Spectropolarimetry of Supernovae

TL;DR

Spectropolarimetry reveals that supernovae are inherently aspherical with different geometries in core-collapse and thermonuclear types, providing insights into explosion mechanisms and implications for cosmology.

Contribution

This review synthesizes spectropolarimetric evidence demonstrating supernova asymmetries and introduces a classification scheme for polarized supernovae behaviors.

Findings

All supernovae show significant asphericity near maximum light

Core-collapse supernovae exhibit stronger inner-layer asymmetry

Outer layers of Type Ia supernovae are more aspherical

Abstract

Overwhelming evidence has accumulated in recent years that supernova explosions are intrinsically 3-dimensional phenomena with significant departures from spherical symmetry. We review the evidence derived from spectropolarimetry that has established several key results: virtually all supernovae are significantly aspherical near maximum light; core-collapse supernovae behave differently than thermonuclear (Type Ia) supernovae; the asphericity of core-collapse supernovae is stronger in the inner layers showing that the explosion process itself is strongly aspherical; core-collapse supernovae tend to establish a preferred direction of asymmetry; the asphericity is stronger in the outer layers of thermonuclear supernovae providing constraints on the burning process. We emphasize the utility of the Q/U plane as a diagnostic tool and revisit SN 1987A and SN 1993J in a contemporary context. An axially-symmetric geometry can explain many basic features of core-collapse supernovae, but significant departures from axial symmetry are needed to explain most events. We introduce a spectropolarimetry type to classify the range of behavior observed in polarized supernovae. Understanding asymmetries in supernovae is important for phenomena as diverse as the origins of gamma-ray bursts and the cosmological applications of Type Ia supernovae in studies of the dark energy content of the universe.