Timescales diagnostics for saving viscous and MHD-driven dusty discs from external photoevaporation

Abstract

The evolution of protoplanetary discs is a function of their internal processes and of their environment. It is unclear if angular momentum is mainly removed viscously or by magnetic winds, or by a combination of the two. While external photoevaporation is expected to influence disc evolution and dispersal, there are observational limitations towards highly irradiated discs. The interplay between these ingredients and their effect on the gas and dust distributions are poorly understood. We investigate the evolution of both the gaseous and solid components of viscous, MHD-wind or hybrid discs, in combination with external FUV-driven mass loss. We test which combinations of parameters protect discs from external irradiation, allowing the solid component to live long enough to allow planet formation to succeed. We run a suite of 1D simulations of smooth discs with varying initial sizes, levels of viscous and MHD-wind stresses modelled via an $\alpha$ parametrisation, and strengths of the external FUV environment. We track disc radii, various lifetime diagnostics, and the amount of dust removed by the photoevaporative wind, as a function of the underlying parameters. The biggest role in determining the fate of discs is played by a combination of its ability to spread radially outwards and the strength of FUV-driven erosion. While MHD wind-driven discs experience less FUV erosion due to the lack of spread, they do not live for longer compared to viscously evolving discs, especially at low-to-moderate FUV fluxes, while higher fluxes yield disc lifetimes that are insensitive to the disc's angular momentum transport mechanism. For the solid component, the biggest role is played by a combination of inward drift and removal by FUV winds. This points to the importance of other physical ingredients, such as disc substructures, even in highly-irradiated disc regions, in order to retain solids.