* Astronomy

Title: Supernova 1998S at 14 years Postmortem: Continuing CSM Interaction and Dust Formation
Authors: Jon Mauerhan, Nathan Smith

We report late-time spectroscopic observations of the Type IIn supernova (SN) 1998S, taken14 years after explosion using the Large Binocular Telescope. The optical spectrum exhibits strong broad emission features of [O I], [O II], and H-alpha, in addition to weaker features of [O III], H-beta and [Fe II]. The last decade of evolution has exhibited a strengthening of the oxygen transitions relative to H-alpha, evidence that the late-time emission is powered by increasingly metal-rich SN ejecta crossing the reverse shock. The H-alpha luminosity of ~8000 L(Sun) 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 ~1000 years before explosion. The emission lines exhibit asymmetric blueshifted profiles, which implies the receding hemisphere of the SN is highly obscured. The effect is wavelength dependent, in a manner consistent with reddening by dust, which implies that dust is not efficiently destroyed by the SN shock. The [O I] 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. This implies that the absent red components of multi-peaked oxygen profiles observed in other supernovae might be obscured by varying degrees of extinction.

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