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

Abstract

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 $R_N=1.8\pm 0.5$ km and a surface color of $g^{\prime }-i^{\prime }=1.15\pm 0.09$. 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 $\sim$4-8 kg s${}^{-1}$. JWST NIRSpec IFU Prism-mode spectra revealed an elongated dust morphology and a symmetric $C{O}_2$ gas distribution in the coma but no $H_{2}O$ or CO emission features, with production rates of $Q_{C{O}_2}=(6.99\pm 0.07)\times 10^{24}$ molec. s${}^{-1}$, $Q_{H_{2}O}\le 1.2\times 10^{24}$ molec. s${}^{-1}$, and $Q_{CO}\le 5.2\times 10^{24}$ molec. s${}^{-1}$. Absorption features at 2.0 and 3.0 $\hat{I}¼$m indicate the presence of water ice, and a subtle 3.1 $\hat{I}¼$m feature is consistent with crystalline water ice in larger grains. A Hapke-style model dominated by large ($D_{eff.}=5.9$ $\hat{I}¼$m) dust grains with a volumetric ice fraction of $f_{ice}=33\%$ fits the spectrum. A thermal model incorporating 450P's orbital history since $\sim$1500 CE aligns with the observed onset of activity driven by $C{O}_2$ outgassing from amorphous water ice crystallization between 140-160 K.