Dust in comet C/1975 V1 (west)

Abstract

We analyse the photopolarimetric response measured in Comet C/1975 V1 (West) using model agglomerated debris particles. Such particles have highly irregular morphology and volume density with properties that are consistent with the samples retrieved by Stardust from Comet 81P/Wild 2. We find that an entire set of observational data including phase function, phase dependence of the degree of linear polarization, photometric, and polarimetric colour in Comet C/1975 V1 (West) can be quantitatively reproduced with a two-component mixture of weakly and highly absorbing particles, having refractive indices m = 1.5+0i or 1.6+0.0005i and 2.43+0.59i, respectively. These refractive indices are consistent with Mg-rich silicates and amorphous carbon, two species considered to be abundant in comets. Our modelling predicts the volume ratio of these materials being about 1:3. We note that in situ mass spectrometry measurements of Comet 1P/Halley suggested a similar ratio of weakly to highly absorbing particles, with a large uncertainty. The best fit to observations is obtained for a polydisperse ensembles of particles that obey the power-law size distribution r-1.5 and r-2. Such a distribution also is in good agreement with in situ studies of Comet 1P/Halley. While our modelling can reproduce the photopolarimetric properties of Comet C/1975 V1 (West), it also is one of the simplest approach, since it has fewer free parameters than other approaches. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.