ATOMS: ALMA three-millimetre observations of massive star-forming regions – XVI. Neutral versus ion line widths.

It has been suggested that the line width of ions in molecular clouds is narrower than that of the co-existing neutral particles, which has been interpreted as an indication of the decoupling of neutral turbulence from magnetic fields within a partially ionized environment. We calculate the principal component analysis (PCA) correlation coefficients of CCH versus H |$^{13}$| CO |$^{+}$| and H |$^{13}$| CN versus H |$^{13}$| CO |$^{+}$|⁠. We find aside from H |$^{13}$| CN, CCH could also be strongly spatial correlated with H |$^{13}$| CO |$^{+}$| in high-mass star-forming regions. CCH and H |$^{13}$| CO |$^{+}$| line emissions are strongly spatial correlated with each other in 48 per cent sources with a PCA correlation coefficient over 0.7. So, we investigate the ambipolar diffusion (AD) effect using CCH and H |$^{13}$| CO |$^{+}$| lines as a neutral/ion pair in a sample of 129 high-mass star-forming clumps. We conduct a careful analysis of line widths of the CCH–H |$^{13}$| CO |$^{+}$| pair pixel-by-pixel in 12 sources, which show a strong correlation in CCH–H |$^{13}$| CO |$^+$| emission and no obvious outflows or multiple velocity components. The mean velocity dispersion of CCH is about the same as H |$^{13}$| CO |$^{+}$| in 12 sources. In low-density regions of most sources, CCH shows a broader velocity dispersion than H |$^{13}$| CO |$^{+}$|⁠. However, the AD effect is not significant from a statistical point of view. [ABSTRACT FROM AUTHOR]