Inferring the physics of protoplanetary disc evolution from the irradiated Cygnus OB2 region

A comparison of viscous and MHD wind-driven scenarios

¹ Division of Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
² Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³ Max-Planck Institute for Astronomy (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁴ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06300 Nice, France
⁵ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 July 2025
Accepted: 29 October 2025

Abstract

Context. Determining the physical processes driving protoplanetary disc evolution is of paramount importance for understanding planet formation. Our current understanding has crystallised around two possible evolution scenarios: turbulent viscosity and magnetohydrodynamic (MHD) wind-driven. Which of these processes dominates, however, remains unclear.

Aims. Our aims are twofold. Firstly, we investigate whether a single set of model parameters can reproduce the observational constraints of non-irradiated and irradiated discs. Secondly, we propose a novel approach to break degeneracies between these two scenarios by studying the relation of stellar accretion rate and externally driven wind mass-loss rates, which evolve differently depending on the mechanism of angular momentum transport in the outer disc, and we test this approach using our models.

Methods. We simulated the evolution of synthetic populations of protoplanetary discs using 1D vertically integrated models for both viscous and MHD wind-driven disc evolution including both internal X-ray and external far ultraviolet (FUV) photoevaporation for both evolution scenarios. We investigated both weak and strong FUV field environments, where the strong FUV field is calculated based on an environment similar to the Cygnus OB2 association. We studied the time evolution of the disc fraction, disc mass–stellar accretion rate relation, the spatial variation of the disc fraction in a highly irradiated cluster, the evolution of disc radii, and the evolution of accretion rates versus wind mass-loss rates.

Results. While both evolution scenarios are capable of reproducing observational constraints, our simulations suggest that different parameters are needed for the angular momentum transport to explain disc lifetimes and the disc mass–stellar accretion rate relation in weakly and strongly irradiated regions. We find that the predicted median disc radii are much larger in low FUV environments compared to Cygnus OB2 but also decrease with time. In the viscous scenario, the median disc radius in a low FUV field environment is ∼100 au larger than for the MHD wind-driven scenario. We further demonstrate that studying stellar accretion rates and externally driven wind mass-loss rates (provided that they can be isolated from internally driven winds, i.e. MHD wind) is indeed a promising way of disentangling the two evolution scenarios.

Conclusions. The fact that a single set of parameters for angular momentum transport is not able to reproduce disc lifetimes in both low and highly irradiated regions at the same time indicates a fundamental difference in the two regions.