Second "Impossible" Ring Around the TNO Quaoar 

 

A recent stellar occultation has revealed the presence of a second ring outside the Roche limit orbiting Quaoar. The discovery, led by Chrystian Luciano Periera, a Ph.D. student at the Observatório Nacional (RJ) and affiliated researcher at Laboratório Interinstitucional de e-Astronomia (LIneA), was published in the journal Astronomy & Astrophysics Letters on April 28, 2023. This study utilized the Canada-France-Hawai’i Telescope and the Gemini North telescope on Maunakea along with robotic telescopes and the members of the United States amateur astronomer community.

Rings around bodies in the Solar System were first observed by Galileo Galilei when he pointed his telescope at Saturn in 1610. In the following centuries, rings would be discovered around the other three giant planets: Jupiter, Uranus, and Neptune.  Until the 2013 discovery of the first ring system around the Centaur object (10199) Chariklo, astronomers were unsure if rings could form around small solar system bodies.  The team demonstrated Chariklo was not alone as a ringed system with the 2017 discovery of a ring surrounding the dwarf planet Haumea. 

In February 2023, the same team led by Bruno Morgado (UFRJ/BR) announced the discovery of the third ring system, now around the Trans-Neptunian object Quaoar.   Further analysis shows Quaoar's system is more complex than previously thought, having a second, innermost ring to the one published earlier this year. These discoveries were made by observing stellar occultations, which occur when an object in the Solar System passes in front of a star and blocks its light for a few moments.  By measuring the duration of the occultation and if there are secondary, fainter occultations, astronomers can determine characteristics of the Solar System object. 

Unlike the rings observed at Chariklo, Haumea, and the four giant planets, Quaoar's rings lies in an unexpected region, well beyond the Roche limit for the body.  The Roche limit is a region where the gravity of the central body (Quaoar) balances with the gravity of the individual ring particles, preventing the particles from forming a moon.  Quaoar’s Roche limit is estimated to be 1,780 km or 1,100 miles from the center of the object.  Another interesting and unusual property of the Quaoar ring is the variability, very narrow and dense in one region while tenuous and extensive in another.

To obtain more information about Quaoar and its curious ring, the team organized an observational campaign for a stellar occultation observed on August 9, 2022, involving amateur and professional telescopes.  The team used the Gemini North and Canada-France-Hawai’i Telescope (CFHT), neighbors on Maunakea. The results of this observational campaign were published in the journal Astronomy & Astrophysics Letters, with Chrystian Luciano Pereira, a doctoral student in astronomy at the Observatório Nacional, Rio de Janeiro, as the first author.

The high performance of the instruments attached to the Gemini North and CFHT telescopes, the 'Alopeke and WIRcam cameras, respectively, combined with their location on Maunakea in Hawaii, allowed for obtaining unique quality light curves. The dense and narrow region of the previously discovered ring (preliminarily named Q1R) was probed by this occultation, revealing a narrow, confined structure approximately 5 km wide.

 "This narrow core of the ring is surrounded by an envelope of dispersed material about 60 km long, resembling in structure the F ring of Saturn or the arc observed in Neptune's rings", said Pereira. This ring's most extensive and tenuous region was also detected, having an average width of 90 km and less than 1% of the density of the thickest region. The calculated distance between Quaoar and this ring is 4,060 km (2,522 miles). 

These data also revealed the presence of a second ring orbiting Quaoar, previously named Q2R. This ring is about 10 km wide and, despite being closer to Quaoar, it is also outside the Roche limit, orbiting 2,520 km (1,565 miles) from the object's center. This reveals how curious and complex Quaoar's system can be.

The outermost ring orbits Quaoar at a distance very close to a stable region Quaoar rotates three times and the ring’s particles orbit once.  This relationship is called a 1:3 spin-orbit resonance, The innermost ring is close to the 5:7 spin-orbit resonance region, i.e., while Quaoar completes seven rotations, the ring particles complete five orbits. This dynamic behavior is observed in the rings around Chariklo and Haumea, which are also close to the 1:3 resonance region. This suggests that resonances may be closely related to the maintenance and location of these rings. Another factor that could cause these rings to be confined is the presence of small “shepherd” satellites that have yet to be discovered.

Future work on the precise determination of Quaoar's shape and new observations of these rings will be important for a better understanding of the dynamic system in which Quaoar and its rings are inserted and the real role of resonances in the maintenance and confinement of these rings.

“WIRCam is an exceptionally stable instrument and observations were taken in a very efficient and fast, almost video-like mode allowing for the high precision measurements needed to detect a short occultation by a faint ring around a distant object in our Solar System.” said Benoit Epinat, WIRCam instrument scientist at CFHT.  “It is incredible to realize we can see a 6 mile ring from over 3 billion miles away.”

This work was carried out as part of the "Lucky Star" project under the leadership of Dr. Bruno Sicardy of the Paris Observatory (Paris, France) and was made possible through a worldwide collaboration involving professional and amateur astronomers. This study had the participation of researchers from several international institutes, such as Instituto de Astrofísica de Andalucía (Granada, Spain), Observatório Nacional (Rio de Janeiro, Brazil), Federal Technological University of Paraná (Curitiba, Brazil), Interinstitutional Laboratory of e-Astronomy (Rio de Janeiro, Brazil), Florida Space Institute (Orlando, Florida), among others.

Gemini North is part of the International Gemini Observatory, operated by NSF’s NOIRLab.

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