Thermal instability and rocky planetesimal formation in the inner regions of protoplanetary disks

Abstract

The inner regions of protoplanetary disks are promising formation sites of rocky planetesimals. Theoretical studies have proposed that dust trapping at the magnetorotational instability (MRI) activation boundary, or the dead-zone inner edge, promotes planetesimal formation. However, the inner disk may be thermally unstable, in which case the dead-zone inner edge may not remain steady, and the associated pressure maximum and dust trap may not be maintained. In this study, we propose a scenario in which planetesimals form in a thermally unstable inner disk through dust self-accumulation driven by the coevolution of dust and disk temperature. To this end, we simultaneously calculate the non-equilibrium thermal evolution, the evolution of the gas and dust surface densities, dust growth, and planetesimal formation. Our results show that thermal instability triggers cyclic MRI activation and deactivation, during which planetesimals form. The MRI is activated in the inner disk, and thermal instability causes the active region to expand outward and then return to an inactive state, producing a periodic cycle. Triggered by a local enhancement in the dust surface density, dust undergoes self-accumulation while migrating inward during the MRI-inactive phase, resulting in planetesimal formation. Once the MRI is reactivated at a smaller radius, the next cycle begins. For a typical accretion rate of $10^{-8}M_{\odot}~{\rm yr^{-1}}$, a planetesimal belt forms near 1 au. Depending on the model parameters, approximately 10$-$80% of the dust flowing into the planetesimal-forming region is converted into planetesimals. This mechanism produces sufficient planetesimal mass for the formation of multiple super-Earths. The resulting planetesimal distribution can serve as a physically motivated initial condition for subsequent planet formation simulations.