Long-period variable subdwarf B stars: Prospects for asteroseismology

We summarize the results of an extensive study aimed at quantitatively interpreting the oscillations detected in long-period variable subdwarf B stars. Our analysis is based on between 300 and 400 hours of time-series photometry obtained for each of three representative targets: PG1627+017, PG1338+481 and PG 0101+039. The former two were the subjects of extensive multi-site campaigns led from the 1.52 m Steward Observatory telescope on Mt. Bigelow, Arizona, while the latter was observed with the 0.15 m Canadian space telescope MOST. We find that, unlike the short-period oscillators, where asteroseismology has been successful in some instances, our understanding of the slow pulsators is somewhat limited due to both observational and conceptual challenges. In particular, the period spectra measured to date are much sparser than those anticipated from models, implying that the indices of the modes observed must be constrained from the outset if asteroseismology is to be achieved. One promising idea is the exploitation of a mode's color-amplitude dependence on its degree index ℓ. through multicolor photometry. Applying that method to the PG 1338+481 data together with other constraints suggests the excitation of ℓ = 1 modes. If confirmed, this would point to a discrepancy between the observed and predicted long-period variable subdwarf B star instability strips of around 7000 K on the blue side, although some of it could be due to incorrect spectroscopic determinations of the effective temperatures of cool sdB stars.