Linear and circular polarization studies of the Eta Carinae Homunculus nebula

The Homunculus nebula, surrounding the massive star system Η Carinae, is a bipolar dust nebula which is outflowing at up to 700 km/s. The bipolar lobes display high linear polarization in the optical and near-IR, which is consistent with an origin in dust scattering from the central source. Extensive imaging and spectropolarimetric studies have not however been able to provide a consistent picture of the dust which has been ejected in the mass loss events, the most important of which occurred in the 1840's. The magnitude of the linear polarization shows very little change with wavelength, suggesting very small grains. On the other hand, models of the IR emission suggest a mixed grain population. The scattering properties of feasible dust mixtures do not however well match the observed optical and near-infrared polarization behaviour. Three possibilities are advanced to explain the dust grain population in the Homunculus: optical depth effects within a clumped distribution; the presence of many small clouds with grain size dependent on depth into the cloud; large-scale grain alignment. The last suggestion is supported by observation of 10μ polarization. Visible light circular polarization observations and refined geometric dust-scattering models are presented to advance the picture of the dust in the Homunculus. Since the dust ejected from Η Carinae is several solar masses, this study is also relevant to the understanding of ISM dust.