Helium ionization driving in Beta Cephei stars

In a series of nonadiabatic linear pulsation analyses, the destabilizing effect of the He(+) ionization edge (which is responsible for an opacity feature near T = 150,000 K) is demonstrated. This mechanism is shown to be most effective in a region of the H-R diagram nearly coincident with that of the observed Beta Cephei variables. Although actual instability is not obtained with standard models, it is shown that this could easily be due to approximations in the models and poor opacity resolution above T = 100,000 K. Slightly modified models that are presented do show an instability strip in good agreement with observed Beta Cepheids.