The external photo-evaporation of planet-forming discs

The external photo-evaporation of planet-forming discs

Planets form within flattened discs of material around young stars. These young stars themselves form in clustered groups of up to hundreds of thousands of stars. Most planet-forming discs therefore find themselves in a situation where they are being shone upon by ultraviolet (UV) radiation from these nearby young stars. This has the effect of stripping material from the disc in what is called a photoevaporative wind. This could play an important role in influencing how the discs evolve and the planets that they form.

Figure 1: An example disc external photo-evaporation model projected. The colour scale is the density and the arrows represent the flow structure of the material

There are two serious issues that limit our understanding of how external photoevaporation of discs. The first issue is that disc evaporation is very hard to model. To get the temperature correct, which determines the structure of the wind and the rate that mass is lost, one has to solve for the chemistry of the gas and how that is affected by the incoming UV radiation. Furthermore, each cell tries to cool by having “line photons” escape and carry away energy. Each region of the calculation therefore depends on every other region, since if the surroundings are very dense these photons won’t escape and vice versa. We have therefore only had 1D models of the process in the past, except in the strongest UV environments where thee modelling is easier.

T. J. Haworth, as part of the DiRAC project dp100, has developed the TORUS-3DPDR code (Bisbas et al. 2015, Harries et al. 2019), to tackle this problem in 2D models for the first time (Haworth & Clarke 2019). This has vastly improved our understanding of how discs are evaporated, including the rate at which they lose mass and where the mass is actually lost from the disc.

The other key challenge to understanding of how external photoevaporation is observing it in action. 1D models are insufficient for this purpose, but with these new 2D models, predictions can be made as to what should be observed (Haworth & Owen 2020)






  • Bisbas et al. (2015), MNRAS, 454, 2828
  • Harries et al. (2019), A&C, 27, 63
  • Haworth & Clarke (2019), MNRAS, 485, 3895
  • Haworth & Owen (2020), MNRAS, 492, 5030