A particle-based approach to dust dynamics in external photoevaporative winds

ABSTRACT Planet-forming discs in sufficiently strong UV environments lose gas in external photoevaporative winds. Dust can also be entrained within these winds, which has consequences for the possible solids reservoir for planet formation, and determines the shielding of the disc by the wind. This has previously been studied in one-dimensional (1D) models, with predictions for the maximum entrained size, as well as a predicted population of stalled dust of decreasing grain size with distance from the disc. We wrote and tested a new dust particle solver to make the first study of the entrainment and dynamics of dust, using steady-state solutions of state-of-the-art 1D and two-dimensional (2D) radiation hydrodynamic simulations of externally photoevaporating discs. In our 1D models, we only consider the outer disc at the mid-plane, verifying previous studies. In our 2D simulations, the wind is launched from the disc surface, as well as the disc edge. In 2D, we find that the maximum entrained grain size varies substantially with angle relative to the plane of the disc, from $\sim 100$ $\mu$m near the disc outer edge down to $\sim 1$ $\mu$m or even sub-micron in the weaker wind from the disc surface. The gradient of stalled dust seen in 1D also only appears near the disc outer edge in 2D, but not from the disc surface. This agrees qualitatively with observations of silhouette discs in the Orion Nebula Cluster. Despite the spatial variation of the dust, the extinction of the UV radiation remains fairly uniform due to the opacity being dominated by the small grains and depends more on the dust distribution within the disc itself.