늙은 산개성단 M67에 대한 운동학적 연구

We present a kinematic study of the old open cluster, M67, using recent Gaia data. The photometric andastrometric data of the 934 members were obtained from the member list of our previous study. The structure of M67was investigated using a surface density map and a radial surface density profile. The apparent and half-number radius ofthis cluster were determined as approximately 30' (7.6pc) and 12.'4 (3.1 pc), respectively. The core radius was determinedto be approximately 4.'0 (1.0 pc) from the King’s empirical density profile fitting. The cluster age was estimated to beapproximately 4 Gyr by fitting the theoretical isochrones to its color-magnitude diagram, which is in good agreement withthose obtained in previous studies. We investigated the proper motion of individual members relative to the systemicmotion of M67. Consequently, the members within the half-number radius appeared to have nearly isotropic motion,whereas the other stars sinked into the cluster center. This indicates that the outskirt of this cluster is contracting. Inaddition, the signature of cluster rotation was detected from radial velocities of stars within rc , with the rotational axislying in an east-west direction (position angle 100o-280o) on the sky plane. This implies that the rotation of the clustercan continue for a long time even after its formation. Assuming virial equilibrium, the virial mass of M67 wasapproximately 1600 M ⊙. The dynamic relaxation time of M67 was estimated to be approximately 190 Myr, indicating thatthe cluster had already been relaxed. Indeed, this cluster shows mass segregation in the radial mass distribution of thestars.