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

Abstract

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.

Hunting for Neighboring Open Clusters with Gaia DR3: 101 New Open Clusters within 500 pc

“…First, we separated all the stars into several searching shell regions along ϖ (>10 mas, 5-10 mas, 3.33-5 mas, 2.5-3,33 mas, 2-2.5 mas), approximately corresponding to the distance (<100 pc, 100-200 pc, 200-300 pc, 300-400 pc, 400-500 pc), which is large enough to cover at least one typical OC. In recent years, many investigations based on Gaia data revealed that besides the high-density inner parts, there might be extended low-density outer halos (Zhong et al 2019;Meingast et al 2021;Zhong et al 2022) or elongated tidal tails (Carrera et al 2019;Zhang et al 2020;Boffin et al 2022;Bai et al 2022) in the outskirt of OCs. To contain the outer halo structure of OCs adequately, we adopted 50 pc as the typical extended spatial scale of OCs (Tarricq et al 2022;Zhong et al 2022).…”

“…In recent years, many investigations based on Gaia data revealed that besides the high-density inner parts, there might be extended low-density outer halos (Zhong et al 2019;Meingast et al 2021;Zhong et al 2022) or elongated tidal tails (Carrera et al 2019;Zhang et al 2020;Boffin et al 2022;Bai et al 2022) in the outskirt of OCs. To contain the outer halo structure of OCs adequately, we adopted 50 pc as the typical extended spatial scale of OCs (Tarricq et al 2022;Zhong et al 2022). As the projected angular size varies with distance, we adopted searching grids with different angular sizes, ranging from 30°at a close distance of 100 pc to about 6°at 500 pc, as listed in Table 1.…”

“…In order to reveal the structural characteristics of the nearby star clusters better, we provided the radii parameters (r c , r t , r o , r e ) of OCSN clusters (Cols. 22-29) according to the two-component model proposed by Zhong et al (2022;hereafter Zhong2022). We listed the reported cluster names in the literature as well as the corresponding reference work .…”

Photometry of the Four Anti-Galactocentric Old Open Clusters: Czernik 30, Berkeley 34, Berkeley 75, and Berkeley 76

“…Among a dozen clustering methods we adopted the Gaussian mixture model, which assumes every cluster follows a Gaussian function. Finally, 137 member stars were found to have membership probability larger than 0.70, which was also used in Zhong et al (2022) as a probability limit for member stars.…”

Assessing the physical reality of Milky Way open cluster candidates

“…Likewise, rt cannot be larger than the derived RJ . Recently, Zhong et al (2022) showed that a two-components model with a King core distribution and a logarithmic Gaussian outer halo distribution describe better the internal and external structural features of open clusters. They found that core, half-mass, tidal and Jacobi radii are statistically linearly related, which suggests that the inner and outer regions of the clusters are interrelated and follow similar evolutionary processes.…”