Measuring elemental abundance ratios in protoplanetary disks at
  millimeter wavelengths
  During the million years of evolution, gas dust and ice in protoplanetary
disks can be chemically reprocessed. There are evidences that the gas-phase
carbon and oxygen abundances are sub-solar in disks belonging to nearby star
forming regions. These findings have a major impact on the composition of the
primary atmosphere of giant planets (but it may also be valid for super-Earths
and sub-Neptunes) as they accrete their gaseous envelopes from the surrounding
material in the disk. In this study, we performed a thermo-chemical modelling
analysis with the aim at testing how reliable and robust are the estimates of
elemental abundance ratios based on (sub-)millimeter observations of molecular
lines. We created a grid of disk models for the following different elemental
abundance ratios: C/O, N/O and S/O, and, we computed the line flux of a set of
carbon-, nitrogen and sulphur-bearing species, namely CN, HCN, NO, C$_{2}$H,
c--C$_{3}$H$_{2}$, H$_{2}$CO, HC$_{3}$N, CH$_{3}$CN, CS, SO, H$_{2}$S and
H$_{2}$CS, that have been detected with present (sub-)millimeter facilities
such as ALMA and NOEMA. We find that the line fluxes, once normalized to the
flux of the $^{13}$CO $J=2-1$ line, are sensitive to the elemental abundance
ratios. On the other hand, the stellar and disk physical parameters have only a
minor effect of the line flux ratios. Our results demonstrate that a
simultaneous analysis of multiple molecular transitions is a valid approach to
constrain the elemental abundance ratio in protoplanetary disks.
