The viscous evolution of circumstellar discs in young star clusters

TL;DR

This study uses simulations to analyze how viscous evolution and dynamical encounters influence circumstellar disk sizes and masses in young star clusters, highlighting the changing dominance of these processes over time and cluster density.

Contribution

The paper introduces a simulation framework that incorporates both viscous evolution and dynamical truncations, comparing their effects under different cluster conditions.

Findings

Viscous evolution primarily drives disk growth in the simulated clusters.

Dynamical truncations become more significant over time, especially before gas expulsion.

The importance of dynamical encounters diminishes after leftover gas is expelled.

Abstract

Stars with circumstellar disks may form in environments with high stellar and gas densities which affects the disks through processes like truncation from dynamical encounters, ram pressure stripping, and external photoevaporation. Circumstellar disks also undergo viscous evolution which leads to disk expansion. Previous work indicates that dynamical truncation and viscous evolution play a major role in determining circumstellar disk 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 which can be present through the whole evolution of the cluster, or expelled after 1 Myr. We compare our simulation results to actual observations of disk sizes, disk masses, and accretion rates in star forming regions. We argue that the relative importance of dynamical truncations and the viscous evolution of the disks 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 disks.