Chemistry and IR emission of acetylene in planet-forming regions of T Tauri disks. Impact of elemental abundances and dust properties

JWST has revealed a broad diversity of spectra, pointing toward a wide range of physical or chemical conditions in the planet-forming regions of disks (<10 au) around T Tauri stars. In particular, acetylene (C_2H_2) is widely detected, but it remains unclear whether the spread in observed line flux is due to variation in the gas-phase C/O ratio or caused by other factors. Our goal is to explore the parameters that influence the mid-infrared (MIR) emission of C_2H_2 and H_2O, and if the spread observed in F /F is tracing a variation in the C/O ratio. _ C_2H_2 _ H_2O Our work is based on the DALI 2D thermochemical model, which self-consistently computes the thermal and chemical structure of the disk, and predicts spectra readily comparable to JWST/MIRI observations. To robustly model organics in inner disks, several improvements have been made: (1) carbon chemistry adapted for warm environments, (2) updated UV shielding treatment, and (3) mutual line overlap in the ray-tracing. With the model improvements, we are able to reproduce the observed C_2H_2 fluxes of T Tauri disks with a realistic disk geometry and solar C/O ratio. Our models show that C_2H_2 is bright and detectable by JWST even with a solar C/O. Its abundance is primarily set by a balance between formation initiated by CO dissociation by X-rays and destruction of carbon chains by atomic oxygen, the latter being generated by X-ray-induced destruction of H_2O and CO. The water UV shielding and hot temperatures (500-1000 K) of the inner disk also favor acetylene formation, as they prevent the destruction of carbon chains and allow the activation barriers of reactions with H_2 to be overcome. C_2H_2 and H_2O emissions are not only sensitive to the C/O ratio but also to the total O/H elemental abundance, supporting recent claims. In particular, we find that enhanced O/H reduces acetylene emission due to an excess of atomic oxygen. F_ C_2H_2 /F_ H_2O is thus a promising tracer of the elemental composition of inner disks. Still, the dust size distribution also plays a key role in this line flux ratio, reflecting the intrinsic link between the gas and dust components. We find that increasing the abundance of small grains relative to large grains favors C_2H_2 flux over H_2O flux. Grain depletion does not affect the F_ C_2H_2 /F_ H_2O ratio as previously suggested by observational works. A preliminary comparison with published JWST observations indicates a gas-phase C/O ratio below unity and suggests that enhanced O/H ratios may be common in T Tauri disks. Thermochemical models are essential for interpreting JWST spectra of inner disks, which ultimately provide information on the composition of exoplanet atmospheres. Yet, robustly estimating the C/O ratio through F_ C_2H_2 /F_ H_2O requires one to constrain dust properties from dust emission and better characterize the carbon chemistry.