Effects of stellar density on the photoevaporation of circumstellar discs

TL;DR

This work addresses how local stellar density governs external photoevaporation of circumstellar discs in young clusters. By coupling disc evolution, stellar dynamics, and UV-driven mass loss in AMUSE across six cluster densities and evolving for $2.0\,\mathrm{Myr}$, the authors quantify how disc survival and mass depend on environment. They find that external photoevaporation rapidly depletes disc masses, with a characteristic break near $n \approx 100\,{\rm stars\,pc^{-2}}$ where masses decline by about a dex toward higher densities, a trend that aligns with observations of regions such as Lupus, ONC, OMC-2, Taurus, and NGC 2024. The results imply that planetary system formation is favored in lower-density pockets within clusters and provide quantitative thresholds for disc survival in dense star-forming environments. Overall, the study demonstrates the critical role of environment in shaping the early stages of planet formation by regulating disc mass loss through external irradiation.

Abstract

Circumstellar discs are the precursors of planetary systems and develop shortly after their host star has formed. In their early stages these discs are immersed in an environment rich in gas and neighbouring stars, which can be hostile for their survival. There are several environmental processes that affect the evolution of circumstellar discs, and external photoevaporation is arguably one of the most important ones. Theoretical and observational evidence point to circumstellar discs losing mass quickly when in the vicinity of massive, bright stars. In this work we simulate circumstellar discs in clustered environments in a range of stellar densities, where the photoevaporation mass-loss process is resolved simultaneously with the stellar dynamics, stellar evolution, and the viscous evolution of the discs. Our results indicate that external photoevaporation is efficient in depleting disc masses and that the degree of its effect is related to stellar density. We find that a local stellar density lower than 100 stars pc$^{-2}$ is necessary for discs massive enough to form planets to survive for \SI{2.0}{Myr}. There is an order of magnitude difference in the disc masses in regions of projected density 100 stars pc$^{-2}$ versus $10^4$ stars pc$^{-2}$. We compare our results to observations of the Lupus clouds, the Orion Nebula Cluster, the Orion Molecular Cloud-2, Taurus, and NGC 2024, and find that the trends observed between region density and disc masses are similar to those in our simulations.

Figures (4)

• Figure 1: Example of cluster evolution for a realisation of the R1.0 model. Black crosses mark the position of the stars at the beginning of the simulation, and the label next to them shows the stellar mass. The sizes of the large, coloured points are proportional to the disc radii, and their colour indicates the total disc mass. The red crosses, when present, show the moment when a disc is dispersed. The thin black lines that follow a red cross indicate the continuing orbit of the star, which keeps moving through the region after its disc has been evaporated. The trajectories of some massive, radiating stars are shown in thin blue lines.
• Figure 2: Disc fractions in time separated by stellar mass. The top panel shows disc fractions for low mass stars ($\mathrm{M}_* \leq 0.5 \mathrm{M}_{\odot}$) and the bottom panel for high mass stars ($0.5 \mathrm{M}_{\odot} < \mathrm{M}_* \leq 1.9 \mathrm{M}_{\odot}$). The lines show the mean for 6 runs of each model, and the shaded areas represent the standard deviation. For clarity, in the bottom panel we plot the standard deviation only for the R0.5 and R5.0 models, but the rest of the models have deviations of similar magnitude.
• Figure 3: Binned mean disc mass versus local stellar number density. The mean mass is calculated using a moving bin spanning 100 stars. The local density is calculated for each star as explained in section \ref{['results:discmass']}. The dotted lines thick represent the binned mean disc mass at $t=0.0Myr$ and the solid thick lines at $t=2.0Myr$. The shaded areas show the standard error. The thin lines represent the binned mean at $0.2Myr$ intervals.
• Figure 4: Binned mean disc mass versus local stellar number density, projected in two dimensions. The mean mass is calculated using a moving bin spanning 100 stars. The local density is calculated for each star as explained in section \ref{['results:discmass']}, but projecting the distances between stars into two dimensions. The dotted lines thick represent the binned mean disc mass at $t=0.0Myr$ and the solid thick lines at $t=2.0Myr$. The shaded areas show the standard error. Diamonds show average disc dust masses and local stellar densities for several observed regions. The different color used for NGC 2024 symbolises the different age of the region.