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arxiv: 2605.24260 · v1 · pith:TGX6UTN3new · submitted 2026-05-22 · 🌌 astro-ph.EP

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

Pith reviewed 2026-06-30 14:11 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords active CentaurCO2 outgassingamorphous ice crystallizationthermal modelingJWST spectroscopyGemini observations450P/LONEOScometary coma
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The pith

A thermal model of 450P's orbit since 1500 CE aligns with the onset of its CO2 outgassing from amorphous ice crystallization at 140-160 K.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper presents Gemini and JWST observations of the active Centaur 450P, including the first likely images of its inactive nucleus at a radius of 1.8 km with a red surface color. Spectra reveal a coma with CO2 gas production but no H2O or CO, along with water ice absorption features best fit by large dust grains containing 33 percent ice by volume. A thermal model that incorporates the object's orbital history back to around 1500 CE matches the timing when activity begins as temperatures allow amorphous water ice to crystallize and release CO2. This case study connects changes in heliocentric distance to the specific trigger for distant cometary activity.

Core claim

The central claim is that a thermal model incorporating 450P's orbital history since ~1500 CE aligns with the observed onset of activity driven by CO2 outgassing from amorphous water ice crystallization between 140-160 K. The nucleus radius is 1.8 km with red color, the coma has low dust production of 4-8 kg per second, and JWST spectra show CO2 at 7e24 molecules per second with water ice features but upper limits on H2O and CO.

What carries the argument

The thermal model of nucleus temperature evolution based on orbital integration since 1500 CE, which predicts when temperatures reach 140-160 K to trigger CO2 release via amorphous ice crystallization.

If this is right

  • Activity in 450P is driven by CO2 release rather than H2O or CO sublimation.
  • The coma contains water ice in grains with effective diameter of 5.9 micrometers and 33 percent volumetric ice fraction.
  • The nucleus color places 450P on the red end of neutral Centaurs, suggesting limited surface processing.
  • Dust production remains low at 4-8 kg per second even as the object moves inward from 7.83 to 7.24 au.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same approach of integrating orbital history into thermal models could be applied to other Centaurs that experienced planetary encounters to test if crystallization thresholds explain their activity onsets.
  • If confirmed, this mechanism implies that some distant objects activate without needing water ice sublimation, affecting models of volatile retention in the outer solar system.
  • Repeated observations of 450P at different orbital positions could directly test whether activity ceases or changes when temperatures drop below the crystallization range.

Load-bearing premise

That the observed activity onset is driven specifically by CO2 outgassing from amorphous water ice crystallization at 140-160 K, which depends on the accuracy of the thermal model and the orbital history integration.

What would settle it

A measurement showing that activity begins at a heliocentric distance where the thermal model predicts temperatures outside the 140-160 K range for ice crystallization would falsify the alignment.

Figures

Figures reproduced from arXiv: 2605.24260 by Adam McKay, Aren Beck, Charles A. Schambeau, Dominique Bockelee-Morvan, Eva Lilly, Geronimo L. Villanueva, James Bauer, Javier Licandro, Kacper Wierzchos, Lori Feaga, Marco Micheli, Maria Womack, Michael A. DiSanti, Michael S. P. Kelley, Noemi Pinilla-Alonso, Olga Harrington Pinto, Sara Faggi, Theodore Kareta, Yanga R. Fernandez.

Figure 1
Figure 1. Figure 1: 450P’s orbital history is visualized with this diagram. The Centaur had a close approach to Saturn in 1992 that changed its long-term trans-Saturnian orbit (black curve) into one with a perihelion closer to Jupiter (blue). 450P is predicted to have a close encounter with Jupiter in July 2026, which will stabilize its orbit for the next ∼ 200 years (gold/orange). These orbital changes increasingly subject t… view at source ↗
Figure 2
Figure 2. Figure 2: Gemini-N GMOS images of 450P. The top-left and top-middle panels display the recovery images when the Centaur appeared inactive and which were used to estimate the nucleus’s radius. The top-right and bottom-left panels display the g′ and i ′ images, respectively, when the Centaur also appeared inactive and which were used to measure the nucleus’s surface color. The bottom-middle and bottom-right panels dis… view at source ↗
Figure 3
Figure 3. Figure 3: Normalized radial surface brightness profiles of the Gemini-N 450P images on 2022-08-21 (r′ ; left panel) and 2022-09-27 (g′ ; right panel). Surrounding field star profiles are overplotted with a gray shaded region representing their range of scatter. 450P’s profiles are similar to those of the field stars, which is consistent with little-to-no dust coma present. from the slice centroid. The skyplane proje… view at source ↗
Figure 4
Figure 4. Figure 4: Wavelength-integrated image panels of 450P extracted from JWST NIRSpec IFU datacube at different bandpasses (the effective wavelength is listed directly above each panel). The panels use the same color scale, ranging from black through blue to white, to indicate increasing surface brightness. The panels are shown with J2000 equatorial north up and east to the left; the sky-plane–projected directions of the… view at source ↗
Figure 5
Figure 5. Figure 5: Left: The spectrum of Centaur 450P at RH=7.16 au obtained with JWST NIRSpec on 2023-09-03. The black line is the spectrum extracted from the median-combined datacube with the coral-red shaded region indicating the spectrum’s 1-σ uncertainty. The spectrum is dominated by reflected solar radiation, a prominent CO2 emission feature at 4.26 µm, and likely water ice absorption features at 2.0 and 3.0 µm. No emi… view at source ↗
Figure 6
Figure 6. Figure 6: Measurements of the dust and CO2 gas emission in 450P’s coma. The left two panels display the dust and CO2 gas surface brightness maps. The location of 450P’s nucleus is indicated by the “x” in each image and the panel orientations and skyplane velocity vector directions are the same as in [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: A close-up of the continuum subtracted JWST spectrum of 450P centered on the CO2 emission band (black line) with the best-fit model spectrum of the CO2 ν3 fluorescence emission band overplotted in a red dashed line using the Planetary Spectrum Generator. The model is consistent with QCO2 = (6.99±0.07)×1024 molec. s−1 and Trot = 60 ± 1 K. The associated 1-sigma uncertainties for the spectrum are shown with … view at source ↗
Figure 8
Figure 8. Figure 8: The continuum subtracted JWST spectrum (black line) for the spectral ranges where H2O (left panel) and CO (right panel) emission bands would appear if present. The 1-σ spectrum uncertainties are shown in gray, and the two red vertical dashed lines show the expected wavelength region of each emission band. Upper limits to H2O and CO production rates were calculated using integrated flux measurements between… view at source ↗
Figure 9
Figure 9. Figure 9: The reflectance spectrum of 450P (black unfilled circles with gray 1-σ error bars) is compared with the maximum likelihood spectral model (red curve) which assumes intimately-mixed grains of amorphous carbon and crystalline water ice. This model is not very sensitive to ice temperatures, so while it assumes Tice = 150 K, ice temperatures within several tens of Kelvin fit the data similarly well. Our modeli… view at source ↗
Figure 10
Figure 10. Figure 10: The evolution of 450P’s perihelion distance (q, blue curve) and semi-major axis (a, gold/orange curve) from 1500 - 2500 AD using 50 clone orbital integrations. The recent a-jump after the 1992 close approach to Saturn is easily seen by the rapid decrease in both a and q. an overestimation of localized heating at the subsolar point, since it receives continuous insolation without diurnal cycling. Thus, our… view at source ↗
Figure 11
Figure 11. Figure 11: These panels display the time evolution of the (a) sub-solar surface temperature, (b) radial profile of nucleus’s interior temperature, and (c) selected nucleus interior radial temperature profiles for dates when dust production estimates are available. The thermal effects due to the a-jump in 1992 are easily seen by the increased temperatures that occurred shortly afterward. In Panel (c), the cyan shaded… view at source ↗
read the original 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 $R_N = 1.8\pm0.5$ km and a surface color of $g' - i' = 1.15\pm0.09$. 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 $CO_2$ gas distribution in the coma but no $H_2O$ or CO emission features, with production rates of $Q_{CO_2} = (6.99\pm0.07)\times10^{24}$ molec. s$^{-1}$, $Q_{H_2O} \leq 1.2\times10^{24}$ molec. s$^{-1}$, and $Q_{CO} \leq 5.2\times10^{24}$ 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 $\sim$1500 CE aligns with the observed onset of activity driven by $CO_2$ outgassing from amorphous water ice crystallization between 140-160 K.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The paper presents Gemini-N and JWST observations of active Centaur 450P/LONEOS, reporting a nucleus radius of 1.8±0.5 km, surface color g'-i'=1.15±0.09, low dust production of ~4-8 kg s^{-1}, CO2 production rate Q_CO2=(6.99±0.07)×10^{24} molec s^{-1} with upper limits on H2O and CO, spectral absorption features indicating water ice (including a 3.1 μm crystalline feature), a Hapke model fit with effective grain diameter 5.9 μm and volumetric ice fraction 33%, and a thermal model using the object's orbital history since ~1500 CE that aligns the onset of activity with CO2 outgassing triggered by amorphous water ice crystallization at 140-160 K.

Significance. If the thermal model result holds, the work provides a well-constrained case study of nucleus and coma properties for an active Centaur whose orbit was perturbed by a 1992 Saturn encounter, with direct spectral and imaging derivations supporting the production rates and ice signatures. The linkage of dynamical history to volatile-driven activity offers a testable framework for Centaur activation mechanisms.

major comments (1)
  1. [thermal model section] Thermal model incorporating orbital history since ~1500 CE (abstract and corresponding modeling section): The reported alignment between the thermal evolution and the observed activity onset at 140-160 K is based on a single nominal backward integration. Centaur orbits are chaotic, and the documented 1992 Saturn encounter implies that small changes in initial conditions or non-gravitational forces can produce divergent trajectories; the manuscript does not report clone integrations, covariance propagation, or sensitivity tests, so the temperature alignment may not be robust.
minor comments (2)
  1. [methods] The abstract and methods description of JWST NIRSpec IFU data reduction should include explicit details on background subtraction, flux calibration, and error budget propagation to support reproducibility of the reported production rates and spectral fits.
  2. [observations] Figure captions and text should clarify the exact epochs of the Gemini and JWST observations relative to the heliocentric distance change from 7.83 to 7.24 au to strengthen the timeline of activity onset.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive comments on our manuscript. Below we provide a point-by-point response to the major comment.

read point-by-point responses
  1. Referee: [thermal model section] Thermal model incorporating orbital history since ~1500 CE (abstract and corresponding modeling section): The reported alignment between the thermal evolution and the observed activity onset at 140-160 K is based on a single nominal backward integration. Centaur orbits are chaotic, and the documented 1992 Saturn encounter implies that small changes in initial conditions or non-gravitational forces can produce divergent trajectories; the manuscript does not report clone integrations, covariance propagation, or sensitivity tests, so the temperature alignment may not be robust.

    Authors: We agree that Centaur orbits are chaotic and that the 1992 Saturn encounter introduces significant uncertainties in the long-term dynamical history. The thermal model in the manuscript was based on a single nominal backward integration, without clone integrations, covariance propagation, or sensitivity tests. We will revise the modeling section (and abstract if needed) to explicitly note that the reported temperature alignment corresponds to the nominal orbit only and to discuss the limitations arising from orbital chaos. This revision will clarify the robustness of the result for readers. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

All reported quantities (nucleus radius, colors, production rates Q_CO2/Q_H2O/Q_CO, ice fraction, grain sizes) are obtained from direct Gemini imaging and JWST NIRSpec spectra via standard reduction and modeling pipelines. The thermal-model alignment statement compares an independent orbital integration against a literature temperature threshold for CO2 release; no equation in the provided text reduces this alignment to a fitted parameter or self-defined input. No self-citation load-bearing steps, ansatzes smuggled via citation, or fitted-input-called-prediction patterns are present.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The central claims rest on standard assumptions in cometary thermal modeling and Hapke reflectance modeling; two fitted parameters appear in the spectral fit.

free parameters (2)
  • effective grain diameter = 5.9 um
    Fitted value of 5.9 micrometers used in the Hapke-style reflectance model to match the observed spectrum.
  • volumetric ice fraction = 33%
    Fitted value of 33 percent ice by volume in the same reflectance model.
axioms (2)
  • domain assumption Standard assumptions of cometary thermal models relating surface temperature to outgassing triggers
    Invoked to link orbital history to the 140-160 K crystallization range that drives CO2 release.
  • domain assumption Hapke reflectance model applicability to mixed ice-dust surfaces at these wavelengths
    Used without additional justification in the spectral fitting section.

pith-pipeline@v0.9.1-grok · 6055 in / 1579 out tokens · 45219 ms · 2026-06-30T14:11:06.874987+00:00 · methodology

discussion (0)

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