A Rapid Ionization Change in the Nebular-Phase Spectra of the Type Ia SN 2011fe

TL;DR

This study presents late-time spectra of SN 2011fe revealing a rapid ionization shift around 500 days post-explosion, challenging existing models and constraining progenitor scenarios.

Contribution

It provides new nebular spectra of SN 2011fe showing an unexpected ionization transition and tests progenitor models using spectral features.

Findings

Rapid ionization shift at ~500 days post-explosion

Non-detections of H and He exclude non-degenerate companions

Non-detections of [OI] disfavor violent white dwarf mergers

Abstract

We present three new spectra of the nearby Type Ia supernova (SN Ia) 2011fe covering $\approx 480-850~$days after maximum light and show that the ejecta undergoes a rapid ionization shift at $\sim 500~$days after explosion. The prominent [FeIII] emission lines at $\approx 4600~$\r{A} are replaced with the permitted FeI+FeII blends at $\sim 4400~$\r{A} and $\sim 5400~$\r{A}. The $\approx 7300~$\r{A} feature, which is produced by [FeII]+[NiII] at $\lesssim 400~$days after explosion, is replaced by broad ($\approx \pm 15\,000~\rm{km}~\rm{s}^{-1}$) symmetric [CaII] emission. Models predict this ionization transition occurring $\sim 100$ days later than what is observed, which we attribute to clumping in the ejecta. Finally, we use the nebular-phase spectra to test several proposed progenitor scenarios for SN2011fe. Non-detections of H and He exclude nearby non-degenerate companions, [OI] non-detections disfavor the violent merger of two white dwarfs, and the symmetric emission-line profiles favor a symmetric explosion.