Late-Time Optical Emission From Core-Collapse Supernovae

TL;DR

This study analyzes late-time optical spectra of ten core-collapse supernovae and Cas A, revealing common features like blueshifted emissions and line profile evolution, which inform understanding of supernova ejecta and circumstellar interactions.

Contribution

It provides new observational data and analysis of late-time emissions in multiple CCSNe, highlighting spectral evolution and potential dust effects in ejecta.

Findings

Blueshifted line emissions are common and long-lasting.

Line profile changes are consistent with ejecta-circumstellar interaction.

Emission line asymmetries may be caused by dust and large-scale ejecta structures.

Abstract

Ground-based optical spectra and Hubble Space Telescope images of ten core-collapse supernovae (CCSNe) obtained several years to decades after outburst are analyzed with the aim of understanding the general properties of their late-time emissions. New observations of SN 1957D, 1970G, 1980K, and 1993J are included as part of the study. Blueshifted line emissions in oxygen and/or hydrogen with conspicuous line substructure are a common and long-lasting phenomenon in the late-time spectra. Followed through multiple epochs, changes in the relative strengths and velocity widths of the emission lines are consistent with expectations for emissions produced by interaction between SN ejecta and the progenitor star's circumstellar material. The most distinct trend is an increase in the strength of [O III]/([O I]+[O II]) with age, and a decline in Halpha/([O I]+[O II]) which is broadly consistent with the view that the reverse shock has passed through the H envelope of the ejecta in many of these objects. We also present a spatially integrated spectrum of the young Galactic supernova remnant Cassiopeia A (Cas A). Similarities observed between the emission line profiles of the 330 yr old Cas A remnant and decades old CCSNe suggest that observed emission line asymmetry in evolved CCSN spectra may be associated with dust in the ejecta, and that minor peak substructure typically interpreted as 'clumps' or 'blobs' of ejecta may instead be linked with large-scale rings of SN debris.