EPoS Contribution

The evolution of velocity dispersion in OB associations - The case of Sco-Cen Josefa Elisabeth Großschedl UzK, Köln, DE
The velocity dispersion (σ) in a young stellar population is a critical parameter, since it is closely tied to the boundness and dispersal of clusters, a process that contributes to the overall structure and evolution of the Galactic field. Velocity dispersion in young stellar populations also offers vital clues to the processes governing star formation, dynamical evolution, and the feedback mechanisms in different star-forming environments. Therefore, understanding the evolution of velocity dispersion in the build-up of a stellar population is crucial for piecing together the larger puzzle of star formation and Galactic evolution. With the development of a new machine-learning-based clustering tool (SigMA) in combination with 5D astrometric data from Gaia DR3, we were recently able to provide a more detailed clustering solution for the nearby OB association Scorpius-Centaurus. We find that the Sco-Cen complex has a larger extent than historically established and that it contains about 34 individual stellar subpopulations with ages from about 2–20 Myr. This updated view now allows a more detailed examination of the formation history of this region. We identify clear spatial-temporal patterns throughout the complex, with older populations being located at the center and younger populations at the outskirts of the region (see Fig. left, and linked paper). By adding auxiliary radial velocity (RV) data, we can study the 3D spatial dynamics of the 34 subpopulations. We find that the whole complex is expanding on the 100-pc scale and that the motions are correlated to the clusters' ages. Moreover, we can now investigate the evolution of velocity dispersion in 3D (σ3D) within a single association; we find that the velocity dispersion monotonically increases with age when successively adding clusters* from old to young for the calculation of σ3D, cumulating in about 4 km/s, while individual clusters only have velocity dispersions on the order of about 1 km/s (see Fig. right). The spatial-temporal patterns and dynamical analysis indicate a feedback-driven formation history, where massive stars (from the older clusters in the center) influenced subsequent star formation, propagating from inside-out. Feedback was likely able to push the surrounding gas, changing the relative motions of the primordial remaining cloud(s), and eventually increasing the average velocity dispersion of the whole association. In conclusion, our research provides a quantifiable assessment of the impact of feedback from massive stars in young complexes like Sco-Cen, where we find relatively “simple” patterns of propagated star formation. (*cluster is used here in a statistical sense; the clusters in Sco-Cen are likely not gravitationally bound)

Caption: Fig. Left: 3D surfaces of 34 stellar populations in the Sco-Cen complex (age-coloured, from about 2–20 Myr, blue to red) as selected with SigMA. A spatial-temporal pattern is visible, with older clusters at the center and young clusters at the outskirts of the complex. In particular, three chains of star formation propagation are visible, partially extending below the Galactic Plane (toward CrA and LCC). Right: Cumulative 3D velocity dispersion, starting from the oldest cluster (e-Lup); at each age-step the stars of the next-younger cluster are added and the velocity dispersion is calculated for all the stars that are in clusters with ages larger than given on the x-axis. The symbols are again color-coded for age (see also left panel), and are scaled by cluster size (of the cluster that was added at that age-step). A monotonic increase in velocity dispersion with age is visible, cumulating in about 4 km/s when all stellar members from the 34 Sco-Cen clusters are combined.
Collaborators: J. Alves, UWien, AT S. Ratzenböck, UWien, AT S. Walch-Gassner, UKöln, DE	Key publication Relevant topic(s): Clustered SF Feedback Triggered SF
Relevant Big Question: What does the evolution of velocity dispersion in a single star-forming complex tell us about its formation history?