Why do hot subdwarf stars pulsate?

TL;DR

This paper reviews the pulsation mechanisms of hot subdwarf B stars, discussing why only some stars with similar properties exhibit pulsations despite well-understood driving forces.

Contribution

It explores potential missing parameters that determine why certain sdB stars pulsate while others do not, despite similar characteristics.

Findings

Theoretical instability regions align with observed pulsators.

Unclear why some similar stars pulsate and others do not.

Possible unidentified factors influence pulsation occurrence.

Abstract

Hot subdwarf B stars (sdBs) are the stripped cores of red giants located at the bluest extension of the horizontal branch. Several different kinds of pulsators are found among those stars. The mechanism that drives those pulsations is well known and the theoretically predicted instability regions for both the short-period p-mode and the long-period g-mode pulsators match the observed distributions fairly well. However, it remains unclear why only a fraction of the sdB stars pulsate, while stars with otherwise very similar parameters do not show pulsations. From an observers perspective I review possible candidates for the missing parameter that makes sdB stars pulsate or not.