Contribution of binary stars to the velocity dispersion inside OB associations with Gaia DR2 data
Abstract
We estimated the contribution of binary systems to the velocity dispersion inside OB-associations derived from Gaia DR2 proper motions. The maximum contribution to the velocity dispersion is given by the systems with the period of revolution of P=5.9 yr whose components shift by a distance of about the diameter of the system during the base-line time of Gaia DR2 observations. We employed two methods to study the motion of the photocenter of the binary system: the first one uses the total displacement between the initial and final visibility periods and the second one is based on solving a system of n equations defining the displacements at the times tn. The first and second methods yield very similar sigmabn values of 0.90 and 0.87 km s-1, respectively. Taking into account the fact that orbits are elliptical slightly decreases the inferred sigmabn. We estimated the eccentricity-averaged sigmabn value to be sigmabn=0.81 km s-1 assuming that the orbital eccentricities of massive binary systems are distributed uniformly in the [0, 0.9] interval. The choice of the exponent gamma in the power-law distribution, pq ~ qgamma, of the component-mass ratios q=M2/M1 of binary systems appears to have little effect on sigmabn. A change of gamma from 0 (flat distribution) to -2.0 (preponderance of systems with low-mass components) changes sigmabn from 0.90 to 1.07 km s-1.