Polarimetry of young stellar objects - II. Circular polarization ofGSS 30

We present results from the first imaging polarimetry measurements of circularly polarized light. These observations, towards the outflow source GSS 30, show degrees of circular polarization in the K_n band of as much as -1.7 per cent, measured in the outflow region, and ~-0.5 per cent towards the source of the outflow, IRS 1. No circular polarization is measured towards the polarization disc, contrary to expectations from current scattering models. The detection of circular polarization in the outflow region means that radiation escaping from the central star must be polarized close to the source before entering the reflection nebula. Monte Carlo simulations are used to show that this may occur with a scattering geometry which contains a dusty envelope surrounding the accretion disc. The relatively high degrees of circular polarization reported here, coupled to our previous linear polarimetry of this object, require that the refractive index of the scattering grains contains an absorptive component, ruling out pure silicates as the only constituent of the grain population.