Late-Time Spectral Observations of the Strongly Interacting Type Ia
  Supernova PTF11kx
  PTF11kx was a Type Ia supernova (SN Ia) that showed time-variable absorption
features, including saturated Ca II H&K lines that weakened and eventually went
into emission. The strength of the emission component of H{\alpha} increased,
implying that the SN was undergoing significant interaction with its
circumstellar medium (CSM). These features were blueshifted slightly and showed
a P-Cygni profile, likely indicating that the CSM was directly related to, and
probably previously ejected by, the progenitor system itself. These and other
observations led Dilday et al. (2012) to conclude that PTF11kx came from a
symbiotic nova progenitor like RS Oph. In this work we extend the spectral
coverage of PTF11kx to 124-680 rest-frame days past maximum brightness. These
spectra of PTF11kx are dominated by H{\alpha} emission (with widths of ~2000
km/s), strong Ca II emission features (~10,000 km/s wide), and a blue
"quasi-continuum" due to many overlapping narrow lines of Fe II. Emission from
oxygen, He I, and Balmer lines higher than H{\alpha} is weak or completely
absent at all epochs, leading to large observed H{\alpha}/H{\beta} intensity
ratios. The broader (~2000 km/s) H{\alpha} emission appears to increase in
strength with time for ~1 yr, but it subsequently decreases significantly along
with the Ca II emission. Our latest spectrum also indicates the possibility of
newly formed dust in the system as evidenced by a slight decrease in the red
wing of H{\alpha}. During the same epochs, multiple narrow emission features
from the CSM temporally vary in strength. The weakening of the H{\alpha} and Ca
II emission at late times is possible evidence that the SN ejecta have
overtaken the majority of the CSM and agrees with models of other strongly
interacting SNe Ia. The varying narrow emission features, on the other hand,
may indicate that the CSM is clumpy or consists of multiple thin shells.
