The opacity mechanism in B-type stars - II. Excitation of high-order g-modes in main-sequence stars

We show that the OPAL opacities, in addition to explaining the origin of the pulsations of β Cep stars, also predict the existence of a large region in the main-sequence band at lower luminosities, where high-order g-modes of low harmonic degree l are unstable. The excitation mechanism remains the same, and is due to the usual κ-effect acting in the metal opacity bump |$(T\approx 2\times {10}^{5}\enspace \text K).$| The new instability domain nearly bridges the gap in spectral types between δ Sct and β Cep stars. The periods of the unstable modes are in the range 0.4–3.5 d for l = 1 and 2. We propose that this excitation mechanism causes photometric variability in the slowly pulsating B-type stars (SPB stars), and perhaps in other B stars for which variability in the same period range has been reported.