To understand which processes are governing the evolution of protoplanetary discs it is fundamental to study different star-forming regions.
It is not yet understood whether the driver for the accretion of the disc material is the viscosity or the disc wind. These two mechanisms lead to a different evolution of the physical sizes of disc populations (Manara et al. PPVII). If viscosity is at play, because of the conservation of angular momentum, discs will get larger with time (Lynden-Bell & Pringle 1974). Instead, if winds remove angular momentum, the disc size should be constant or decrease with time (Tabone et al. 2021).
This leads to a different evolution of the disc radius if measured using the gas as a tracer.
On the contrary, due to the presence of radial drift (Weidenschilling 1977), the disc radius measured using dust is always shrinking in time.
I will show an analytic solution for evaluating the gas disc sizes of populations of discs and a suite of models to probe the secular evolution of the ratio between the dust and gas radius (Toci et al. 2023, 2021). Our disc models evolve considering viscous evolution and two populations of dust, including grain growth, fragmentation, dust back-reaction, and radial drift. The flux of the models is evaluated using the radiative transfer code RADMC3D (Dullemond et al. 2012).
We tested different values of initial conditions, and we compared our results with the observational findings of the Lupus star-forming region.
For a wide range of models, the ratio between the dust and gas radii quickly becomes very large (>5) due to the presence of radial drift, pointing out that radial drift is too efficient in the models. To solve this issue, substructures, routinely resolved in bright, large discs, must be present in most of the sources, although unresolved.