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The Not-Quite-Synchronous Rotation Periods of Eris and Dysnomia

Presentation #307.06 in the session “Centaurs and Kuiper Belt Objects: Multiples”.

Published onOct 26, 2020
The Not-Quite-Synchronous Rotation Periods of Eris and Dysnomia

The Kuiper Belt dwarf planet (136199) Eris is the most massive object identified beyond the orbit of Neptune (e.g., Brown & Butler, 2018; Holler et al., submitted) and has one known satellite, Dysnomia, which is itself a large Kuiper Belt Object (KBO; Brown & Butler, 2018). The origin of Dysnomia from either a giant impact or capture is still debated. Characterization of the tidal state of the system from rotation light curves of both bodies provides valuable information about the origin of the system. Ground-based V-band observations of Eris sampling 58 nights between August 2015 and January 2016 with the Palomar 60-inch (P60) robotic telescope (Cenko et al., 2006) were obtained to evaluate the rotation period of Eris. These data were reduced using the PHOTOMETRYPIPELINE (Mommert, 2017). Hubble Space Telescope observations (GO 15171) with the WFC3/UVIS instrument and the F606W filter over 7 orbits in January and February 2018 were used to determine the rotation period of Dysnomia. The results indicate that Eris’ most likely rotation period is 14.56±0.10 days, while Dysnomia’s rotation period is synchronous with its orbital period of 15.785899 days (Holler et al., submitted). This discrepancy indicates that the system is not yet fully tidally evolved. Dynamical model results differ depending on whether the initial semi-major axis of Dysnomia was greater than or less than the current value, corresponding to origin by capture and giant impact, respectively. While Dysnomia is expected to de-spin over a time period much shorter than the age of the solar system, regardless of its initial semi-major axis, Eris is not expected to fully de-spin for even a reasonably dense Dysnomia (~1.2 g cm-3) and an initially fast (a few hours; giant impact) or slow (a few days to weeks; capture) rotation period. The fact that the average rotation period for KBOs with diameters greater than 200 km is ~9 hours (Lacerda & Luu, 2006; Thirouin & Sheppard, 2019), and not multiple days, favors the giant impact mechanism for the origin of Dysnomia.

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