First detection of CO$_2$ emission in a Centaur: JWST NIRSpec observations of 39P/Oterma

TL;DR

This study reports the first detection of CO₂ emission in a Centaur, 39P/Oterma, using JWST NIRSpec, revealing insights into its outgassing composition and activity mechanisms, with implications for understanding Centaur evolution.

Contribution

First detection of CO₂ gas emission in a Centaur, providing new insights into its composition and activity drivers compared to previous observations.

Findings

CO₂ emission detected with a production rate of (5.96 ± 0.80) × 10²³ molecules s⁻¹

CO and H₂O were not detected, setting upper limits on their abundance

The CO₂ to H₂O ratio suggests CO₂ and/or CO drive activity, not water

Abstract

Centaurs are minor solar system bodies with orbits transitioning between those of Trans-Neptunian Scattered Disk objects and Jupiter Family comets. 39P/Oterma 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, CO$_2$ gas emission was detected in a Centaur, with a production rate of Q$_{CO_2}$ = (5.96 $\pm$ 0.80) $\times$ 10$^{23}$ molecules s$^{-1}$. This is the lowest detection of CO$_2$ of any Centaur or comet. CO and H$_2$O were not detected down to constraining upper limits. Derived mixing ratios of Q$_{CO}$/Q$_{CO_2}$ $\leq$2.03 and Q$_{CO_2}$/Q$_{H_2O}$ $\geq$0.60 are consistent with CO$_2$ 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 Jupiter-family comet orbit. To help contextualize the JWST data we also acquired visible CCD imaging data on two dates in July (Gemini North) and September (Lowell Discovery Telescope) 2022. 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 to 2.49~km.