JWST Observations of the Active Centaur 423P/Lemmon: Gas and Dust Comae Characterizations

Centaurs are small bodies that, compared to age of the Solar System, are located on dynamically unstable, giant planet-crossing orbits between Jupiter and Neptune. Their orbits place them between the icy, distant trans-Neptunian objects (TNOs) and the rapidly sublimating Jupiter-family comets (JFCs), and it is thought that centaurs are one step in the evolution of the former to the latter (e.g., [1, 2, 3]). With this in mind, cometary activity in centaurs is of particular interest for a number of reasons. Perturbed out of the cold storage of the TNO region, these closer, warmer orbits could mean that this is the first time these objects are undergoing phase transitions of key volatile species (e.g., CO, CH₄, CO₂, amorphous water ice) in their interiors since their formations [4]. While centaurs are generally understood to have an abundance of volatile inventories, the observed diversity of centaur activity behaviors beyond the water ice sublimation line is not well understood. One possible mechanism for this distant activity is the sublimation of other volatile ices, such as CO and CO₂ [5]; another one is the crystallization of porous amorphous water ice, which could have stored various amounts of non-water volatiles that are released upon crystallization [6]. Whatever the mechanism at work, one of the keys to understanding the gas-driven mass loss of these objects—the gas itself—has been difficult to observe in many centaurs directly until recently. With its sensitivity, the James Webb Space Telescope (JWST) has made it possible to observe volatile gas species in more centaurs, many of which for the first time [5, 7]. Through these new observations it is possible to analyze both components—the gas and the dust—of the comae around these objects directly through multi-wavelength observing campaigns.   

We present JWST near-infrared (NIR) spectral and Gemini North Telescope visible imaging data of the active centaur 423P/Lemmon (hereafter, 423P). The UT 2022 November 28 JWST spectrum was collected with Near Infrared Spectrograph (NIRSpec) in integral field unit (IFU) mode with the low-resolution PRISM/CLEAR disperser-filter combination [8]. The target and dedicated background observations were collected with the 4-point dither pattern. The data span the entire 0.6 - 5.3μm wavelength range of the NIRSpec instrument, which includes regions where CO, CO₂, and H₂O (common gas comae species) are known for fluorescence. The Gemini data were collected between 2019 - 2024 using the Gemini Multi-Object Spectrographs (GMOS) in imaging mode to monitor the dust coma’s secular lightcurve behavior. The combination of long baseline dust activity monitoring with snapshot gas comae measurements has been limited for active centaurs, especially for a relatively small one such as 423P (radius ~ 1.5 km) [9]. A multi-wavelength characterization of 423P such as this, in combination with similar studies of more active centaurs in general utilizing JWST observations (see, e.g., [5]), opens the door for future detailed thermophysical modeling investigations targeted at understanding the nature of the interiors of these objects.  

Our results focus on analysis of the gas species in the JWST data, with the Gemini data providing us with a long-term baseline understanding of the dust characteristics and behavior. We present an extracted NIR reflectance spectrum of 423P and provide estimates of the gas production rates (or their upper limits, in the case of non-detections) of common gas species.  Preliminary analysis shows that CO₂ emission is present in the spectrum, with CO and H₂O emission being more ambiguous. The format of the JWST datacubes allow us the unique opportunity to see the spatial distribution of gas and dust around the centaur through the examination of individual datacube slices at specific wavelengths that those particles are known to contribute at. We present preliminary analysis of gas and dust coma morphologies, the latter in the context of the Gemini data. We discuss how these morphologies could relate to activity mechanisms occurring on the nucleus. We also place this study of 423P in context with previous observations as well as compare it to other centaurs, TNOs, and JFCs.  

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