The Kinematics of the Young Stellar Population in the W5 Region of the Cassiopeia OB6 Association: Implication for the Formation Process of Stellar Associations

Abstract

The star-forming region W5 is a major part of the Cassiopeia OB6 association. Its internal structure and kinematics may provide hints of the star formation process in this region. Here, we present a kinematic study of young stars in W5 using the Gaia data and our radial velocity data. A total 490 out of 2000 young stars are confirmed as members. Their spatial distribution shows that W5 is highly substructured. We identify a total of eight groups using the k-means clustering algorithm. There are three dense groups in the cavities of H ii bubbles, and the other five sparse groups are distributed at the edges of the bubbles. The three dense groups have almost the same age (5 Myr) and show a pattern of expansion. The scale of their expansion is not large enough to account for the overall structure of W5. The three northern groups are, in fact, 3 Myr younger than the dense groups, which indicates independent star formation events. Only one of these groups shows the signature of feedback-driven star formation as its members move away from the eastern dense group. The other two groups might have formed in a spontaneous way. On the other hand, the properties of two southern groups are not understood as those of a coeval population. Their origins can be explained by dynamical ejection of stars and multiple star formation. Our results suggest that the substructures in W5 formed through multiple star-forming events in a giant molecular cloud.

Dynamics of Star Cluster Formation: Mergers in Gas-rich Environments

“…The dynamics of stars in young clusters are important observables used to probe their formation history. One example is cluster expansion, which has been observed in many young star clusters (e.g., Kuhn et al 2019Kuhn et al , 2022Kounkel et al 2022;Della Croce et al 2023) and associations (e.g., Lim et al 2023). Recent work has found that expansion can be anisotropic around young clusters (e.g., Wright et al 2019Wright et al , 2023 implying that the cluster structure before expansion may not be spherical.…”