Probing Dust and Gas Properties Using Ringed Disks

Abstract: How rapidly a planet grows in mass and how far they may park from the host star depend sensitively on two non-dimensional parameters: Stokes number St and turbulent $\alpha$. Yet, these parameters remain highly uncertain being difficult or impossible to measure directly. Here, we demonstrate how the ringed disks can be leveraged to obtain St and $\alpha$ separately by constructing a simple toy model that combines dust radial equation of motion under aerodynamic drag and coupling to gas motion with the measured distribution of dust masses in Class 0/I disks. Focusing on known systems with well-resolved dust rings, we find that the range of St and $\alpha$ that are consistent with the measured properties of the rings are small: $10^{-4} \lesssim {\rm St} \lesssim 10^{-2}$ and $10^{-5} \lesssim \alpha \lesssim 10^{-3}$. These low St and $\alpha$ ensure the observed rings are stable against clumping. Even in one marginal case where the formation of bound clumps is possible, further mass growth by pebble accretion is inhibited. Furthermore, the derived low $\alpha$ is consistent with the nearly inviscid regime where Type I migration can be prematurely halted. Our analysis predicts minimal planet population beyond $\sim$10s of au where we observe dust rings and significantly more vigorous planet formation inside $\sim$10 AU, consistent with current exo-giant statistics. We close with discussions on the implications of our results on small planet statistics at large orbital distances.