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

Abstract

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 \textit{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 $\sim10\sigma$ confidence and find that Typhon's rotation pole is $\gtrsim20^\circ$ 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.