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

## TL;DR

This study links the properties of protoplanetary discs in Cygnus OB2 to its star formation history by modeling FUV-driven disc erosion, revealing the timing of gas expulsion and the impact of external photoevaporation.

## Contribution

It introduces a novel method to constrain star-forming environment history using disc properties and models FUV-induced mass loss to infer gas expulsion timing and initial conditions.

## Key findings

- Gas was expelled approximately 0.5 Myr ago under certain assumptions.
- Disc mass scales steeply with stellar mass, M_disc ∝ m_star^β, with β ≥ 2.7.
- Cygnus OB2 likely formed from massive clumps of about 10^4 solar masses.

## Abstract

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.