On the Motion of Multiple Stars in the Pleiades Based on Gaia DR2

Abstract

We estimated a number of parameters for small groups of stars in the open Pleiades star cluster using Gaia DR2 coordinate and proper motion data for stars in the region of radius d = 2.5° relative to the center of the cluster on the celestial sphere. We selected 36 wide pairs of stars with magnitudes m_G < 15^m and velocities of the stars in these pairs with respect to their geometric centers close to the circular velocity relative to their centers of mass. The average mass ratio of the components in such pairs is q$$ \simeq $$ 0.67 ± 0.04, and the median ratio is q$$ \simeq $$ 0.78. The number of such pairs increases with q and has a maximum near q$$ \simeq $$ 0.8–0.9. The mutual distances r_1,2 between individual stars in the pairs increase with distance r from the cluster center, and a maximum number of such pairs are located in the r = 3–4 pc interval (median r$$ \simeq $$ 3.64). We note the absence of dynamically active binaries in the cluster. The formulas for the potential energy of a binary star are written with account for the force fields of the cluster and Galaxy. Only one of the identified wide binaries has a negative total energy. The "(BP–RP) color index—$$~{{m}_{{\text{G}}}}$$" diagram for m_G < 15^m was used to select 62–70 unresolved multiple stars in the Pleiades. The spatial-kinematic parameters of the considered groups of wide binaries and unresolved multiple stars coincide within the margin of error, which indicates a common origin of these groups in this cluster for m_G < 15^m. For unresolved multiple stars of arbitrary multiplicity, we derived formulas for estimating the masses of the components from the luminosity of the star, considered as a single star on the main sequence (MS). Using these formulas for unresolved multiple stars in the Pleiades leads to an average increase in the masses of such "single MS stars" in binary, triple, quadruple stars by factors of 1.61, 2.19, 2.74 correspondingly. The shape of the stellar velocity dispersion $$\sigma _{v}^{2}$$ dependence on r in the small groups is determined by a combined influence of gravitational instability and violent relaxation in the cluster. We determined the sizes of such groups (several pc) and decay times (several million years). The degree of correlation of $$\sigma _{v}^{2}$$ in small groups of stars increases near the center of the cluster and in the 2 < r < 5.7 pc interval.