SN 2014C: VLBI Images of a Supernova Interacting with a Circumstellar Shell

TL;DR

This paper presents VLBI observations of supernova 2014C over several years, revealing its expansion dynamics and interaction with a dense circumstellar shell, which explains its unusual spectral evolution.

Contribution

First VLBI measurements of SN 2014C over multiple epochs, providing detailed insights into its expansion and circumstellar interaction.

Findings

SN 2014C's outer radius reached 6.4 x 10^{16} cm at 384 days

Expansion velocity decreased from ~19,300 km/s to ~13,600 km/s over time

Supernova shock interacted with a dense circumstellar shell within the first year

Abstract

We report on VLBI measurements of supernova 2014C at several epochs between $t = 384$ and 1057 days after the explosion. SN 2014C was an unusual supernova that initially had Type Ib optical spectrum, but after $t = 130$ d it developed a Type IIn spectrum with prominent H$\alpha$ lines, suggesting the onset of strong circumstellar interaction. Our first VLBI observation was at $t = 384$ d, and we find that the outer radius of SN 2014C was $(6.40 \pm 0.26) \times 10^{16}$ cm (for a distance of 15.1 Mpc), implying an average expansion velocity of $19300 \pm 790$ \kms\ up to that time. At our last epoch, SN 2014C was moderately resolved and shows an approximately circular outline but with an enhancement of the brightness on the W side. The outer radius of the radio emission at $t = 1057$ d is $(14.9 \pm 0.6) \times 10^{16}$ cm. We find that the expansion between $t = 384$ and 1057 d is well described by a constant velocity expansion with $v = 13600 \pm 650$ \kms. SN 2014C had clearly been substantially decelerated by $t = 384$ d. Our measurements are compatible with a scenario where the expanding shock impacted upon a shell of dense circumstellar material during the first year, as suggested by the observations at other wavelengths, but had progressed through the dense shell by the time of the VLBI observations.