2007 TY430: A cold classical kuiper belt type binary in the plutino population

Kuiper Belt object 2007TY430 is the first wide, equal-sized, binary known in the 3:2 mean motion resonance with Neptune. The two components have a maximum separation of about 1arcsec and are on average less than 0.1mag different in apparent magnitude with identical ultra-red colors (g - i = 1.49 0.01mag). Using nearly monthly observations of 2007TY430 from 2007 to 2011, the orbit of the mutual components was found to have a period of 961.2 4.6days with a semi-major axis of 21000 160km and eccentricity of 0.1529 0.0028. The inclination with respect to the ecliptic is 15.68 0.22deg and extensive observations have allowed the mirror orbit to be eliminated as a possibility. The total mass for the binary system was found to be 7.90 0.21 × 10 ¹⁷kg. Equal-sized, wide binaries and ultra-red colors are common in the low-inclination "cold" classical part of the Kuiper Belt and likely formed through some sort of three-body interactions within a much denser Kuiper Belt. To date 2007TY430 is the only ultra-red, equal-sized binary known outside of the classical Kuiper Belt population. Numerical simulations suggest 2007TY430 is moderately unstable in the outer part of the 3:2 resonance and thus 2007TY430 is likely an escaped "cold" classical object that later got trapped in the 3:2 resonance. Similar to the known equal-sized, wide binaries in the cold classical population, the binary 2007TY430 requires a high albedo and very low density structure to obtain the total mass found for the pair. For a realistic minimum density of 0.5gcm ^-3 the albedo of 2007TY430 would be greater than 0.17. For reasonable densities, the radii of either component should be less than 60km, and thus the relatively low eccentricity of the binary is interesting since no tides should be operating on the bodies at their large distances from each other. The low prograde inclination of the binary also makes it unlikely that the Kozai mechanism could have altered the orbit, making the 2007TY430 binary orbit likely one of the few relatively unaltered primordial binary orbits known. Under some binary formation models, the low-inclination prograde orbit of the 2007TY430 binary indicates formation within a relatively high velocity regime in the Kuiper Belt.