Large-Scale Kinematics, Astrochemistry and Magnetic Field Studies of
  Massive Star-forming Regions through HC3N, HNC and C2H Mappings
  We have mapped 27 massive star-forming regions associated with water masers
using three dense gas tracers: HC3N 10-9, HNC 1-0 and C2H 1-0. The FWHM sizes
of HNC clumps and C2H clumps are about 1.5 and 1.6 times higher than those of
HC3N, respectively, which can be explained by the fact that HC3N traces more
dense gas than HNC and C2H. We found evidence for increase in optical depth of
C2H with `radius' from center to outer regions in some targets, supporting the
chemical model of C2H. The C2H optical depth is found to decline as molecular
clouds evolve to later stage, suggesting that C2H might be used as "chemical
clock" for molecular clouds. Large-scale kinematic structure of clouds was
investigated with three molecular lines. All these sources show significant
velocity gradients. The magnitudes of gradient are found to increase towards
the inner region, indicating differential rotation of clouds. Both the ratio of
rotational to gravitational energy and specific angular momentum seem to
decrease toward the inner region, implying 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 uniformly magnetic
sphere model. The derived magnetic field strengths range from 3 to 88 \mu G,
with a median value of 13 \mu G. The mass-to-flux ratio of molecular cloud is
calculated to be much higher than critical value with derived parameters, which
agrees well with numerical simulations.
