Linear analyses for the stability of radial and non-radial oscillations of massive stars

In order to understand the periodic and semiperiodic variations of luminous OBA-type stars, linear non-adiabatic stability analyses for radial and non-radial oscillations have been performed for massive evolutionary models (8-90M_⊙). In addition to radial and non-radial oscillations excited by the kappa-mechanism and strange mode instability, we discuss the importance of low-degree oscillatory convection (non-adiabatic g^-) modes. Although their kinetic energy is largely confined to the convection zone generated by the Fe-opacity peak near 2 × 10⁵K, the amplitude can emerge into the photosphere and should be observable in a certain effective temperature range. They have periods longer than those of the radial strange modes so that they seem to be responsible for some of the long-period microvariations of Luminous Blue Variables (LBVs; S Dor variables) and α Cyg variables. Moreover, monotonically unstable radial modes are found in some models whose initial masses are greater than or equal to 60M_⊙ with Z= 0.02. The monotonic instability probably corresponds to the presence of an optically thick wind. The instability boundary roughly coincides with the Humphreys-Davidson limit.