The Gaia-ESO Survey: 3D dynamics of young groups and clusters from GES and Gaia EDR3

TL;DR

This study uses Gaia and Gaia-ESO data to analyze the 3D kinematics of young star groups, revealing their dynamical states, anisotropic velocity dispersions, and expansion or contraction behaviors, providing insights into their formation and evolution.

Contribution

It presents the first large-scale 3D kinematic analysis of young stellar groups combining Gaia astrometry and spectroscopy, highlighting their dynamical states and expansion patterns.

Findings

Most groups are not dynamically relaxed and are expanding.

Velocity dispersions correlate with group mass, but no strong virial relation is observed.

One group, Rho Oph, is contracting and sub-virial considering molecular cloud mass.

Abstract

We present the first large-scale 3D kinematic study of ~2000 spectroscopically-confirmed young stars (<20 Myr) in 18 star clusters and OB associations (hereafter groups) from the combination of Gaia astrometry and Gaia-ESO Survey spectroscopy. We measure 3D velocity dispersions for all groups, which range from 0.61 to 7.4 km/s (1D velocity dispersions of 0.35 to 4.3 km/s). We find the majority of groups have anisotropic velocity dispersions, suggesting they are not dynamically relaxed. From the 3D velocity dispersions, measured radii and estimates of total mass we estimate the virial state and find that all systems are super-virial when only the stellar mass is considered, but that some systems are sub-virial when the mass of the molecular cloud is taken into account. We observe an approximately linear correlation between the 3D velocity dispersion and the group mass, which would imply that the virial state of groups scales as the square root of the group mass. However, we do not observe a strong correlation between virial state and group mass. In agreement with their virial state we find that nearly all of the groups studied are in the process of expanding and that the expansion is anisotropic, implying that groups were not spherical prior to expansion. One group, Rho Oph, is found to be contracting and in a sub-virial state (when the mass of the surrounding molecular cloud is considered). This work provides a glimpse of the potential of the combination of Gaia and data from the next generation of spectroscopic surveys.