UPPER LIMIT TO THE MASS OF PULSATIONALLY STABLE STARS WITH UNIFORM CHEMICAL-COMPOSITION

Nuclear-energized pulsational instability is a well-known feature of models of chemically homogeneous stars above a critical mass. With the new Rogers-Iglesias opacities, the instability occurs above 120-150 M. for normal Galactic Population I chemical compositions, and above approximately 90 M. for stars in metal-poor environments like the outer Galaxy and the Small Magellanic Cloud. Models of homogeneous helium-burning stars are unstable above masses of 19 and 14 M., respectively. These significant increases of the critical masses, in the normal metallicity cases, over the values derived previously with the Los Alamos opacities can explain the stability of the brightest observed O-type stars, but they do not exclude the possibility that the most luminous hydrogen-deficient Wolf-Rayet stars are experiencing this type of instability.