The β Cephei instability strip

We carried out a series of linear stability analyses of the radial and low-degree non-radial p modes for stellar models with initial masses of 6-30 M-circle dot. The stellar models were computed by using convective overshoot distance d(over) = 0.10, 0.25 and 0.40 H-p. Our numerical results show that the beta Cephei instability strip forms a horn-shaped region pointing upwards near the main sequence on the Hertzsprung-Russell diagram (HRD). The lower part of the instability strip for the radial modes join the zero-age main-sequence (ZAMS) at M approximate to 8.0-10.5 M-circle dot, while the top of the instability strip extends up to M approximate to 15.5 M-circle dot. The instability strip for the non-radial modes is even wider. The overall instability strip is dominated by the radial and non-radial fundamental modes. The first overtone (the radial-order index n=1) is also pulsationally unstable. We have shown that the beta Cephei stability is almost independent of the overshoot parameter d(over) used for the stellar models, while it depends critically on the metal abundance. With decreasing metal abundance, the instability region shrinks and eventually disappears for Z less than or similar to 0.005.