Dust in comet C/2007 N3 (LULIN)

We report optical imaging, optical and near-infrared polarimetry, and Spitzer mid-infrared spectroscopy of comet C/2007 N3 (Lulin). Polarimetric observations were obtained in R (0.676 μm) at phase angles from 0°44 to 21° with simultaneous observations in H (1.65 μm) at 4°0, exploring the negative branch in polarization. Comet C/2007 N3 (Lulin) shows a typical negative polarization in the optical as well as a similar negative branch at near-infrared wavelengths. The 10 μm silicate feature is only weakly in emission and, according to our thermal models, is consistent with emission from a mixture of silicate and carbon material. We argue that large, low-porosity (akin to ballistic particle cluster aggregates), rather absorbing aggregate dust particles best explain both the polarimetric and the mid-infrared spectral energy distribution.