2021ApJ...912....7P

Dense cores are the final place where turbulence is dissipated. It has been proposed from theoretical arguments that the nonthermal velocity dispersion should be narrower both for molecular ions (compared to neutrals) and for transitions with higher critical densities. To test these hypotheses, we compare the velocity dispersion of N₂H⁺ (1-0) (n_crit = 6 x 10⁴ cm^–3) and NH₃ (n_crit = 2 x 10³ cm^–3), in the dense core Barnard 5. We analyze well-resolved and high signal-to-noise observations of NH₃ (1,1) and (2,2) obtained with combining Robert C. Byrd Green Bank Telescope (GBT) and Very Large Array (VLA) data, and N₂H⁺ (1-0) obtained with GBT Argus, which present a similar morphology. Surprisingly, the nonthermal velocity dispersion of the ion is systematically higher than that of the neutral by 20%. The derived sonic Mach number, M_s=σ_NT/c_s, has peak values M_s,N2H⁺=0.59 and M_s,NH3_=0.48 for N₂H⁺ and NH₃, respectively. This observed difference may indicate that the magnetic field even deep within the dense core is still oscillating, as it is in the turbulent region outside the core. The ions should be more strongly dynamically coupled to this oscillating field than the neutrals, thus accounting for their broader line width. If corroborated by further observations, this finding would shed additional light on the transition to quiescence in dense cores.