Massive Star Formation Starts in Subvirial Dense Clumps Unless Resisted by Strong Magnetic Fields

Knowledge of the initial conditions of high-mass star formation is critical for theoretical models, but are not well observed. Built on our previous characterization of a Galaxy-wide sample of 463 candidate high-mass starless clumps (HMSCs), here we investigate the dynamical state of a representative subsample of 44 HMSCs (radii 0.13–1.12 pc) using Green Bank Telescope NH 3 (1,1) and (2,2) data from the Radio Ammonia Mid-Plane Survey pilot data release. By fitting the two NH 3 lines simultaneously, we obtain velocity dispersion, gas kinetic temperature, NH 3 column density and abundance, Mach number, and virial parameter. Thermodynamic analysis reveals that most HMSCs have Mach number