Suppression of Resonant Overstability at Sharp Migration Gradients

Abstract

Mean-motion resonances are expected to frequently arise at the inner edges of protoplanetary disks, where planet-disk interactions facilitate large-scale orbital convergence. Under certain conditions, however, the same dissipative forces that promote resonant capture can drive resonant librations overstable, ultimately breaking commensurabilities. Here we examine the onset of overstability near disk torque reversals and show that it can be subdued when the transition is sufficiently sharp. Adopting the dissipative circular restricted three-body problem as a paradigm, we present a WKB-style analysis that reduces the resonant dynamics to a damped, driven harmonic oscillator. Within this framework, we obtain an effective frictional term that is proportional to the local migration-rate gradient, parameterized by a dimensionless coefficient $\beta$ that encodes the steepness of the local torque reversal. Our analytical theory predicts that overstability is quenched once $\beta \gtrsim \tau_a/\tau_e$, where $\tau_a$ and $\tau_e$ denote the characteristic disk-driven evolution timescales of semi-major axis and eccentricity. We verify and refine our analytic results with direct $N$-body integrations. Simple estimates based on conventional type-I scalings suggest that the competition between overstability and its mitigation at disk inner edges is a borderline outcome that is sensitive to the detailed structure of planet-disk interactions.