2012ApJ...745...47L

We have mapped 27 massive star-forming regions associated with water masers using three dense gas tracers: HC₃ N 10-9, HNC 1-0, and C₂H 1-0. The FWHM sizes of HNC clumps and C₂ H clumps are about 1.5 and 1.6 times higher than those of HC₃N, respectively, which can be explained by the fact that HC₃ N traces more dense gas than HNC and C₂H. We found evidence for an increase in the optical depth of C₂ H with a "radius" from the center to the outer regions in some targets, supporting the chemical model of C₂H. The C₂ H optical depth is found to decline as molecular clouds evolve to a later stage, suggesting that C₂ H might be used as a "chemical clock" for molecular clouds. The large-scale kinematic structure of clouds was investigated with three molecular lines. All of these sources show significant velocity gradients. The magnitudes of gradient are found to increase toward the inner region, indicating the differential rotation of clouds. Both the ratio of rotational to gravitational energy and the specific angular momentum seem to decrease toward the inner region, implying the obvious angular momentum transfer, which might be caused by magnetic braking. The average magnetic field strength and number density of molecular clouds is derived using the uniform magnetic sphere model. The derived magnetic field strengths range from 3 to 88 µG, with a median value of 13 µG. The mass-to-flux ratio of the molecular cloud is calculated to be much higher than the critical value with derived parameters, which agrees well with numerical simulations.