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

N. Sameshima, T. Miyata, T. Kamizuka, Y. Aikawa, M. Honda, L. I. Cleeves, N. P. Ballering, M. J. Colmenares, C. González-Ruilova, V. V. Guzman T. J. Haworth, C. J. Law, J. P. Williams

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 $\le$ 18~$\mathrm{μm}$ 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~$\mathrm{μm}$ 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.

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~ 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~ 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.
Paper Structure (18 sections, 5 equations, 15 figures, 4 tables)

This paper contains 18 sections, 5 equations, 15 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations and Data
  3. Targets
  4. SST IRS
  5. JWST MIRI MRS
  6. Spectra
  7. Fitting
  8. Model
  9. Dust mass absorption coefficient
  10. Fitting
  11. Results
  12. Discussion
  13. The discrepancy between the best-fitting model and the observed data
  14. Temperature of crystalline silicate
  15. Mechanism of Variability
  16. ...and 3 more sections

Figures (15)

  • Figure 1: The SST (capped error bars) and JWST (vertical error bars) spectra and their ratio (JWST/SST) for Sz 96 (top) and IP Tau (bottom). The JWST data have been binned to a resolution of $R=500$.
  • Figure 2: An illustration of the model used in our study. The star, rim and mid-plane are optically thick and the surface is optically thin. Silicate features are seen in the radiation from the surface.
  • Figure 3: Fitting results of Sz 96: overall fitting results (top), contributions from each dust component (middle), and residual of the best-fitting model (bottom) for SST (left) and JWST (right). In the middle panel, the emissions from the star, rim, and mid-plane, as well as components contributing less than 0.5% to the total flux, are not shown.
  • Figure 4: Fitting results of IP Tau: overall fitting results (top), contributions from each dust component (middle), and residual of the best-fitting model (bottom) for SST (left) and JWST (right). In the middle panel, the emissions from the star, rim, and mid-plane, as well as components contributing less than 0.5% to the total flux, are not shown.
  • Figure 5: The temperature parameters for Sz 96 (filled symbols) and IP Tau (open symbols). The x-axis represents SST temperatures, while the y-axis represents JWST temperatures. The error bars represent the 0.025 and 0.975 quantiles of the posterior distributions of the parameters. The dashed line shows where the SST and JWST temperatures would be equal. $T^\mathrm{cr}_\mathrm{sur}$ for the SST data of IP Tau is unconstrained.
  • ...and 10 more figures