On the Crystallinity of the Protoplanetary Dust Disks Based on JWST MIRI MRS Data

Dust in protoplanetary disks is the main building blocks of planet formation. The spatial distribution of crystalline silicates provides important information about dust evolution, reflecting the physical and chemical properties of dust across different regions. We obtained spectroscopic data for 20 protoplanetary disks by using the Mid-Infrared Instrument (MIRI/MRS) on the James Webb Space Telescope (JWST) and divided these disks into inner and outer disk. By analyzing the mid-infrared spectra of these regions and fitting the silicate features with a dust emission model based on the Bruggeman modle, our statistical analysis shows the observations: The silicate dust in the inner protoplanetary disk has lower crystallinity and smaller size; whereas the dust in the outer disk has higher crystallinity and larger size relatively. The result suggests that crystalline silicates may have been efficiently transported from their formation regions to the whole disk, while large-sized and high-crystallinity silicate features are absent in the surface layer of the inner disk. To explain this absence, we proposed a three-layered silicate growth and sedimentation model and analyzed the reasons for the formation of this model: the compact low-crystallinity silicates in the surface layer shield the settled larger higher-crystallinity silicates, leading to the observed “low crystallinity” in the inner disk”. Future studies based on higher resolution mid-IR spectroscopy could display the spatial distribution of silicate crystallinity more precisely and help us to understand the evolution and formation of protoplanetary disk.