Excitation of g modes in Wolf–Rayet stars by a deep opacity bump

We examine the stability of = 1 and = 2 g modes in a pair of nitrogen-rich Wolf–Rayet stellar models characterized by differing hydrogen abundances. We find that modes with intermediate radial orders are destabilized by a κ mechanism operating on an opacity bump at an envelope temperature of log T ≈ 6.25. This ‘deep opacity bump’ is due primarily to L-shell bound–free transitions of iron. Periods of the unstable modes span ∼11–21 h in the model containing some hydrogen, and ∼3–12 h in the hydrogen-depleted model. Based on the latter finding, we suggest that self-excited g modes may be the source of the 9.8-h periodic variation of WR 123 recently reported by Lefèvre et al.