Author 1: Kirsanova M.S., Pavlyuchenkov Y.N.(INASAN)

Title: Observations and Analysis of CH₃OH, CO, and HCN Line Emission in the Oort-Cloud Comet C/2017 K2 (PANSTARRS)

Abstracts:

We present pre-perihelion observations of methanol, CO, and HCN in the Oort-cloud comet C/2017 K2 (PANSTARRS), obtained with the APEX 12-m telescope and the Onsala 20-m telescope from April to July 2022. As the comet’s heliocentric distance decreased from 3.4 to 2.7 au, the intensities of CH₃OH lines increased substantially (by factors of 1.1–4.0), with the strongest enhancement observed for transitions whose upper-level energies exceed 40 K. In contrast, the brightness of CO and HCN lines remained constant, indicating possible depletion of these hypervolatiles. 
We estimate a coma-gas kinetic temperature of 250–1000 K and a water production rate of Q(H₂O) = (7–10)×10²⁸ s⁻¹. The inferred methanol-to-water abundance ratio is 1.3–1.7%. The observed depletion of hypervolatiles is consistent with the loss of an original surface ice layer, possibly during a previous passage near the Sun. Our results show that non-LTE effects dominate in the coma and must be accounted for to accurately derive molecular production rates. We also find indications of weak non-thermal excitation, including potential maser activity in the CH₃OH 8₀–7₁ line.