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

作　　者：Ajoy Barman Himadri Sekhar Das 

机构地区：[1]Department of Physics, Assam University

出　　处：《Research in Astronomy and Astrophysics》2015年第7期953-962,共10页天文和天体物理学研究（英文版）

基　　金：funded by the National Aeronautics and Space Administration and the National Science Foundation;supported by the Science and Engineering Research Board (SERB), a statutory body under the Department of Science and Technology (DST), Government of India;the Fast Track scheme for Young Scientists (SR/FTP/PS-092/2011)

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.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.

关 键 词：polarization -- ISM： clouds -- dust extinction -- ISM： individual ob-jects： CB4 

分 类 号：P144[天文地球—天体物理]