New Insights into the Structure of Open Clusters in the Gaia Era

With the help of Gaia data, it is noted that in addition to the core components, there are low-density outer halo components in the extended region of open clusters. In order to study the extended structure beyond the core radius of the cluster (~10 pc), based on Gaia Early Data Release 3, taking up to 50 pc as the searching radius, we use the pyUPMASK algorithm to redetermine the member stars of the open cluster within 1-2 kpc. We obtain the member stars of 256 open clusters, especially those located in the outer halo region of open clusters. Furthermore, we find that the radial density profile in the outer region for most open clusters deviates from the Kings profile. In order to better describe the internal and external structural characteristics of open clusters, we propose a two-component model: a core component with a King model distribution and an outer halo component with a logarithmic Gaussian distribution, and then suggest using four radii (r _c , r _t , r _o , r _e ) for describing the structure and distribution profile of star clusters, where r _t and r _e represent the boundaries of the core component and outer halo component, respectively. Finally, we provide a catalog of 256 clusters with structural parameters. In addition, our study shows the sizes of these radii are statistically linearly related, which indicates that the inner and outer regions of the cluster are interrelated and follow similar evolutionary processes. Further, we show that the structure of two components can be used to better trace the cluster evolution properties at different stages.