Dust delivery and entrainment in photoevaporative winds

TL;DR

This paper models dust dynamics in photoevaporative winds of protoplanetary discs, providing a semi-analytic method to determine dust entrainment and delivery, revealing maximum grain sizes and the influence of turbulence and wind properties.

Contribution

It introduces a semi-analytic approach to rapidly estimate dust properties in photoevaporative winds, including maximum grain size for entrainment, without complex hydrodynamic simulations.

Findings

Photoevaporation creates vertical gas flows aiding dust delivery.

Maximum grain size for entrainment is significantly larger than for delivery by advection.

Most entrained dust follows gas trajectories, with turbulence influencing dust transport.

Abstract

We model the gas and dust dynamics in a turbulent protoplanetary disc undergoing extreme-UV photoevaporation in order to better characterise the dust properties in thermal winds (e.g. size distribution, flux rate, trajectories). Our semi-analytic approach allows us to rapidly calculate these dust properties without resorting to expensive hydrodynamic simulations. We find that photoevaporation creates a vertical gas flow within the disc that assists turbulence in supplying dust to the ionisation front. We examine both the delivery of dust to the ionisation front and its subsequent entrainment in the overlying wind. We derive a simple analytic criterion for the maximum grain size that can be entrained and show that this is in good agreement with the results of previous simulations where photoevaporation is driven by a range of radiation types. We show that, in contrast to the case for magnetically driven winds, we do not expect large scale dust transport within the disc to be effected by photoevaporation. We also show that the maximum size of grains that can be entrained in the wind ($s_{\rm max}$) is around an order of magnitude larger than the maximum size of grains that can be delivered to the front by advection alone ($s_{\rm crit} \lesssim 1$ $\mu{\rm m}$ for Herbig Ae/Be stars and $\lesssim 0.01$ $\mu{\rm m}$ for T Tauri stars). We further investigate how larger grains, up to a limiting size $s_{\rm limit}$, can be delivered to the front by turbulent diffusion alone. In all cases, we find $s_{\rm max} \gtrsim s_{\rm limit}$ so that we expect that any dust that is delivered to the front can be entrained in the wind and that most entrained dust follows trajectories close to that of the gas.