Polarimetry of young stellar objects .1. Linear polarization of GSS 30

We present broad-band, near-infrared (JHK(n)) imaging linear polarimetry of the young stellar object GSS 30. The overall polarization pattern is centrosymmetric about the illuminating source, IRS 1. The model by Clark & McCall is used to derive the wavelength dependence of the degrees of polarization of the scattered radiation in the nebula. The results suggest that the axis of the nebula is inclined at an angle of similar to 25 degrees-30 degrees to the plane of the sky, with the northern lobe towards us. The grain size distribution is best fitted by grains covering the range 0.005 to 0.35 mu m. However, the model cannot reproduce the observed polarization pattern across the nebula, which is seen to decrease towards its edge. A polarization disc, i.e. a band of aligned polarization vectors, is seen close to the source. This pattern breaks down to the east of the source where the polarization pattern reverts back to a centrosymmetric one. We speculate that a binary system could disrupt the density gradient across the circumstellar disc such that photons can easily escape in this direction and are then scattered (singly) towards us. To the west of the source the pattern of the polarization disc remains intact. The degrees of polarization, both in the line of sight to the source and along the disc, are higher at J and H than they are at K-n. We propose that the wavelength dependence of the polarization here may be produced by a combination of scattering and dichroism, with the grains aligned by a magnetic field parallel to the plane of the disc.