2019ApJ...884..137H

Probing magnetic fields in giant molecular clouds is often challenging. Fortunately, recent simulations show that analysis of velocity gradients (the velocity gradient technique; VGT) can be used to map out the magnetic field morphology of different physical layers within molecular clouds when applied to CO isotopologues with different optical depths. Here, we test the effectiveness of the VGT 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.