Cosmic Kitchen: How Carbon and Oxygen Shape the Look of Disks express
When a chef prepares a dish, not only the proportions of ingredients matter but also their total amount. Two pies with the same sugar-to-flour ratio but made from different batch sizes will turn out differently. So it is with disks around young stars: carbon and oxygen are the key 'spices,' and their absolute amounts change the look of the cosmic 'kitchen' no less than their ratio.
Astronomers modeled 25 variations of such disks, changing the carbon and oxygen content. It turned out that the infrared emission of molecules — for example, water, carbon dioxide, and acetylene — can differ by more than a factor of ten even when the C/O ratio is the same. The reason is that the total amount of elements determines where in the disk molecules get heated and start to shine.
The scientists found convenient 'light cues': the brightness ratio of carbon dioxide to water indicates the carbon abundance, while the ratio of water to acetylene indicates the oxygen abundance. Combining these two cues allows us to recover the C/O ratio itself. This works even if the disk's structure is only roughly known.
Recent observations by the James Webb Space Telescope confirm that disks around young T Tauri stars indeed have different compositions. Perhaps the transport of matter within the disk and gaps in it mix the 'ingredients,' creating a unique recipe for each future planetary system.
Original abstract
as written by the authors arXiv:2602.16030
Mid-infrared observations of planet-forming disks reveal a wide diversity in molecular spectra. Carbon and oxygen abundances play a central role in setting the chemical environment of the inner disk and the spectral appearance. We aim to systematically explore how variations in elemental carbon and oxygen abundances affect the mid-infrared spectra of planet-forming disks, and to identify robust mid-infrared molecular diagnostics of C/H, O/H, and the C/O ratio. Using the thermochemical disk code ProDiMo and the line radiative transfer code FLiTs, we construct a grid of 25 models with varying carbon and oxygen abundances, covering a broad range of C/O ratios. We analyze the resulting mid-infrared molecular emission, including species such as $$\rm H_2O$$, $$\rm CO$$, $$\rm CO_2$$, $$\rm C_2H_2$$, $$\rm OH$$. We find that the mid-infrared molecular spectra are highly sensitive not only to the C/O ratio, but also to the absolute abundances of carbon and oxygen. Despite the same disk structure and C/O ratios, molecular fluxes (e.g., $$\rm C_2H_2$$, $$\rm CO_2$$) 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 identify diagnostic molecular flux ratios - such as $$\rm CO_2$$/$$\rm H_2O$$ and $$\rm H_2O$$/$$\rm C_2H_2$$ - that can serve as tracers of C/H and O/H respectively. By combining these diagnostics, we demonstrate a method to infer the underlying C/O ratio. Our model grid provides a framework for interpreting mid-infrared molecular emission from disks, allowing 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 seen in a sample of T Tauri disks, possibly shaped by disk transport processes and the presence of gaps.
In one of the modeled disks, acetylene turned out to be ten times more abundant than in another, even though the carbon-to-oxygen ratio was the same — simply the total amount of elements differed.