Chemistry and IR emission of acetylene in planet-forming regions of T Tauri disks. Impact of elemental abundances and dust properties
Abstract
(Abridged) We aim to explore the parameters that influence the mid-infrared emission of C2H2 and H2O, and if the spread observed in FC2H2/FH2O is tracing a variation of the C/O ratio. Our work is based on the DALI 2D thermochemical model to predict 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 raytracing. We are able to reproduce the observed C2H2 fluxes of T Tauri disks with a solar C/O ratio. Acetylene 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 H2O and CO. The water UV shielding and hot temperatures of the inner disk also favor acetylene formation, as they prevent the destruction of carbon chains and allow overcoming activation barriers of reactions with H2. C2H2 and H2O 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. FC2H2/FH2O 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. We find that increasing the abundance of small grains relative to large grains favors C2H2 flux over H2O flux. Grain depletion does not affect the line flux 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.
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