OB associations and their origins

TL;DR

OB associations are more complex and structured than previously thought, with new data revealing their detailed stellar content and challenging old ideas about their origins and evolution.

Contribution

This paper reviews recent observational advances that significantly revise the understanding of OB associations' structure, dynamics, and formation history.

Findings

OB associations have more substructure than previously believed.

New data from Gaia and other surveys have expanded knowledge of their stellar populations.

Theories of their origins are being fundamentally revised.

Abstract

OB associations are unbound groups of young stars made prominent by their bright OB members, and have long been thought to be the expanded remnants of dense star clusters. They have been important in astrophysics for over a century thanks to their luminous massive stars, though their low-mass members have not been well studied until the last couple of decades. This has changed thanks to data from X-ray observations, spectroscopic surveys and astrometry from Gaia that allows their full stellar content to be identified and their dynamics to be studied, which in turn is leading to changes in our understanding of these systems and their origins, with the old picture of Blaauw (1964) now being superseded. It is clear now that OB associations have considerably more substructure than once envisioned, both spatially, kinematically and temporally. These changes have implications for the star formation process, the formation and evolution of planetary systems, and the build-up of stellar populations across galaxies.