Spectropolarimetry of the Type Ia Supernova 2012fr

TL;DR

This study uses spectropolarimetry to analyze the 3D geometry of Supernova 2012fr, revealing polarization patterns that support the pulsating delayed detonation explosion model over merger scenarios.

Contribution

First spectropolarimetric analysis of SN 2012fr at multiple epochs, providing insights into its explosion geometry and supporting the pulsating delayed detonation model.

Findings

High velocity components are highly polarized at early times.

Polarization angles indicate axial symmetry in the explosion.

Low continuum polarization suggests non-merger explosion scenario.

Abstract

Spectropolarimetry provides the means to probe the 3D geometries of Supernovae at early times. We report spectropolarimetric observations of the Type Ia Supernova 2012fr at four epochs: -11, -5, +2 and +24 days, with respect to B-lightcurve maximum. SN 2012fr is a normal Type Ia SN, similar to SNe 1990N, 2000cx and 2005hj (that all exhibit low velocity decline rates for the principal Si II line). The SN displays high velocity components at -11 days that are highly polarized. The polarization of these features decreases as they become weaker from -5 days. At +2 days, the polarization angles of the low velocity components of silicon and calcium are identical and oriented at 90 degrees relative to the high velocity Ca component. In addition to having very different velocities, the high and low velocity Ca components have orthogonal distributions in the plane of the sky. The continuum polarization for the SN at all four epochs is low <0.1%. We conclude that the low level of continuum polarization is inconsistent with the merger-induced explosion scenario. The simple axial symmetry evident from the polarization angles of the high velocity and low velocity Ca components, along with the presence of high velocity components of Si and Ca, are consistent with the pulsating delayed detonation model. We predict that, during the nebular phase, SN 2012fr will display blue-shifted emission lines of Fe-group elements.