Analysis of Type IIn SN 1998S: Effects of Circumstellar Interaction on Observed Spectra

We present spectral analysis of early observations of the Type IIn supernova 1998S using the general non-local thermodynamic equilibrium atmosphere code PHOENIX. We model both the underlying supernova spectrum and the overlying circumstellar interaction region and produce spectra in good agreement with observations. The early spectra are well fitted by lines produced primarily in the circumstellar region itself, and later spectra are due primarily to the supernova ejecta. Intermediate spectra are affected by both regions. A mass-loss rate of order ~ 0.0001-0.001 M ☉ yr -1 is inferred for a wind speed of 100-1000 km s -1 . We discuss how future self-consistent models will better clarify the underlying progenitor structure.