Molecular diagnostics for the mid-infrared emission of planet-forming disks

Context . Mid-infrared (MIR) observations of planet-forming disks reveal a broad diversity among molecular spectra. Carbon and oxygen abundances play a central role in setting the chemical environment of the inner disk and the spectral appearance. Aims . We aim to systematically explore how variations in elemental carbon and oxygen abundances affect the MIR spectra of planet-forming disks. We also aim to identify robust MIR molecular diagnostics of the C/H, O/H, and C/O ratios. Methods . Using the thermochemical disk code P ro D i M O and the line radiative transfer code FLiTs, we constructed a grid of 25 models with varying carbon and oxygen abundances, covering a broad range of C/O ratios. We analyzed the resulting MIR molecular emission, including species such as H 2 O, CO, CO 2 , C 2 H 2 , and OH. Results . We find that the MIR molecular spectra are highly sensitive not only to the C/O ratio, but also to the absolute abundances of carbon and oxygen. Despite sharing the same disk structure and C/O ratios, the molecular fluxes (e.g., C 2 H 2 , CO 2 ) vary by more than an order of magnitude. This variation stems from the differences in excitation conditions and emitting regions caused by the elemental abundances of oxygen and carbon. We identified diagnostic molecular flux ratios (such as CO 2 /H 2 O and H 2 O/C 2 H 2 ) that can serve as tracers of C/H and O/H, respectively. By combining these diagnostics, we have been able to demonstrate the use of our method for inferring the underlying C/O ratio. Conclusions . Our model grid provides a framework for interpreting MIR molecular emission from disks, allowing for estimates of elemental abundances if the disk properties and structure are known. Comparisons with recent JWST observations suggest that a variety in C and O abundances is observed in a sample of T Tauri disks, possibly shaped by disk transport processes and the presence of gaps.