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

¹ Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
e-mail: sdr@aao.gov.au
² Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN, UK
³ Department of Physics and Astronomy, Rice University, 6100 South Main, MS-108, Houston, TX 77251-1892, USA
⁴ Istituto di Radioastronomia, INAF, via P. Gobetti, 40129 Bologna, Italy
⁵ International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6102, Australia
⁶ Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia

Received: 21 September 2016
Accepted: 6 October 2016

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^-1. These observations provide independent evidence of an extremely dense circumstellar medium surrounding the progenitor star.