# Continuum Foreground Polarization and Na~I Absorption in Type Ia SNe

## TL;DR

This study analyzes continuum polarization in 19 Type Ia supernovae, revealing correlations with Na I D lines, color, and extinction, and suggesting interstellar dust influences observed polarization.

## Contribution

It provides new insights into the relationship between polarization, Na I D absorption, and interstellar dust in supernova host galaxies.

## Key findings

- Continuum polarization varies widely among SNe Ia.
- Polarization correlates with color excess and extinction.
- Na I D lines are linked to non-galactic interstellar matter.

## Abstract

We present a study of the continuum polarization over the 400--600 nm range of 19 Type Ia SNe obtained with FORS at the VLT. We separate them in those that show Na I D lines at the velocity of their hosts and those that do not. Continuum polarization of the sodium sample near maximum light displays a broad range of values, from extremely polarized cases like SN 2006X to almost unpolarized ones like SN 2011ae. The non--sodium sample shows, typically, smaller polarization values. The continuum polarization of the sodium sample in the 400--600 nm range is linear with wavelength and can be characterized by the mean polarization (P$_{\rm{mean}}$). Its values span a wide range and show a linear correlation with color, color excess, and extinction in the visual band. Larger dispersion correlations were found with the equivalent width of the Na I D and Ca II H & K lines, and also a noisy relation between P$_{\rm{mean}}$ and $R_{V}$, the ratio of total to selective extinction. Redder SNe show stronger continuum polarization, with larger color excesses and extinctions. We also confirm that high continuum polarization is associated with small values of $R_{V}$. The correlation between extinction and polarization -- and polarization angles -- suggest that the dominant fraction of dust polarization is imprinted in interstellar regions of the host galaxies. We show that Na I D lines from foreground matter in the SN host are usually associated with non-galactic ISM, challenging the typical assumptions in foreground interstellar polarization models.