Turbulent thermal diffusion: a way to concentrate dust in protoplanetary discs

Turbulence acting on mixes of gas and particles generally diffuses the latter evenly through the former. However, in the presence of background gas temperature gradients, a phenomenon known as turbulent thermal diffusion appears as a particle drift velocity (rather than a diffusive term). This process moves particles from hot regions to cold ones. We re-derive turbulent thermal diffusion using astrophysical language and demonstrate that it could play a major role in protoplanetary discs by concentrating particles by factors of tens. Such a concentration would set the stage for collective behaviour such as the streaming instability and hence planetesimal formation.