Possible ring material around centaur (2060) Chiron

Abstract: We propose that several short duration events observed in past stellar occultations by Chiron were produced by rings material. From a reanalysis of the stellar occultation data in the literature we determined two possible orientations of the pole of Chiron's rings, with ecliptic coordinates l=(352+/-10) deg, b=(37+/-10) deg or l=(144+/-10) deg, b=(24+/-10) deg . The mean radius of the rings is (324 +/- 10) km. One can use the rotational lightcurve amplitude of Chiron at different epochs to distinguish between the two solutions for the pole. Both imply lower lightcurve amplitude in 2013 than in 1988, when the rotational lightcurve was first determined. We derived Chiron's rotational lightcurve in 2013 from observations at the 1.23-m CAHA telescope and indeed its amplitude is smaller than in 1988. We also present a rotational lightcurve in 2000 from images taken at CASLEO 2.15-m telescope that is consistent with our predictions. Out of the two poles the l=(144+/-10) deg, b=(24+/-10) deg solution provides a better match to a compilation of rotational lightcurve amplitudes from the literature and those presented here. We also show that using this preferred pole, Chiron's long term brightness variations are compatible with a simple model that incorporates the changing brightness of the rings as the tilt angle with respect to the Earth changes with time. Also, the variability of the water ice band in Chiron's spectra in the literature can be explained to a large degree by an icy ring system whose tilt angle changes with time and whose composition includes water ice, analogously to the case of Chariklo. We present several possible formation scenarios for the rings from qualitative points of view and speculate on the reasons why rings might be common in centaurs. We speculate on whether the known bimodal color distribution of centaurs could be due to presence of rings and lack of them.

Possible Ring Material Around Centaur (2060) Chiron

The paper explores the existence of potential ring material around the minor planet and centaur (2060) Chiron. The authors base their investigation on historical stellar occultation data which exhibited short duration dimming events that are similar to the features typical of ring systems, specifically those observed in another centaur, Chariklo. The findings contribute to understanding the nature and prevalence of ring systems around small solar bodies beyond the major planets and within the centaur population.

Stellar Occultation Reanalysis

The authors reanalyzed past stellar occultations, focusing on the dimming events occurring before and after Chiron’s main body occultation. These secondary events were symmetric, bringing into question the traditional jet interpretation. Instead, the data suggests the presence of ring-like material with dimensions and separations common to rings observed around Chariklo. From interpreting these occultation measurements, two potential pole orientations were derived for Chiron’s rings, with ecliptic coordinates being $\lambda$=(352$\pm$10)$^{$, $\beta$=(37$\pm$10)$^{$ or $\lambda$=(144$\pm$10)$^{$, $\beta$=(24$\pm$10)$^{$, and a mean radius of approximately 324 km.

Spectral Water Ice Feature Variability

Chiron’s spectra exhibit variable water ice absorption features. A proposed explanation correlates this variability to the changing aspect angle of the icy ring system. The diminishing of the water ice band would logically follow when the rings are edge-on relative to Earth, minimizing the observable cross-section. These aspects and alignments can explain episodic disappearance of spectroscopic features when Chiron was inactive and devoid of cometary activity.

Photometric Modeling of Brightness Variability

A substantial contribution of the paper is modeling Chiron’s brightness over time, revealing correlations between historical brightness measurements and the aspect angle of the ring system. The model incorporates a simple geometric interpretation of brightness contribution from both Chiron’s body and rings. Chiron’s long-term brightness can be broadly understood in terms of this model, acknowledging the diminishment in brightness when rings are edge-on. This supports the selection of the second pole solution as compatible, consistent with observed brightness minima.

Rotational Lightcurve Amplitude Analysis

The amplitude of Chiron's rotational lightcurve, observed at different epochs, shows variations consistent with expected contributions from the ring system under the preferred pole orientation scenario. Historical data, alongside new observational findings, assert varying amplitudes corresponding to the predicted brightness contributions from the proposed ring configuration. The reduced amplitude variations are accounted for by the ring system's changing brightness angle.

Implications and Speculation

The potential ring material around Chiron, analogous to Chariklo’s rings, suggests ring systems might be common among centaurs, potentially linked to specific formation mechanisms in the outer solar system. The paper speculates on possible formation scenarios for rings, including material liberation through collisions or disintegration of satellite structures, emphasizing further theoretical exploration and observational efforts that could unveil more centaurs with such features.

Moreover, it suggests that the known bimodal color distribution of centaurs might correlate with the presence or absence of rings, proposing a theoretical lens through which future color-related centaur studies can be approached.

Conclusion

The evidence suggests that Chiron shares significant occlusion and spectral characteristics with Chariklo, specifically through ring-like structures, calling into question intrinsic assumptions about small-body environment complexity and ring formation in the outer solar system. Further observational and theoretical exploration is essential to elucidate the processes governing such phenomena and ascertain the prevalence of ring systems across different solar system bodies. This contributes to a broader understanding of how collisions and environmental configurations influence the evolution of solar system objects in dynamically active regions like the centaur belt.