Study of grain alignment efficiency and a distance estimate for small globule CB4

摘要

We study the polarization efficiency(defined as the ratio of polarization to extinction) of stars in the background of the small, nearly spherical and isolated Bok globule CB4 to understand the grain alignment process. A decrease in polarization efficiency with an increase in visual extinction is noticed. This suggests that the observed polarization in lines of sight which intercept a Bok globule tends to show dominance of dust grains in the outer layers of the globule. This finding is consistent with the results obtained for other clouds in the past. We determined the distance to the cloud CB4 using near-infrared photometry(2MASS J H KScolors) of moderately obscured stars located at the periphery of the cloud. From the extinction-distance plot,the distance to this cloud is estimated to be(459 ± 85) pc.