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 and K lines that weakened and eventually went into emission. The strength of the emission component of H alpha gradually increased, implying that the SN was undergoing significant interaction with its circumstellar medium (CSM). These features, and many others, 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. 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. The late-time spectra of PTF11kx are dominated by Ha emission (with widths of full width at half-maximum intensity approximate to 2000 km s(-1)), strong Ca II emission features (similar to 10,000 km s(-1) 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 Ha is weak or completely absent at all epochs, leading to large observed H alpha/H beta intensity ratios. The H alpha emission appears to increase in strength with time for similar to 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.