A new Gaia census of OB associations within 1 kpc

TL;DR

This study leverages Gaia DR3 to construct a complete census of OB associations within 1 kpc, identifying 57 highly coherent groups (2232 members) with 5D HDBSCAN clustering and rigorous contamination/incompleteness testing. It provides physical and kinematic characterizations, including a median of ~83 B-type stars per association and a typical total initial mass near 2000 M☉, along with substantial expansion signatures in ~75% of the sample. The authors validate the catalog through extensive crossmatching with historical OB catalogs, star clusters, and young stellar groups, and relate the associations to local Galactic structures such as the Radcliffe Wave and multiple superclouds. The results effectively double the known OB association census in the solar neighborhood and establish a framework for future work with WEAVE/4MOST RVs and lower-mass members to derive more precise isochronal/kinematic ages and 3D orbital histories.

Abstract

OB associations are primordial tracers of star formation and Galactic structure. Originally defined about 80 years ago, their historical membership lists have been superseded thanks to the precise astrometry from ESA's \textit{Gaia}'s satellite. Recent studies have however been mostly focused on individual OB associations or limited by the coverage of spectroscopic surveys. In this paper, we exploit a complete census of $\sim$25,000 O- and B-type stars within 1 kpc of the Sun to produce a highly-reliable catalogue of 57 OB associations using the HDBSCAN clustering algorithm, increasing the number of known OB associations by a factor of two within this volume. We assess the validity of this catalogue by crossmatching our OB association members with other catalogues of OB associations, star clusters and young stellar groups, confirming the high-confidence of our census of OB associations. We characterize these OB associations physically (total initial stellar mass, number of OB stars, ...) and kinematically (velocity dispersion, linear expansion ages, ...). Nearly three quarters of the OB associations (41 out of 57) exhibit a significant expansion pattern in at least one direction, including 13 in both plane-of-the-sky directions, though differences in expansion velocity suggest anisotropical expansion patterns. We compare the locations of these OB associations with superclouds and features in the local Milky Way such as the Radcliffe Wave and discuss the implications for star formation in the solar neighbourhood.

Figures (6)

• Figure 1: Galactic Cartesian coordinates of the 57 new OB associations, centered on the Sun's position. On the top panel have been labeled each OB association, displayed on top of the background extinction map from Edenhofer2024. The colours used for each association are the same between panels.
• Figure 2: Transverse velocity distribution of the 57 new OB associations within 1 kpc. The colours used for each association are the same as in Fig. \ref{['Map_OBAssociations']}.
• Figure 3: Fitted linear gradient (and the corresponding kinematic age) between the Galactic longitude and the proper motions in Galactic longitude (corrected for the LSR) for the Ori OB1b members. 100 random samples have been drawn from the posterior distribution of the linear regression and have been displayed in light orange to illustrate the uncertainty on the fitted gradient.
• Figure 4: Same as the top panel from Fig. \ref{['Map_OBAssociations']} but with the superclouds from Kormann2025, which includes the Radcliffe Wave (in red, Alves2020 and the Split (in blue, Lallement2019). The other newly identified superclouds on this figure are the Malpolon Cloud (in orange), Natrix Cloud (in cyan), Vela Ridge Cloud (in green) and SSE (in pink).
• Figure 5: 2D projection on the Z/Y' plane of the four other superclouds from Kormann2025 within 1 kpc, rotated clockwise, with the OB associations displayed on top and rotated with the same angle. The location of the Sun is indicated with the corresponding symbol, using $Z_{\odot} = 20.8 \pm 0.3$ pc from BennettBovy2019.