Molecular Outgassing in Centaur 29P/Schwassmann–Wachmann 1 during Its Exceptional 2021 Outburst: Coordinated Multiwavelength Observations Using nFLASH at APEX and iSHELL at the NASA-IRTF

Abstract The extraordinary 2021 September–October outburst of Centaur 29P/Schwassmann–Wachmann 1 afforded an opportunity to test the composition of primitive Kuiper disk material at high sensitivity. We conducted nearly simultaneous multiwavelength spectroscopic observations of 29P/Schwassmann–Wachmann 1 using iSHELL at the NASA Infrared Telescope Facility (IRTF) and nFLASH at the Atacama Pathfinder EXperiment (APEX) on 2021 October 6, with follow-up APEX/nFLASH observations on 2021 October 7 and 2022 April 3. This coordinated campaign between near-infrared and radio wavelengths enabled us to sample molecular emission from a wealth of coma molecules and to perform measurements that cannot be accomplished at either wavelength alone. We securely detected CO emission on all dates with both facilities, including velocity-resolved spectra of the CO ( J = 2–1) transition with APEX/nFLASH and multiple CO ( v = 1–0) rovibrational transitions with IRTF/iSHELL. We report rotational temperatures, coma kinematics, and production rates for CO and stringent (3 σ ) upper limits on abundance ratios relative to CO for CH 4 , C 2 H 6 , CH 3 OH, H 2 CO, CS, and OCS. Our upper limits for CS/CO and OCS/CO represent their first values in the literature for this Centaur. Upper limits for CH 4 , C 2 H 6 , CH 3 OH, and H 2 CO are the most stringent reported to date, and are most similar to values found in ultra CO-rich Oort cloud comet C/2016 R2 (PanSTARRS), which may have implications for how ices are preserved in cometary nuclei. We demonstrate the superb synergy of coordinated radio and near-infrared measurements, and advocate for future small-body studies that jointly leverage the capabilities of each wavelength