Pulsation of high luminosity helium stars

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Dynamic Stability, Helium, R Coronae Borealis Stars, Stellar Envelopes, Stellar Luminosity, Abundance, Astronomical Models, Brightness Temperature, Carbon, Light Curve, Thermal Stability

Scientific paper

The paper studies the stability of radial-pulsation modes of the helium envelopes of HdC stars, which are high-luminosity helium stars found in the region of the H-R diagram occupied by the R Coronae Borealis stars and other hydrogen-deficient carbon stars. The analysis is performed on models with the following properties: luminosities of 5000 and 10,000 times the solar luminosity, masses of 0.6 and 1 solar mass, and effective temperatures (T eff) of 5000 to 10,000 K, 12,000 K, and 15,000 K. The periods of linear adiabatic and nonadiabatic pulsations are determined, and nonlinear calculations are carried out for the unstable modes. The results show that: (1) the fundamental mode is unstable for log T eff less than 3.88, (2) a region of overtone instability occurs for log T eff between 3.93 and 4.12, (3) nonadiabatic effects are so strong that there is no one-to-one correspondence between adiabatic and nonadiabatic modes, and (4) the light amplitudes of nonlinear pulsations for certain unstable modes are of the order of 1.0 magnitude (bolometric).

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