Inferring the physics of protoplanetary disc evolution from the irradiated Cygnus OB2 region -- A comparison of viscous and MHD wind-driven scenarios

TL;DR

This work compares viscous and MHD wind-driven protoplanetary disc evolution under internal X-ray and external FUV photoevaporation, using 1D population syntheses in weak and Cygnus OB2-like irradiation fields. It shows that both scenarios can reproduce broad observational constraints, but cannot be described by a single set of angular-momentum-transport parameters across environments, implying fundamental regional differences. A key contribution is a proposed diagnostic based on the relation between stellar accretion rate and externally driven wind mass-loss, which, if external winds can be isolated observationally, could decisively distinguish the dominant angular-momentum transport mechanism. The findings highlight that disc radii and lifetimes, particularly in strongly irradiated regions, are strongly shaped by external photoevaporation and that combining disc fractions, Mdust–Mdot_acc,star relations, and wind diagnostics has the potential to break model degeneracies in planet-forming environments.

Abstract

Our current understanding has crystallised around two possible evolution scenarios for protoplanetary discs (turbulent viscosity and magnetohydrodynamic (MHD) wind-driven) - but which dominates remains uncertain. 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 the two evolution 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. We evolve 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 investigate both weak and strong FUV field environments, where the strong FUV field is calculated based on an environment similar to the Cygnus OB2 association. While both scenarios are able to reproduce observational constraints, our simulations suggest that different parameters are needed for the angular momentum transport to explain disc lifetimes and 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 decreasing with time. In the viscous scenario, the median disc radius in a low FUV field environment is ~100au larger than for the MHD wind-driven scenario. We further show 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.

Figures (10)

• Figure 1: Conceptual illustration of disc evolution scenarios and expected accretion-outflow correlation. Top panels: Conceptual representations of the two evolution scenarios. The protoplanetary disc is shown with arrows indicating the direction of the accretion flow. Outflows emerging from the disc are coloured by different processes i.e. internal photoevaporation (green), external photoevaporation (violet) and MHD wind (blue). Note that in the MHD wind-driven scenario, internal photoevaporation is shielded by the emerging MHD wind in an early phase and only acts in a later phase when the MHD wind has decreased. Bottom panels: Expected correlations of the stellar accretion rate versus the outflow rates from all processes (red) or external PEW only (blue), based on theoretical considerations.
• Figure 2: View of FUV field in the synthetic cluster after the formation phase ($t=\mathrm{2\,Myr}$). Stars are coloured by the projected FUV field $|\mathcal{F}_\mathrm{FUV,CygnOB2}|_\mathrm{2D}$. Stars contributing to the local FUV field are highlighted by black ($\mathrm{2\,M_\odot}<M_\star<\mathrm{16\,M_\odot}$) or red ($16\,\mathrm{M_\odot}<M_\star$) markers. The size of the markers scale with stellar mass.
• Figure 3: Considered initial accretion timescale distributions $\tau_\mathrm{acc,0,short}$ and $\tau_\mathrm{acc,0,long}$.
• Figure 4: Time evolution of the disc fraction for different evolution scenarios, FUV field strengths $\mathcal{F}_\mathrm{FUV,10G_0}$ and $\mathcal{F}_\mathrm{FUV,CygnOB2}$, varying $\tau_\mathrm{acc}$ and $f_\mathrm{PAH}$. Note that in the strongly irradiated cases, a duration of star formation of 2 Myr is accounted for while the low irradiated cases stars are set at $t=0$. Panels a) and b) show results from simulations with $\mathcal{F}_\mathrm{FUV,10G_0}$ for $\tau_\mathrm{acc,0,short}$ and $\tau_\mathrm{acc,0,long}$, alongside with a selected sample of observed disc fractions for low mass star forming regions. Panels c) and d) show the results of the strongly irradiated cases with $\mathcal{F}_\mathrm{FUV,CygnOB2}$ for $\tau_\mathrm{acc,0,short}$ and $\tau_\mathrm{acc,0,long}$, alongside with observed disc fraction in Cygnus OB2 and two additional high mass star forming regions. See Appendix \ref{['app:observed_disc_fractions']} for a Table \ref{['tab:disc_fractions']} and discussion of the observational constraints.
• Figure 5: Stellar accretion rate $\dot{M}_\mathrm{acc,\star}$ versus disc masses $M_\mathrm{disc}$ for simulations with $f_\mathrm{PAH}=0.1$ and $\tau_\mathrm{acc,0,short}$ (left panel) and $\tau_\mathrm{acc,0,long}$ (right panel). A subset of $\mathrm{1000}$ simulations is shown at $\mathrm{3\,Myr}$. Lines of constant $M_\mathrm{disc}/\dot{M}_\mathrm{acc,\star}$ (i.e. $\tau_\mathrm{acc}$) are shown for 0.1, 1 and 10 Myr (dotted lines). The coloured areas show the central 60% of the $\tau_\mathrm{acc}$ distribution for close-in low mass star forming regions Lupus and Chamaeleon I, obtained from Manara2022. The large difference between our low FUV simulations and observations from Chamaeleon I and Lupus point towards strongly variable, time-dependent and heterogeneous angular momentum transport between different regions.