradial instability strips for hot, evolved stars

Three different interior compositions are used in the present radial and nonradial, linear, nonadiabatic pulsation analyses of model stellar envelopes in the effective temperature range of 80,000-150,000 K, with total masses of 0.6 solar masses and radii chosen so that they line up along a prewhite dwarf cooling curve: (1) 100 percent C; (2) 50 percent C-12 and 50 percent O-16 by mass; and (3) 10 percent C-12 and 90 percent O-16 by mass. Radial and nonradial instability strips are found for each composition, caused the partial ionization of C, or O, or both. Attention is given to the cause of the pulsations of the hot, evolved star PG 1159-035, which, although perhaps too hot to lie within the proposed instability strips, is in some measure of agreement with models having significant amounts of O-16 near the stellar surface. This implies a greater production of O-16 through He burning than previously supposed.