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

TL;DR

This study analyzes late-time optical and ultraviolet spectra of SN 2011fe, revealing that recombination-driven fluorescence dominates spectrum formation and that the transition to the nebular phase is complex and wavelength-dependent.

Contribution

It provides detailed spectral modeling showing the dominance of recombination processes and the complexity of the nebular transition in Type Ia supernovae at late times.

Findings

Forbidden lines dominate optical spectra after 205 days

Recombination, not collisional excitation, drives late-time spectra

Ultraviolet spectra remain optically thick at 360 days

Abstract

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 ultraviolet spectrum obtained with Hubble Space Telescope at 360 days post-maximum light. We compare these observations with synthetic spectra produced with the radiative transfer code PHOENIX. The day +100 spectrum can be well fit with models which neglect collisional and radiative data for forbidden lines. Curiously, including this data and recomputing the fit yields a quite similar spectrum, but with different combinations of lines forming some of the stronger features. At day +205 and later epochs, forbidden lines dominate much of the optical spectrum formation; however, our results indicate that recombination, not collisional excitation, is the most influential physical process driving spectrum formation at these late times. Consequently, our synthetic optical and UV spectra at all epochs presented here are formed almost exclusively through recombination-driven fluorescence. Furthermore, our models suggest that the ultraviolet spectrum even as late as day +360 is optically thick and consists of permitted lines from several iron-peak species. These results indicate that the transition to the "nebular" phase in Type Ia supernovae is complex and highly wavelength-dependent.