Revisiting the "radial-drift barrier" of planet formation and its relevance in observed protoplanetary discs

TL;DR

This study analytically and numerically investigates dust grain migration in protoplanetary discs, showing that most grains can survive radial drift under observed disc conditions, challenging the traditional radial-drift barrier concept.

Contribution

The paper provides a combined analytical and numerical analysis of dust migration, applying it to observed discs to assess the radial-drift barrier's relevance in real systems.

Findings

Most grains survive radial migration in observed discs.

Conditions p+q+1/2 ≤ 0 and q ≤ 2/3 prevent the radial-drift barrier.

The radial-drift barrier may not be a significant obstacle in planet formation.

Abstract

Context. To form metre-sized pre-planetesimals in protoplanetary discs, growing grains have to decouple from the gas before they are accreted onto the central star during their phase of fast radial migration and thus overcome the so-called "radial-drift barrier" (often inaccurately referred to as the "metre-size barrier"). Aims. To predict the outcome of the radial motion of dust grains in protoplanetary discs whose surface density and temperature follow power-law profiles, with exponent p and q respectively. We investigate both the Epstein and the Stokes drag regimes which govern the motion of the dust. Methods. We analytically integrate the equations of motion obtained from perturbation analysis. We compare these results with those from direct numerical integration of the equations of motion. Then, using data from observed discs, we predict the fate of dust grains in real discs. Results. When a dust grain reaches the inner regions of the disc, the acceleration due to the increase of the pressure gradient is counterbalanced by the increase of the gas drag. We find that most grains in the Epstein (resp. the Stokes) regime survive their radial migration if-p+q+1/2 \leq0 (resp. if q\leq 2/3). The majority of observed discs satisfies both-p+q+ 1/2 \leq0 and q\leq 2/3: a large fraction of both their small and large grains remain in the disc, for them the radial drift barrier does not exist.