2023A&A...674A.206K

We report a new imaging spectroscopic observation of Oort cloud comet C/2017 K2 (hereafter K2) with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope on its way to perihelion at 2.53 au, around a heliocentric distance where H₂O ice begins to play a key role in comet activation. Normalized reflectances over 6500-8500 Å for its inner (cometocentric distance ρ ≈ 10³ km) and outer (ρ ≈ 2 × 10⁴ km) comae are 9.7 ± 0.5 and 7.2 ± 0.3 % (10³ Å)^–1, respectively, the latter being consistent with the slope observed when the comet was beyond the orbit of Saturn. The dust coma of K2 at the time of observation appears to contain three distinct populations: millimeter-sized chunks prevailing at ρ ≲ 10³ km; a 10⁵ km steady-state dust envelope; and fresh anti-sunward jet particles. The dust chunks dominate the continuum signal and are distributed over a similar radial distance scale as the coma region with redder dust than nearby. They also appear to be co-spatial with OI¹D, suggesting that the chunks may accommodate H₂O ice with a fraction (≳1%) of refractory materials. The jet particles do not colocate with any gas species detected. The outer coma spectrum contains three significant emissions from C₂(0,0) Swan band, OI¹D, and CN(1,0) red band, with an overall deficiency in NH₂. Assuming that all OI¹D flux results from H₂O dissociation, we compute an upper limit on the water production rate Q_H2O of ∼7 × 10²⁸ molec s^–1 (with an uncertainty of a factor of two). The production ratio log[Q_C2/Q_CN] of K2 suggests that the comet has a typical carbon chain composition, with the value potentially changing with distance from the Sun. Our observations suggest that dust chunks (>0.1 mm) containing water ice and near K2's nucleus emitted beyond 4 au may be responsible for its very low gas rotational temperature and the discrepancy between its optical and infrared lights reported at similar heliocentric distances.