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

Abstract

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 Shakura & Sunyaev α-viscosity parameter is α = 10 −2 (corresponding to a viscous timescale of τ visc ≈ 0.5 Myr for a disc of scale radius 40 au around a 1 M star). This value of α 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 ∼10 4 M. 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 ∝ m βstar , where β exceeds ∼2.7 – steeper than correlations observed in regions of moderate FUV flux (1 < β < 1.9). This difference offers a promising diagnostic of the influence of external photoevaporation in a given region.