A new Gaia census of OB associations within 1 kpc

TL;DR

This study provides a comprehensive and reliable catalog of 56 OB associations within 1 kpc, doubling the known associations and analyzing their physical and kinematic properties using Gaia data.

Contribution

It introduces a new, complete census of OB associations within 1 kpc, utilizing Gaia astrometry and the HDBSCAN clustering algorithm to identify and characterize these groups.

Findings

Identified 56 OB associations within 1 kpc, doubling previous counts.

Most associations (38/56) show significant expansion patterns.

Many associations exhibit anisotropic expansion velocities.

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 Originally defined about 80 years ago, their historical membership lists have been superseded thanks to the precise astrometry from ESA's \textit{Gaia}'s satellite.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 56 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, ...). The majority of the OB associations (38 out of 56) exhibit a significant expansion pattern in at least one direction, including 12 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.