First Detection of CO2 Emission in a Centaur: JWST NIRSpec Observations of 39P/Oterma

Centaurs are minor solar system bodies with orbits transitioning between those of trans-Neptunian scattered disk objects and Jupiter-family comets (JFCs). 39P/Oterma (39P) is a frequently active centaur that has recently held both centaur and JFC classifications and was observed with the JWST NIRSpec instrument on 2022 July 27 UTC while it was 5.82 au from the Sun. For the first time, CO2 gas emission was detected in a centaur, with a production rate of = (5.96 +/- 0.80) x 10(23) molecules s(-1). This is the lowest detection of CO2 of any centaur or comet. CO and H2O were not detected down to constraining upper limits. Derived mixing ratios of Q(CO)/Q <= 2.03 and /Q >= 0.60 are consistent with CO2 and/or CO outgassing playing large roles in driving the activity, but not water, and show a significant difference between the coma abundances of 29P/Schwassmann-Wachmann 1, another centaur at a similar heliocentric distance, which may be explained by thermal processing of 39P's surface during its previous JFC orbit. To help contextualize the JWST data we also acquired visible CCD imaging data on two dates in 2022 July (Gemini-North) and September (Lowell Discovery Telescope). Image analysis and photometry based on these data are consistent with a point-source detection and an estimated effective nucleus radius of 39P in the range of R-nuc = 2.21-2.49 km.

Automated summary

The observation of 39P/Oterma, an active centaur, has detected the lowest level of CO2 emission among centaurs and comets. The presence of CO2 and/or CO suggests their significant roles in driving the activity of this centaur, while water does not play a major role. The thermal processing of 39P's surface during its previous orbit may explain the notable difference in coma abundances between 39P and other similar centaurs.