MINDS survey of silicates in T Tauri disks: Correlation between dust and gas

Context. Silicates are key constituents of planet-forming disks and are among the most important building blocks of rocky planets. Mid-infrared spectral features of micron-sized silicate grains are powerful tracers of grain growth, mineralogy, and disk chemistry.
Aims. We characterized the dust mineralogy in T Tauri disks using James Webb Space Telescope (JWST)/Mid-Infrared Instrument (MIRI) observations. A further aim of ours was to investigate the connections between the dust and molecular gas compositions.
Methods. We analyzed JWST/MIRI spectra of 26 disks as part of the MIRI mid-Infrared Disk Survey (MINDS). We employed spectral decomposition with our new DustComp tool to derive the mass fractions of individual dust species. We included in our fits Mg2SiO4 (forsterite), MgSiO3 (enstatite), and SiO2 (silica) together with amorphous silicates of corresponding stoichiometry.
Results. We find that Mg-rich (and Fe-poor) silicates represent our data well. Fit residuals are typically within ±3%. Grain size distributions are skewed toward larger sizes (>2 µm), indicating significant growth. Large (∼5 µm-sized) amorphous Mg-silicates were robustly detected, whereas the presence of large crystalline grains could not be firmly established. The average dust composition is dominated by grains of Mg2SiO4 stoichiometry (∼60%, including amorphous and crystalline state), followed by MgSiO3 (∼30%) and SiO2 (∼10%). The mass fractions of crystalline grains are typically in the 5–24% range, with a mean of 14%. We robustly detected annealed silica in nine objects, with cristobalite as the main polymorph. We found a correlation between dust and molecular gas composition: disks with strong annealed silica features show relatively strong CO2 emission, while forsterite-rich disks display stronger H2O emission. Disks with annealed silica features may also have
elevated gas-phase C/O ratios, suggesting a process, such as dust sublimation and recondensation, that establishes thermo-chemical equilibrium between solids and gas.
Conclusions. The correlation between dust and gas may provide the first indication that the molecular gas composition regulates the availability of dust species in the inner disk.