Measuring elemental abundance ratios in protoplanetary disks at millimeter wavelengths (2005.03891v2)

Abstract: 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.