Scattering polarization of 3-$\mu$m water-ice feature by large icy grains

TL;DR

This study explores how the polarization of light scattered by water-ice grains at 3 micrometers reveals details about grain size and ice abundance, aiding understanding of icy cosmic environments.

Contribution

It demonstrates that polarization enhancement at the water-ice feature depends on grain size and ice abundance, providing a new method to characterize ice properties in space.

Findings

Polarization enhancement is sensitive to micron-sized grains and ice abundance.

High water-ice absorption causes surface reflection to dominate internal scattering.

Models with micron-sized grains and low ice abundance match observations of protostar L1551 IRS 5.

Abstract

Water ice has a strong spectral feature at a wavelength of approximately $3~\mu$m, which plays a vital role in our understanding of the icy universe. In this study, we investigate the scattering polarization of this water-ice feature. The linear polarization degree of light scattered by $\mu$m-sized icy grains is known to be enhanced at the ice band; however, the dependence of this polarization enhancement on various grain properties is unclear. We find that the enhanced polarization at the ice band is sensitive to the presence of $\mu$m-sized grains as well as their ice abundance. We demonstrate that this enhancement is caused by the high absorbency of the water-ice feature, which attenuates internal scattering and renders the surface reflection dominant over internal scattering. Additionally, we compare our models with polarimetric observations of the low-mass protostar L1551 IRS 5. Our results show that scattering by a maximum grain radius of a few microns with a low water-ice abundance is consistent with observations. Thus, scattering polarization of the water-ice feature is a useful tool for characterizing ice properties in various astronomical environments.