Kinematic Properties of Open Star Clusters with Data from the Gaia DR2 Catalogue

Abstract

We consider open star clusters (OSCs) with the proper motions, parallaxes, and line-of-sight velocities calculated by various authors from Gaia DR2 data. The distance scale factor has been found by analyzing the separate solutions of the basic kinematic equations to be p =1.00 ± 0.04. It shows that the distances calculated using the parallaxes from the Gaia DR2 catalogue do not need any correction factor. We have investigated the solutions obtained from various OSC samples differing in both age and accuracy of their parallax and line-of-sight velocity measurements. The solution obtained from a sample of 930 OSCs satisfying the constraints on the age log t < 9.0 and the relative trigonometric parallax error <30% is recognized to be the best one, with 384 OSCs in this sample having the mean line-of-sight velocities calculated from at least three probable members of the corresponding clusters. As a result of the simultaneous solution of all the basic kinematic equations, we have found the following kinematic parameters from this sample: (U, V, W)_⊙ = (8.53, 11.22, 7.83) ± (0.38, 0.46, 0.32) km s⁻¹, Ω₀ = 28.71 ± 0.22 km s⁻¹ kpc⁻¹, Ω′₀ = −4.100 ± 0.058 km s⁻¹ kpc⁻², and Ω″₀ 0.736 ± 0.033 km s⁻¹ kpc⁻³. The linear rotation velocity at the adopted at the adopted solar distance R₀ = 8.0 ± 0.15 kpc is V₀ = 229.7 ± 4.6 km s⁻¹. An analysis of the proper motions for these 930 OSCs has shown that, apart from the rotation around the Galactic z axis, there is rotation of the entire sample around the x axis with an angular velocity of 0.48 ± 0.15 km s⁻¹ kpc⁻¹ differing significantly from zero.