Spectropolarimetry of Type II supernovae (I) Sample, observational data and interstellar polarization

TL;DR

This study analyzes polarization spectra of Type II supernovae to distinguish intrinsic and interstellar polarization components, revealing insights into dust properties in host galaxies and demonstrating a new method for ISP analysis.

Contribution

Introduces an improved method to analyze interstellar polarization in supernovae, enabling better study of dust properties in external galaxies.

Findings

Most SNe showed a steady ISP component with polarization ≤ 1.0%.

SN 2001dh and possibly SN 2012aw had non-Milky-Way-like ISP.

Method can be applied to larger samples for interstellar dust studies.

Abstract

We investigate polarization spectra of hydrogen-rich core-collapse supernovae (Type~II SNe). The polarization signal from SNe contains two independent components: intrinsic SN polarization and interstellar polarization (ISP). From these components, we can study the SN explosion geometry and the dust properties in their host galaxies or in the Milky Way. In this first paper, using a new improved method, we investigate the properties of the ISP components of 11 well-observed Type~II SNe. As a result of our analysis, we find that 10 out of these 11 SNe showed a steady ISP component with a polarization degree $\lesssim 1.0$ \%, while one SN was consistent with zero ISP. As for the wavelength dependence, SN~2001dh (and possibly SN~2012aw) showed a non-Milky-Way-like ISP likely originating from the interstellar dust in their respective host galaxies: their polarization maxima were located at short wavelengths ($\lesssim4000$~\AA). Similar results have been obtained previously for highly reddened SNe. The majority of the SNe in our sample had too large uncertainties in the wavelength dependence of their ISP components to consider them further. Our work demonstrates that, by applying this method to a larger SN sample, further investigation of the ISP component of the SN polarization can provide new opportunities to study interstellar dust properties in external galaxies.