Whimper of a Bang: Documenting the Final Days of the Nearby Type Ia
  Supernova 2011fe
  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 \AA{} quasi-bolometric
luminosity is just ($420 \pm 20$) $L_{\odot}$. By measuring the late-time
quasi-bolometric light curve, we present the first confident detection of
$^{57}$Co decay in a SN Ia light curve and estimate a mass ratio of $\log
(^{57}\mathrm{Co}/^{56}\mathrm{Co}) = -1.59^{+ 0.06 }_{- 0.07 }$. We do not
have a clean detection of $^{55}\mathrm{Fe}$, but find a limit of
$^{55}\mathrm{Fe}/^{57}\mathrm{Co} < 0.22$ 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 ($^{55}\mathrm{Fe}/^{57}\mathrm{Co} = 0.27$) over the
higher central densities of near-Chandrasekhar-mass single-degenerate models
($^{55}\mathrm{Fe}/^{57}\mathrm{Co} = 0.68$). 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 $\sim 600$ days with HST and possibly beyond.
