Non-linear radial pulsations of hot extreme helium stars

Abstract

Hydrodynamic models of helium stars, with masses of 0.7 and 0.6 M⊙, bolometric magnitudes in the range – 4 to – 6 mag and effective temperatures in the range 9000 to 3 × 104 K, are considered. The vertical part of the blue boundary of the pulsation instability region is found to shift from |${T}_\text {eff}\approx 7400\enspace \text K$| to |${T}_\text {eff}\approx 1.1 \times {10}^{4}\enspace \text K$| when the stellar mass changes from 1 to 0.7 M⊙. In the stars located along the vertical part of the instability region boundary, the pulsation instability is driven by both the κ- and γ-mechanisms, the order of the principal pulsation mode increasing with decreasing luminosity. The stars with |${T}_\text {eff}\ge 1.2 \times {10}^{4}\enspace \text K$| are found to be unstable only for |$L\ge 1.28 \times {10}^{4}\enspace \text {L}_{\odot}$|⁠. In the stars located along the horizontal part of the instability region boundary, the role of the κ-mechanism decreases with increasing effective temperature, so that for Teff ≳ 2 × 104 K the pulsations are mostly due to the γ-mechanism. The pulsation motions of the hot extreme helium stars with Teff ≳ 2 × 104 K can be represented by a superposition of the non-adiabatic running waves.