Beyond Point Masses. VI. Spin-Orbit Evolution of the Centaur Binary Typhon-Echidna

Only three binaries have been identified among the Centaur population. Because their perihelia are significantly closer than those of other trans-Neptunian binaries (TNBs), these systems allow a detailed look at tight binaries in the broader TNO population and provide critical insight into the disruption of binaries as they enter the Centaur population. Using recent and archival Hubble Space Telescope (HST) observations, along with Keck data, we present a spin-orbit study of Typhon-Echidna. We find that the binary's mutual orbit is inconsistent with a Keplerian orbit; more detailed non-Keplerian fits show that the mutual orbit is rapidly precessing. We measure Typhon's dynamical oblateness, J 2, at about10 sigma confidence and find that Typhon's rotation pole is 20 misaligned with the binary's mutual orbit. Assuming Typhon has a triaxial shape, our results, combined with rotational light curves and thermal measurements from the literature, suggest ellipsoidal semi-axes of a=93+8-6 km, b=84+6-6 km, and c=65+9-8 km. We further investigate the observational consequences of the complex spin-orbit dynamics, including light curve alteration by axial precession of Typhon and substantial changes to the system's mutual event season. Based on the system's dynamically excited state, we suggest a recent encounter with a giant planet may have substantially altered the system, potentially consistent with a binary in an early stage of disruption. This hypothesis can be tested with resolved photometric observations of the system. Our investigation highlights how non-Keplerian dynamics enhances our understanding of TNB systems and motivates ongoing observations of TNBs with astrometry, photometry, and stellar occultations.