Using simultaneous mass accretion and external photoevaporation rates for d203-504 to constrain disc evolution processes

We cannot understand planet formation without understanding disc evolutionary processes. However, there is currently ambiguity about how protoplanetary discs transport angular momentum (e.g. via viscosity or winds) and the relative contributions and interplay of different dispersal mechanisms. A key difficulty is that for any given system only a handful of disc parameters are usually available to constrain theoretical models. Recent observations of the d203-504 disc in Orion, have yielded values of the stellar accretion rate, external photoevaporative mass loss rate, stellar mass and the disc size and mass. In particular, having the combination of accretion rate and external photoevaporative rate is new. Using this unique combination of observables, we run a suite of disc evolution simulations to constrain which scenarios can match the observed values. We explore both viscous and MHD wind-driven discs, finding that they best match observations when the angular momentum transport $α$ parameter is $3\times10^{-4}\leqα_ν\leq2\times10^{-3}$ for viscous discs, and $2\times10^{-3}\leqα_{\rm DW}\leq10^{-2}$ for MHD wind-driven discs, consistent with other estimates in the literature. As well constraining the disc properties and evolution, the d203-504 disc allows us to define a new irradiation age, since in order to match observations, it was required that the disc had only just appeared in the extreme UV environment it is currently exposed to (a known issue for proplyds referred to as the proplyd lifetime problem). This indicates that it is either very young, i.e. <0.1 Myr, or it has been shielded until recently, which would have protected the planet forming reservoir and helped facilitate planet growth despite it now residing in a harsh UV environment.

💡 Research Summary

This paper presents a comprehensive observational and theoretical study of the protoplanetary disc d203‑504 in the Orion Nebula, exploiting the unique availability of simultaneous measurements of the stellar accretion rate (≈ 8 × 10⁻⁹ M⊙ yr⁻¹) and the external photoevaporative mass‑loss rate (≈ 3 × 10⁻⁷ M⊙ yr⁻¹). Together with estimates of the stellar mass (0.72 M⊙), disc radius (31 au), and dust mass (4–18 M⊕), d203‑504 provides five key constraints that are rarely available for a single system. The authors employ a 1‑D viscous disc evolution code that can operate in either a pure α‑viscosity mode (Shakura & Sunyaev 1973) or a magneto‑hydrodynamic (MHD) wind‑driven mode (Tabone et al. 2022). In both cases the surface density evolution follows a diffusion equation that includes (i) viscous angular‑momentum transport parameterised by α_ν, (ii) angular‑momentum extraction by a magnetocentrifugal wind characterised by α_DW and a lever arm λ = 3, (iii) internal X‑ray driven photoevaporation (Picogna et al. 2021) and (iv) external far‑UV (FUV) driven photoevaporation (Haworth et al. 2023b). The total photoevaporative loss at each radius is taken as the maximum of the internal and external contributions, allowing both processes to act simultaneously.

A broad grid of models is explored: initial disc masses from 0.02 to 1 M⊙, characteristic radii of 20–50 au, and α parameters spanning α_ν = 10⁻⁴–10⁻² and α_DW = 10⁻³–10⁻¹. Each model is evolved for up to 1.5 Myr, and at each timestep the simulated accretion rate, external photoevaporation rate, disc radius and dust mass are compared to the observed values. The key findings are:

1. Viscous‑only models match the observations only when the turbulent α lies in the narrow range 3 × 10⁻⁴ ≤ α_ν ≤ 2 × 10⁻³. This range is consistent with previous estimates from disc population studies and suggests that modest turbulence can sustain the measured accretion while allowing external photoevaporation to dominate the mass loss.

2. MHD‑wind‑only models reproduce the data for wind‑driven angular‑momentum extraction efficiencies 2 × 10⁻³ ≤ α_DW ≤ 10⁻². In this regime the wind supplies the bulk of the accretion torque, and the disc’s surface density profile evolves in a way that still yields the observed external photoevaporation rate.

3. The two mechanisms are not mutually exclusive; hybrid models with both modest viscosity and a weak wind can also satisfy the constraints, but the parameter space collapses onto the same α ranges identified above.

4. By tracking when the simulated disc first attains the observed radius and mass under the measured UV field (FUV ≈ 8 × 10⁴ G₀), the authors define an “irradiation age”. The models require that the disc has been exposed to the current extreme‑UV environment for less than ~0.1 Myr. This short exposure time resolves the classic “proplyd lifetime problem”, which predicts disc lifetimes of only a few × 10⁴ yr under constant strong UV illumination.

Two plausible evolutionary scenarios emerge: (i) d203‑504 is an extremely young disc that formed and was immediately bathed in the strong UV field, or (ii) the disc spent most of its early life shielded within the molecular cloud and only recently (within the last 0.1 Myr) emerged into the harsh radiation field. Both scenarios imply that a substantial reservoir of solid material can survive long enough for planet formation, even in environments traditionally considered hostile.

The paper’s broader implications are significant. First, it demonstrates that simultaneous constraints on both mass inflow (accretion) and outflow (photoevaporation) can tightly bound the angular‑momentum transport efficiency, a parameter that is otherwise difficult to measure. Second, it shows that external photoevaporation does not necessarily preclude planet formation; instead, its impact depends sensitively on the disc’s transport physics and exposure history. Finally, the methodology provides a template for future studies of other proplyds where multi‑wavelength observations can deliver comparable sets of observables, opening a path toward a statistically robust understanding of disc evolution across diverse star‑forming environments.