JWST and Gemini Observations of the Active Centaur 450P/LONEOS: Nucleus and Coma Characterizations

Between 2019 and 2024, we used the Gemini-N and JWST observatories to conduct a detailed case study of the active Centaur 450P/LONEOS, whose orbit was significantly altered by a close Saturn encounter in 1992. Gemini-N GMOS optical images likely captured the first views of 450P's inactive nucleus, indicating a relatively small radius of R N = 1.8 plus or minus0.5 km and a surface color of g' - i' = 1.15 plus or minus0.09. This places 450P on the red end of the neutral/gray Centaur population and may indicate comparatively limited solar-driven surface processing relative to other known active Centaurs. A coma developed as 450P changed its heliocentric distance, R H, from 7.83 au to 7.24 au, with an estimated low dust production rate of about 4-8 kg s-1. JWST NIRSpec IFU Prism-mode spectra revealed an elongated dust morphology and a symmetric CO 2 gas distribution in the coma but no H 2O or CO emission features, with production rates of Q CO 2 = (6.99 plus or minus0.07) times10 24 molec. s-1, Q H 2O at most 1.2 times10 24 molec. s-1, and Q CO at most 5.2 times10 24 molec. s-1. Absorption features at 2.0 and 3.0 mu m indicate the presence of water ice, and a subtle 3.1 mu m feature is consistent with crystalline water ice in larger grains. A Hapke-style model dominated by large (D eff. = 5.9 mu m) dust grains with a volumetric ice fraction of f ice = 33% fits the spectrum. A thermal model incorporating 450P's orbital history since about 1500 CE aligns with the observed onset of activity driven by CO 2 outgassing from amorphous water ice crystallization between 140-160 K.