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Study of sdO models: pulsation analysis




We have explored the possibility of driving pulsation modes in models of subluminous O-stars (sdOs), in which the effects of element diffusion, gravitational settling and radiative levitation have been neglected so that the distribution of iron-peak elements remains uniform throughout the evolution. The stability of these models was determined using a non-adiabatic oscillations code. We analysed 27 sdO models from 16 different evolutionary sequences and discovered the first ever sdO models capable of driving high-radial-order g modes. In one model, the driving is by a classical κ mechanism due to the opacity bump from iron-peak elements at temperature ∼200 000 K. In a second model, the driving results from the combined action of κ mechanisms operating in three distinct regions of the star: (i) a carbon–oxygen partial ionization zone at temperature ∼2 × 106 K; (ii) a deeper region at temperature ∼2 × 107 K, which we attribute to ionization of argon, and (iii) at the transition from radiative to conductive opacity in the core of the star.