SN 1987A - The evolution from red to blue

Abstract

Envelope models in thermal and dynamic equilibrium are used to explore the nature of the transition of Sk -69^deg^202, the progenitor of SN 1987A, from the Hayashi track to its final blue position in the H-R diagram. Loci of possible thermal equilibrium solutions are presented as a function of T_eff_ and M_c/o_, the mass of the carbon/oxygen core interior to the helium burning shell, assuming the hydrogen burning shell to be extinct. These solutions have only a red (Hayashi track) solution for small M_c/o_, only a blue solution for large M_c/o_, and triple-valued solutions for T_eff_ at intermediate values of M_c/o_. We proposed that the only legitimate means to evolve from red to blue is to develop a C/O core in excess of the maximum value, M^II^_c/o_, beyond which only a blue solution exists, where ^II^_c/o_ is a function of the luminosity, helium core mass, envelope mass, convective mixing length, and the envelope helium distribution. We find that uniform helium enrichment of the envelope results in red-blue evolution but that the resulting blue solution is much hotter than Sk -69^deg^202. Solutions in which the only change is to redistribute the portion of the envelope enriched in helium during main-sequence convective core contraction into a step function with Y ~ 0.5 at a mass cut of ~10 M_sun_ give a natural transition from red to blue and a final value of T_eff_ in agreement with observations. We argue that Sk -69^deg^202 probably fell on a post-Hayashi track sequence at moderate T_eff_ and discuss the possible connection of this sequence to the step distribution in the H-R diagram of the Large Magellanic Cloud (LMC) recently defined by Garmany and Fitzpatrick.