The Initial Masses of the Red Supergiant Progenitors to Type-II Supernovae

There are a growing number of nearby SNe for which the progenitor star is detected in archival pre-explosion imaging. From these images it is possible to measure the progenitor's brightness a few years before explosion, and ultimately estimate its initial mass. Previous work has shown that II-P and II-L supernovae (SNe) have Red Supergiant (RSG) progenitors, and that the range of initial masses for these progenitors seems to be limited to $<$17M$_\odot$. This is in contrast with the cutoff of 25-30M$_\odot$ predicted by evolutionary models, a result which is termed the 'Red Supergiant Problem'. Here we investigate one particular source of systematic error present in converting pre-explosion photometry into an initial mass, that of the bolometric correction (BC) used to convert a single-band flux into a bolometric luminosity. We show, using star clusters, that RSGs evolve to later spectral types as they approach SN, which in turn causes the BC to become larger. Failure to account for this results in a systematic underestimate of a star's luminosity, and hence its initial mass. Using our empirically motivated BCs we reappraise the II-P and II-L SNe that have their progenitors detected in pre-explosion imaging. Fitting an initial mass function to these updated masses results in an increased upper mass cutoff of $M_{\rm hi}$=$19.0^{+2.5}_{-1.3}$M$_\odot$, with a 95% upper confidence limit of $<$27M$_\odot$. Accounting for finite sample size effects and systematic uncertainties in the mass-luminosity relationship raises the cutoff to $M_{\rm hi}$=25M$_\odot$ ($<$33M$_\odot$, 95% confidence). We therefore conclude that there is currently no strong evidence for `missing' high mass progenitors to core-collapse SNe.