Evolution of circumstellar discs in young star-forming regions

TL;DR

This study investigates how the environment of young star-forming regions shapes circumstellar disc evolution by coupling molecular-cloud collapse to disc dynamics. It employs a two-stage model (Stage 1: cloud collapse and star formation; Stage 2: evolving discs with viscous spreading, external and internal photoevaporation, dust losses, and truncations) using AMUSE and the FRIED grid to capture gas and dust processes alongside cluster dynamics. The key finding is that an extended star-formation history ($2$–$5\ \mathrm{Myr}$) allows late-formed discs to survive in UV-rich environments, solving the proplyd lifetime problem and yielding a population where gas is depleted but solids persist, enabling rocky-planet formation across a wide range of disc masses and radii. This work highlights the critical role of birth-time, dynamical history, and local density in governing disc survival and the potential diversity of planetary systems formed in clustered environments.

Abstract

The evolution of circumstellar discs is influenced by their surroundings. The relevant processes include external photoevaporation due to nearby stars, and dynamical truncations. The impact of these processes on disc populations depends on the star-formation history and on the dynamical evolution of the region. Since star formation history and the phase-space characteristics of the stars are important for the evolution of the discs, we start simulating the evolution of the star cluster with the results of molecular cloud collapse simulations. In the simulation we form stars with circumstellar discs, which can be affected by different processes. Our models account for the viscous evolution of the discs, internal and external photoevaporation of gas, external photoevaporation of dust, and dynamical truncations. All these processes are resolved together with the dynamical evolution of the cluster, and the evolution of the stars. An extended period of star formation, lasting for at least 2 Myr, results in some discs being formed late. These late formed discs have a better chance of survival because the cluster gradually expands with time, and a lower local stellar density reduces the effects of photoevaporation and dynamical truncation. Late formed discs can then be present in regions of high UV radiation, solving the proplyd lifetime problem. We also find a considerable fraction of discs that lose their gas content, but remain sufficiently rich in solids to be able to form a rocky planetary system.

Figures (12)

• Figure 1: The evolution of the molecular cloud collapse and star formation process in run #4. The second panel shows the moment before the first stars form, and the center panel shows the region during star formation. The fourth panel shows the moment just before gas expulsion, and the rightmost panel shows the region close to the end of the simulation.
• Figure 2: Number of stars in time for each simulation run.
• Figure 3: Cluster half-mass radius of the simulations in time. The solid lines correspond to the half-mass radius while star formation is still ongoing and the dotted lines after it has ended.
• Figure 4: Q parameter of our simulations in time, and values for observed star-forming regions. The Q parameter in our simulations considers only stars with masses $M_* > 0.5 \mathrm{\ M}_{\odot}$. The Q parameters are computed at times corresponding to the ages of the observed star-forming regions and the end of the simulation, and shown as points connected by linear dashed lines to guide the eye.
• Figure 5: Fractal dimension of our simulations in time, and values for observed star-forming regions. The fractal dimension in our simulations considers only stars with masses $M_* > 0.5 \mathrm{\ M}_{\odot}$. The fractal dimensions are computed at times corresponding to the ages of the observed star-forming regions and the end of the simulation, and shown as points connected by linear dashed lines to guide the eye.