Theory predicts that we should find fast , ejected ( runaway ) stars of all masses around dense , young star-forming regions . N -body simulations show that the number and distribution of these ejected stars could be used to constrain the initial spatial and kinematic substructure of the regions .
We search for runaway and slower walkaway stars within 100 pc of the Orion Nebula Cluster ( ONC ) using Gaia DR2 astrometry and photometry .
We compare our findings to predictions for the number and velocity distributions of runaway stars from simulations that we run for 4 Myr with initial conditions tailored to the ONC .
In Gaia DR2 , we find 31 runaway and 54 walkaway candidates based on proper motion , but not all of these are viable candidates in three dimensions .
About 40 per cent are missing radial velocities , but we can trace back 9 3D-runaways and 24 3D-walkaways to the ONC , all of which are low/intermediate-mass ( < 8 M _ { \sun } ) .
Our simulations show that the number of runaways within 100 pc decreases the older a region is ( as they quickly travel beyond this boundary ) , whereas the number of walkaways increases up to 3 Myr .
We find fewer walkaways in Gaia DR2 than the maximum suggested from our simulations , which may be due to observational incompleteness .
However , the number of Gaia DR2 runaways agrees with the number from our simulations during an age of \sim 1.3–2.4 Myr , allowing us to confirm existing age estimates for the ONC ( and potentially other star-forming regions ) using runaway stars .