On the Motion of Stars in the Pleiades According to Gaia DR2 Data

Abstract

Abstract—Several parameters of the Pleiades cluster were estimated. We used Gaia DR2 data on the coordinates, proper motions, and radial velocities of stars in regions with radius d° =2.5° and size 60° × 60° around the cluster center. Based on data on stars with magnitudes mG ≤ 18m, we constructed a map and profile of the density, luminosity and mass functions of the cluster, determined the cluster radius, 10.9° ± 0.3° (26.3 ± 0.7 pc), and the radius of its core, 2.62° (6.24 pc), and obtained estimates for the number of stars in the cluster, 1542 ± 121, and their mass, 855 ± 104M⊙; numbers of stars in the core of the cluster, 1097 ± 77, and their mass 665 ± 71M⊙. Distribution of stars with mG< 16m at distances rs from the cluster center in three-dimensional space of rs< 1 pc and at rs ∼ 1.4–5 pc contains radial density waves. Based on the data on stars with mG< 16m, we determined the average rotation velocity of the core of the cluster vc = 0.56 ± 0.07 km s−1 at distances d in the sky plane d ≤ 4.6 pc from its center. The rotation is “prograde”, the angle between the projection of the axis of rotation of the cluster core onto the sky plane and the direction to the North Pole of the Galaxy is ϕ = 18.8° ± 4.4°, the angle between the axis of rotation of the cluster core and the sky plane is ϑ = 43.2° ± 4.9°, the rotation velocity of the cluster core at a distance of d ≃ 5.5pc from its center is close to zero: vc = 0.1 ± 0.3 km s−1. According to the data on stars with mG< 17m, the velocity of the “retrograde” rotation of the cluster at a distance of d ≃ 7.1 pc from its center is vc = 0.48 ± 0.20 km s−1, the angle ϕ = 37.8° ± 26.4°. The dependences of moduli of the tangential and radial components of the velocity field of the stars of the cluster core in the sky plane on the distance d to the center of the cluster contain a number of periodic oscillations. The dispersions of the velocities of the stars in the cluster core σv increase on average with an increase in rs, which, like the radial density waves and the waves of oscillations of the velocity field in the sky plane, indicates the nonstationarity of the cluster in the field of regular forces. The Jeans wavelength in the cluster core decreases, and the velocity dispersion of the stars in the core under the Jeans instability increases after taking into account the influence of the external field of the Galaxy on the cluster. The region of gravitational instability in the Pleiades cluster is located in the interval rs = 2.2–5.7 pc and contains 39.4–60.5% of the total number of stars in the considered samples of cluster stars. Estimates of the Pleiades dynamic mass and tidal radius are obtained. © 2020, Pleiades Publishing, Ltd.