Evidence for Radial Expansion at the Core of the Orion Complex with Gaia
  EDR3
  We present a phase-space study of two stellar groups located at the core of
the Orion complex: Brice\~no-1 and Orion Belt Population-near (OBP-near). We
identify the groups with the unsupervised clustering algorithm, Shared Nearest
Neighbor (SNN), which previously identified twelve new stellar substructures in
the Orion complex. For each of the two groups, we derive the 3D space motions
of individual stars using Gaia EDR3 proper motions supplemented by radial
velocities from Gaia DR2, APOGEE-2, and GALAH DR3. We present evidence for
radial expansion of the two groups from a common center. Unlike previous work,
our study suggests that evidence of stellar group expansion is confined only to
OBP-near and Brice\~no-1 whereas the rest of the groups in the complex show
more complicated motions. Interestingly, the stars in the two groups lie at the
center of a dust shell, as revealed via an extant 3D dust map. The exact
mechanism that produces such coherent motions remains unclear, while the
observed radial expansion and dust shell suggest that massive stellar feedback
could have influenced the star formation history of these groups.
