The polarimetric properties of cometary dust and a possible effect of dust aging by the Sun

Abstract

After the last apparition of comet Halley in 1985-86, a number of other comets were observed in polarimetry with IHW-continuum filters. From the in situ dust measurements of Halley, dust size distribution functions were obtained (Lamy et al. 1987, AA Mazets et al. 1986, Nature, 321, 276), which were later used by several authors to interpret polarisation data of Halley. In the present work, polarimetric data of various comets have been analyzed, using Mie theory and assuming that the composition of dust particles does not differ from comet to comet (Delsemme 1987, ESA SP-278, 19). The individual grain size distribution functions so obtained for various comets suggest different values for the abundance of coarser grains. Introducing a relative abundance of coarser index g, we study any possible dependence of g on the dynamical age of a comet, where the dynamical age can be defined in terms of some of the orbital parameters of the comet. For the four non-periodic comets available for our analysis, we find a clear empirical relation g = −2.5.q 2/3 emerging from our work. This relation strengthens the concept that comets whose grains are processed more by the solar radiation (these comets may be dynamically older) contain a relatively smaller number of finer grains. The case for other periodic comets is also discussed here. Since the work has been carried out using Mie scattering theory meant for perfect compact spheres, it is also suggested to repeat the calculations with more realistic porous grains in a follow-up paper.