Progressive Red Shifts in the Late-Time Spectra of Type Ia Supernovae

TL;DR

This study analyzes the evolution of late-time spectral features in Type Ia supernovae, revealing a consistent red shift in nebular emission lines due to decreasing velocity and optical depth, challenging previous interpretations.

Contribution

It provides a comprehensive analysis of late-time spectral shifts in SNe Ia, showing that red shifts are due to physical changes in line properties rather than blueshift decreases.

Findings

The 4700 A feature shifts from ~4600 A to longer wavelengths up to day +300.

The width of the 4700 A feature increases from ~170 A to over 200 A by day +350.

Red shifts are attributed to decreasing velocity and optical depth, not blueshift halts.

Abstract

We examine the evolution of late-time, optical nebular features of Type Ia supernovae (SNe Ia) using a sample consisting of 160 spectra of 27 normal SNe Ia taken from the literature as well as unpublished spectra of SN 2008Q and ASASSN-14lp. Particular attention was given to nebular features between 4000-6000 A in terms of temporal changes in width and central wavelength. Analysis of the prominent late-time 4700 A feature shows a progressive central wavelength shift from ~4600 A to longer wavelengths out to at least day +300 for our entire sample. We find no evidence for the feature's red-ward shift slowing or halting at an [Fe III] blend centroid of 4701 A as has been proposed. The width of the feature also steadily increases with a FWHM ~170 A at day +100 growing to 200 A or more by day +350. Two weaker adjacent features around 4850 and 5000 A exhibit very similar red shifts to that of the 4700 A feature but show no change in width until very late times. We discuss possible causes for the observed red-ward shifting of these late-time optical features including contribution from [Co II] emission at early nebular epochs and the emergence of additional features at later times. We conclude that the ubiquitous red shift of these common late-time, nebular SN Ia spectral features is not mainly due to a decrease in a blueshift of forbidden Fe lines but the result, in part, of decreasing velocity and/or optical depth of permitted Fe lines.