Constraining the Progenitor Companion of the Nearby Type Ia SN 2011fe with a Nebular Spectrum at +981 Days

TL;DR

This study presents a late-time nebular spectrum of SN 2011fe, analyzing spectral features and searching for signs of a companion star, ultimately constraining progenitor models and excluding many single-degenerate scenarios.

Contribution

First observational constraints on a potential remnant star in SN 2011fe at +981 days, providing limits on its temperature and luminosity, and testing progenitor models.

Findings

No evidence of narrow Hα emission.

Constraints on remnant star temperature and luminosity.

Spectral features consistent with nebular emission lines.

Abstract

We present an optical nebular spectrum of the nearby Type Ia supernova 2011fe, obtained 981 days after explosion. SN 2011fe exhibits little evolution since the +593 day optical spectrum, but there are several curious aspects in this new extremely late-time regime. We suggest that the persistence of the $\sim5800$~\AA\ feature is due to Na I D, and that a new emission feature at $\sim7300$~\AA\ may be [Ca II]. Also, we discuss whether the new emission feature at $\sim6400$~\AA\ might be [Fe I] or the high-velocity hydrogen predicted by Mazzali et al. The nebular feature at 5200~\AA\ exhibits a linear velocity evolution of $\sim350$ $\rm km\ s^{-1}$ per 100 days from at least +220 to +980 days, but the line's shape also changes in this time, suggesting that line blending contributes to the evolution. At $\sim 1000$ days after explosion, flux from the SN has declined to a point where contribution from a luminous secondary could be detected. In this work we make the first observational tests for a post-impact remnant star and constrain its temperature and luminosity to $T \gtrsim 10^4$ $\rm K$ and $L \lesssim 10^4$ $\rm L_{\odot}$. Additionally, we do not see any evidence for narrow H$\alpha$ emission in our spectrum. We conclude that observations continue to strongly exclude many single-degenerate scenarios for SN 2011fe.