IRON AND MOLECULAR OPACITIES AND THE EVOLUTION OF POPULATION-I STARS

Effects of recent opacity revisions on the evolution of Population I stars are explored over the mass range 1.5-60 M.. Opacities are due to Iglesias, Rogers, and Wilson (high temperatures) and to Sharp and to Alexander, Johnson, and Rypma (low temperatures). Opacity parameters considered include the angular momentum coupling scheme for iron, the relative iron abundance, the total metal abundance, and diatomic and triatomic molecular sources. Surprisingly, only the total metal abundance exerts an important control over the evolutionary tracks. Blue loops on the H-R diagram during core helium burning can be very sensitive to opacity, but only insofar as the simple formation or suppression of a blue loop is concerned. The blue loops are most robust for stellar masses around 10 ME.. We confirm, from a comparison of stellar models with observational data, that the total metal abundance is close to solar and that convective core overshooting is likely to be very slight. The new models predict the existence of an iron convection zone in the envelope and a great widening of the main-sequence band in the H-R diagram at luminosities brighter than 10(5) L..