Post-perihelion photometry of dust grains in the coma of 67P Churyumov–Gerasimenko
2 Dipartimento di Fisica e Astronomia "Galileo Galilei"
3 IFN - CNR Institute for Photonics and Nanotechnologies
4 OFFIS - Carl Von Ossietzky Universität Oldenburg = Carl von Ossietzky University of Oldenburg
5 MPS - Max-Planck-Institut für Sonnensystemforschung = Max Planck Institute for Solar System Research
6 LAM - Laboratoire d'Astrophysique de Marseille
7 ISSI - International Space Science Institute [Bern]
8 CAB - Centro de Astrobiologia [Madrid]
9 ESTEC - European Space Research and Technology Centre
10 CBK - Space Research Centre of Polish Academy of Sciences
11 Department of Physics and Astronomy [Uppsala]
12 IGEP - Institut für Geophysik und Extraterrestrische Physik [Braunschweig]
13 Department of Astronomy [College Park]
14 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
15 PLANETO - LATMOS
16 Department of Industrial Engineering [University of Padua]
17 UNITN - Università degli Studi di Trento = University of Trento
18 CISAS - Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”
19 OAT - INAF - Osservatorio Astronomico di Trieste
20 IAA-CSIC - Instituto de Astrofísica de Andalucía = Institute of Astrophysics of Andalusia
21 DLR Institut für Planetenforschung
22 IANCU - Institute of Astronomy [Taiwan]
23 ESAC - European Space Astronomy Centre
24 Dipartimento di Geoscienze [Padova]
25 DEI - Department of Information Engineering [Padova]
26 ARC - NASA Ames Research Center
27 Physikalisches Institut [Bern]
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Résumé
We present a photometric analysis of individual dust grains in the coma of comet 67P/Churyumov–Gerasimenko using OSIRIS images taken from 2015 July to 2016 January. We analysed a sample of 555 taken during 18 d at heliocentric distances ranging between 1.25 and 2.04 au and at nucleocentric distances between 80 and 437 km. An automated method to detect the tracks was specifically developed. The images were taken by OSIRIS NAC in four different filters: Near-IR (882 nm), Orange (649 nm), FarOrange (649 nm) and Blue (480 nm). It was not always possible to recognize all the grains in the four filters, hence we measured the spectral slope in two wavelengths ranges: in the interval [480–649] nm, for 1179 grains, and in the interval [649–882] nm, for 746 grains. We studied the evolution of the two populations’ average spectral slopes. The data result scattered around the average value in the range [480–649] nm, while in the [649–882] nm we observe a slight decreasing moving away from the Sun as well as a slight increasing with the nucleocentric distance. A spectrophotometric analysis was performed on a subsample of 339 grains. Three major groups were defined, based on the spectral slope between [535–882] nm: (i) the steep spectra that may be related with organic material, (ii) the spectra with an intermediate slope, likely a mixture of silicates and organics and (iii) flat spectra that may be associated with a high abundance of water ice.
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