Explosion imminent: the appearance of red supergiants at the point of core-collapse

Abstract

ABSTRACT From the early radiation of type II-P supernovae (SNe), it has been claimed that the majority of their red supergiant (RSG) progenitors are enshrouded by large amounts of circumstellar material (CSM) at the point of explosion. The inferred density of this CSM is orders of magnitude above that seen around RSGs in the field, and is therefore indicative of a short phase of elevated mass-loss prior to explosion. It is not known over what time-scale this material gets there: is it formed over several decades by a ‘superwind’ with mass-loss rate $\dot{M} \sim 10^{-3}\, {\rm M_\odot \, yr^{-1}}$; or is it formed in less than a year by a brief ‘outburst’ with $\dot{M}\sim 10^{-1}\, {\rm M_\odot \, yr^{-1}}$? In this paper, we simulate spectra for RSGs undergoing such mass-loss events, and demonstrate that in either scenario, the CSM suppresses the optical flux by over a factor of 100, and that of the near-IR by a factor of 10. We argue that the ‘superwind’ model can be excluded as it causes the progenitor to be heavily obscured for decades before explosion, and is strongly at odds with observations of II-P progenitors taken within 10 yr of core-collapse. Instead, our results favour abrupt outbursts < 1 yr before explosion as the explanation for the early optical radiation of II-P SNe. We therefore predict that RSGs will undergo dramatic photometric variability in the optical and infrared in the weeks-to-months before core-collapse.