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

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.