Light Echoes from Eta Carinae's Great Eruption: Spectrophotometric
  Evolution and the Rapid Formation of Nitrogen-rich Molecules
  We present follow-up optical imaging and spectroscopy of one of the light
echoes of $\eta$ Carinae's 19th-century Great Eruption discovered by Rest et
al. (2012). By obtaining images and spectra at the same light echo position
between 2011 and 2014, we follow the evolution of the Great Eruption on a
three-year timescale. We find remarkable changes in the photometric and
spectroscopic evolution of the echo light. The $i$-band light curve shows a
decline of $\sim 0.9$ mag in $\sim 1$ year after the peak observed in early
2011 and a flattening at later times. The spectra show a pure-absorption early
G-type stellar spectrum at peak, but a few months after peak the lines of the
[Ca II] triplet develop strong P-Cygni profiles and we see the appearance of
[Ca II] 7291,7324 doublet in emission. These emission features and their
evolution in time resemble those observed in the spectra of some Type IIn
supernovae and supernova impostors. Most surprisingly, starting $\sim 300$ days
after peak brightness, the spectra show strong molecular transitions of CN at
$\gtrsim 6800$ \AA. The appearance of these CN features can be explained if the
ejecta are strongly Nitrogen enhanced, as it is observed in modern
spectroscopic studies of the bipolar Homunculus nebula. Given the spectroscopic
evolution of the light echo, velocities of the main features, and detection of
strong CN, we are likely seeing ejecta that contributes directly to the
Homunculus nebula.
