A theoretical exploration of the pulsational stability of subdwarf B stars

In this paper, we first review the main steps of a purely theoretical exploration of the pulsation properties of subdwarf B (sdB) stars that led us, ultimately, to postulate the existence of a new class of pulsating stars. Using both detailed evolutionary and static models of sdB stars, we were able to establish that a potent oscillation driving mechanism exists in these objects. This mechanism results from the kappa -effect associated with a local 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 should show observable pulsational instabilities, a theoretical prediction that was confirmed observationally by the independent discovery of real sdB pulsators by a team of astronomers at the South African Astronomical Observatory. We also review the current status of sdB star seismology, a field that has been growing at a fast pace following this key discovery. For that purpose, we present sample results obtained from more recent pulsation computations based on improved stellar models-our so-called second-generation models-which include a detailed treatment for gravitational settling and radiative levitation of iron. These clearly reveal that the theoretical expectations built upon the recognition of the iron driving mechanism in pulsating sdB stars reproduce remarkably well the observational data currently available. Such results confirm the basic ideas that we developed and that explain the origin of pulsations in sdB stars. They also pave the way for the future exploitation of the full asteroseismological potential of these stars.