Discovery of Carbon Monoxide in Distant Comet C/2017 K2 (PANSTARRS)

The Oort cloud comet C/2017 K2 (hereafter K2) is remarkable for developing a coma at large heliocentric distances (up to 35 au; see abstract by Jewitt et al.). The detection of K2 beyond the orbit of Uranus offers an unprecedented opportunity for us to examine the development of activity in a pristine comet arriving from near-interstellar temperatures. The distant activity of K2 cannot be explained by either the sublimation or the crystallization of water ice. Supervolatile sublimation, most likely of carbon monoxide (CO), has been proposed as a plausible driver of the observed mass loss. In this presentation, we present the detection of the J = 2-1 rotational transition in outgassed CO from K2 when at heliocentric distance r_H = 6.72 au, using the James Clerk Maxwell Telescope. The CO line is blue-shifted by 0.20±0.03 km s^-1 with an area and width of 8.3±2.3 mK km s^-1and 0.28±0.08 km s^-1, respectively. The CO production rate is Q_CO = (1.6±0.5)×10²⁷s^-1. These are the first observations of a gaseous species in K2 and provide observational confirmation of the role of supervolatile sublimation in this comet.