Single-dish observations and non-LTE analysis of CH3OH, HCN, and CO line emission in the Oort cloud comet C/2017 K2 (PANSTARRS)

Journal article, 2026

We present pre-perihelion observations of methanol, carbon monoxide, and hydrogen cyanide in the Oort cloud comet C/2017 K2 (PANSTARRS), performed with the APEX 12-m and Onsala 20-m telescopes from 2022 April to July. As the comet’s heliocentric distance decreased from 3.4 to 2.7 au, CH(Formula presented) OH line intensities increased substantially (by factors of 1.1–4.0), with the most pronounced enhancement in lines with upper-level energies (Formula presented) K. In contrast, the brightness of the CO and HCN lines remained constant. We estimate the best-fitting gas kinetic temperatures (Formula presented)  K and water production rate of (Formula presented) s(Formula presented). The derived methanol-to-water abundance ratio (Formula presented), depending on the observed period. Our results demonstrate that non-LTE effects are dominant in the coma and must be accounted for to accurately derive molecular production rates. We also report weak non-thermal excitation, including potential maser activity in the CH(Formula presented) OH (Formula presented) line.