19 - A hole in Ia? Spectroscopic and polarimetric signatures of SN Ia asymmetry due to a companion star

Summary

Abstract

In the popular progenitor scenario, Type Ia supernova are the result of a white dwarf exploding in a binary system. The presence of a nearby companion star could cause a substantial asymmetry in the supernova ejecta — according to the models of Marietta et. al. (2000), the companion carves out an hole in the ejecta. The opening angle of the hole is as large as 40°. Such an asymmetry would leave signatures in the supernova flux and polarization spectra. We explore this possibility using a three-dimensional Monte-Carlo LTE radiative transfer code which includes gamma ray transport and a temperature correction procedure. We calculate synthetic spectra and polarization levels from multiple lines of sight to see how an ejecta hole model compares to observations.

Introduction

While some Type Ia supernovae (SNe Ia) are known to be aspherical, the exact nature of the asymmetry is unknown. The direct evidence of the asphericity is the detection of non-zero intrinsic polarization in, for example, SN 1999by [4] and SN 2001el [17]. In both cases, the polarization level was rather low (∼0.7% for SN 1999by, ∼0.4% for SN 2001el), which indicates a mild asymmetry along the line of sight. In addition, the polarization angle was fairly constant across the majority of line features, indicating that the bulk of the ejecta obeyed a near axial symmetry. The exact shape of the supernova ejecta is an important question, as it must be closely tied to the explosion processes and progenitor systems of SNe Ia.