Instability of radial modes at the Wolf-Rayet evolutionary stage

The evolution of a Population-I star with an initial mass M _ZAMS = 60 M _⊙ has been calculated. At the stage when a red giant turns into an early-type helium star, the vast bulk of the stellar mass is concentrated in a compact core surrounded by an extended envelope that is unstable with respect to radial oscillations. The range of effective temperatures within which the instability arises extends to T _eff ? 10⁵ K. For the models corresponding to the Wolf-Rayet evolutionary stage (5 × 10⁴ K ≤ T _eff ≤ 1.05 × 10⁵ K), hydrodynamic calculations of self-exciting radial stellar pulsations have been performed. The pulsational instability develops in a time interval comparable to the dynamic timescale. Once the amplitude has ceased to grow, the pulsational motions are nonlinear traveling waves propagating from the core boundary to the stellar surface. The velocity amplitude of the outer layers is 500 km s^?1 < ΔU < 10³ km s^?1, depending on the effective temperature. During the evolution of a helium star, the mean ratio of the maximum expansion velocity of the outer layers to the local escape velocity decreases and lies within the range 0.25 < U _max/v _esc < 0.6 for the models considered. The nonlinearity of the stellar pulsations is responsible for the increase in the mean radius (bar r) of the Lagrangian layers compared to the equilibrium radius r _eq. The effect of the increase in mean radius decreases with rising effective temperature from(bar r)/r ～ 10 at T _eff = 7 × 10⁴ K to (bar r)/r ≈ 2 at T _eff = 10⁵ K. The radial pulsation periods for the models considered lie within the range 0.1 day ≤ Π ≤ 1.6 day and the amplitude of the bolometric magnitude variations does not exceed 0 _. ^m 2.