Radiation shielding of protoplanetary discs in young star-forming regions

TL;DR

This study investigates how the ambient gas in young star-forming clusters shields protoplanetary discs from external photoevaporation (EPE) and dynamic truncation. By coupling the Torch star-formation framework (AMUSE) with a disc-population model, the authors simulate disc evolution under external irradiation with and without extinction, revealing that shielding can prolong disc lifetimes by up to an order of magnitude and preserve solids, especially around low-mass stars. EPE remains the dominant mass-loss mechanism globally, but dynamic truncations can dominate for a minority (~10%) of discs; shielding also alters disc-radius evolution and the relationship between disc mass and distance to massive stars. The work highlights the environment’s pivotal role in disc and planet formation, while noting limitations such as neglected protostellar outflows and pre-main sequence feedback, and suggesting pathways for reconciling model predictions with observations in varied star-forming regions.

Abstract

Protoplanetary discs spend their lives in the dense environment of a star forming region. While there, they can be affected by nearby stars through external photoevaporation and dynamic truncations. We present simulations that use the AMUSE framework to couple the Torch model for star cluster formation from a molecular cloud with a model for the evolution of protoplanetary discs under these two environmental processes. We compare simulations with and without extinction of photoevaporation-driving radiation. We find that the majority of discs in our simulations are considerably shielded from photoevaporation-driving radiation for at least 0.5 Myr after the formation of the first massive stars. Radiation shielding increases disc lifetimes by an order of magnitude and can let a disc retain more solid material for planet formation. The reduction in external photoevaporation leaves discs larger and more easily dynamically truncated, although external photoevaporation remains the dominant mass loss process. Finally, we find that the correlation between disc mass and projected distance to the most massive nearby star (often interpreted as a sign of external photoevaporation) can be erased by the presence of less massive stars that dominate their local radiation field. Overall, we find that the presence and dynamics of gas in embedded clusters with massive stars is important for the evolution of protoplanetary discs.

Figures (22)

• Figure 1: Left column: three projections of gas column density when star formation starts (in m1-5r/g), one at the end of m6r, and a slice of the FUV flux around the most massive star at the end of m5r. Middle, right columns: Zooms of the gas column density at the end of runs m1r-m5r and m1g-m5g, respectively. Black points are massive stars (size corresponds to mass), white points are non-massive stars.
• Figure 2: The star formation history of all runs. The lines indicate the cumulative mass of formed stars. Symbols (the run number for the main runs and m6r, $l$ for l3r, and $h$ for h3r) show the mass and formation time of massive stars.
• Figure 3: The evolution of the stellar virial ratio of all runs. Solid lines include the gravitational potential of the gas and stars, dotted lines only that of the stars. The horizontal black lines mark virial ratios of 1 (virial equilibrium) and 2 (gravitational boundedness).
• Figure 4: The fraction of stars in a local stellar density greater than some value, at three moments in time, for the main radiative and geometric runs. Black lines are aggregated over all simulations that have data at that moment.
• Figure 5: The cumulative mass loss through time for all runs, through different mass loss channels. These channels are external photoevaporation (EPE), internal photoevaporation (IPE) plus accretion, and dynamic truncation. In the top two panels, we emphasise runs m1r and m1g as representative examples for clearer viewing.