Coma environment of comet C/2017 K2 around the water ice sublimation boundary observed with VLT/MUSE

Abstract

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 QH2O of ~7 × 1028 molec s−1 (with an uncertainty of a factor of two). The production ratio log[QC2/QCN] 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.