Investigating fragmentation of gas structures in OB cluster-forming molecular clump G33.92+0.11 with 1000 AU resolution observations of ALMA

We report new, $\sim$1000 AU spatial resolution observations of 225 GHz dust continuum emission towards the OB cluster-forming molecular clump G33.92+0.11. On parsec scales, this molecular clump presents a morphology with several arm-like dense gas structures surrounding the two central massive ($\gtrsim$100 $M_{\odot}$) cores. From the new, higher resolution observations, we identified 28 localized, spatially compact dust continuum emission sources, which may be candidates of young stellar objects. Only one of them is not embedded within known arm-like (or elongated) dense gas structures. The spatial separations of these compact sources can be very well explained by Jeans lengths. We found that G33.92+0.11 may be consistently described by a marginally centrifugally supported, Toomre unstable accretion flow which is approximately in a face-on projection. The arm-like overdensities are natural consequence of the Toomre instability, which can fragment to form young stellar objects in shorter time scales than the timescale of the global clump contraction. On our resolved spatial scales, there is not yet evidence that the fragmentation is halted by turbulence, magnetic field, or stellar feedback.
Comment: 24 pages, 18 figures. Accepted to publish on December 04, 2018; updated to arXiv on December 05, 2018