Rattling Resonant Chains with Dynamical Tides and Parametric Resonance Passage

Abstract

Close-in sub-Neptunes are born in mean-motion resonant chains that break apart on a timescale of $\sim$100 Myr. If the chains are first ''rattled'' --- have their free eccentricities excited to a few percent --- then breaking occurs on the required timescale. We propose that chains are rattled by internal oscillations tidally driven by the star in the innermost planet. Unlike other proposals, there is no need to invoke any reservoir of planetesimals or external perturbers --- the chain unravels from within. We develop a Hamiltonian theory (and an accompanying toy model) which predicts a parametric instability whereby chain librations exponentially amplify when the tidal forcing frequency equals the sum of the internal mode frequency and the chain libration frequency. We also introduce a new eigenmode solver for rotating planets, $\texttt{MAQAM}$, that in combination with the evolutionary code $\texttt{ORCHARD}$ enables us to track the frequency evolution of pressure, gravity, and inertial modes in cooling sub-Neptunes. Inertial modes in the gas envelope and fluid mantle can evolve to become momentarily parametrically resonant. Passage through a single parametric resonance can raise the free eccentricities in the chain to a few percent within a few tens of Myrs.