Nonradial instability strips based on carbon and oxygen partial ionization in hot, evolved stars

The authors have calculated periods and growth rates for a new class of nonradially pulsating variable stars in the linear, nonradial, nonadiabatic theory. The best observed member of this class is PG 1159-035 which is observed to be pulsating in a number of periods around 500 s and may well have an effective temperature exceeding 105K. Stellar envelopes constructed for a 0.6 M_sun; star are in the effective temperature range 7×104K ≤ Te ≤ 1.5×105K following the white dwarf cooling track at radii 0.016 - 0.030 R_sun;. These envelopes have compositions of half carbon and half oxygen by mass. The authors present their nonradial results and discuss the newly discovered instability strip in some detail. All unstable nonradial modes in the period range from ≡25 s to ≡1200 s and their growth rates are described. The implications of these results for stellar evolution studies are examined.