A binary merger product as the direct progenitor of a Type II-P supernova

Type II-P supernovae (SNe II-P) are the most common type of core-collapse SNe in the local Universe. The progenitors of SNe II-P have been directly observed in fortuitous pre-explosion images with effective temperatures and luminosities consistent with red supergiants (RSGs). For a long time such RSGs were believed to originate from single stars. Recent binary population synthesis, however, predict that many observed SNe II-P should actually arise from interacting binaries. This binary progenitor channel still lacks observational evidence since it is difficult to distinguish whether a detected progenitor comes from the single or binary star system simply based on its appearance in pre-explosion images. In this work, we report the discovery of a RSG progenitor of the Type II-P SN 2018gj. While the effective temperature and luminosity are similar to those of other SN II-P progenitors, it is located in a very old environment and the short plateau of the SN light curve suggests a very low-mass H-rich envelope. These characteristics are best explained if the progenitor is the merger product of a close binary system, in which the progenitor increased its He-core mass but lost much of its H-rich envelope during the back-and-forth mass transfer with a binary companion before merging. Similar analysis for a sample of nearby SNe II-P shows that at least 18–24% of them are likely to originate from binary progenitors. Our discovery provides the first compelling observational evidence that interacting binaries are a viable and potentially significant channel for SN II-P progenitors. *These authors contributed equally: Zexi Niu, Ning-Chen Sun, Emmanouil Zapartas.