2020ApJ...900L...4P

We identify structures of the young star cluster NGC 2232 in the solar neighborhood (323.0 pc) and a newly discovered star cluster, LP 2439 (289.1 pc). Member candidates are identified using the Gaia DR2 sky position, parallax, and proper-motion data by an unsupervised machine-learning method, STARGO. Member contamination from the Galactic disk is further removed using the color-magnitude diagram. The four identified groups (NGC 2232, LP 2439, and two filamentary structures) of stars are coeval with an age of 25 Myr and were likely formed in the same giant molecular cloud. We correct the distance asymmetry from the parallax error with a Bayesian method. The 3D morphology shows the two spherical distributions of clusters NGC 2232 and LP 2439. Two filamentary structures are spatially and kinematically connected to NGC 2232. Both NGC 2232 and LP 2439 are expanding. The expansion is more significant in LP 2439, generating a loose spatial distribution with shallow volume number and mass density profiles. The expansion is suggested to be mainly driven by gas expulsion. With 73% of the cluster mass bound, NGC 2232 is currently experiencing a process of revirialization, However, LP 2439, with 52% of the cluster mass unbound, may fully dissolve in the near future. The different survivability traces the different dynamical states of NGC 2232 and LP 2439 prior to the onset of gas expulsion. While NGC 2232 may have been substructured and subvirial, LP 2439 may have either been virial/supervirial or experienced a much faster rate of gas removal.