Detection of Six Trans-Neptunian Binaries with NICMOS: A High Fraction of Binaries in the Cold Classical Disk

We have analyzed a homogeneous set of observations of 81 trans-Neptunian objects obtained with NIC2 on the Hubble Space Telescope with the goal of identifying partially resolved binaries. Using PSF fitting we have identified six likely binaries in addition to the three new binaries already found in this data set. We find that 11 % of trans-Neptunian objects are binaries at the separation and brightness limits of NIC2. The identification of these new binaries significantly increases the known lower limit to the binary fraction among trans-Neptunian objects. The origin of such a high fraction of binaries remains to be determined. Most interestingly, detectable binaries appear to be about 4 times more common among the cold classical disk than in the dynamically excited populations.