Spatial distribution of crystalline silicates in protoplanetary disks: How to interpret mid-infrared observations

TL;DR

This study models the spatial distribution of crystalline silicates in protoplanetary disks to interpret mid-infrared observations, revealing how dust evolution and transport affect spectral features and matching observations with improved models.

Contribution

It introduces a detailed modeling approach combining dust crystallization, radial mixing, and spectral synthesis to interpret mid-IR spectra of protoplanetary disks.

Findings

Crystalline dust distribution varies with grain size and disk region.

Vertical mixing distributes crystalline dust throughout the disk.

Models with reduced crystallinity better match observed spectra.

Abstract

Crystalline silicates are an important tracer to the dust evolution in protoplanetary disks. In the inner disk, amorphous silicates are annealed by the high temperatures. These crystalline silicates are radially and vertically distributed in the disk. We aim to model the spatial distribution of crystalline silicate in the disk and its mid-IR spectra to study the effect on dust spectral features and to compare these to observations. We modeled a T-Tauri protoplanetary disk and defined the crystallization region from the crystallization and residence timescales. Radial mixing and drift were compared to find a vertically mixed region. We used the DISKLAB code to obtain the spatial distribution of the crystalline silicates, and MCMax code to model the mid-infrared spectrum. In our modeled disk, different grain sizes get crystallized in different regions in the disk. Crystallized dust in the disk surface is well mixed with the midplane due to vertical mixing and gets distributed to the outer disk by radial transport. Our model shows different contributions of the disk zones to the dust spectral features. Feature strengths change when varying the spatial distribution of crystalline dust. Our modeled spectra qualitatively agree with observations, but the modeled 10 $\mu$m feature is strongly dominated by crystalline dust. Models with reduced crystallinity and depletion of small crystalline dust in the inner disk show a better match with observations. Mid-IR observations of the disk surface represent the radial distribution of small dust in the midplane and provide us with dust properties in the inner disk. The inner and outer disks contribute more to shorter and longer wavelength features, respectively. Amorphization, sublimation, and dust evolution have to be considered to match observations. This study could interpret the spectra of protoplanetary disks taken with the MIRI on board the JWST.