Asteroseismological studies of long- and short-period variable subdwarf B stars

The thesis presented here lays the observational and theoretical foundations for the asteroseismological interpretation of the newly discovered class of long-period variable subdwarf B stars. In addition, we further our understanding of the short-period variable subdwarf B stars and develop a method of verifying the analyses achieved for these more extensively studied objects.

After summarising the status quo on hot subdwarfs and introducing basic pulsation theory, we carry out an asteroseismological analysis of the rapid oscillator EC 20117-4014. We then describe a theoretical exploration into the potential of multi-colour photometry for partial mode identification in both the short- and the long-period variables. In the case of the former, the tools developed should provide a means of verifying the mode identification inferred from asteroseismology for a number of objects, while for the latter they provide necessary a priori constraints. The remainder of the thesis is devoted to a pioneering study aimed at quantitatively extracting and interpreting the oscillations exhibited by 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: PG 1627+017, PG 1338+481 and PG 0101+039. We find that their oscillatory properties are in agreement with predictions from non-adiabatic pulsation calculations at the relative but not the absolute level. In particular, the experimental blue edge of the instability strip exceeds that predicted by up to 8000 K, indicating that the subdwarf B star models currently used are subject to deficiencies. Quantitative interpretations of the period spectra uncovered were possible for two of the targets and indicate the preferential excitation of modes in well-defined frequency ranges, attributed to an unknown energy channelling mechanism. The ensuing underabundance of observed pulsations compared to those predicted together with the near-asymptotic behaviour of the modes greatly complicates the asteroseismological exercises attempted. Nevertheless, the results achieved are very promising and will in all likelihood pave the way for future studies of long-period variable subdwarf B stars.