The HR diagram of massive stars : widening of the main sequence band and WR formation.

Plausible physical causes of the observed main sequence band widening are first discussed separately and then their combined effect is evaluated. These are mass loss by stellar wind, overshooting from convective cores, effect of stellar winds on hydrostatic radii, effect of opacity in the CNO ionization zone. It is found that, while each term taken separately cannot produce stellar models matching the observational requirement, only a suitable combination of overshooting, mass loss by stellar wind all over the major stages of central nuclear burning, and a moderate opacity enhancement in the outer layers may lead theoretical models into satisfactory agreement with the observational data. The problem of formation, statistics, and distribution of WR stars across the galactic plane is discussed in the light of these new stellar models. It is found that the dependence of opacity rather than mass loss rate on the ambient metallicity plays the dominant role in accounting for the properties of WR stars.