ALMA RESOLVES THE SPIRALING ACCRETION FLOW IN THE LUMINOUS OB CLUSTER-FORMING REGION G33.92+0.11

Abstract

How rapidly collapsing parsec-scale massive molecular clumps feed high-mass\nstars, and how they fragment to form OB clusters, have been outstanding\nquestions in the field of star-formation. In this work, we report the resolved\nstructures and kinematics of the approximately face-on, rotating massive\nmolecular clump, G33.92+0.11. Our high resolution Atacama Large\nMillimeter/submillimeter Array (ALMA) images show that the spiral arm-like gas\noverdensities form in the eccentric gas accretion streams. First, we resolved\nthat the dominant part of the $\\sim$0.6 pc scale massive molecular clump\n(3.0$^{+2.8}_{-1.4}$$\\cdot$10$^{3}$ $M_{\\odot}$) G33.92+0.11 A is tangled with\nseveral 0.5-1 pc size molecular arms spiraling around it, which may be\nconnected further to exterior gas accretion streams. Within G33.92+0.11 A, we\nresolved the $\\sim$0.1 pc width gas mini-arms connecting with the two central\nmassive (100-300 $M_{\\odot}$) molecular cores. The kinematics of arms and cores\nelucidate a coherent accretion flow continuing from large to small scales. We\ndemonstrate that the large molecular arms are indeed the cradles of dense\ncores, which are likely current or future sites of high-mass star formation.\nSince these deeply embedded massive molecular clumps preferentially form the\nhighest mass stars in the clusters, we argue that dense cores fed by or formed\nwithin molecular arms play a key role in making the upper end of the stellar\nand core mass functions.\n