On the origin of kinematic structure in the young association Serpens OB2

ABSTRACT The Serpens OB2 association ($\ell \sim 18{_{.}^{\circ}} 5$, $b\sim 1{_{.}^{\circ}} 9$, $d=1950\pm 30$ pc) is a large star-forming complex $\sim$65 pc above the Galactic mid-plane, with a clumpy, elongated structure extending $\sim$50 pc parallel to the plane. We analyse probable association members, including OB stars and low-to-intermediate-mass young stellar objects (YSOs) from the Spitzer/IRAC Candidate YSO (SPICY) catalogue. We use $^{13}$CO Milky Way Imaging Scroll Painting Survey (MWISP) data to trace the molecular clouds. The OB stars are concentrated towards the centre of the association, coincident with a gap in the molecular clouds, and towards the side nearest the Galactic plane. The YSOs are distributed throughout the association, but cluster around molecular-cloud clumps. Using Gaia DR3 proper motions to probe the association’s internal kinematics, we find aligned stellar velocities on length scales $\lesssim$2 pc, two-point statistics that show increasing velocity differences and predominantly divergent motions at larger separations, and distinct velocities for star clusters within the association. Finally, the association exhibits gradual but statistically significant global expansion perpendicular to the Galactic plane, with a spatial gradient of $0.10\pm 0.02$ km s$^{-1}$ pc$^{-1}$. The clumpy stellar distribution, correlated velocities on small scales, and increasingly divergent motions on larger scales are consistent with an initial velocity field inherited from a turbulent molecular cloud modified by stellar feedback. The global vertical expansion may arise from large-scale turbulence or feedback-driven shell expansion, with the H ii region Sh 2-54 preferentially pushing the molecular gas away from the Galactic plane. Ser OB2 demonstrates that the multiscale expansion of an OB association can begin even while star formation is still ongoing throughout the complex.