The theoretical spectra of g-modes in subdwarf B stars

Observations by Kepler and TESS have revealed a complex situation for the g-modes present in subdwarf B (sdB) stars, which are He-core burning objects having lost most of their envelope on the Red Giant Branch. Some sdB stars exhibit trapped modes interposing in the asymptotic period sequences of regular period spacing, while others do not and have “smoother” pulsation patterns with more regular spacing. In order to clarify the situation, we thoroughly computed the theoretical pulsation spectra of g-modes obtained from several types of our models: firstly, from two types of static (parametric) models having different prescriptions for their chemical and thermal internal structures, and secondly from evolutionary models, which include an original prescription for the treatment of semiconvection. We show that g-mode pulsation spectra and the appearance of trapped modes are highly dependent on the chemical and thermal structures in the models as the star evolves, particularly in the region just above the He-burning core. Depending on the prescriptions and type of models, we observe mainly three types of spectra for mid to high radial-order g-modes (the ones observed in sdB stars): “flat” spectra of nearly constant period spacing; spectra with deep minima of the period spacing interposing between modes with more regular spacing (which correspond to trapped modes); and spectra showing a “wavy” pattern in period spacing. For the two latter cases, we have identified the region where the modes are trapped in the star. Comparison with observations must now be carried out to disentangle the situation and constrain the internal structures of core-He burning stars by asteroseismology.