Contribution of binary stars to the velocity dispersion inside OB associations with Gaia DR2 data

TL;DR

This study quantifies how binary star systems influence the observed velocity dispersion within OB associations using Gaia DR2 data, revealing that binaries significantly contribute to the measured velocities.

Contribution

It introduces two methods to estimate binary contributions to velocity dispersion and assesses the impact of orbital eccentricity and mass ratio distributions.

Findings

Binary systems with a 5.9-year period dominate the velocity dispersion contribution.

Estimated binary contribution to velocity dispersion is approximately 0.81 km/s.

Orbital eccentricity and mass ratio distribution have minimal effect on the contribution estimate.

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 t_n. The first and second methods yield very similar sigma_bn values of 0.90 and 0.87 km s-1, respectively. Taking into account the fact that orbits are elliptical slightly decreases the inferred sigma_bn. We estimated the eccentricity-averaged sigma_bn value to be sigma_bn=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, p_q ~ q^gamma, of the component-mass ratios q=M_2/M_1 of binary systems appears to have little effect on sigma_bn. A change of gamma from 0 (flat distribution) to -2.0 (preponderance of systems with low-mass components) changes sigma_bn from 0.90 to 1.07 km s-1.