Planetesimal formation during protoplanetary disk buildup

TL;DR

This study explores the conditions under which planetesimals can form during the early stages of protoplanetary disk buildup, challenging the common assumption that formation occurs only after the disk is fully developed.

Contribution

It couples a planetesimal formation model with a disk formation model to identify scenarios where planetesimals form during disk buildup, highlighting the role of specific turbulence parameters.

Findings

Planetesimals typically form after disk buildup when the disk cools and loses mass.

Under certain turbulence conditions, planetesimals can form during the disk buildup stage.

Layered accretion scenarios may facilitate early planetesimal formation.

Abstract

Models of dust coagulation and subsequent planetesimal formation are usually computed on the backdrop of an already fully formed protoplanetary disk model. At the same time, observational studies suggest that planetesimal formation should start early, possibly even before the protoplanetary disk is fully formed. In this paper, we investigate under which conditions planetesimals already form during the disk buildup stage, in which gas and dust fall onto the disk from its parent molecular cloud. We couple our earlier planetesimal formation model at the water snow line to a simple model of disk formation and evolution. We find that under most conditions planetesimals only form after the buildup stage when the disk becomes less massive and less hot. However, there are parameters for which planetesimals already form during the disk buildup. This occurs when the viscosity driving the disk evolution is intermediate ($\alpha_v \sim 10^{-3}-10^{-2}$) while the turbulent mixing of the dust is reduced compared to that ($\alpha_t \le 0.03 \cdot \alpha_v$), and with the assumption that water vapor is vertically well-mixed with the gas. Such $\alpha_t \ll \alpha_v$ scenario could be expected for layered accretion, where the gas flow is mostly driven by the active surface layers, while the midplane layers, where most of the dust resides, are quiescent.