THE H-R DIAGRAM OF THE LARGE MAGELLANIC CLOUD AND IMPLICATIONS FOR STELLAR EVOLUTION

In this paper we produce an H-R diagram for the Large Magellanic Cloud using a sample of 1375 "normal" spectral class O through G supergiants. Our definition of "normal" includes all O stars that fit into the standard two-dimensional spectral classification framework and all later type stars that do not exhibit optical emission lines. The sample thus excludes Wolf-Rayet stars and the luminous blue variables. The upper luminosity limit observed for this sample agrees with previous estimates and indicates that only stars initially less massive than ∼40 M⊙ evolve to the red supergiant region. Between luminosities of Mbol = -8.5 and Mbol = -6.40 we find a discontinuity in the density distribution of stars in the region of the blue supergiants. On the hotter side of a line we refer to as the "ledge," the density of stars is over 5 times greater than on the cooler side. We identify this feature with an evolutionary time scale change occurring during the core heliumburning phase of the evolution of intermediate mass stars (20-30 M⊙ > M > 10 M⊙). We suggest that this feature is best reproduced by stellar evolution models which contain "blue loops." With this scenario, many of the observed blue supergiants are expected to be in a post-red supergiant phase of evolution. This hypothesis is consistent with suggestions that many supergiants show evidence for contamination of their surface chemical composition by the products of nuclear burning. Such contamination would occur during the red supergiant phase through the action of a surface convection zone.