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

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 $\approx 4600$Å are replaced with the permitted FeI+FeII blends at $\sim 4400$Å and $\sim 5400$Å. The $\approx 7300$Å feature, which is produced by [FeII]+[NiII] at $\lesssim 400$days after explosion, is replaced by broad ($\approx \pm 15000\mathrm{km}{\mathrm{s}}^{-1}$) 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.

arXiv comment8 pages, 1 table, 6 figures. Accepted by ApJL

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