Internal motions in OB-associations with Gaia DR2
  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
sigma_v =4.5 km s-1. The median virial and stellar masses of OB-associations
are M_vir=8.9 x 10^5 and M_st=8.1 x 10^3 Ms, respectively. The median
star-formation efficiency in parent giant molecular clouds appears to be
epsilon=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 analyzed 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.
