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

Abstract

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