Left: the 3D positions and velocities of stars alongside L20's dust map in gray. Molecular cloud spines are shown in green, cyan, and magenta for Orion A, B, and λ, respectively. Stars are colored by their expansion velocity relative to the expansion center of the cluster. The expansion velocities are obtained by subtracting off the median LSR velocity of the cluster, which is (v _x , v _y , v _z ) = (−7.3 km s⁻¹, 2.7 km s⁻¹, 2.5 km s⁻¹) in Galactic Cartesian coordinates. The orange point at the end of the orange line indicates the expansion center. The orange line shows the direction to the Sun. Right: radial profile of velocities for all the stars in OBP-B1. The blue line corresponds to the median velocity at that distance from the expansion center, and the shaded blue region corresponds to the 1σ uncertainty (representing the dispersion in the 100 mock positions and 3D velocities calculated in Section 5.1). The dashed purple line indicates where the turn-off occurs, which matches the distance from the expansion center to the small foreground dust clouds. The interactive version of the left panel can be found here.