Equilibrium models for luminous helium stars

Equilibrium models for helium stars have been calculated in order to obtain mass-luminosity relations and to estimate the rate of the evolutionary change of the effective temperature. A luminous helium star is assumed to consist of a degenerate carbon-oxygen or oxygen-neon-magnesium core, a helium burning shell and a helium envelope. The envelope mass, which is obtained as the eigenvalue of the model for a given luminosity, is smaller for the larger core mass and for the higher effective temperature. The estimated rate of the evolutionary change of the effective temperature is found to be consistent with the observed period change of a pulsating R Coronae Borealis star, RY Sgr, if the core mass is greater than about 0.7 solar masses. A better constraint on the mass of the helium stars may be obtained if the period change of a hotter pulsating helium star is detected. (author)