The HST Non-Detection of SN Ia 2011fe 11.5yr After Explosion Further Restricts Single-Degenerate Progenitor Systems

We present deep Hubble Space Telescope imaging of the nearby Type Ia supernova (SN Ia) 2011fe obtained 11.5yr after explosion. No emission is detected at the SN location to a $1\sigma$ ($3\sigma$) limit of $F555W>30.2(29.0)$mag, or equivalently $M_V>1.2(-0.1)$mag, neglecting the distance uncertainty to M101. We constrain the presence of donor stars impacted by the SN ejecta with the strictest limits thus far on compact (i.e., $\log g\gtrsim 4$) companions. H-rich zero-age main-sequence companions with masses $\ge 2{\mathrm{M}}_{\odot }$ are excluded, a significant improvement upon the pre-explosion imaging limit of $\approx 5{\mathrm{M}}_{\odot }$. Main-sequence He stars with masses $\ge 1{\mathrm{M}}_{\odot }$ and subgiant He stars with masses $\lesssim 0.8{\mathrm{M}}_{\odot }$ are also disfavored by our late-time imaging. Synthesizing our limits on post-impact donors with previous constraints from pre-explosion imaging, early-time radio and X-ray observations, and nebular-phase spectroscopy, essentially all formation channels for SN2011fe invoking a non-degenerate donor star at the time of explosion are unlikely.