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How rapidly collapsing parsec-scale massive molecular clumps feed high-massstars, and how they fragment to form OB clusters, have been outstandingquestions in the field of star-formation. In this work, we report the resolvedstructures and kinematics of the approximately face-on, rotating massivemolecular clump, G33.92+0.11. Our high resolution Atacama LargeMillimeter/submillimeter Array (ALMA) images show that the spiral arm-like gasoverdensities form in the eccentric gas accretion streams. First, we resolvedthat the dominant part of the $\sim$0.6 pc scale massive molecular clump(3.0$^{+2.8}_{-1.4}$$\cdot$10$^{3}$ $M_{\odot}$) G33.92+0.11 A is tangled withseveral 0.5-1 pc size molecular arms spiraling around it, which may beconnected further to exterior gas accretion streams. Within G33.92+0.11 A, weresolved the $\sim$0.1 pc width gas mini-arms connecting with the two centralmassive (100-300 $M_{\odot}$) molecular cores. The kinematics of arms and coreselucidate a coherent accretion flow continuing from large to small scales. Wedemonstrate that the large molecular arms are indeed the cradles of densecores, which are likely current or future sites of high-mass star formation.Since these deeply embedded massive molecular clumps preferentially form thehighest mass stars in the clusters, we argue that dense cores fed by or formedwithin molecular arms play a key role in making the upper end of the stellarand core mass functions.

ALMA resolves the spiraling accretion flow in the luminous OB cluster forming region G33.92+0.11