Unveiling the ice and gas nature of active centaur (2060) Chiron using the James Webb Space Telescope (2407.07761v1)

Abstract: (2060) Chiron is a large centaur that has been reported active on multiple occasions including during aphelion passage. Studies of Chirons coma during active periods have resulted in the detection of C(triple)N and CO outgassing. Significant work remains to be undertaken to comprehend the activation mechanisms on Chiron and the parent molecules of the gas phases detected. This work reports the study of the ices on Chirons surface and coma and seeks spectral indicators of volatiles associated with the activity. Additionally, we discuss how these detections could be related to the activation mechanism for Chiron and, potentially, other centaurs. In July 2023, the James Webb Space Telescope (JWST) observed Chiron when it was active near its aphelion. We present JWST/NIRSpec spectra from 0.97 to 5.27 microns with a resolving power of 1000, and compare them with laboratory data for identification of the spectral bands. We report the first detections on Chiron of absorption bands of several volatile ices, including CO2, CO, C2H6, C3H8, and C2H2. We also confirm the presence of water ice in its amorphous state. A key discovery arising from these data is the detection of fluorescence emissions of CH4, revealing the presence of a gas coma rich in this hyper-volatile molecule, which we also identify to be in non-local thermal equilibrium (nonLTE). CO2 gas emission is also detected in the fundamental stretching band at 4.27 microns. We argue that the presence of CH4 emission is the first proof of the desorption of CH4 due to a density phase transition of amorphous water ice at low temperature in agreement with the estimated temperature of Chiron during the JWST observations (61 K). Detection of photolytic and proton irradiation products of CH4 and CO2 on the surface, in the coma ice grains, or in the ring material is also detected via a forest of absorption features from 3.5 to 5.3 microns.

• The paper reveals that Chiron's volatile ice detections, including CO₂, CO, and other ices, indicate a low dust-to-ice ratio that facilitates sublimation.
• The paper identifies a methane-rich gas coma through fluorescence emissions, demonstrating active desorption from amorphous water ice at around 61 K.
• The paper shows a fan-shaped coma structure, suggesting solar-driven activity that shapes the evolving surface chemistry of Centaur Chiron.

Unveiling the Composition of Centaur Chiron with JWST Observations

The paper titled "Unveiling the ice and gas nature of active centaur (2060) Chiron using the James Webb Space Telescope" by Pinilla-Alonso et al. presents an in-depth analysis of the volatile composition of Chiron, offering valuable insights into its activation mechanisms. Utilizing the advanced observational capabilities of the James Webb Space Telescope (JWST), specifically the NIRSpec instrument, the researchers undertook spectroscopic observations of Chiron in July 2023 when the centaur exhibited activity near its aphelion.

Observational Methodology

The JWST observations covered the spectral range of 0.97 to 5.27 μm with a resolving power of approximately 1000. The data reduction process was meticulous, adhering to calibration protocols to assure data integrity. By employing an empirical PSF fitting technique, the team extracted Chiron's reflectance spectrum and identified key absorption features indicative of its surface and coma composition.

Key Findings

1. Volatile Ice Detections: The paper reports the detection of several volatile ices, including CO₂, CO, C₂H₆, C₃H₈, and C₂H₂. These findings are crucial as they signify the presence of a reservoir of volatile ices accessible for sublimation on Chiron's surface, which aligns with the hypothesis of a low dust-to-ice ratio. The detection of amorphous water ice and its association with methane desorption highlights Chiron's intriguing low-temperature surface processes.
2. Gas Coma Observations: Notably, the presence of a methane-rich gas coma was confirmed through the detection of fluorescence emissions, a novel finding for a centaur at such heliocentric distances. This detection of CH₄ emission, revealing the desorption of CH₄ from a density phase transition of amorphous water ice, aligns with estimated surface temperatures around 61 K.
3. Morphological Analysis: The spectral data also revealed an extended surface brightness around the nucleus, suggesting an active coma structure responsive to solar influence, with evident sunward activity in the form of a fan-shaped coma.

Theoretical and Practical Implications

The comprehensive detection of both ice and gas phases on Chiron provides critical insights into the compositional and thermal characteristics of centaurs, expanding our understanding of their behavior as transitional objects in the Solar System. The presence of -CH- and -CO- irradiation products implicates ongoing surface processes influenced by solar radiation-induced chemistry. This evidence contributes to evolving models of centaur and cometary activity, especially in terms of activation mechanisms driven by surface crystallization and other phase changes in mixed ices.

Future Directions

Further spectroscopic studies with JWST and ground-based observatories are essential to unravel the detailed composition of Chiron's nucleus, coma, and evolving debris ring. Additionally, modeling the thermal evolution and irradiation effects on centaur surfaces will provide profound insights into their transitional nature and the broader context of icy bodies in the outer Solar System.

Through its pioneering use of advanced spectroscopy and comprehensive analysis, this paper significantly advances the field of centaur research, offering foundational data that will inform future theoretical models and observational strategies within the astrophysical community.