A Dense Molecular Ringlike Structure in Gaseous CO Depletion Region G34.74−0.12

Abstract We report the discovery of a dense molecular ringlike structure in a dense (10 5 cm −3 ), cold (parsec-scale CO depletion at a factor of 5), and young (10 4 yr) 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. The ringlike structure is redshifted with respect to the clump, spanning from V sys,lsr + 0.9 to V sys,lsr + 2.9 km s −1 , with a total mass of 109 M ⊙ . It is spatially coincident with 1.3 and 3.0 mm dust continuum emission from the cores, and several protostellar outflows. However, no free–free emission or H 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 windblown 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.