Comet C/2017 K2 gas production rates

We derived the production rates of the daughter species, OH, NH, CN, C_3_, and C_2_, using a Haser model as well as the dust proxy parameter A(0)f{rho}. We used CRIRES+, the high-resolution infrared echelle spectrometer of the ESO VLT, and UVES, its high-resolution ultraviolet-visual echelle spectrograph, to study simultaneously the parent and daughter species, at three different epochs from May to September 2022, when the comet crossed the water sublimation region.

C/2017 K2 (PanSTARRS) light curve shows a complex evolution of its brightness, with several slopes and a plateau around perihelion, revealing the onset and competition of various species. The photometric analysis shows constant coma colors across the whole heliocentric range, indicating similar properties of the dust grains released during the survey, and in agreement 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 oxygen forbidden lines shows the transition between CO and CO2 dominating the comet's activity to the onset of the water sublimation below 3.0au. Molecular abundance analysis from the IR spectra classified C/2017 K2 as a typical-to-enriched comet, with HCN identified as the main parent molecule of CN, and C_2_ probably originating from C_2_H_2_ rather than C_2_H_6_.