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

We present three new spectra of the nearby Type Ia supernova (SN Ia) 2011fe covering approximate to 480-850 days after maximum light and show that the ejecta undergoes a rapid ionization shift at similar to 500 days after explosion. The prominent Fe iii emission lines at approximate to 4600 angstrom are replaced with Fe i+Fe ii blends at similar to 4400 angstrom and similar to 5400 angstrom. The approximate to 7300 angstrom feature, which is produced by [Fe ii]+[Ni ii] at less than or similar to 400 days after explosion, is replaced by broad (approximate to +/- 15,000 km s(-1)) symmetric [Ca ii] emission. Models predict this ionization transition occurring similar to 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 SN 2011fe. Nondetections of H and He exclude nearby nondegenerate companions, [O i] nondetections disfavor the violent merger of two white dwarfs, and the symmetric emission-line profiles favor a symmetric explosion.