Variations in the volatile-driven activity of comet C/2017 K2 (PanSTARRS) revealed by long-term multiwavelength observations

Context. By conducting a comprehensive study of comets across a broad range of heliocentric distances, we can improve our understanding of the physical mechanisms that trigger their activity at various distances from the Sun. At the same time, we can identify possible differences in the composition of these outer Solar System bodies that belong to various dynamical groups. C/2017 K2 (PanSTARRS) is a dynamically new Oort cloud comet (DNC) that has exhibited activity at an extremely large heliocentric distance of 23.75 au, was found to have a CO-rich coma at 6.72 au, and attracted the interest of the community by becoming a bright DNC at perihelion. Aims. Through photometry and spectroscopy, we study the activity evolution and chemical composition of C/2017 K2 over a long-term monitoring from October 2017 (rh = 15.18 au) pre-perihelion to April 2025 (rh = 8.46 au) post-perihelion. Methods. We used the two TRAPPIST telescopes to monitor the activity with broadband and cometary narrowband filters. We produced an 8-year light curve and colors from the broadband images and computed the activity slopes. We derived the production rates of the daughter species, OH, NH, CN, C3, and C2, using a Haser model as well as the dust proxy parameter A(0)fρ. We used CRIRES+, the high-resolution infrared echelle spectrometer of the ESO VLT, and UVES, its high-resolution ultraviolet-visual echelle spectrograph, to simultaneously study the parent and daughter species at three different epochs from May to September 2022, when the comet crossed the water sublimation region. Results. The light curve of C/2017 K2 (PanSTARRS) shows a complex evolution of its brightness, with several slopes and a plateau around perihelion that reveal the onset and competition of various species. The photometric analysis shows constant coma colors throughout the heliocentric range, which indicates similar properties of the dust grains that are released during the survey, and agrees with the colors of other active long-period comets. The production rates indicate a typical C2/CN composition and a high dust-to-gas ratio. The analysis of the forbidden oxygen lines showed that the transition between CO and CO2 dominates the cometary activity to the onset of the water sublimation below 3.0 au. The molecular abundance analysis from the infrared spectra classified C/2017 K2 as a typical-to-enriched comet, with HCN identified as the main parent molecule of CN, and C2 probably originating from C2H2 rather than C2H6.