Supernova 1998S at 14 years postmortem: Continuing circumstellar interaction and dust formation

We report late-time spectroscopic observations of the Type IIn supernova (SN) 1998S, taken 14 years after explosion using the Large Binocular Telescope. The optical spectrum exhibits strong, broad emission features of [Oi], [Oii] and Hα, in addition to weaker features of [Oiii], Hβ and [Feii]. The last decade of evolution has exhibited a strengthening of the oxygen transitions relative to Hα, evidence that the late-time emission is powered by increasingly metal-rich SN ejecta crossing the reverse shock. The Hα luminosity of ≈8000 L_⊙ requires that SN 1998S is still interacting with relatively dense circumstellar material (CSM), probably produced by the strong wind of a red supergiant progenitor at least ~10³ years before explosion. The emission lines exhibit asymmetric blueshifted profiles, which implies that the receding hemisphere of the SN is obscured by dust. The [Oiii] λ5007 line, in particular, exhibits a complete suppression of its red wing. This could be the result of the expected wavelength dependence for dust extinction or a smaller radial distribution for [Oiii]. In the latter case, the red wing of [Oiii] could be absorbed by core dust, while both the blue and red wings are absorbed by dust within the cool dense shell between the forward and reverse shocks; this interpretation could explain why late-time [Oiii] emission from SNe is often weaker than models predict. The [Oi] line exhibits double-peaked structure on top of the broader underlying profile, possibly due to emission from individual clumps of ejecta or ring-like structures of metal-rich debris. The centroids of the peaks are blueshifted and lack a red counterpart. However, an archival spectrum obtained on day 1093 exhibits a third, redshifted peak, which we suspect has become extinguished by dust that formed over the last decade, after day 1093. This implies that the 'missing' red components of multi-peaked oxygen profiles observed in other SNe might be obscured by varying degrees of dust extinction.