Journal Article

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

Jon Mauerhan and Nathan Smith

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 424, issue 4, pages 2659-2666
Published in print August 2012 | ISSN: 0035-8711
Published online August 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.21325.x
Supernova 1998S at 14 years postmortem: continuing circumstellar interaction and dust formation

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Abstract

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 [O i], [O ii] and Hα, in addition to weaker features of [O iii], Hβ; and [Fe ii]. 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 [O iii] 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 [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, 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.

Keywords: supernovae: general; supernovae: individual: SN 1998S

Journal Article.  6314 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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