Substructure and halo population of Double Cluster h and χ Persei

¹ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
e-mail: jzhong@shao.ac.cn, chenli@shao.ac.cn
² Department of Mathematical Sciences, Xi’an Jiaotong-Liverpool University, 111 Ren’ai Rd., Suzhou Dushu Lake Science and Education Innovation District, Suzhou Industrial Park, Suzhou 215123, PR China
³ School of Astronomy and Space Science, University of Chinese Academy of Sciences, No. 19A, Yuquan Road, Beijing 100049, PR China
⁴ Shanghai Key Lab for Astrophysics, 100 Guilin Road, Shanghai 200234, PR China

Received: 27 September 2018
Accepted: 18 February 2019

Abstract

Context. Gaia DR2 provides an ideal dataset to study the stellar populations of open clusters at larger spatial scales because the cluster member stars can be well identified by their location in the multidimensional observational parameter space with high precision parameter measurements.

Aims. In order to study the stellar population and possible substructures in the outskirts of Double Cluster h and χ Persei, we use Gaia DR2 data in a sky area of about 7.5° in radius around the Double Cluster cores.

Methods. We identified member stars using various criteria, including their kinematics (namely, proper motion), individual parallaxes, and photometric properties. A total of 2186 member stars in the parameter space were identified as members.

Results. Based on the spatial distribution of the member stars, we find an extended halo structure of h and χ Persei about six to eight times larger than their core radii. We report the discovery of filamentary substructures extending to about 200 pc away from the Double Cluster. The tangential velocities of these distant substructures suggest that they are more likely to be remnants of primordial structures, instead of a tidally disrupted stream from the cluster cores. Moreover, internal kinematic analysis indicates that halo stars seem to experience a dynamic stretching in the RA direction, while the impact of the core components is relatively negligible. This work also suggests that the physical scale and internal motions of young massive star clusters may be more complex than previously thought.