SIMBAD references

We study the motions inside 28 OB associations with the use of Gaia DR2 proper motions. The average velocity dispersion calculated for 28 OB associations including more than 20 stars with Gaia DR2 proper motion is σ_v = 4.5 km s^–1. The median virial and stellar masses of OB associations are M_vir = 8.9 x 10⁵ and M_st = 8.1 x 10³ M_☉, respectively. The median star-formation efficiency in parent giant molecular clouds appears to be ε = 1.2 per cent. Gaia DR2 proper motions confirm the expansion in the Per OB1, Car OB1, and Sgr OB1 associations found earlier with Gaia DR1 data. We also detect the expansion in Gem OB1, Ori OB1, and Sco OB1 associations, which became possible for the first time now when analysed with Gaia DR2 proper motions. The analysis of the distribution of OB stars in the Per OB1 association shows the presence of a shell-like structure with the radius of 40 pc. Probably, the expansion of the Per OB1 association started with the velocity greater than the present-day expansion velocity equal to 5.0 ± 1.7 km s^–1.