Protoplanetary disks: Sensitivity of the chemical composition to various
  model parameters
  Protoplanetary disks are challenging objects for astrochemical models due to
strong density and temperature gradients and due to the UV photons 2D
propagation. In this paper, we have studied the importance of several model
parameters on the predicted column densities of observed species. We
considered: 1) 2-phase (gas and homogeneous grains) or 3-phase (gas, surface,
and bulk of grains) models, 2) several initial compositions, 3) grain growth
and dust settling, and 4) several cosmic-ray ionization rates. Our main result
is that dust settling is the most crucial parameter. Including this effect
renders the computed column densities sensitive to all the other model
parameters, except cosmic-ray ionization rate. In fact, we found almost no
effect of this parameter for radii larger than 10 au (the minimum radius
studied here) except for N2H+. We also compared all our models with all the
column densities observed in the protoplanetary disk around DM Tau and were not
able to reproduce all the observations despite the studied parameters. N2H+
seems to be the most sensitive species. Its observation in protoplanetary disks
at large radius could indicate enough N2 in the gas-phase (inhibited by the
3-phase model, but boosted by the settling) and a low electron abundance
(favored by low C and S elemental abundances).
