Circular polarimetry and the line of sight to the Becklin¿Neugebauer object

The 3.1 µm absorption feature of water-ice has been observed spectroscopically in many molecular clouds and, when it has been observed spectropolarimetrically, usually a corresponding polarization feature is seen. Typically on these o ccasions, and particularly for the BN object, a distinct position angle shift between the featu re and continuum is seen, which indicates both a fractionation of the icy material and a chan ging 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, to gether with the sign of the position angle shift, indicates where along the line of sight the icy m aterial 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 ic e 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.