Velocity and Line Profile Variations in Massive Stars, WR-Stars and Their Origin

Abstract

The stability of massive stars with respect to infinitesimal radial perturbations is reexamined. The analysis is based on stellar models obtained by standard stellar evolution calculations for stars with initial masses above 20M ⊙, where the effect of mass loss is accounted for by adopting an empirical mass loss rate. Three types of instability are identified: 1. Dynamical instabilities associated with mode resonances and the occurrence of “strange modes”. 2. Instability of low order acoustic modes driven by the к—mechanism. 3. Instability of the fundamental mode driven by the e—mechanism for stellar models close to the main sequence. Both with respect to their strength and the range of unstable stellar models instabilities driven by the к— and e— mechanisms are negligible when compared with resonance and strange mode instabilities. The dependence on opacity and metallicity Z of the range of unstable stellar models in the HRD is studied in detail. If the metallicity is low (Z=0.004), the stability boundary closely resembles the blue edge of the observed Humphreys-Davidson (HD) limit. With increasing metallicity the blue edge of the domain of instability shifts to higher effective temperatures thus covering the range where LBVs are observed. Dynamical instabilities on the main sequence are found for masses M > 80 M ⊙ (Z=0.02) and M > 58 M ⊙ (Z=0.03). For details see [1],[2]. Extremely strong dynamical instabilities associated with the occurrence of “strange modes” are found in WR-stars.