OUP accepted manuscript

Abstract

Recent detections of g-mode pulsations in subdwarf B (sdB) stars from the space missions CoRoT and Kepler have verified that nearly all of these pulsators have regular period spacings. This discovery has been extensively applied to mode identification for the observed g-mode sdB pulsators. However, the discovery of regular period spacings apparently contradicts the strong g-mode trapping structure expected in canonical sdB models, which is caused by steep chemical gradients in such evolved stars. Although the mode trapping efficiency could be reduced somewhat by taking into account the diffusion effect during the sdB phase, it is still difficult to explain the regular period spacings of g-mode sdB pulsators, especially in the short-period range. We suggest that the regular period spacings may arise earlier in the evolution of sdB stars rather than during the sdB stage itself. As for a canonical sdB star, it is evolved from a low-mass star and is considered to have gone through the He flash. The He flash causes extensive and aggressive convection that extends very close to the He/H transition zone. When considering a proper convective overshoot during the He-flash stage, the chemical profile in the He/H transition zone becomes smoother. Detailed model calculations show that the mode trapping efficiency could be reduced to approximately the level of observations throughout the observed period range by taking the He-flash overshoot into account.