Proper motions of OB stars in the far Carina Arm

In large scale maps of the Galactic disc, the Carina Arm stands out as a clear spiral feature, hosting prominent star clusters and associations rich in massive stars. We study the proper motions of 4199 O and early B most likely in the far Carina Arm, at distances mainly in excess of 4 kpc from the Sun, within the sky region, $282^{\circ} 50$ OB stars at $\ell \sim 286^{\circ}$, $b \sim -1^{\circ}.4$ behind the main run of the far arm. An analysis of relative proper motions is performed that yields an incidence of runaway O stars of at least 10\% (potentially $>20$\% when full space motions become available). To map where runaways have run away from, we set up simulations for the region that assume linear trajectories and test for trajectory impact parameter in order to identify likely ejection hot spots. We find the method currently gives good results for times of flight of up to $\sim$4 Myr. It shows convincingly that only NGC 3603 and Westerlund 2 have ejected OB stars in significant numbers. Indeed, both clusters have experienced intense spells of ejection between 0.6--0.9 and 0.5--0.8 Myr ago, respectively.