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We introduce a new technique to determine the gas turbulence and surfacedensity in bright disc rings, under the assumption that dust growth is limitedby turbulent fragmentation at the ring centre. We benchmark this prescriptionin HD 163296, showing that our measurements are consistent with availableturbulence upper limits and agree with independent estimates of the gas surfacedensity within a factor of two. We combine our results with literaturemeasurements of the dust surface density and grain size to determine thedust-to-gas ratio and Stokes number in the 67 au and 100 au rings. Ourestimates suggest that particle clumping is taking place under the effect ofstreaming instability (SI) in the 100 au ring. Even though in the presence ofexternal isotropic turbulence this process might be hindered, we provideevidence that turbulence is non-isotropic in both rings and likely originatingfrom mechanisms (such as ambipolar diffusion) that could ease particle clumpingunder SI. Finally, we determine the mass accretion rate under the assumptionthat the disc is in steady state and turbulence regulates angular momentumtransport. Our results are in tension with spectroscopic measurements andsuggest that other mechanisms might be responsible for accretion, inqualitative agreement with the detection of a magneto-centrifugal wind in thissystem. Applying our method to larger samples can be used to statisticallyassess if SI is a viable mechanism to form planetesimals in bright rings.

Observing planetesimal formation under streaming instability in the rings of HD 163296