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The Trapezium Cluster in Orion teaches us that massive stars often form together in compact systems—but how such dense OB groupings are assembled remains an open question. In this talk, I will present high-resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) that reveal an exceptionally young, deeply embedded massive quadruple OB system in the making. The four massive protostars are arranged in an off-center belt at radii of 0.02–0.06 pc. By decomposing the ionized and neutral gas components, we trace the dynamic interplay between stars and gas down to thousand-AU scales. The gas kinematics and orbital energetics provide direct evidence for how material transitions from unbound, parabolic trajectories to bound orbits, assembling onto the forming protostars. These observations offer a rare glimpse into the early dynamical assembly of a Trapezium-like massive cluster.

BIO

Fengwei Xu is a postdoctoral fellow at the Max Planck Institute for Astronomy in Heidelberg, Germany. He received his Ph.D. in astronomy from the Kavli Institute for Astronomy and Astrophysics, Peking University, in June 2025, following a B.S. in Physics from Peking University in 2020. His research focuses on Galactic molecular clouds, massive star and cluster formation, and stellar feedback, using millimeter and submillimeter facilities including ALMA, JVLA, SMA, NOEMA, the IRAM 30m telescope, and APEX. He has led several ALMA and JVLA observational surveys aimed at understanding the dynamical assembly of massive stars, spanning spatial scales from parsec-sized clumps down to hundred-AU disks. His PhD thesis was selected as a 2025 Peking University Outstanding Doctoral Dissertation.

Host: Fanyi Meng