The Evolution of Massive Stars: The Influence of Initial Composition and Mass Loss
We have studied the evolution of 15, 30, and 40 M(,(CIRCLE)) stars with compositions Y = 0.28 and Z = 0.02, 0.01, 0.001 and 0.0002, and of 50 M(,(CIRCLE)) stars with Y = 0.28, Z = 0.0002. These models have been evolved through core helium burning 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(,i(, ))<(, )30 M(,(CIRCLE)) evolve redward very slowly and spend less than 1% of their total lifetimes as red supergiants. Above 30 M(,(CIRCLE)), 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(,i) (LESSTHEQ) 30 M(,(CIRCLE)) and Z < 0.001 do not become red supergiants prior to carbon ignition. Mass loss causes all models to evolve redward more rapidly during core helium burning. The effects of mass loss and variations in composition on the internal structure, lifetimes and surface chemical abundances are discussed. Comparisons are made with observational data on blue to red ratios for luminous stars and Cepheid period-frequency distributions. Our results are found to produce good agreement with the general trends in these data.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign