2015ApJ...806..213S

The well-studied Type IIn supernova (SN IIn) 1998S is often dubbed the prototypical SN IIn, and it provides a unique opportunity to study its progenitor star from within as the supernova (SN) lights up dense circumstellar material 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 (Δλ < 1.0 Å) spectrum published of a SN IIn and the earliest spectrum published 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 (SNe) 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 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 x 10^–3M_☉/yr during the ∼15 years before core collapse, while other studies indicate a much lower rate at earlier times (>15 years 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.