Interstellar extinction, polarization, and grain alignment in the Sh 2-185 (IC 59 and IC 63) region

TL;DR

This study investigates how radiation influences dust grain alignment in nebulae by analyzing polarization and extinction, confirming a linear relationship between polarization efficiency and stellar illumination intensity.

Contribution

It extends the understanding of grain alignment by confirming the proportional relationship at high radiation intensities in the Sh 2-185 region, supporting previous findings from the Local Bubble wall.

Findings

Polarization efficiency correlates linearly with radiation intensity.

The same relationship applies in high radiation environments like Sh 2-185.

Supports the Radiative Alignment Torques theory across different regions.

Abstract

Optical and infrared continuum polarization from the interstellar medium is driven by radiative processes aligning the grains with the magnetic field. While a quantitative, predictive theory of Radiative Alignment Torques (RAT) exists and has been extensively tested, several parameters of the theory remain to be fully constrained. In a recent paper, \citet{medan2019} showed that the polarization efficiency (and therefore grain alignment efficiency) at different locations in the wall of the Local Bubble (LB) could be modeled as proportional to the integrated light intensity from the surrounding stars and OB associations. Here we probe that relationship at high radiation field intensities by studying the extinction and polarization in the two reflection nebulae IC\,59 and IC\,63 in the Sh 2-185 H II region, illuminated by the B0 IV star $\gamma$ Cassiopeia. We combine archival visual polarimetry with new 7-band photometry in the Vilnius system, to derive the polarization efficiency from the material. We find that the same linear relationship seen in the Local Bubble wall also applies to the Sh 2-185 region, strengthening the conclusion from the earlier study.