JWST-DECO: Temporal Variations in the Mid-IR Silicate Features of Two T Tauri Discs Based on Spitzer and JWST Observations

Abstract Mid-infrared spectra of planet-forming discs commonly show prominent silicate emission, whose spectral shape is sensitive to the disc temperature distribution as well as its mineralogical composition. We report new James Webb Space Telescope (JWST) observations of the discs around Sz 96 and IP Tau and find that their silicate emission significantly changes in the 20 years since they were observed with the Spitzer Space Telescope (SST). Significant differences between the SST and JWST spectra are found for both sources, with flux variations of 10–15 % in Sz 96 and 30–35 % in IP Tau. Sz 96 dimmed at ≤ 18 μm and did not change significantly at longer wavelengths, whereas IP Tau became brighter across the entire wavelength range, with a particularly strong enhancement around 10 μm in the JWST data compared to the SST data. We propose that this large degree of variability is explained by structural changes in the inner regions of the discs. Specifically, we also find that crystalline silicates exhibit lower temperatures than amorphous silicates in the JWST data of both sources. This result supports the idea that crystalline grains, formed through high-temperature annealing in the inner disc regions, have been transported outward, leading to their presence in cooler regions of the disc. While similar behavior had been reported in previous SST-based studies, the much higher spectral resolution of JWST enables clearer identification of the crystalline features.