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The well-studied Type IIn supernova (SN) 1998S is often dubbed theprototypical SN IIn, and it provides a unique opportunity to study itsprogenitor star from within as the SN lights up dense circumstellar material(CSM) launched from the progenitor. Here we present a Keck HIRES spectrum of SN1998S taken within a few days after core collapse - both the earliesthigh-resolution ($\Delta\lambda < 1.0\,\AA$) spectrum published of a SN IIn andthe earliest published spectrum of SN 1998S. Modern SN studies achieveimpressively short turn-around times between SN detection and the firstobserved spectrum, but high-resolution spectra of very young supernovae arerare; the unique spectrum presented here provides a useful case study forobservations of other young SN systems including SN 2013cu, which displayed aremarkably similar spectrum when very young. We examine the fully resolvedemission-line profiles of SN 1998S, finding evidence for extreme mass loss fromthe progenitor at velocities much less than those characteristic of Wolf-Rayet(WR) stars. We model our high-resolution SN 1998S spectrum using theradiative-transfer code CMFGEN and explore the composition, density, andvelocity 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 beforecore collapse). A comparison with a spectrum of SN 2013cu indicates manysimilarities, though SN 2013cu was of Type IIb - indicating that very differentsupernovae can arise from progenitors with extreme mass loss in the last fewyears before explosion.

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