EARLY EMISSION FROM THE TYPE IIn SUPERNOVA 1998S AT HIGH RESOLUTION

The well-studied Type IIn supernova (SN) 1998S is often dubbed the\nprototypical SN IIn, and it provides a unique opportunity to study its\nprogenitor star from within as the SN lights up dense circumstellar material\n(CSM) launched from the progenitor. Here we present a Keck HIRES spectrum of SN\n1998S taken within a few days after core collapse - both the earliest\nhigh-resolution ($\\Delta\\lambda < 1.0\\,\\AA$) spectrum published of a SN IIn and\nthe earliest published spectrum of SN 1998S. Modern SN studies achieve\nimpressively short turn-around times between SN detection and the first\nobserved spectrum, but high-resolution spectra of very young supernovae are\nrare; the unique spectrum presented here provides a useful case study for\nobservations of other young SN systems including SN 2013cu, which displayed a\nremarkably similar spectrum when very young. We examine the fully resolved\nemission-line profiles of SN 1998S, finding evidence for extreme mass loss from\nthe progenitor at velocities much less than those characteristic of Wolf-Rayet\n(WR) stars. We model our high-resolution SN 1998S spectrum using the\nradiative-transfer code CMFGEN and explore the composition, density, and\nvelocity gradients within the SN system. We find a mass-loss rate of $6.0\n\\times 10^{-3} M_{\\odot}$ per year during the $\\sim$15 yr before core collapse,\nwhile other studies indicate a much lower rate at earlier times (>15yr before\ncore collapse). A comparison with a spectrum of SN 2013cu indicates many\nsimilarities, though SN 2013cu was of Type IIb - indicating that very different\nsupernovae can arise from progenitors with extreme mass loss in the last few\nyears before explosion.\n