Dense Molecular Ring-like structure in gaseous CO depletion region G34.74-0.12

Abstract: We report the discovery of a dense molecular ring-like structure in a dense (10$^5$ cm$^{-3}$), cold (pc-scale CO depletion at a factor of 5), and young (10$^4$ year) star-forming region G34.74-0.12, revealed by C$^{18}$O (2-1), HNC (1-0), and N$2$H$^+$ (1-0) observations with the Atacama Large Millimeter/submillimeter Array (ALMA). The ring-like structure is redshifted with respect to the clump, spanning from $V{\rm sys,lsr} + 0.9$ to $V_{\rm sys,lsr} + 2.9$ km s$^{-1}$, with a total mass of 109 $M_{\odot}$. It is spatially coincident with 1.3 mm and 3.0 mm dust continuum emission from cores, and several protostellar outflows. However, no free-free emission or H\textsc{ii} region is detected in association with this structure. With a slow expansion speed indicated by the position-velocity diagram, this ring structure differs from rings previously identified in more evolved star-forming regions. Possible explanations for the ring-like structure include a relic wind-blown bubble produced by a deeply embedded young stellar object, a hollow cavity formed by cloud-cloud interactions, a gas ring resulting from a temperature gradient, or a line-of-sight superposition of multiple outflows or dense clouds. This discovery offers a rare observational glimpse into the earliest dynamical processes involved in massive star formation.