Supernova 1998S at 14 years Postmortem: Continuing Circumstellar Interaction and Dust Formation

TL;DR

This study presents 14-year post-explosion spectroscopic observations of SN 1998S, revealing ongoing circumstellar interaction, dust formation, and evolving emission line profiles, providing insights into late-stage supernova evolution.

Contribution

It provides the first long-term spectroscopic analysis of SN 1998S, showing continued CSM interaction and dust effects that influence emission line profiles and nebular evolution.

Findings

SN 1998S still interacts with dense circumstellar material.

Dust obscuration affects emission line profiles, especially red wings.

Late-time oxygen emission lines show complex structures and dust effects.

Abstract

We report late-time spectroscopic observations of the Type IIn SN 1998S, taken 14 years after explosion using the Large Binocular Telescope. The optical spectrum exhibits broad emission features of [O I], [O II], [O III], H-alpha, H-beta, and [Fe II]. The last decade of evolution has exhibited a strengthening of the oxygen transitions, evidence that the late-time emission is powered by increasingly metal-rich SN ejecta crossing the reverse shock. The H-alpha luminosity requires that SN 1998S is still interacting with 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 that the receding hemisphere of the SN is obscured by dust. The [O III] 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 [O III]. In the latter case, the red wing of [O III] 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 [O III] emission from SNe is often weaker than models predict. 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 "missing" red components of multi-peaked oxygen profiles observed in other SNe might be obscured by varying degrees of dust extinction.