Massive star evolution and SN 1987A

Our understanding of the evolution of massive stars has been challenged by SN 1987A, which may well have been the normal evolution of a single star of ∼20 M_⊙ of LMC abundances; there seems to be no argument based upon stellar evolution which contradicts that simple hypothesis. A set of stellar evolutionary sequences, for mass M/M_⊙ = 15, 20, and 25 and metallicity z = 0.002, 0.005, 0.007, 0.010, and 0.20 are presented; semiconvection is restricted to operating slower than the local thermal time scale (as previously done). Using these sequences, simple models of the massive star content of the LMC agree moderately well with the new observational data of Fitzpatrick and Garmany. They are at least as successful as interpretations based upon use of the Schwarzschild criterion to describe semiconvection; they imply that we have found few of the main-sequence supergiants in the LMC. Untangling the effects of various uncertainties will require more rigor in both observation and theory. Despite the varieity of envelope behavior, the structure of the core at collapse is rather similar for the stars of a given mass. Variations due to different rates of mass loss are likely to be larger than those due to composition. There is an increasing tendency for dredge-up of helium core material with increasing heavy-element abundance.