DR 21 South Filament: A Parsec-sized Dense Gas Accretion Flow onto the DR 21 Massive Young Cluster

Abstract

DR 21 south filament (DR21SF) is a unique component of the giant network of filamentary molecular clouds in the north region of the Cygnus X complex. Unlike the highly fragmented and star-forming active environment wherein it resides, DR21SF exhibits a coherent profile in the column density map with very few star formation signposts, even though the previously reported linear density of the filament is an order of magnitude higher than the thermal stable threshold. We derive the size (3.6 pc by 0.13 pc), temperature (10-15 K), and mass (1048 M_⊙) of DR21SF from our single-dish observations of NH₃ (1, 1) and (2, 2) inversion lines in conjunction with the column density map from our previous work. Star-forming sites are identified along the filament where gas temperature is excessive. We find clear gradients in radial velocity and intrinsic line width along the spine of the filament. The gradients can be well interpreted by a scenario of an accretion flow feeding DR 21 at a mass transfer rate of 1.1 × 10^-3M_⊙ yr^-1. Based on the analysis of its kinematic temperature, intrinsic line width, and mass distribution, we conclude that DR21SF is in an overall trans-critical status, which indicates an early evolutionary stage.