A pilot study of the two OB associations Cygnus OB2 and Carina OB1 using the Gaia data

We present a kinematic study of the two young OB associations Cygnus OB2 and Carina OB1 using the recently released Gaia astrometric data. The unimodal distributions of parallaxes of stars indicate that these associations are real stellar systems, rather than line-of-sight coincidences. The associations are found to comprise dense star clusters and a sparse halo which have different proper motions. Clusters have small spatial sizes with small dispersions in proper motion, while the haloes extending to tens of parsecs have a large dispersion in proper motion. We speculate that this aspect is related to that found in molecular clouds, the so-called "line width-size" relation. In this talk, the formation process of these associations is discussed, based on our findings.