Improved opacities and pulsation stability in subluminous B and O stars

We reconsider the excitation of radial oscillations in low-mass early-type stars by the so-called iron-bump opacity mechanism. Our models provide an indication of stability and periods for low-order radial and, by extension, non-radial p modes. The models are important for understanding pulsations in subluminous B and O stars and other evolved stars of low mass. Following the recent computation of new data by the Opacity Project, the position of the opacity peak due to iron-group elements was found to have shifted to higher temperatures. At these temperatures and for solar abundances of both species, nickel was found to contribute nearly as much opacity as iron. We have examined the stability of low-mass B stars over a wide range of effective temperature, luminosity-to-mass ratio and hydrogen abundance. We have increased the abundance of iron and nickel by factors ranging from 1 to 10. We have shown how the new opacities shift the 'iron-bump' instability strip towards slightly higher effective temperature, and that they give an instability strip of size comparable to that obtained with the older OPAL opacities for an enrichment factor three times smaller than previously. Thus the new opacities can better match the observed distribution of EC14026 stars with a lower concentration of iron and nickel. We have briefly examined radial pulsation models for the subluminous O star SDSS J160043.6+074802.9. We find no unstable radial modes with periods in the range reported; such stars would have log L/M similar to 2.5. We do find that low-order modes in sdO stars with log L/M greater than or similar to 3 can be unstable due to Fe + Ni opacity. Accurate spectroscopic parameters are required to progress this further.