The velocity field of the Scorpius-Centaurus OB association

TL;DR

The paper presents a non-parametric reconstruction of the Scorpius-Centaurus OB association's 3D velocity field using Information Field Theory applied to Gaia DR3 data on a 70-by-70-by-50 grid with 3 pc resolution. It introduces a two-field velocity model (main and delta) and a robust forward model linking fields to stellar observables, solved via geometric variational inference to handle a high-dimensional posterior. The results reveal coherent inside-out expansion with typical speeds of 1–2 km s^-1 and local maxima near 10 km s^-1, with power spectra indicating excess small-scale structure from stellar feedback and a predominantly expanding divergence pattern; Galactic rotation plays a subordinate role. This framework enables quantitative descriptions of the dynamical state and feedback history of Sco-Cen and provides a basis for applying similar analyses to other nearby OB associations and ISM-kinematic studies.

Abstract

We present a non-parametric reconstruction of the three-dimensional velocity field of the Scorpius-Centaurus OB association (Sco- Cen). Using Gaia DR3 astrometry and radial velocities, we infer the velocity field using information field theory on a 70 x 70 x 50 grid at 3 pc resolution. Our model suggests the existence of a primary stellar velocity field with a secondary field that accounts for an additional young kinematic component in Upper Scorpius and Lupus. We find clear tracers of a feedback-driven expansion of the association, while Galactic rotation appears to play a subordinate role. The results confirm the existence of cluster chains and reveal coherent large-scale expansion with characteristic speeds of 1-2 km s$^{-1}$ and local maxima of about 10 km s$^{-1}$. Power spectra indicate an excess of small-scale structure and slopes shallower than Kolmogorov, consistent with energy injection from stellar feedback. Maps of the divergence reveal net positive values, implying an approximate dispersal timescale of 10-15 Myr. A comparison with molecular gas in Lupus and Ophiuchus shows broadly consistent patterns but systematic velocity offsets of several km s$^{-1}$, suggesting partial decoupling for optically visible young stars and gas. The framework presented provides a physically motivated description of the Sco-Cen velocity field and a basis for quantifying the dynamical state and feedback history of OB associations in the local Galaxy.

Figures (16)

• Figure 1: Illustration of the different categories within our stellar selection.
• Figure 2: Orientation plot. This image shows the location and ages of the Sco-Cen clusters 2023Ratzenboeck1, 2023Ratzenboeck2, 2025MiretRoig. The clusters that contain the stars that we refer to as the main sub-selection are shown as spheres, while the secondary clusters are shown as diamonds. We shade the volume where the secondary selection was performed in gray. The black square indicates the geometric center of Sco-Cen 2025Grossschedl. Additionally, we show three cluster chains as lines. A 3D interactive version of this plot is available https://shutsch.github.io/sco_cen_data.html. We note that not all features mentioned are visible in this projection; we refer the reader to the online version for best visibility. Furthermore, individual clusters are identifiable in the online version.
• Figure 3: Posterior mean of the flow fields in the velocity reference frame of 2025Grossschedl. For each plot, we have defined a mask using the density of stars to highlight only parts of the volume that were directly informed by data as described in Sect. \ref{['subsec:vel_field']}. Fig. \ref{['fig:flow_field_main']} depicts the main field, while Figs. \ref{['fig:flow_field_us']} and \ref{['fig:flow_field_add']} depict the secondary and $\delta$ field. These projections of the vector fields can naturally only provide a first impression and might locally be misleading; for a better depiction, we refer to the 3D interactive version of these plots available https://shutsch.github.io/sco_cen_data.html.
• Figure 4: The power spectra of the main and $\delta$ velocity field components, which are part of the forward model. The black line indicates the Kolmogorov slope $-11/3$, while the posterior samples are indicated as thin colored lines. We have scaled the line indicating the Kolmogorov spectrum to the average of the three component spectra at $k=0.005\,\mathrm{pc}^{-1}$ in each case, for illustrational purposes.
• Figure 5: Posterior mean of the divergence of the main flow field. A 3D interactive version of this plot is available https://shutsch.github.io/sco_cen_data.html.