THE OPACITY MECHANISM IN B-TYPE STARS .1. UNSTABLE MODES IN BETA CEPHEI STAR MODELS

An improved version of the OPAL opacities, with the metal abundance parameter Z having values of 0.02 and 0.03, has been used in a stability survey for models of 7- to 16-M. stars in the main-sequence (MS) and early post-MS evolutionary phases. All beta Cep stars lie within the domain of the HR diagram where pulsational instability is caused by the opacity mechanism acting in the metal opacity bump at T almost-equal-to 2 x 10(5) K. For most of the objects, a value of Z = 0.02 suffices to account for the pulsation. The positions of beta Cep stars in the HR diagram correspond to models in an advanced phase of core hydrogen burning, constructed without allowing for effects of the convective overshooting. The fundamental radial mode (p1) is unstable in all sequences of stellar models considered, except the one with M = 7 M. and Z = 0.02. Instability of the overtones, up to p3 for Z = 0.03, appears with mass increase. Non-radial modes of low degrees are unstable in a wider frequency range encompassing low-order g-modes. All of these modes may be associated with the beta Cep phenomenon. In addition, for higher spherical harmonic degrees (l greater-than-or-equal-to 6), there is a domain of high-order g-mode instability. Excitation of such modes may be responsible for long-period variability of line profiles observed in some B-type stars.