Type IIP Supernova Progenitors. III. Blue to Red Supergiant Ratio in Low-metallicity Models with Convective Overshoot

The distribution of stars in the Hertzsprung–Russell diagram (HRD) for a stellar conglomeration represents a snapshot of its evolving stellar population. Some of the supergiant stars may transit the HRD from blue to red and then again to blue during their late evolutionary stages, as exemplified by the progenitor of SN 1987 A. Others may transit a given part of the HRD more than twice in a “blue loop” and end up as red supergiants before they explode. Since stars in blue loops spend a considerable part of their lives there, these stages may change the relative number of modeled supergiants in the HRD. Their lifetimes, in turn, depend upon the initial mass of the star, how convection in its interior is modeled, and how much mass loss takes place during its evolution. The observed ratio of the number of blue to red supergiants and yellow to red supergiants sensitively tests the stellar evolution theory. We compare modeled number ratios of these supergiants with observed data from the Large Magellanic Cloud, as it has a metallicity very similar to that of the environment of SN 2013ej. We successfully model these by taking into account moderate (exponential) convective overshooting. We explore its effect on the final radius and mass of the star prior to core collapse. The radius differs dramatically with overshoot. These factors controlling presupernova structure may affect the postexplosion optical/IR light curves and spectral development.