Red Supergiant Stars as Supernova Progenitors - the X-ray Perspective

Red Supergiants (RSGs) have for decades been assumed to be the progenitors of Type IIP supernovae (SNe). They are expected to have dense winds with mass-loss rates up to 10^-4 M_⊙ yr^-1. We have created a database of available X-ray lightcurves of SNe. Type IIP SNe are found to have the lowest X-ray luminosities among all classes, which is surprising given the high mass-loss rate winds expected from their red supergiant progenitors, and therefore the high density medium into which Type IIP SNe are expected to expand into. We show that the low X-ray luminosity sets a limit on the mass-loss rate of the progenitor star which can collapse to become a RSG, which is about 10^-5 M_⊙ yr^-1. This in turn can be used to set a limit on the initial mass of a RSG star which can become a Type IIP progenitor, which is about 19 M_⊙. This is consistent with the limit obtained via direct optical progenitor identification. Optically identified progenitors of Type IIP SNe are found to be RSGs with masses less than about 17 M_⊙ (Smartt (2009)). We discuss the implications of this result for stellar evolution, theorize on the fate of RSG stars with initial mass > 19 M_⊙, and discuss what type of SNe they will produce at the end of their lifetime.