External photoevaporation of circumstellar discs constrains the time-scale for planet formation

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 ∼ 100 M pc −3 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.Key words: key 1 key 2 1 G 0 is the FUV field measured in units of the Habing flux, 1.6 × 10 −3 erg s −1 cm −2 (Habing 1968).

The observational anatomy of externally photoevaporating planet-forming discs – I. Atomic carbon

“…Model A is a 10 M jup , 100AU disc and model B a 20 M jup , 200AU disc. The disc size can evolve quickly when external photoevaporation operates (Haworth et al 2017(Haworth et al , 2018aWinter et al 2018;Concha-Ramírez et al 2019;Winter et al 2020), and certainly would for these models which have mass loss rates ∼ 10 −6 M yr −1 , but the flow morphology and observable characteristics are similar regardless, so long as the disc is still large enough to drive an externally driven photoevaporative wind (Haworth & Clarke 2019). An illustrative example of the density, temperature and velocity for model A is given in Figure 1.…”

Time-domain Study of the Young Massive Cluster Westerlund 2 with the Hubble Space Telescope. I

“…Over the years several authors have investigated how dynamical truncation (Vinke et al 2015;Portegies Zwart 2016;Vincke & Pfalzner 2016), stellar winds (Pelupessy & Portegies Zwart 2012), dynamical interaction (Olczak et al 2008;Reche 2009;de Juan Ovelar et al 2012) and photoevaporation from OB stars can affect the evolution of circumstellar disks. Recently, in simulating the effects of FUV radiation on circumstellar disks in clusters of different stellar densities, Concha-Ramiŕez et al (2019) estimated that in regions of high stellar density about 80% of the disks can be destroyed by external photoevaporation in less then 2 Myr while, in comparison, mass loss caused by dynamical encounters is negligible.…”

The birth environment of planetary systems

“…Clearly, in very dense environments, we would not expect large (100 AU) discs to retain gas beyond an age of 1 Myr if there were massive stars present [ 127 , 131 ]. This has two very interesting implications for giant planet formation: Gas giant planets must form extremely quickly (within 1 Myr), and within 10 AU of the host star where the effects of external photoevaporation will be limited.…”

“…There is, therefore, a significant tension here; on the one hand, enrichment from massive stars is required to apparently drive the internal evolution of Earth [ 226 ], but photoionizing radiation is likely to significantly disrupt the formation of Jupiter and Saturn. Resolving this tension remains an active topic of research in the field [ 2 , 127 , 131 , 227 ], with the solution being that perhaps the Sun’s disc was truncated to a radius that allowed the formation of Jupiter and Saturn, but severely depleting the outer regions of the disc.…”