Large magellanic cloud helium-rich peculiar blue supergiants and SN 1987A

The theoretical distribution of massive stars in the H-R diagram is compared to the revised data of Fitzpatrick and Garmany for the LMC. Preferred models of ∼20 M_⊙ undergo a thermal contraction at T_eff∼ 35,000 K at the end of core hydrogen burning but reestablish thermal equilibrium to the red of the main sequence at T_eff ∼20,000 K after ignition of a hydrogen-burning shell. They then evolve on a nuclear time scale to T_eff ∼6000 K where they lose thermal equilibrium and jump to the Hayashi track. The theoretical and observed distributions agree with two significant exceptions: the blue thermal contraction "gap" is overpopulated compared to the theory, and there is a "ledge" crossing the center of the H-R diagram. We explore the hypothesis that some of the observed stars in the blue "gap" are secondaries that have accreted heliumrich matter from deep within the hydrogen envelope of a red supergiant primary. Some preliminary observational justification is given. Other features of the observed H-R diagram and alternative modes of evolution are also discussed along with implications for Type Ib supernovae and SN 1987A.