Identification of the Red Supergiant Progenitor of Supernova 2005cs: Do the Progenitors of Type II‐P Supernovae Have Low Mass?

The stars that end their lives as supernovae (SNe) have been directlyobserved in only a handful of cases, due mainly to the extreme difficulty inidentifying them in images obtained prior to the SN explosions. Here we reportthe identification of the progenitor for the recent Type II-plateau(core-collapse) SN 2005cs in pre-explosion archival images of the WhirlpoolGalaxy (M51) obtained with the Hubble Space Telescope (HST) Advanced Camera forSurveys (ACS). From high-quality ground-based images of the SN from theCanada-France-Hawaii Telescope, we precisely determine the position of the SNand are able to isolate the SN progenitor to within 0".04 in the HST/ACSoptical images. We further pinpoint the SN location to within 0".005 fromHST/ACS ultraviolet images of the SN, confirming our progenitor identification. From photometry of the SN progenitor obtained with the pre-SN ACS images, andalso limits to its brightness in pre-SN HST/NICMOS images, we infer that theprogenitor is a red supergiant star of spectral type K0--M3, with initial mass7--9 Msun. We also discuss the implications of the SN 2005cs progenitoridentification and its mass estimate. There is an emerging trend that the mostcommon Type II-plateau SNe originate from low-mass supergiants 8--15 Msun.