SIMBAD references

We present three new spectra of the nearby Type Ia supernova (SN Ia) 2011fe covering ≃480-850 days after maximum light and show that the ejecta undergoes a rapid ionization shift at ∼500 days after explosion. The prominent Fe III emission lines at ≃4600 Å are replaced with Fe I+Fe II blends at ∼4400 Å and ∼5400 Å. The ≃7300 Å feature, which is produced by [Fe II]+[Ni II] at <=400 days after explosion, is replaced by broad (≃±15,000 km s^–1) symmetric [Ca II] 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 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.