An opacity mechanism for the pulsations of OB stars

It is proposed that the sudden appearance of a tremendous number of same-shell transition iron lines, as the temperature rises above 100,000 K, gives a high sensitivity of the opacity to temperature at the very low densities found in OB giants. This produces kappa effect pulsations. The reason not all B stars pulsate could be that a slight primordial deficit in the iron abundance in the surface layer reduces the opacity and its sensitivity to temperature and reduces the kappa effect driving. A slight amount of iron concentration by radiative levitation could cause a star to pulsate even if it did not originally have enough primordial iron to cause this mechanism to operate. Then any slow slight mixing caused by the unstable shearing nonradial pulsations could restabilize the pulsations as actually observed in Alpha Vir and Beta CMa. Rapid levitation and mixing for the very luminous B stars with their very low density envelopes could even explain luminous blue variables that pulsate only a few cycles before they stabilize again. Large amplitude pulsations like those seen in BW Vul would indicate a somewhat large primordial iron abundance compared to all other B stars. Multimode behavior is theoretically expected for thismore » pulsation mechanism, and for some B stars only a few nonradial modes (possibly selected by rotation) may survive to observable amplitudes. 39 refs.« less