2024ApJ...974L...6W

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 NH₃ (1,1) and (2,2) data from the Radio Ammonia Mid-Plane Survey pilot data release. By fitting the two NH₃ lines simultaneously, we obtain velocity dispersion, gas kinetic temperature, NH₃ column density and abundance, Mach number, and virial parameter. Thermodynamic analysis reveals that most HMSCs have Mach number <5, inconsistent with what have been considered in theoretical models. All but one (43 out of 44) of the HMSCs are gravitationally bound with virial parameter α_vir < 2. Either these massive clumps are collapsing or magnetic field strengths of 0.10-2.65 mG (average 0.51 mG) would be needed to prevent them from collapsing. The estimated B-field strength correlates tightly with density, ${B}_{\mathrm{est}}/\mathrm{mG}=0.269\,{({n}_{{{\rm{H}}}_{2}}/{10}^{4}\,{\mathrm{cm}}^{-3})}^{0.61}$, with a similar power-law index as found in observations but a factor of 4.6 higher in strength. For the first time, the initial dynamical state of high-mass formation regions has been statistically constrained to be subvirial, in contradiction to theoretical models in virial equilibrium and in agreement with the lack of observed massive starless cores. The findings urge future observations to quantify the magnetic field support in the prestellar stage of massive clumps, which has rarely been explored so far, toward a full understanding of the physical conditions that initiate massive star formation.