Contribution of Binary Stars to the Velocity Dispersion inside OB Associations with Gaia DR2 Dataстатья
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Аннотация: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 thedisplacements at the times tn. The first and second methods yield very similar σbn values of 0.9 and0.87 km/s, respectively. Taking into account the fact that orbits are elliptical slightly decreases the inferred σbn. We estimated the eccentricity-averaged σbn value to be sigma_bn = 0.81 km/s assuming that the orbital eccentricities of massive binary systems are distributed uniformly in the e ∈ [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 = M2 /M1 of binary systems appears to have little effect on sigma_bn. A change of γ from 0 (flat distribution) to −2.0 (preponderance of systems with low-mass components) changes σbn from 0.9 to 1.07 km/s. The paper is based on a presentation made at the conference “Astrometry yesterday, today, tomorrow” (SAI MSU, Oct 14–16, 2019).