How the microphysical properties of external photoevaporation influence the global evolution of protoplanetary discs

Abstract: External photoevaporation is one of the dominant mechanisms for mass loss from protoplanetary discs. However this mass loss is theoretically expected to depend upon the microphysical properties of protoplanetary discs, which are currently poorly constrained in observations. In this work we explore the impact of microphysics on the bulk evolution of discs. The polycyclic aromatic hydrocarbon (PAH) abundance, and the extent to which grain growth has occurred in the disc have profound effects on the strength of mass loss rates due to external photoevaporation, which in turn can have a significant impact on the disc evolution, impacting disc radii and accretion rates over time. The strongest sensitivity is to whether grain growth has occurred in the disc, which reduces the amount of dust entrained in the wind to shield the disc, thus increasing the rate at which gas is lost. Additionally, larger PAH abundances result in stronger heating and higher mass loss rates, but to a lesser extent than grain growth. We find that plausible variations in the PAH abundance and disc dust evolution can leave observable differences in disc populations. This work highlights the importance of obtaining observational constraints of the microphysical properties of protoplanetary discs. Future observations from JWST should soon be able to provide these constraints.