Circular Polarimetry and the Line of Sight to the BN Object

Abstract

The 3.1 micron absorption feature of water-ice has been observed spectroscopically in many molecular clouds and, when it has been observed spectropolarimetrically, usually a corre- sponding polarization feature is seen. Typically on these occasions, and particularly for the BN object, a distinct position angle shift between the feature and continuum is seen, which indicates both a fractionation of the icy material and a changing alignment direction along the line of sight. Here the dependence of circular polarimetry on fractionation along the line of sight is investigated and it is shown that the form of its spectrum, together with the sign of the position angle shift, indicates where along the line of sight the icy material lies. More specifically a coincidence between the sign of the position angle displacement in the ice feature, measured north through east, and that of the circular polarization ice feature means that the icy grains are overlaid by bare grains. Some preliminary circular polarimetry of BN has this characteristic and a similar situation is found in the only two other cases for which relevant observations so far exist.