INTERACTING MASSIVE BINARIES, H-R DIAGRAM OF THE LARGE MAGELLANIC CLOUD, AND SN-1987A

The observational constraints on massive star models for the LMC arise essentially from (1) the distribution of stars in the H-R diagram of the LMC, (2) the observations of SN 1987A and its progenitor, and (3) from the determinations of surface abundances of certain blue supergiants in the LMC which are seen to be helium- and nitrogen-enriched. It is shown that the standard physical inputs used in single star evolutionary models do not give good agreement with these observational constraints. The effect of helium enrichment of secondary stars in binary systems and the presence of a substantial fraction of such stars in the blue supergiant (BSG) region is explored. It is shown that the resulting distribution of stars in the BSG region gives good agreement with all the observational features, especially those emphasized by Fitzpatrick & Garmany. We show that mass transfer to the secondary while it is still within its main sequence could give an evolutionary model at 20 M. which stays in the blue throughout its lifetime and yet could show helium enrichment and CNO processing in the circumstellar material and the envelope of the star consistent with the observations of SN 1987A. It is argued, however, that a more consistent model for the progenitor star is obtained if the component masses in the binary system are nearly equal and the secondary star accretes matter from the primary after it evolves to the Hayashi track. The presence of highly helium- and nitrogen-enriched stars in the BSG region is shown to arise as a natural consequence of interacting binary star evolution, a feature in general difficult to reconcile with all the existing evolutionary models in the literature. We predict that these stars should be abundant at high temperatures close to the main sequence, while later temperature regions should be populated by stars with moderate helium and nitrogen enrichment.