SN 1978K: An evolved supernova outside our Local Group detected at millimetre wavelengths

TL;DR

This paper reports late-time millimetre wavelength detections of supernova SN 1978K, revealing its high luminosity and dense circumstellar environment, and compares its evolution to other supernovae, challenging existing templates.

Contribution

It presents the first millimetre detections of SN 1978K at late times and provides new VLBI measurements of its size and expansion velocity, offering insights into its dense circumstellar medium.

Findings

SN 1978K is >400 times more luminous than SN 1987A at millimetre wavelengths.

The supernova's expansion velocity is <1500 km/s.

SN 1978K's dense circumstellar medium is confirmed by VLBI observations.

Abstract

Supernova 1978K is one of the oldest-known examples of the class of Type IIn supernovae that show evidence for strong interaction between the blast wave and a dense, pre-existing circumstellar medium. Here we report detections of SN 1978K at both 34 GHz and 94 GHz, making it only the third extragalactic supernova (after SN 1987A and SN 1996cr) to be detected at late-times at these frequencies. We find SN 1978K to be >400 times more luminous than SN 1987A at millimetre wavelengths in spite of the roughly nine year difference in ages, highlighting the risk in adopting SN 1987A as a template for the evolution of core-collapse supernovae in general. Additionally, from new VLBI observations at 8.4 GHz, we measure a deconvolved diameter for SN 1978K of ~5 milli-arcsec, and a corresponding average expansion velocity of <1500 km/s. These observations provide independent evidence of an extremely dense circumstellar medium surrounding the progenitor star.