2002ApJ...581..285K

New high-resolution polarization images of the η Car Homunculus have been obtained with the Hubble Space Telescope (HST) at several wavelengths spanning the near-UV to the near-IR. With these images, taken with WFPC2 and NICMOS, we have been able to compare the polarization properties of the Homunculus at different wavelengths. We confirm that the Homunculus is primarily a reflection nebula and review the evidence that the inner region within 2" of η Car has nonstellar hydrogen recombination emission. Additionally, we detect nebular [N II] emission in the inner 2" region. We confirm that the large-scale morphology of the polarization appears to be independent of wavelength from the near-UV to the near-IR. The smooth images of polarization and the very structured images of intensity show that there is no correlation between the surface brightness and polarization. We examine the polarization percentages and vector angles of small-scale features in the Homunculus, such as the paddle, the skirt, the spot, and the southern ridge, to speculate on whether the features are composed of scattering particles similar to those in the lobes as a whole. We construct three-dimensional radiative transfer models of the Homunculus. The models have a hollow ``figure eight'' with a central light source. Radiation from the central source is Mie-scattered off dust grains in the ``figure eight'' into our line of sight. Patchy extinction lining the outer shell of the ``figure eight'' is used to explain surface brightness inhomogeneities, but the patchy extinction is not modeled. We find that a combination of small and large silicate dust grains reproduces the high polarization and independence with wavelength of the data much better than a model with a single distribution of standard ISM-type silicate dust grains. Finally, two different geometrical forms of the Homunculus from the literature were used in the models and found to produce very similar polarization features.