Velocity centroid gradients for absorbing media
  We explore how the velocity gradient technique (VGT) can be applied to
absorbing media in the case of$^{13}$CO 2-1 emission. The VGT is a new way to
trace magnetic fields in the plane of the sky using only spectroscopic
observations. We apply the VGT to magnetohydrodynamic turbulence simulations
that have been post-processed to include $^{13}$CO 2-1 emission and we
calculate the velocity centroid gradients. We find that the velocity centroid
gradients trace the projected magnetic field in media with different $^{13}$CO
abundances, densities and optical depths. Our study opens up the possibility of
using velocity centroid gradients to trace magnetic fields in molecular clouds
using 13CO emission.
