Study of background star polarization and polarization efficiency of three selected Bok globules CB56, CB60 and CB69

TL;DR

This study maps the polarization of background stars in three Bok globules to analyze magnetic field alignment and polarization efficiency, revealing a decrease in efficiency with increasing extinction and insights into dust grain alignment.

Contribution

It provides the first polarization maps of CB56, CB60, and CB69 using V-band CCD polarimetry, and quantifies the relationship between polarization efficiency and extinction in these clouds.

Findings

Magnetic field in CB56 aligns with the galactic field, unlike in CB60 and CB69.

Polarization efficiency decreases with increasing extinction, following a power law.

Linear polarization is more efficient in low extinction regions.

Abstract

We present the polarization maps of three selected Bok globules CB56, CB60 and CB69 constructed using a V-band data from a CCD imaging polarimeter. The aim of this work is to measure the optical polarization ($p_v$) of background field stars in order to determine the polarization efficiency, $p_v/A_v$. We find that the local magnetic field of the cloud CB56 is almost aligned with the galactic field, but not in CB60 and CB69. A trend of decreasing polarization efficiency with increasing extinction ($A_v$) is observed: it can be well represented by a power law, $p_v/A_v \propto A_v^{-\alpha}$, where $\alpha = -0.56 \pm 0.36$, $-0.59 \pm 0.51$ and $-0.52 \pm 0.49$ for CB56, CB60 and CB69 respectively. This indicates that the linear polarization of the starlight due to aligned dust grains in these clouds is produced more efficiently in low extinction regions, compared with high obscured lines of sight.