The evolution of massive stars. II - The influence of initial composition and mass loss

Abstract

We have studied the evolution of 15, 30, and 40 M/sub sun/ stars with compositions Y = 0.28 and Z = 0.01, 0.001, and 0.0002, and of 50 M/sub sun/ stars with Y = 0.28, Z = 0.0002. These models have been evolved from the zero-age main sequence through core helium burning and up to the point of carbon ignition. All models were evolved with and without the inclusion of mass loss. We have used a moderate mass loss rate with different efficiency factors for main-sequence and blue supergiant stars. Our results indicate that all massive stars ignite helium as blue supergiants. Stars with initial masses M/sub i/< or approx. =30 M/sub sun/ evolve redward very slowly and spend less than 1% of their total lifetimes as red supergiants. Above 30 M/sub sun/, the rate of redward evolution increases dramatically. The effect of a reduction in the initial metal content relative to solar abundances is to cause the models to be bluer and, in general, slightly more luminous at comparable stages of evolution. In the absence of mass loss, models with M/sub i/< or =30 M/sub sun/ and Z< or =0.001 do not become red supergiants prior to carbon ignition.more » Mass loss causes all models to evolve redward more rapidly during core helium burning.« less