OB-runaway peculiar tangential velocity threshold. Left panel: tangential velocity of the members (blue) of the young cluster RCW38 with respect to the LSR velocity: the peculiar tangential velocity $v_{\text{LSR}}^t$. The red histogram shows $v_{\text{LSR}}^t$ for all OB runaways identified in this region. Right panel: cumulative distribution of $v_{\text{LSR}}^t$ for the members of RCW38. We note that 98% of the members have a peculiar tangential velocity less than 15 km s⁻¹. We take this value as the threshold for the tangential peculiar velocity to identify the star as a runaway.