External photoevaporation of protoplanetary discs in Cygnus OB2: linking
  discs to star formation dynamical history
  Many stars form in regions of enhanced stellar density, wherein the influence
of 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. In this work, 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 how long ago
primordial gas was expelled from the region; $ 0.5$ Myr ago if the Shakura &
Sunyaev $\alpha$-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. With this gas expulsion timescale we are
able to produce 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 $\beta \gtrsim 2.7$ (depending on disc initial
conditions and FUV exposure). This is steeper than observed correlations in
regions of moderate FUV flux ($1 < \beta <1.9$), and offers a promising
diagnostic to establish the influence of external photoevaporation in a given
region.
