Enhancing the local dust-to-gas ratio in protoplanetary discs is a necessary first step to planetesimal formation .
In laminar discs , dust settling is an efficient mechanism to raise the dust-to-gas ratio at the disc midplane .
However , turbulence , if present , can stir and lift dust particles , which ultimately hinders planetesimal formation .
In this work , we study dust settling in protoplanetary discs with hydrodynamic turbulence sustained by the vertical shear instability .
We perform axisymmetric numerical simulations to investigate the effect of turbulence , particle size , and solid abundance or metallicity on dust settling .
We highlight the positive role of drag forces exerted onto the gas by the dust for settling to overcome the vertical shear instability .
In typical disc models we find particles with a Stokes number \sim 10 ^ { -3 } can sediment to \raisebox { -3.44 pt } { $ \buildrel \textstyle < \over { \sim } $ } 10 \% of the gas scale-height , provided that \Sigma _ { \mathrm { d } } / \Sigma _ { \mathrm { g } } \raisebox { -3.44 pt } { $ \buildrel% \textstyle > \over { \sim } $ } 0.02 —0.05 , where \Sigma _ { \mathrm { d,g } } are the surface densities in dust and gas , respectively .
This coincides with the metallicity condition for small particles to undergo clumping via the streaming instability .
Super-solar metallicities , at least locally , are thus required for a self-consistent picture of planetesimal formation .
Our results also imply that dust rings observed in protoplanetary discs should have smaller scale-heights than dust gaps , provided that the metallicity contrast between rings and gaps exceed the corresponding contrast in gas density .