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

TL;DR

This study uses Gaia DR2 data to analyze the Pleiades cluster, estimating its parameters, star distribution, rotation, and gravitational stability, revealing non-stationary dynamics and regions of gravitational instability.

Contribution

First detailed Gaia DR2-based analysis of Pleiades cluster parameters, star distribution, rotation, and gravitational stability, highlighting non-stationary behavior and internal wave phenomena.

Findings

Cluster radius: 10.9±0.3 deg (26.3±0.7 pc)

Number of stars: 1542±121, total mass: 855±104 solar masses

Region of gravitational instability: 2.2-5.7 pc, containing 39.4-60.5% of stars

Abstract

We used Gaia DR2 data in order to estimate parameters of the Pleiades. With the data on stars with m_G<=18 mag, we constructed the density maps and profile, luminosity and mass functions, determined the cluster radius, 10.9+/-0.3 deg (26.3+/-0.7 pc), and the radius of its core, 2.62 deg (6.24 pc), and obtained estimates for the number of stars in the cluster, 1542+/-121, and their mass, 855+/-104 solar mass. Distribution of stars with m_G<16 mag at distances r_s from the cluster center in three-dimensional space of r_s<1 pc and at r_s~1.4-5 pc contains radial density waves. We determined the average `prograde' rotation velocity of the core of the cluster v_c=0.56+/-0.07 km/s at distances d in the sky plane d<=4.6 pc from its center. The rotation velocity of the cluster core at a distance of d~5.5 pc from its center is close to zero: v_c=0.1+/-0.3 km/s. The velocity of the `retrograde' rotation of the cluster at a distance of d~7.1 pc from its center is v_c=0.48+/-0.20 km/s. The dependences of moduli of the tangential and radial components of the cluster core star velocities in the sky plane on the distance d to the cluster center contain the periodic oscillations. The velocity dispersions of the cluster core stars increase on average with an increase in r_s, which, like the radial density waves and the waves of oscillations of the velocity field in the sky plane, indicates the non-stationarity of the cluster in the regular field. The Jeans wavelength in the cluster core decreases, and the velocity dispersion of the core stars under the Jeans instability increases after taking into account the Galactic gravitational field. The region of gravitational instability in the Pleiades cluster is located in the interval r_s=2.2-5.7 pc and contains 39.4-60.5% of the total number of stars in the considered samples.