Tracing Multi-Scale Magnetic Field Structure Using Multiple Chemical
  Tracers in Giant Molecular Clouds
  Probing magnetic fields in Giant Molecular Clouds is often challenging.
Fortunately, recently simulations show that analysis of velocity gradients (the
Velocity Gradient Technique) can be used to map out the magnetic field
morphology of different physical layers within molecular clouds when applied CO
isotopologues with different optical depths. Here, we test the effectiveness of
the Velocity Gradient Technique in reconstructing the magnetic field structure
of the molecular cloud Vela C, employing seven chemical tracers that have
different optical depths, i.e. 12CO, 13CO, C18O, CS, HNC, HCO+, and HCN. Our
results show good correspondence between the magnetic field morphology inferred
from velocity gradients using these different molecular tracers and the
magnetic field morphology inferred from BLASTPol polarization observations. We
also explore the possibility of using a combination of velocity gradients for
multiple chemical tracers to explain the structure of the magnetic field in
molecular clouds. We search for signatures of gravitational collapse in the
alignment of the velocity gradients and magnetic field and conclude that
collapsing regions constitute a small fraction of the cloud.
