Thirty years of SN 1980K: Evidence for light echoes

TL;DR

This study analyzes optical and infrared data of SN 1980K over six years, modeling light echoes from circumstellar dust to explain its slow brightness decline and spectral features, providing insights into the supernova's environment.

Contribution

The paper introduces a semi-analytic dust radiative-transfer model for light echoes, explaining SN 1980K's evolution with a thin circumstellar shell and proposing a new origin for high-velocity spectral lines.

Findings

Light echoes from a circumstellar shell explain the slow fading.

Dust properties match those observed in SN 1987A.

High-velocity lines originate from shocked ejecta clumps.

Abstract

We report optical and mid-infrared photometry of SN 1980K between 2004 and 2010, which show slow monotonic fading consistent with previous spectroscopic and photometric observations made 8 to 17 years after outburst. The slow rate-of-change over two decades suggests that this evolution may result from scattered and thermal light echoes off of extended circumstellar material. We present a semi- analytic dust radiative-transfer model that uses an empirically corrected effective optical depth to provide a fast and robust alternative to full Monte-Carlo radiative transfer modeling for homogenous dust at low to intermediate optical depths. We find that unresolved echoes from a thin circumstellar shell 14-15 lt-yr from the progenitor, and containing about 0.02 Msun of carbon-rich dust, can explain the broadband spectral and temporal evolution. The size, mass and dust composition are in good agreement with the contact discontinuity observed in scattered echoes around SN 1987A. The origin of slowly-changing high-velocity [O I] and Halpha lines is also considered. We propose an origin in shocked high-velocity metal-rich clumps of ejecta, rather than arising in the impact of ejecta on slowly-moving circumstellar material, as is the case with hot spots in SN 1987A.