A Gaia view of the two OB associations Cygnus OB2 and Carina OB1: The signature of their formation process

TL;DR

This study uses Gaia DR2 data to analyze the kinematic structures of Cygnus OB2 and Carina OB1, revealing substructures that suggest their formation was driven by turbulence in their natal clouds rather than cluster evolution.

Contribution

It provides the first detailed kinematic analysis of these OB associations, linking their substructures to turbulence-driven formation processes.

Findings

OB associations consist of dense clusters and extended halos with distinct kinematic properties.

Clusters have small proper motion dispersions and sizes of 5-8 parsecs.

Halos are more extended with larger dispersions, indicating different formation dynamics.

Abstract

OB associations are the prime star forming sites in galaxies. However the detailed formation process of such stellar systems still remains a mystery. In this context, identifying the presence of substructures may help tracing the footprints of their formation process. Here, we present a kinematic study of the two massive OB associations Cygnus OB2 and Carina OB1 using the precise astrometry from the Gaia Data Release 2 and radial velocities. From the parallaxes of stars, these OB associations are confirmed to be genuine stellar systems. Both Cygnus OB2 and Carina OB1 are composed of a few dense clusters and a halo which have different kinematic properties: the clusters occupy regions of 5-8 parsecs in diameter and display small dispersions in proper motion, while the halos spread over tens of parsecs with a 2-3 times larger dispersions in proper motion. This is reminiscent of the so-called "line width-size" relation of molecular clouds related to turbulence. Considering that the kinematics and structural features were inherited from those of their natal clouds would then imply that the formation of OB associations may result from structure formation driven by supersonic turbulence, rather than from the dynamical evolution of individual embedded clusters.