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

Carbon and oxygen elemental abundances

¹ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700AV Groningen, The Netherlands
² Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
³ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁴ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁵ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
⁶ Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
⁷ Department of Astronomy, University of Michigan, 1085 S. University, Ann Arbor, MI 48109, USA
⁸ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 July 2025
Accepted: 17 February 2026

Abstract

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 ProDiMO 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 H₂O, CO, CO₂, C₂H₂, 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., C₂H₂, CO₂) 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 CO₂/H₂O and H₂O/C₂H₂) 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.