Dust Accumulation near the Magnetospheric Truncation of Protoplanetary Discs around T Tauri Stars

Abstract

The prevalence of short-period super-Earths that are independent of host metallicity challenges the theoretical construction of their origin. We propose that dust trapping in the global pressure bump induced by magnetospheric truncation in evolved protoplanetary discs (PPDs) around T Tauri stars offers a promising formation mechanism for super-Earths, where the host metallicity is already established. To better understand this planet forming scenario, we construct a toy inner disc model and focus on the evolution of dust trapped in the bump, taking into account the supply from drifting pebbles and loss due to funnel flows. We develop an implicit coagulation-fragmentation code, $\mathtt{Rubble}$, and perform a suite of simulations to evolve the local dust size distributions. Our study for the first time considers dust feedback effect on turbulent diffusion in this kind of model. We report that efficient dust growth and significant accumulation of dust mass is possible in less turbulent disc with sturdier solids and with faster external supply, laying out a solid foundation for further growth towards planetesimals and planetary embryos. We further find that, depending on the dominant process, solid mass may predominantly accumulate in cm-sized grains or particles in runaway growth, indicating different ways of forming planetesimals. Furthermore, these various outcomes show different efficiencies in saving dust from funnel flows, suggesting that they may be distinguishable by constraining the opacity of funnel flows. Also, these diverse dust behaviours may help explain the observed dipper stars and rapidly varying shadows in PPDs.