A theoretical exploration of the pulsational stability of subdwarf B stars

In this paper, we first review the main steps of a purely theoretical explo ration of the pulsation properties of subdwarf B (sdB) stars that led us, u ltimately, to postulate the existence of a new class of pulsating stars. Us ing both detailed evolutionary and static models of sdB stars, we were able to establish that a potent oscillation driving mechanism exists in these o bjects. This mechanism results from the kappa -effect associated with a loc al enrichment of iron in the stellar envelope caused by diffusion. On this basis, we reached the conclusion that a fraction of hot B subdwarf stars sh ould show observable pulsational instabilities, a theoretical prediction th at was confirmed observationally by the independent discovery of real sdB p ulsators by a team of astronomers at the South African Astronomical Observa tory. We also review the current status of sdB star seismology, a field tha t has been growing at a fast pace following this key discovery. For that pu rpose, we present sample results obtained from more recent pulsation comput ations based on improved stellar models-our so-called second-generation mod els-which include a detailed treatment for gravitational settling and radia tive levitation of iron. These clearly reveal that the theoretical expectat ions built upon the recognition of the iron driving mechanism in pulsating sdB stars reproduce remarkably well the observational data currently availa ble. Such results confirm the basic ideas that we developed and that explai n the origin of pulsations in sdB stars. They also pave the way for the fut ure exploitation of the full asteroseismological potential of these stars.