Post-common envelope binary stars, radiative levitation, and blue large-amplitude pulsators

Abstract

Following the discovery of blue large-amplitude pulsators (BLAPs), single star evolution models of post-red giant branch stars that have undergone a common envelope (CE) ejection in the form of a high mass-loss rate have been constructed and analysed for pulsation stability. The effects of atomic diffusion, particularly radiative levitation, have been examined. Two principal models were considered, being post-CE stars of 0.31 and 0.46 M_{⊙}. Such stars are likely, in turn, to become either low-mass helium white dwarfs or core helium-burning extreme horizontal branch stars. The inclusion of radiative levitation leads to opacity driven pulsations in both types of post-CE objects when their effective temperatures are comparable to those of BLAPs, with similar periods. The extent of the instability region for models in these simulations, which are not in thermal balance, is larger than that found for static models, in agreement with previous theory. By comparing to observations, and making some simple evolutionary assumptions, we conclude the 0.31 M_{⊙} star is the more likely candidate for BLAPs. The rate of period change is negative for both cases, so the origin of BLAPs with positive rates of period change remain uncertain.