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The evolution of velocity dispersion in OB associations - The case of Scorpius-Centaurus

Figure 1. Left: 3D surfaces of 34 stellar populations in the Sco-Cen stellar 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). See also the link to the INTERACTIVE version. 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.

Josefa Großschedl

The velocity dispersion (σ) in a young stellar population is a critical parameter, since it is closely tied to the boundedness 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 association 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, Ratzenböck et al. 2023a) 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 (distance ~ 100-150 pc). 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 (Ratzenböck et al. 2023b, see Figure 1). By adding auxiliary radial velocity (RV) data, we also get information on 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 (including all stellar members from all 34 clusters), while individual clusters only have velocity dispersions on the order of about 1 km/s.

The spatial-temporal patterns and dynamical analysis indicate a feedback-driven formation history of Sco-Cen, 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 to the total velocity dispersion of the whole association, as observed today. 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.

(*The term cluster is used here in a statistical sense; the clusters in Sco-Cen are likely not gravitationally bound)