Numerical estimation of the capture ability of Neptunian mean motion resonances

Resonant populations of trans-Neptunian objects serve as crucial dynamical archives for unraveling the early migratory history of the Solar System. A quantitative assessment of the capture efficiency into various mean motion resonances (MMRs) during migration is essential for understanding the origins of these populations, constraining migration parameters, and reconstructing of the primordial planetesimal disk. Using numerical simulations, this study systematically investigates the capture capability of exterior MMRs during Neptune's outward migration in a planar model. For a specific p:q MMR, the small bodies can be captured only when their eccentricities surpass a certain threshold, e_min, which increases with faster migration rates, greater distances of MMRs, and higher resonance orders. We also find that 1:q-type MMRs exhibit notably higher e_min due to their unique dynamical structure. On the other hand, as long as a particle's eccentricity is suitable, its capture efficiency shows little dependence on the migration rate; instead, it mainly depends on the p value and heliocentric distance, decaying exponentially as either parameter increases. Based on our simulation results, we derive for the first time a simple empirical expression to calculate e_min and the capture efficiency. From beyond the 1:2 MMR out to approximately the 1:4 MMR, the theoretically predicted capture numbers follow a trend that resembles what is seen in observations, suggesting that migration capture could be the primary source of resonant populations in these regions. However, in more distant regions, the theoretical predictions fall significantly short of observational estimates, implying that other mechanisms (e.g., resonant sticking) might be necessary. This research provides a systematic quantitative framework for understanding capture into Neptunian MMRs during migration. Future integrations of more comprehensive observational data will facilitate a more precise reconstruction of the Solar System's early dynamical evolution.