2017ApJ...841...48S

Using the Hubble Space Telescope (HST) and the Large Binocular Telescope, we followed the evolution of the Type Ia supernova (SN Ia) 2011fe for an unprecedented 1840 days past B-band maximum light and over a factor of 7 million in flux. At 1840 days, the 4000-17000 A quasi-bolometric luminosity is just (420±20) L_☉. By measuring the late-time quasi-bolometric light curve, we present the first confident detection of ⁵⁷Co decay in a SN Ia light curve and estimate a mass ratio of log(⁵⁷Co/⁵⁶Co)=-1.59_–0.07^+0.06. We do not have a clean detection of ⁵⁵Fe, but find a limit of ⁵⁵Fe/⁵⁷Co 0.22 with 99% confidence. These abundance ratios provide unique constraints on the progenitor system because the central density of the exploding white dwarf(s) dictates these nucleosynthetic yields. The observed ratios strongly prefer the lower central densities of double-degenerate models (⁵⁵Fe/⁵⁷Co=0.27) over the higher central densities of near-Chandrasekhar-mass single-degenerate models (⁵⁵Fe/⁵⁷Co=0.68). However, additional theoretical studies predicting isotopic yields from a broader range of progenitor systems are motivated by these unique observations. We will continue to observe SN 2011fe for another ∼600 days with HST and possibly beyond.