Runaway origins of a disc mass gradient in $\sigma$ Orionis

Radiation from massive stars is known to significantly affect the evolution of protoplanetary discs around surrounding stars by driving external photoevaporative winds. Typically most studies assume that the massive stars driving these winds are comoving with their associated clusters. However, it is also known that massive stars can be runaways, after being violently ejected from their birth environment through interactions with other massive stars. In this letter, we show that the well studied system $\sigma~{\rm Ori~AB}$ is actually a runaway system, only now passing through $\sigma~{\rm Orionis}$. There are multiple observable features that indicate this is the case, including significantly larger proper motions for $\sigma~{\rm Orionis}$ than the surrounding stars, an infrared arc of ionising gas along the predicted velocity vector, and a disparity in protoplanetary disc masses across $\sigma~{\rm Orionis}$. We finally use protoplanetary disc evolution models to explain the observed disparity in disc masses, showing that those discs downstream of $\sigma~{\rm Ori~AB}$, i.e. those yet to encounter it, have larger masses than those upstream, consistent with observations. Overall, our work highlights the importance of understanding the dynamical history of star forming regions, since the time varying UV fields provided by runway stars results in a complex history for the evolution of the protoplanetary discs.