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

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 0.5Myr ago if the Shakura & Sunyaev alpha-viscosity parameter is alpha = 10(-2) (corresponding to a viscous time-scale of tau(visc) approximate to 0.5Myr for a disc of scale radius 40 au around a 1M(circle dot) star). This value of a is effectively an upper limit, since it assumes efficient extinction of FUV photons throughout the embedded phase. This gas expulsion time-scale 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 similar to 10(4)M(circle dot). Finally we predict that in regions of efficient FUV-induced mass-loss, disc mass M-disc as a function of stellar host mass m(star) follows a power law with M-disc proportional to m(star)(beta), where beta exceeds similar to 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.