Many stars form in regions of enhanced stellar density , where stellar neighbours can have a strong influence on a protoplanetary disc ( PPD ) population .
In particular , far ultraviolet ( FUV ) flux from massive stars drives thermal winds from the outer edge of PPDs , accelerating disc destruction .
Here , we present a novel technique for constraining the dynamical history of a star forming environment using PPD properties in a strongly FUV irradiated environment .
Applying recent models for FUV induced mass loss rates to the PPD population of Cygnus OB2 , we constrain the time since primordial gas expulsion .
This is 0.5 Myr ago if the \citeauthor Sha73 \alpha -viscosity parameter is \alpha = 10 ^ { -2 } ( corresponding to a viscous timescale of \tau _ { \mathrm { visc } } \approx 0.5 Myr for a disc of scale radius 40 au around a 1 M _ { \odot } star ) .
This value of \alpha is effectively an upper limit , since it assumes efficient extinction of FUV photons throughout the embedded phase .
This gas expulsion timescale is consistent with a full dynamical model that fits kinematic and morphological data as well as disc fractions .
We suggest Cygnus OB2 was originally composed of 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 mass loss , disc mass M _ { \mathrm { disc } } as a function of stellar host mass m _ { \mathrm { star } } follows a power law with M _ { \mathrm { disc } } \propto m _ { \mathrm { star } } ^ { \beta } , where \beta exceeds \sim 2.7 – steeper than correlations observed in regions of moderate FUV flux ( 1 < \beta < 1.9 ) .
This difference offers a promising diagnostic of the influence of external photoevaporation in a given region .