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

ABSTRACT 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 magnetogydrodynamic (MHD) wind-driven discs, finding that they best match observations when the angular momentum transport $\alpha$ parameter is $3\times 10^{-4}\le \alpha _{\nu }\le 2\times 10^{-3}$ for viscous discs, and $2\times 10^{-3}\le \alpha _{\rm DW}\le 10^{-2}$ 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 ultraviolet environment.