MINDS. A transition from H_2O to C_2H_2 dominated disk spectra with decreasing stellar luminosity

The chemical composition of the inner regions of disks around young stars will largely determine the properties of planets that form in these regions. Many physical processes in the disks drive their chemical evolution, and some of them depend on and/or correlate with the stellar properties. We explore the connection between stellar properties and the chemistry of the inner disk in protoplanetary disks as traced by mid-infrared spectroscopy. We used JWST-MIRI observations of a large diverse sample of sources to explore trends between the carbon-bearing molecule C_2H_2 and the oxygen-bearing molecule H_2O. Additionally, we calculated the average spectrum for the T Tauri ( and very low-mass star (VLMS; samples from JWST-MIRI MRS data and used slab models to determine the properties of the average spectra in each subsample. We find a significant anticorrelation between the flux ratio of C_2H_2/H_2O and the stellar luminosity. The F_ C_2H_2 /F_ H_2O flux ratios of disks around VLMSs are significantly higher than the fluxes in their higher-mass counterparts. This is driven by the generally weak H_2O and strong C_2H_2 in disks around low-mass hosts. We also explored trends with the strength of the 10 μm silicate feature, the stellar accretion rate, and the disk dust mass. They are all correlated with F_ C_2H_2 /F_ H_2O which may be related to processes that drive the carbon enrichment in disks around VLMSs, but are also degenerate with the system properties (i.e., the and relations). Slab model fits to the average spectra show that H_2O emission in the VLMS sample is quite similar in temperature and column density to a warm (∼600 K) H_2O component in the T Tauri spectrum. This indicates that the high C/O gas-phase ratio in these disks is not due to oxygen depletion alone. Instead, the many hydrocarbons, including some with high column densities, suggest that carbon enhancement occurs in the disks around VLMSs. The observed differences in the chemistry of the inner disk as a function of host properties are likely to be accounted for by differences in the disk temperatures, stellar radiation field, and the evolution of dust grains.