Observations of compact pulsators: the subdwarf B variables

Abstract

Following the successes of white dwarf asteroseismology made possible through the instigation of appropriate infrastructure for multi-site observational campaigns, novel kinds of multi-periodic nonradial pulsators among compact evolved objects recently have received a lot of attention. Since the discovery of the first prototype a decade ago, these subdwarf B variables have developed into a major new focus due to two reasons:(1) Although their current evolutionary status appears to be solved (subdwarf B stars can be identified with extreme horizontal branch models), the corresponding proposed formation scenarios still await validation or falsification through decisive evidence, and in the meantime continue to raise fundamental questions related to late stellar evolution. Probing the internal structure of these objects with asteroseismological methods therefore represents a valuable potential resource, for which the observational basis must be provided.(2) In an amusing course of matters, the theoretical interpretation of the pulsational behaviour of the variety of pulsational properties found in subdwarf B stars seems to have grown more difficult with time: From the successes of an initial simultaneous prediction of the p-mode pulsations by theory at the time of discovery of the first variables, to the current difficult task to reproduce the complex behaviour observed especially in the g-mode domain theoretically. The observed overlap area (hybrid objects with both p- and g-modes) in particular seems to constitute a major challenge to theory and has triggered vivid activity with some surprising first results.This obviously has significant impact on how to best exploit the wealth of observational data that is currently available, which spans time-resolved photometric and spectroscopic data from the optical (ground-based) to the FUV (space-based) domain. In the course of briefly reviewing some important observational campaigns on subdwarf B pulsators conducted world-wide in the last years, as well as a subjective selection of specific targets, I will identify some key objects.