Evolution of Stars with M8 M⊙

Abstract

A review of research in the field of evolution of massive stars during the last three years is presented. The analysis of computed stellar models in the helium-burning stage provides evidence for the existence of neutrino emission predicted by the theory of universal Fermi-interaction. The criterion for convective stability in the zone of variable molecular weight still remains uncertain despite several possibilities for a unique choice of the stability criterion that were recently suggested.Evolutionary computations of massive star models in the carbon, oxygen, neon and silicon-burning stages provide opportunities to obtain models of presupernovae of different types and to investigate the cause of instability responsible for the implosion as a function of the initial mass of the star.If the neutrino emission is included in computations the core of a star with M≳90 M⊙ loses stability due to the positron-electron pair formation in the oxygen-burning stage. A star with 16 M⊙ ≲ M ≲ 90 M⊙ collapses because of photodissociation of the iron nuclei but a core of a star with 9 M⊙ ≲ M ≲ 16 M⊙ collapses due to neutronization of silicon or iron nuclei.Attention is paid to the possibility of circulation mixing of matter within the boundary of a carbon-oxygen core, due to the fast rotation of the core in advanced stages of evolution, if the angular momentum is conserved in the course of evolution. As the molecular weight barrier is low in late stages of chemical evolution, it can not prevent the penetration of circulation flows outside the convective core.Difficulties of the present theory of convection in stars are stressed and the necessity of a two-dimensional approach to the solution of the stellar constitution equations is emphasized.