Photometric study of eight distant comets

ABSTRACT Our primary task is to search for rapid dust colour variations of comets beyond 3 au from the Sun. We use an archive of photometric observations obtained at the Skalnaté Pleso Observatory since 2015 and data collected recently. This work describes the results gained for four hyperbolic, C/2021 A1 (Leonard), C/2020 S4 (PanSTARRS), C/2014 A4 (SONEAR), C/2013 V2 (Borisov), and four long-period, C/2017 T2 (PanSTARRS), C/2016 N4 (MASTER), C/2015 O1 (PanSTARRS), C/2014 W2 (PanSTARRS), comets observed at heliocentric distances further than 3 au. The cometary activity and dust colour were relatively steady and typical of other long-period and hyperbolic comets. However, some estimations strongly deviated from the average. Moreover, three of the eight studied comets revealed dust colour variations. During 2 d, the change was between red and slightly blue or neutral for C/2017 T2 (PanSTARRS) and C/2016 N4 (MASTER). The third comet, C/2020 S4 (PanSTARRS), revealed slow variations from a strong red colour closer to neutral values. The study of coma morphology revealed jet structures in comae of four comets: C/2020 S4 (PanSTARRS), C/2017 T2 (PanSTARRS), C/2015 O1 (PanSTARRS), and C/2014 W2 (PanSTARRS). We used the geometric model of jets in a cometary coma to reproduce the found ejections and estimate nuclei rotation characteristics and locations of active areas. The results indicate a possible connection between the strong activity and the formation of morphological structures and colour variations.