ИСТИНА

We use Gaia stellar proper motions to study the kinematics of OB-associations. The average one-dimensional velocity dispersion inside 18 OB-associations with more than 10 Gaia stars is sigmav=3.9 km s-1. The median virial and stellar masses of OB-associations equal 7.1 105 and 9.0 103 solar masses, respectively. Thus OB-associations must be unbound objects provided they do not include a lot of dense gas. The median star-formation efficiency is epsilon=2.1%. Nearly one third of stars of OB-associations must lie outside their tidal radius. We found that the Per OB1 and Car OB1 associations are expanding with the expansion started in a small region of 11--27 pc 7--10 Myr ago. The average expansion velocity is 6.3 km s-1. The location of the Sun near the OLR of the bar creates conditions which stimulate formation of giant molecular clouds.