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

TL;DR

This study uses ALMA observations to resolve spiral accretion streams and dense cores within a massive molecular clump, shedding light on the process of high-mass star formation and cluster development.

Contribution

First detailed ALMA imaging of spiral accretion flows and dense cores in a massive molecular clump, revealing the structure and kinematics of star-forming regions.

Findings

Spiral arm-like overdensities form in eccentric gas streams.

Massive molecular arms are connected to dense cores and accretion flows.

Dense cores within arms are likely sites of high-mass star formation.

Abstract

How rapidly collapsing parsec-scale massive molecular clumps feed high-mass stars, and how they fragment to form OB clusters, have been outstanding questions in the field of star-formation. In this work, we report the resolved structures and kinematics of the approximately face-on, rotating massive molecular clump, G33.92+0.11. Our high resolution Atacama Large Millimeter/submillimeter Array (ALMA) images show that the spiral arm-like gas overdensities form in the eccentric gas accretion streams. First, we resolved that 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 with several 0.5-1 pc size molecular arms spiraling around it, which may be connected further to exterior gas accretion streams. Within G33.92+0.11 A, we resolved the $\sim$0.1 pc width gas mini-arms connecting with the two central massive (100-300 $M_{\odot}$) molecular cores. The kinematics of arms and cores elucidate a coherent accretion flow continuing from large to small scales. We demonstrate that the large molecular arms are indeed the cradles of dense cores, which are likely current or future sites of high-mass star formation. Since these deeply embedded massive molecular clumps preferentially form the highest mass stars in the clusters, we argue that dense cores fed by or formed within molecular arms play a key role in making the upper end of the stellar and core mass functions.