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K. S. Noll, D. C. Stephens (STScI), W. M. Grundy (Lowell Obs.), D. J. Osip (OCIW), I. Griffin (Mus. Science & Industry)
We have determined the orbits of two transneptunian binary systems, (58534) 1997 CQ29 and (66652) 1999 RZ253, using data from the Hubble Space Telescope and the Magellan Obseratory. The relative astrometry derived from these observations allows us to find complete orbit solutions that determine the period, semimajor axis, eccentricity, and the orientation of the orbit plane. In both cases we have been able to determine the system mass to10% or better.
By combining the orbit-derived mass with measured photometry of resolved components, it is possible to derive the albedo as a function of density. For densities of 1000 kg m-3 the red geometric albedos of both objects are remarkably high, pR = 0.18 and 0.37 for RZ253 and CQ29 respectively. While it has long been recognized that the assignment of cometary albedos to all Kuiper Belt objects is a convenient fiction, the presence of objects with such high albedos suggests that Kuiper Belt albedos may be more diverse than previously thought. Because albedo is used to translate observed brightnesses to mass, the implication of a significantly higher average albedo for the Kuiper Belt is the reduction of the total mass of the belt by as much as an order of magnitude.
Combining mass and photometry also makes it possible to extract the object’s diameters, again as a function of density. The components of CQ29 are particularly noteworthy for their small sizes; the primary component has a diameter of 80 km and the smaller secondary only 66 km. This is approaching the size of the largest cometary nuclei such as Hale-Bopp.
With accurately determined masses, semimajor axes and diameters it is possible to look at the orbits in terms of the Hill radius. The transneptunian binaries appear to orbit at a few percent of the Hill radius, similar to binaries found in the main belt and to regular satellite systems but different than captured satellites.
This work was supported by grants from the Space Telescope Science Institute operated by AURA under NASA contract NAS 5-26555.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.