Molecular diagnostics for the mid-infrared emission of planet-forming disks. Carbon and oxygen elemental abundances

Abstract

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.

Figures (21)

• Figure 1: C/O ratios explored in the grid. The change in the carbon and oxygen elemental abundances relative to their fiducial abundances are shown in the x- and y-axes. The fiducial carbon and oxygen elemental abundances are listed in Table \ref{['tab:elementsTable']}. Each square represents a model in the grid with the C/O ratio written on the square. The color of the square corresponds to the C/O ratio of the corresponding model.
• Figure 2: Disk structure of the fiducial model. Only the innermost 12 au and $z/r$$>$0.1 region, which is relevant for MIR emission, is shown. The left panels (a and d) show the gas density and the gas-to-dust mass ratio. The middle panels (b and e) show the dust and the gas temperatures. The right panels (c and f) show the dominant heating and cooling processes, which are labeled to the right of the respective panels. The black dotted line in each panel indicates the vertical extinction A$\rm _v$=1 line. The black dashed and dash-dotted lines show the 300 K and 1000 K gas temperature contours (gray contours in panel b indicate dust temperatures).
• Figure 3: Continuum subtracted FLiTs spectrum (black, shifted vertically for clarity) of reference model using a spectral resolving power of 3000 between 13 and 17 $\mu$m. The different colors indicate different molecular/atomic emission highlighted in colored text.
• Figure 4: Difference in gas temperatures with respect to the fiducial model. White contours (solid lines) are gas temperatures of 300 K and 1000 K. The white dotted lines show the 300 K and 1000 K gas temperatures, and white dashed lines show A$\rm _v$=1 of the reference model. Since the gas and dust densities are the same for each model, the location of the A$\rm _v$=1 contour is the same in all panels. The center panel shows the actual gas temperature of the fiducial model, which is the same as panel e of Fig. \ref{['fig:fiducial_summary']}. The text in each panel denotes $\rm \Delta log_{10}(\varepsilon_C)$, $\rm \Delta log_{10}(\varepsilon_O)$, and the C/O ratio.
• Figure 5: Comparison of molecular emission across the grid. The text in each panel denotes $\rm \Delta log_{10}(\varepsilon_C)$, $\rm \Delta log_{10}(\varepsilon_O)$, and the C/O ratio. The molecular spectra are centered at 5.05 $\mu$m, 6.6 $\mu$m, 13.7 $\mu$m, 14.98 $\mu$m, and 23.86 $\mu$m from left to right. Subscripts 'rv' and 'ro' refer to ro-vibrational and pure rotational line emission.