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We present optical spectra of the nearby Type Ia supernova SN 2011fe at 100,205, 311, 349, and 578 days post-maximum light, as well as an ultravioletspectrum obtained with Hubble Space Telescope at 360 days post-maximum light.We compare these observations with synthetic spectra produced with theradiative transfer code PHOENIX. The day +100 spectrum can be well fit withmodels which neglect collisional and radiative data for forbidden lines.Curiously, including this data and recomputing the fit yields a quite similarspectrum, but with different combinations of lines forming some of the strongerfeatures. At day +205 and later epochs, forbidden lines dominate much of theoptical spectrum formation; however, our results indicate that recombination,not collisional excitation, is the most influential physical process drivingspectrum formation at these late times. Consequently, our synthetic optical andUV spectra at all epochs presented here are formed almost exclusively throughrecombination-driven fluorescence. Furthermore, our models suggest that theultraviolet spectrum even as late as day +360 is optically thick and consistsof permitted lines from several iron-peak species. These results indicate thatthe transition to the "nebular" phase in Type Ia supernovae is complex andhighly wavelength-dependent.

Optical and ultraviolet spectroscopic analysis of SN 2011fe at late times