Radial pulsations of helium stars with masses from 1 to 10 M⊙

We present the results of our hydrodynamic calculations of radial pulsations in helium stars with masses 1 M ⊙ ≤ M ≤ 10 M ⊙ , luminosity-to-mass ratios 1 × 10 3 L ⊙ / M ⊙ ≤ L / M ≤ 2 × 10 4 L ⊙ / M ⊙ , and effective temperatures 2 × 10 4 K ≤ T eff ≤ 10 5 K for mass fractions of helium Y =0.98 and heavy elements Z =0.02. We show that the lower boundary of the pulsation-instability region corresponds to L / M ∼ 10 3 L ⊙ / M ⊙ and that the instability region for L / M ≲ 5 × 10 3 L ⊙ / M ⊙ is bounded by effective temperatures T eff ≲ 3 × 10 4 K. As the luminosity rises, the instability boundary moves into the left part of the Hertzsprung-Russell diagram and radial pulsations can arise in stars with effective temperatures T eff ≲ 10 5 K at L / M ≳ 7 × 10 3 L ⊙ / M ⊙ . The velocity amplitude for the outer boundary of the hydrodynamic model increases with L/M and lies within the range 200 ≲ Δ U ≲ 700 km s −1 for the models under consideration. The periodic shock waves that accompany radial pulsations cause a significant change of the gas-density distribution in the stellar atmosphere, which is described by a dynamic scale height comparable to the stellar radius. The dynamic instability boundary that corresponds to the separation of the outer stellar atmospheric layers at a superparabolic velocity is roughly determined by a luminosity-to-mass ratio L / M ∼ 3 × 10 4 L ⊙ / M ⊙ .