The stability of massive stars and its dependence on metallicity and opacity

Linear Strange Modes in Massive Stars

“…The spectrum of the outer cavity is identified with strange modes, that of the inner cavity with ordinary modes. A shift of the barrier during stellar evolution causes the spectra to cross thus leading to multiple resonances which in an adiabatic environment unfold into avoided crossings (see Kiriakidis et al 1993). Except for extremely low irrelevant effective temperatures the stability analysis based on the adiabatic approximation does not reveal any instability (Glatzel & Kiriakidis 1998;see, however, Stothers & Chin 1993, 1994, 1995.…”

“…Phenomenologically, for massive stars SMs and SMIs come in three groups apparently related to the three opacity maxima (see, e.g., Iglesias et al 1992 and caused by the contribution of heavy elements ("Fe") and the ionization of helium ("He") and hydrogen/helium ("H/He") respectively (see Kiriakidis et al 1993). The run of the growth rates of the instabilities directly reflects the run of the opacity.…”

“…On the basis of a standard LNA analysis for radial perturbations domains of instability in the HRD have been determined by Kiriakidis et al (1993). Instabilities associated with He-SMs and H/He-SMs define a largely metallicity independent unstable region in the upper right corner of the HRD, whose boundaries approximately coincide with the observed Humphreys -Davidson limit (Humphreys & Davidson 1979).…”

Pulsations in O Stars

“…Among O stars, however, only very few pulsators are known. Fullerton et al (1996) listed 3 confirmed and 6 suspected pulsating 0 stars and noticed that all of them are located in the instability strip predicted by Kiriadikis et al (1993).…”

“…To compare the occupied domain in the HRD with the predicted location of strange-mode occurrence in these massive stars (Kiriadikis et al (1993), Glatzel & Mehren (1996)) a conversion from [My, Sp. type] to [L, T e ff] is needed.…”