Carbon Monoxide in the Distantly Active Centaur (60558) 174P/Echeclus at 6 au

(60558) 174P/Echeclus is an unusual object that belongs to a class of minor planets called Centaurs, which may be intermediate between Kuiper Belt objects and Jupiter family comets. It is sporadically active throughout its orbit at distances too far for water ice, the source of activity for most comets, to sublimate. Thus, its coma must be triggered by another mechanism. In 2005, Echeclus had a strong outburst with peculiar behavior that raised questions about the nucleus' homogeneity. To test nucleus models, we performed the most sensitive search to date for the highly volatile CO molecule via its J = 2-1 emission toward Echeclus during 2016 May–June (at 6.1 astronomical units from the Sun) using the Arizona Radio Observatory 10 m Submillimeter Telescope. We obtained a 3.6σ detection with a slightly blueshifted (δv = −0.55 ± 0.10 km s⁻¹) and narrow (Δv _FWHM = 0.53 ± 0.23 km s⁻¹) line. The data are consistent with emission from a cold gas from the sunward side of the nucleus, as seen in two other comets at 6 au. We derive a production rate of Q(CO) = (7.7 ± 3.3)$\times {10}^{26}$ mol s⁻¹, which is capable of driving the estimated dust production rates. Echeclus' CO outgassing rate is ∼40 times lower than what is typically seen for another Centaur at this distance, 29P/Schwassmann–Wachmann 1. We also used the IRAM 30 m telescope to search for the CO J = 2-1 line, and derive an upper limit that is above the SMT detection. Compared with the relatively unprocessed comet C/1995 O1 (Hale–Bopp), Echeclus produces significantly less CO, as do Chiron and four other Centaurs.