Early Emission from the Type IIn Supernova 1998S at High Resolution

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