Radiative levitation and opacity driving: The potential of hot subdwarf pulsators for testing diffusion and other competing processes in stars
Résumé
The hot and compact subdwarf (sdB and sdO) stars harbor three classes of nonradial pulsators: the rapid p-mode EC14026-type sdB pulsators discovered in 1997, the slow g-mode sdB pulsators known as the V1093 Her variables found in 2003, and the rapid sdO pulsators identified in 2006. The oscillations in these stars are singular in that the driving mechanism a classical kappa effect triggered by the Z-bump, a region where partial ionisation of heavy elements from the iron group significantly increases the gas opacity is truly effective only through local abundance enhancements of such elements (in particular of iron itself ) caused by microscopic diffusion. They are the only pulsating stars, among those currently known across the HR diagram, that show oscillations so tightly linked to these diffusive phenomena. In this paper, we present a short review of our present understanding of the various components that drive pulsations in subdwarf stars and we outline the remarkable potential of these stars as laboratories where diffusion and other competing processes may be tested.