ATOMS: ALMA Three-millimeter 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 H¹³CO⁺ and H¹³CN versus H¹³CO⁺. We find aside from H¹³CN, CCH could also be strongly spatial correlated with H¹³CO⁺ in high-mass star-forming regions. CCH and H¹³CO⁺ 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 H¹³CO⁺ 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–H¹³CO⁺ pair pixel-by-pixel in 12 sources, which show a strong correlation in CCH–H¹³CO⁺ emission and no obvious outflows or multiple velocity components. The mean velocity dispersion of CCH is about the same as H¹³CO⁺ in 12 sources. In low-density regions of most sources, CCH shows a broader velocity dispersion than H¹³CO⁺. However, the AD effect is not significant from a statistical point of view.