Thinkshop 2020

ALMA is showing that most proto-planetary discs are highly sub-structured and that the most frequent structure consists in azimuthally symmetric "gaps and rings". Rings have attracted a lot of attention since they might be the signature of young planets. But rings are extremely important for another reason: they provide us with a privileged window inside disc physics. Indeed, as shown by the DSHARP team, their finite dust width shows that there must be some level of dust diffusion, or else the dust would only collect at the pressure maximum. However, the DSHARP team was only able to place a lower limit, and not to measure, the amount of diffusion, because they did not have information on the gas distribution. I will show how the analysis of the gas rotation curve, another breakthrough enabled by ALMA observations of molecular lines, is a powerful way to measure the width of rings in the gas and therefore allows us to measure the dust-gas coupling, which controls the dust ring width. Formally, the relevant parameter is the ratio between the Shakura-Sunyaev α and the dust Stokes number St. I will also discuss the impact of the disc 3D structure on this analysis and show that the measurement of ring width is robust towards the details of the vertical structure. At the moment, there are only two objects with good enough S/N to perform these measurements. In these objects, I will report gas widths larger than in the dust, consistently with the idea of dust trapping. I will show how the data point to a relatively high degree of dust-gas coupling (typical α/St ~ 0.1). Scenarios with very low levels of turbulence and high levels of grain growth can therefore be rejected. Future constraints on the dust grain size in the rings will help in breaking the degeneracy between St and alpha.