ИСТИНА

We study the motions inside 28 OB-associations with the use of {\it Gaia} DR2 proper motions. The average velocity dispersion calculated for 28 OB-associations including more than 20 stars with {\it Gaia} DR2 proper motion is $\sigma_v =4.5$ km s$^{-1}$. The median virial and stellar masses of OB-associations are $M_{vir}=8.9 \times 10^5$ and $M_{st}=8.1 \times 10^3$ M$_\odot$, respectively. The median star-formation efficiency in parent giant molecular clouds appears to be $\epsilon=1.2$ per cent. {\it Gaia} DR2 proper motions confirm the expansion in the Per OB1, Car OB1 and Sgr OB1 associations found earlier with {\it 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 {\it 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 $5.0\pm1.7$ km s$^{-1}$. (a) Elliptical orbits of components 1 and 2 of the binary system are shown as the small and large ellipses, respectively. The foci of the orbits and the system mass center are located at the point $O$. The points P and A indicate the positions of the pericenter and apocenter of the orbit of the secondary component. The angles theta1 and theta_2 are counted from the pericenter P and correspond to the positions of the secondary component at the initial t1 and final t2 visibility periods of Gaia DR2 observations, respectively. (b) Dependence of the additional velocity Vbn averaged over the parameters q and t1 (t1 in [0, P]) on log P calculated for orbits with the eccentricity e=0.5 (the red line) and e=0 (the blue line)