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Weywot: the darkest known satellite in the trans-Neptunian region

Presentation #202.04 in the session TNO Theory and Physical Properties (Oral Presentation)

Published onOct 23, 2023
Weywot: the darkest known satellite in the trans-Neptunian region

Several stellar occultations by Weywot, the only known satellite of the TNO Quaoar, were predicted to happen during May/June 2023. Weywot had been previously detected, although fortuitously, through stellar occultations with one chord from Namibia (Kretlow, 2019) and two chords from the US (although no data from that event have yet been published). Even with the two previous events, observations of the first two stellar occultations predicted to happen in May 2023 were unsuccessful. This required us to obtain new HST data in order to update the predicted path with the focus on the last stellar occultation that was crossing the continental US. HST data and a third successful, single-chord stellar occultation detected from New Zeland on May 26 helped us to confirm the predicted path and on June 22, 2023 at 07:52 UT, Weywot occulted the Gaia DR3 star 4103952409464066176 (V = 15.8 mag). We obtained four positive chords and several negative chords south of the body. Our preliminary results indicate that Weywot is a large satellite, with effective diameter at almost 200 km, which is consistent with the single-chord from Kretlow (2019). This is much larger than the estimated measurements from Herschel data, which indicated a diameter of ~80 km (Fornasier et al., 2013). Our new, larger estimate of Weywot’s diameter requires a visual geometric albedo of ~4%, considering that it is 5 mag fainter than Quaoar (Fraser & Brown, 2013). Quaoar’s albedo, on the other hand, is about three times larger (12.4 ± 0.6%, Pereira et al., 2023).

Weywot has turned out to be even more interesting than previously thought. While most TNO satellites have circularized orbits, Weywot’s orbital eccentricity, e, is somewhere between 0 and 0.1 (Vachier’s private communication). If the satellite was formed through a giant impact during the massive disk phase in which collisional activity was very intense (Morbidelli and Nesvorny, 2020), then the satellite’s orbit should have been circularized through tidal evolution (as is the case for Dysnomia, Eris’ satellite, and Hi’iaka, Haumea’s largest satellite) with e ~ 0. The circularization timescale for an object formed via giant impact is only a few million years (Brunini, 2020) but should be longer for an object that was captured, since it would initially be on an orbit with a larger semi-major axis. Also, the large difference in the albedo between Weywot and Quaoar supports the possibility of Weywot being a captured satellite. However, an effective diameter of almost 200 km places its size in a similar range to those of other dwarf planets satellites, which are believed to have very different formation scenarios.

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