Investigating the Unusual Spectroscopic Time-Evolution in SN 2012fr

TL;DR

SN 2012fr exhibits unusual spectroscopic evolution with features indicating a shell-like density enhancement, challenging typical supernova models and providing new insights into Type Ia supernova diversity.

Contribution

The study models SN 2012fr's spectral evolution using SYNOW and PHOENIX to identify a shell-like density structure as the cause of its peculiar features.

Findings

Unusual spectral features linked to a shell-like density in outer ejecta

Spectral modeling suggests density enhancements influence Si II and Ca II features

SN 2012fr's properties are similar to the peculiar SN 2000cx

Abstract

The type Ia supernova (SN) 2012fr displayed an unusual combination of its Si II {\lambda}{\lambda}5972, 6355 features. This includes the ratio of their pseudo equivalent widths, placing it at the border of the Shallow Silicon (SS) and Core Normal (CN) spectral subtype in the Branch diagram, while the Si II {\lambda}6355 expansion velocities places it as a High-Velocity (HV) object in the Wang et al. spectral type that most interestingly evolves slowly, placing it in the Low Velocity Gradient (LVG) typing of Benetti et al. Only 5% of SNe Ia are HV and located in the SS+CN portion of the Branch diagram and less than 10% of SNe Ia are both HV and LVG. These features point towards SN 2012fr being quite unusual, similar in many ways to the peculiar SN 2000cx. We modeled the spectral evolution of SN 2012fr to see if we could gain some insight into its evolutionary behavior. We use the parameterized radiative transfer code SYNOW to probe the abundance stratification of SN 2012fr at pre-maximum, maximum, and post-maximum light epochs. We also use a grid of W7 models in the radiative transfer code PHOENIX to probe the effect of different density structures on the formation of the Si II {\lambda}6355 absorption feature at post-maximum epochs. We find that the unusual features observed in SN 2012fr are likely due to a shell-like density enhancement in the outer ejecta. We comment on possible reasons for atypical Ca II absorption features, and suggest that they are related to the Si II features.