External photoevaporation of circumstellar disks constrains the timescale for planet formation

TL;DR

This work demonstrates that external photoevaporation, driven by FUV radiation from nearby massive stars in young clusters, is a dominant mechanism destroying protoplanetary disks on timescales of a few million years. By coupling viscous disk evolution (VADER) with dynamical truncations and an external photoevaporation model (FRIED grid) in an AMUSE framework, the authors show that in dense environments ($\rho\sim100\,M_\odot\,\mathrm{pc}^{-3}$) about $\sim80\%$ of disks are dispersed within $<2\mathrm{Myr}$, while in less dense regions ($\rho\sim50$) the fraction is lower but still substantial. They find mass loss is dominated by external photoevaporation rather than dynamical encounters, with mean FUV fields around $\sim475$ G0 for dense clusters and $\sim56$ G0 for less dense ones; disks with sufficient solid mass ($M_{dust}>10\,M_{\oplus}$) to form planet cores are rare by a few Myr, implying planet formation must initiate very early, typically before $\sim0.1-1$ Myr. The results highlight the critical role of initial cluster conditions and imply that many planetary systems in dense environments may form under severe external constraints, shaping the diversity of exoplanet populations observed today.

Abstract

Planet-forming circumstellar disks are a fundamental part of the star formation process. Since stars form in a hierarchical fashion in groups of up to hundreds or thousands, the UV radiation environment that these disks are exposed to can vary in strength by at least six orders of magnitude. This radiation can limit the masses and sizes of the disks. Diversity in star forming environments can have long lasting effects in disk evolution and in the resulting planetary populations. We perform simulations to explore the evolution of circumstellar disks in young star clusters. We include viscous evolution, as well as the impact of dynamical encounters and external photoevaporation. We find that photoevaporation is an important process in destroying circumstellar disks: in regions of stellar density $ρ\sim 100 \mathrm{\ M}_\odot \mathrm{\ pc}^{-3}\mathrm{\ }$ around 80% of disks are destroyed before 2 Myr of cluster evolution. Our findings are in agreement with observed disk fractions in young star forming regions and support previous estimations that planet formation must start in timescales < 0.1 - 1 Myr.

Figures (14)

• Figure 1: Definition of disk radius. The black lines show the disk profile and the corresponding red lines show the measured disk radius. Solid lines represent a disk initialized with $R = 100au$. The dashed lines show the same disk after $0.1Myr$ of isolated evolution, and the dotted lines after $1.0Myr$ of isolated evolution.
• Figure 2: FUV luminosity vs. stellar mass fit calculated from the ZAMS spectra. $\mathrm{M}_* = 1.9 \mathrm{\ M}_\odot$ is the lower mass limit of the FRIED grid, and the lower mass limit for the stars to be considered emitting FUV radiation in our simulations (see Section \ref{['grid']} for details).
• Figure 3: Operations performed in each macroscopic time step. Within each macroscopic time step $\mathrm{t}_{\mathrm{n}}$, internal evolutionary processes such as stellar evolution and gravitational dynamics proceed in their own internal time steps.
• Figure 4: Flowchart of the photoevaporation process.
• Figure 5: Mean mass loss in time due to external photoevaporation (blue) and dynamical truncations (red). The solid and dashed lines correspond to the $\rho_{100} \mathrm{\ }$ and $\rho_{50} \mathrm{\ }$ regions, respectively. The shaded areas indicate the standard deviation of the simulations.