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Many stars form in regions of enhanced stellar density, wherein the influenceof stellar neighbours can have a strong influence on a protoplanetary disc(PPD) population. In particular, far ultraviolet (FUV) flux from massive starsdrives thermal winds from the outer edge of PPDs, accelerating discdestruction. In this work, we present a novel technique for constraining thedynamical history of a star forming environment using PPD properties in astrongly FUV irradiated environment. Applying recent models for FUV inducedmass loss rates to the PPD population of Cygnus OB2, we constrain how long agoprimordial gas was expelled from the region; $ 0.5$ Myr ago if the Shakura &Sunyaev $\alpha$-viscosity parameter is $\alpha = 10^{-2}$ (corresponding to aviscous timescale of $\tau_\mathrm{visc} \approx 0.5$ Myr for a disc of scaleradius $40$ au around a $1\, M_\odot$ star). This value of $\alpha$ iseffectively an upper limit, since it assumes efficient extinction of FUVphotons throughout the embedded phase. With this gas expulsion timescale we areable to produce a full dynamical model that fits kinematic and morphologicaldata as well as disc fractions. We suggest Cygnus OB2 was originally composedof distinct massive clumps or filaments, each with a stellar mass $\sim 10^4 \,M_\odot$. Finally we predict that in regions of efficient FUV induced massloss, disc mass $M_\mathrm{disc}$ as a function of stellar host mass$m_\mathrm{star}$ follows a power law with $M_\mathrm{disc} \proptom_\mathrm{star}^\beta$, where $\beta \gtrsim 2.7$ (depending on disc initialconditions and FUV exposure). This is steeper than observed correlations inregions of moderate FUV flux ($1 < \beta <1.9$), and offers a promisingdiagnostic to establish the influence of external photoevaporation in a givenregion.

External photoevaporation of protoplanetary discs in Cygnus OB2: linking discs to star formation dynamical history