Spectropolarimetry of the 3-μm water-ice feature towards young stellar objects

Abstract

We present spectropolarimetry of the 3-μm water-ice feature towards five young stellar objects embedded in molecular clouds, including the Becklin–Neugebauer object, with wavelength range and spectral resolution much improved over previous studies. There is ice-feature polarization excess in four of the five sources and our observations indicate that the polarization is caused by the dichroic absorption of aligned grains in at least three of these. The ice-feature polarization excess is always accompanied by a systematic variation in the position angle of polarization, indicating that the ice-mantled grains are fractionated in the line of sight through a changing magnetic-field orientation. The results are compared with a recently published mid-infrared survey and we find good correlations between the polarization of the 3-μm ice feature and the 10-μm silicate feature, compelling evidence for the presence of water-ice mantled silicate grains, and which suggests that the core/mantle ratio does not differ widely between objects, an important result for grain models.