Evidence of Stellar Expansion at the Core of the Orion Complex

We present a 3D-dynamics study of two groups of stars located at the core of the Orion complex. We used the results of the unsupervised clustering algorithm, Shared Nearest Neighbor (SNN), that revealed 21 stellar groups using Gaia DR2 positions (ra, dec, parallax) and proper motions. We focus on two of the most stable and massive SNN-recovered groups, namely Briceño-1 and OBP-near, a newly discovered group. For each group, the 3D-velocity vectors of individual stars are obtained by Gaia DR2 proper motions and radial velocities and, where present, radial velocities are adopted by APOGEE-2 data. Our study of the 3D-dynamics of this region shows evidence of expansion of the stars from a central region consistent with their stellar age. Unlike previous work, our results suggest that expansion within the complex is confined to these two groups while the rest of the complex shows a more complex dynamical history. Finally, recent results of 3D dust mapping indicate that both stellar groups lie in the center of a dust shell.