Instability strips of slowly pulsating B stars and β Cephei stars: the effect of the updated OP opacities and of the metal mixture

The discovery of β Cephei stars in low-metallicity environments, as well as the difficulty in theoretically explaining the excitation of the pulsation modes observed in some β Cephei and hybrid slowly pulsating B–β Cephei pulsators, suggest that the ‘iron opacity bump’ provided by stellar models could be underestimated. We analyze the effect of uncertainties in the opacity computations and in the solar metal mixture on the excitation of pulsation modes in B-type stars. We carry out a pulsational stability analysis for four grids of main-sequence models with masses between 2.5 and 12 M⊙ computed with OPAL and OP opacity tables and two different metal mixtures. We find that in a typical β Cephei model the OP opacity is 25 per cent larger than OPAL in the region where the driving of pulsation modes occurs. Furthermore, the difference in the Fe mass fraction between the two metal mixtures considered is of the order of 20 per cent. The implication on the excitation of pulsation modes is non-negligible: the blue border of the slowly pulsating B star (SPB) instability strip is displaced at higher effective temperatures, leading to a larger number of models being hybrid SPB-β Cephei pulsators. Moreover, higher overtone p-modes are excited in β Cephei models and unstable modes are found in a larger number of models for lower metallicities, in particular β Cephei pulsations are also found in models with Z= 0.01.