Narrow transient absorptions in late-time optical spectra of type Ia supernovae: evidence for large clumps of iron-rich ejecta?

An examination of late-time, optical spectra of type Ia supernovae (SNe Ia) revealed surprisingly narrow absorption features which only become visible a few months after maximum light. These features, most clearly seen in the late-time spectra of the bright, recent SNe Ia ASASSN-14lp and SN 2017bzc, appear as narrow absorptions at ∼4840 Å, ∼5000 Å, and as a sharp inflection at ∼4760 Å on the red side of the prominent late-time 4700 Å feature. A survey of online archival data revealed similar features present in the spectra of 10 other normal and 91T-like SNe Ia, including SN 2011fe. Unlike blue spectral features which exhibit progressive redward shifts, these narrow absorptions remain at the same wavelength from epoch to epoch for an individual SN, but can appear at slightly different wavelengths for each object. These features are also transient, appearing and then fading in 1-3 months. After ruling out instrumental, data reduction, and atmospheric affects, we discuss possible explanations including progenitor mass-loss material, interaction with material from previous novae events, and absorption by large discrete clumps of high-velocity Fe-rich ejecta.