Formation of close-in super-Earths: dust enrichment of the inner disk due to the MRI

Do the ubiquitous short-period super-Earths/mini-Neptunes form in situ, in the inner protoplanetary disk? If so, the key step to planet formation is dust enrichment of the inner disk. In the outer disk the dust particles are expected to quickly grow by coagulation and drift inwards due to gas drag. The inner disk structure, governed by viscous accretion due to the magneto-rotational instability (MRI), features a local gas pressure maximum that could potentially trap the drifting dust particles. We consider evolution of dust, including growth, radial drift and fragmentation, in a steady-state gas disk, with the inner disk gas structure self-consistently determined from MRI criteria (Mohanty et al. 2017). We find that in the inner disk the dust particle size is limited by fragmentation due to relative turbulent velocities. The particles remain too small to feel significant gas drag and therefore do not accumulate at the local gas pressure maximum. However, as a decrease in particle size also implies less radial drift for the particles from the outer disk, this ultimately leads to dust enhancement throughout the inner disk, interior to the pressure maximum