Identification of OB Associations Using the LAMOST–Gaia OB Star Sample

Abstract OB associations, as an intermediate stage between Galactic clusters and field stars, play an important role in understanding the star formation process, early stellar evolution, and Galactic evolution. In this work, we construct a large sample of OB stars with six-dimensional (6D) phase space parameters ( l , b , d , V los , pmra, pmdec) by combining the distances from C. A. L. Bailer-Jones et al., radial velocities derived from low-resolution spectra of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, and proper motions from Gaia Data Release 3 (DR3). This sample includes 19,933 OB stars, most of which are located within 6 kpc of the Sun. Using 6D phase-space parameters and a friends-of-friends clustering algorithm, we identify 67 OB associations and 112 OB association candidates; among these, 49 OB associations and 107 OB association candidates are newly identified. The Galactic rotation curve derived using 67 OB association members is relatively flat in the range of Galactocentric distances 7 < R < 13 kpc. The angular rotation velocity at a solar Galactocentric distance of R ⊙ = 8.34 kpc is Ω 0 = 29.05 ± 0.55 km s −1 kpc −1 . The spatial distribution of the 67 OB associations indicates that they are mainly located at low Galactic latitudes and near spiral arms of the Milky Way. Additionally, we estimate the velocity dispersions and sizes of these 67 OB associations. Our results show that the velocity dispersions decrease as Galactocentric distances increase, while their sizes increase as Galactocentric distances increase.