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The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A tidally-evolved binary Centaur |
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| Authors: | WM Grundy JA Stansberry DC Stephens DE Trilling JR Spencer HF Levison |
| Institution: | a Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA b Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA c Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA d Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA e Southwest Research Institute, 1050 Walnut St. #400, Boulder, CO 80302, USA f NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, USA |
| Abstract: | Hubble Space Telescope observations of Uranus- and Neptune-crossing object (65489) Ceto/Phorcys (provisionally designated 2003 FX128) reveal it to be a close binary system. The mutual orbit has a period of 9.554±0.011 days and a semimajor axis of 1840±48 km. These values enable computation of a system mass of (5.41±0.42)×1018 kg. Spitzer Space Telescope observations of thermal emission at 24 and 70 μm are combined with visible photometry to constrain the system's effective radius  and geometric albedo  . We estimate the average bulk density to be  , consistent with ice plus rocky and/or carbonaceous materials. This density contrasts with lower densities recently measured with the same technique for three other comparably-sized outer Solar System binaries (617) Patroclus, (26308) 1998 SM165, and (47171) 1999 TC36, and is closer to the density of the saturnian irregular satellite Phoebe. The mutual orbit of Ceto and Phorcys is nearly circular, with an eccentricity ?0.015. This observation is consistent with calculations suggesting that the system should tidally evolve on a timescale shorter than the age of the Solar System. |
| Keywords: | Centaurs Kuiper belt Trans-neptunian objects Satellites |
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