Interpreting high spatial resolution line observations of planet-forming
  disks with gaps and rings -- The case of HD 163296
  Spatially resolved continuum observations of planet-forming disks show
prominent ring and gap structures in their dust distribution. However, the
picture from gas observations is much less clear and constraints on the radial
gas density structure (i.e. gas gaps) remain rare and uncertain. We want to
investigate the importance of thermo-chemical processes for the interpretation
of high-spatial-resolution gas observations of planet-forming disks and their
impact on derived gas properties. We apply the radiation thermo-chemical disk
code ProDiMo (PROtoplanetary DIsk MOdel) to model self-consistently the dust
and gas disk of HD 163296, using the DSHARP gas and dust observations. With
this model we investigate the impact of dust gaps and gas gaps, considering
chemistry and heating/cooling processes, on the observables and the derived gas
properties. We find distinct peaks in the radial line intensity profiles of the
CO line data of HD 163296 at the location of the dust gaps. Our model indicates
that those peaks are not only a consequence of a gas temperature increase
within the gaps but are mainly caused by the absorption of line emission from
the back side of the disk by the dust rings. For two of the three prominent
dust gaps in HD 163296, we find that thermo-chemical effects are negligible for
deriving density gradients via measurements of the rotation velocity. However,
for the gap with the highest dust depletion, the temperature gradient can be
dominant and needs to be considered to derive accurate gas density profiles.
Self-consistent gas and dust thermo-chemical modelling in combination with
high-quality observations of multiple molecules are necessary to accurately
derive gas gap depths and shapes. This is crucial to determine the origin of
gaps and rings in planet-forming disks and to improve the mass estimates of
forming planets if they are the cause of the gap.
