The Albedo, Size, and Density of Binary Kuiper Belt Object (47171) 1999 TC36

We measured the system-integrated thermal emission of the binary Kuiper BeltObject 1999 TC36 at wavelengths near 24 and 70 microns using the Spitzer spacetelescope. We fit these data and the visual magnitude using both the StandardThermal Model and thermophysical models. We find that the effective diameter ofthe binary is 405 km, with a range of 350 -- 470 km, and the effective visiblegeometric albedo for the system is 0.079 with a range of 0.055 -- 0.11. Thebinary orbit, magnitude contrast between the components, and system mass havebeen determined from HST data (Margot et al., 2004; 2005a; 2005b). Oureffective diameter, combined with that system mass, indicate an average densityfor the objects of 0.5 g/cm3, with a range 0.3 -- 0.8 g/cm3. This density islow compared to that of materials expected to be abundant in solid bodies inthe trans-Neptunian region, requiring 50 -- 75% of the interior of 1999 TC36 betaken up by void space. This conclusion is not greatly affected if 1999 TC36 is``differentiated'' (in the sense of having either a rocky or just a non-porouscore). If the primary is itself a binary, the average density of that(hypothetical) triple system would be in the range 0.4 -- 1.1 g/cm3, with aporosity in the range 15 -- 70%.Comment: ApJ, in press (May, 2006