Spectropolarimetry of the 2012 outburst of SN 2009ip: a bi-polar explosion in a dense, disk-like CSM

TL;DR

This study uses spectropolarimetry to reveal a bipolar explosion geometry and a dense, disk-like circumstellar medium around SN 2009ip during its 2012 outburst, providing insights into its asymmetric ejecta and CSM structure.

Contribution

First spectropolarimetric monitoring of SN 2009ip's 2012 outburst revealing its bipolar explosion and disk-like CSM structure through detailed polarisation analysis.

Findings

Continuum polarisation indicates 10-15% deviation from spherical symmetry.

High-velocity line polarisation shows elements form in the same ejecta region.

Monte Carlo simulations suggest a disk-like CSM inclined by 14±2 degrees.

Abstract

We present a sequence of eight spectropolarimetric observations monitoring the geometric evolution of the late phase of the major 2012 outburst of SN 2009ip. These were acquired with the FORS2 polarimeter mounted on ESO VLT. The continuum was polarised at 0.3-0.8 per cent throughout the observations, showing that the photosphere deviated substantially from spherical symmetry by 10-15 per cent. Significant line polarisation is detected for both hydrogen and helium at high velocities. The similarity in the polarised signal between these elements indicates that they form in the same location in the ejecta. The line polarisation (p$\sim$1-1.5 per cent) at low velocities revealed the presence of a highly-aspherical hydrogen and helium rich circumstellar medium (CSM). Monte Carlo simulations of the observed polarimetry were performed in an effort to constrain the shape of the CSM. The simulations imply that the polarimetry can be understood within the framework of a disk-like CSM inclined by 14$\pm$2 degrees out of the line of sight, obscuring the photosphere only at certain epochs. The varying temporal evolution of polarisation at high and low velocities indicated that the fast-moving ejecta expanded with a preferred direction orthogonal to that of the CSM.