We present three new spectra of the nearby Type Ia supernova (SN Ia) 2011fe covering ≈480−850days after maximum light and show that the ejecta undergoes a rapid ionization shift at ∼500days after explosion. The prominent [FeIII] emission lines at ≈4600\r{A} are replaced with the permitted FeI+FeII blends at ∼4400\r{A} and ∼5400\r{A}. The ≈7300\r{A} feature, which is produced by [FeII]+[NiII] at ≲400days after explosion, is replaced by broad (≈±15000kms−1) symmetric [CaII] emission. Models predict this ionization transition occurring ∼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.
@article{arxiv.2111.00016,
title = {A Rapid Ionization Change in the Nebular-Phase Spectra of the Type Ia SN 2011fe},
author = {M. A. Tucker and C. Ashall and B. J. Shappee and C. S. Kochanek and K. Z. Stanek and P. Garnavich},
journal= {arXiv preprint arXiv:2111.00016},
year = {2022}
}