Evolving into a remnant: optical observations of SN 1978K at three decades

TL;DR

This study presents optical observations of SN 1978K over three decades, revealing persistent emission lines and slow evolution, indicating ongoing interaction with dense circumstellar material and a transition towards remnant status.

Contribution

It provides the first long-term optical monitoring of SN 1978K, demonstrating its slow evolution and detailed analysis of its circumstellar environment and progenitor mass loss.

Findings

SN 1978K has not faded significantly after three decades.

The supernova interacts with dense, likely non-spherical circumstellar material.

SN 1978K's evolution resembles that of SN 1987A, indicating a transition to a remnant.

Abstract

We present new optical observations of the supernova SN 1978K, obtained in 2007 and 2014 with the Very Large Telescope. We discover that the supernova has not faded significantly, even more than three decades after its explosion. The spectrum exhibits numerous narrow (FWHM $\lesssim600$ km s$^{-1}$) emission lines, indicating that the supernova blastwave is persistently interacting with dense circumstellar material (CSM). Evolution of emission lines indicates that the supernova ejecta is slowly progressing through the reverse shock, and has not expanded past the outer edge of the circumstellar envelope. We demonstrate that the CSM is not likely to be spherically distributed, with mass of $\lesssim$ 1 M$_\odot$. The progenitor mass loss rate is estimated as $\gtrsim 0.01$ M$_\odot$ yr$^{-1}$. The slowly fading late-time light curve and spectra show striking similarity with SN 1987A, indicating that a rate at which the CSM is being swept-up by the blastwave is gradually decaying and SN 1978K is undergoing similar evolution to become a remnant. Due to its proximity (4 Mpc), SN 1978K serves as the next best example of late-time supernova evolution after SN 1987A.