We identify hierarchical structures in the Vela OB2 complex and the cluster pair Collinder 135 and UBC 7 with Gaia EDR3 using the neural network machine learning algorithm StarGO. Five second-level substructures are disentangled in Vela OB2, which are referred to as Huluwa 1 (Gamma Velorum), Huluwa 2, Huluwa 3, Huluwa 4 and Huluwa5. Based on our finding on the age spread and mass stratification in Vela OB2, among Huluwa 1–5, we propose an alternative scenario in which Huluwa 1–5 have originated from sequential star formation while supernova explosion quenched the star formation in Huluwa 4–5. Please check for details in https://ui.adsabs.harvard.edu/abs/2021arXiv210607658P/abstract

{"references": ["https://ui.adsabs.harvard.edu/abs/2021arXiv210607658P/abstract", "https://ui.adsabs.harvard.edu/abs/2021ApJ...912..162P/abstract", "https://ui.adsabs.harvard.edu/abs/2020ApJ...900L...4P/abstract"]}