The viscous evolution of circumstellar discs in young star clusters

Stars with circumstellar discs may form in environments with high stellar and gas densities that affects the discs through processes like truncation from dynamical encounters, ram pressure stripping, and external photoevaporation. Circumstellar discs also undergo viscous evolution that leads to disc expansion. Previous work indicates that dynamical truncation and viscous evolution play a major role in determining circumstellar disc size and mass distributions. However, it remains unclear under what circumstances each of these two processes dominates. Here, we present results of simulations of young stellar clusters taking viscous evolution and dynamical truncations into account. We model the embedded phase of the clusters by adding leftover gas as a background potential that can be present through the whole evolution of the cluster, or expelled after |$1\, \mathrm{Myr}$|⁠. We compare our simulation results to actual observations of disc sizes, disc masses, and accretion rates in star-forming regions. We argue that the relative importance of dynamical truncations and the viscous evolution of the discs changes with time and cluster density. Viscous evolution causes the importance of dynamical encounters to increase in time, but the encounters cease soon after the expulsion of the leftover gas. For the clusters simulated in this work, viscous growth dominates the evolution of the discs.