Interpretation of photo-polarimetric observations of comet 17P/Holmes

We present multispectral photo-polarimetric observations of comet 17P/Holmes taken at three different dates. These observations show the evolution of the negative polarization branch (NPB) as a function of time and wavelength. We perform discrete-dipole approximation (DDA) simulations on agglomerated debris particles of various sizes and refractive indices. Our simulations show that the observations are consistent with the cloud being composed of agglomerated debris particles having refractive indices of approximately m=1.5-1.6+0.1i. Our results are also consistent with the particles obeying a power-law size distribution r-a and having a lower particle-radius cut-off of approximately 0.6 μm, where the index a∼3.5 for the early observations and shrinks to a∼1.5 for the later observations. This is consistent with the smaller, more accelerated particles in the distribution being propelled out of the field of view.

Research highlights► In October of 2007, comet 17P/Holmes revealed “blue color” of linear polarization. ► In November of 2007, the wavelength dependence of polarization was neutralized. ► Material of dust particles needs to be highly absorbing (Im(m)∼0.1). ► Size distribution of dust particles is required to have a bottom limit at 1.2 μm. ► Contribution of small particles was decreasing between October and November 2007.