Evidence for asymmetric distribution of circumstellar material around Type Ia Supernovae

TL;DR

This study reveals an asymmetric distribution of circumstellar material around Type Ia Supernovae, affecting observed absorption lines and colors, with implications for understanding supernova progenitors and explosion mechanisms.

Contribution

It provides evidence that circumstellar material around SNe Ia is asymmetrically distributed, influencing observed properties and suggesting post-explosion origins for this asymmetry.

Findings

Stronger absorption lines in SNe Ia with positive nebular velocity shifts.

Color differences in SNe Ia are partly due to extinction from asymmetric CSM.

Asymmetry likely originates after explosion due to ionizing flux differences.

Abstract

We study the properties of low-velocity material in the line of sight towards nearby Type Ia Supernovae (SNe Ia) that have measured late phase nebular velocity shifts (v_neb), thought to be an environment-independent observable. We have found that the distribution of equivalent widths of narrow blended Na I D1 & D2 and Ca II H & K absorption lines differs significantly between those SNe Ia with negative and positive v_neb, with generally stronger absorption for SNe Ia with v_neb > 0. A similar result had been found previously for the distribution of colors of SNe Ia, which was interpreted as a dependence of the temperature of the ejecta with viewing angle. Our work suggests that: 1) a significant part of these differences in color should be attributed to extinction, 2) this extinction is caused by an asymmetric distribution of circumstellar material (CSM) and 3) the CSM absorption is generally stronger on the side of the ejecta opposite to where the ignition occurs. Since it is difficult to explain 3) via any known physical processes that occur before explosion, we argue that the asymmetry of the CSM is originated after explosion by a stronger ionizing flux on the side of the ejecta where ignition occurs, probably due to a stronger shock breakout and/or more exposed radioactive material on one side of the ejecta. This result has important implications for both progenitor and explosion models.