Photometry of dust grains of comet 67P and connection with nucleus regions

Abstract

Aims. Multiple couple of high resolution images of the dust coma of comet 67P have been collected by Osiris on board Rosetta and allowed to extract and to analyze dust grain tracks. We have performed the photometric analysis of 70 dust grain tracks observed on two different images ob- tained in the two filters F24 and F28, centered at λ = 480.7 nm and at λ = 743.7 nm respectively, allowing to derive the color and the reddening of each one. We have then extracted several images of the nucleus observed with the same filters and with the same phase angle to be compared with the dust grain reddening. Methods. We have developed a quasi automatic method to recognize and to extract dust tracks in the Osiris images providing size, FWHM and photometric data. The dust tracks characterized by low signal to noise ratio have been checked manually. Results. Most of the dust grain reddening are very similar to the nucleus values, confirming they come from the surface or sub-surface layer. The histogram of the dust grain reddening has a secondary peak at negative values and shows some grains having values higher than the nucleus suggesting a different composition from the surface grains.