Variation of the disk thickness across ice bands: A method to determine ice abundances in highly inclined protoplanetary disks

TL;DR

The study tackles the difficulty of measuring ice abundances in highly inclined protoplanetary disks where ice-band saturation limits spectroscopy. It introduces a thickness-based method that uses the disk’s apparent thickness, $d_ ext{neb}$, as a function of wavelength to trace ice opacity, validating the approach with radiative-transfer models via MCFOST and JWST/NIRSpec data from four edge-on disks. The results show a consistent thickness decrease with wavelength except at ice-band positions where a pronounced bump occurs; the bump height, $ abla d_ ext{neb}/d_ ext{neb}$, scales with ice abundance and does not saturate, with additional CO$_2$ and CO ice features detected. This method provides a saturation-free way to constrain ice abundances in highly inclined disks through disk-model fitting, broadening the applicability of ice studies to systems where two scattering surfaces are resolvable.

Abstract

The James Webb Space Telescope provides unprecedented information to study ices in protoplanetary disks. However, the saturation of ice bands in highly inclined disks hinders the measurement of ice abundances using classical spectroscopy. This is unfortunate as the presence and more importantly abundance of ices plays a key role in, e.g., the evolution of dust (because it modifies the sticking properties) and the composition of planetesimals and exoplanetary atmospheres. To overcome this issue and quantify the ice abundance within disks, we introduce a new method based on measuring the changes in the apparent disk thickness as a function of wavelength, which is directly and quantitatively related to the grain opacity. Specifically, we expect i) that the increased opacity within ice bands should result in a thicker disk than in the adjacent continuum, and ii) the thickness variations to be proportional to the abundance of ice. We extracted the disk thickness in model images of edge-on disks containing different abundances of water ice, as well as in James Webb Space Telescope spectral imaging of four edge-on disks. For both models and observations, the disk thickness decreases toward longer wavelengths except across the positions of ice absorption features where the thickness is enhanced across the band. In the model images, we demonstrate that this effect increases with ice abundance without any hint of saturation. This definitely demonstrates the presence of the ice species within each disk and confirms our expectation that this method can be applied to estimate ice abundances. Thanks to this method, it will thus be possible to constrain the ice abundance in highly inclined disks with disks model fitting. Unlike spectroscopic analysis, this method is not subject to saturation and should therefore be more robust and applicable to all disks for which the two surfaces can be resolved.

Figures (3)

• Figure 1: JWST/NIRSpec image of Tau 042021 centered at 3.1 $\mu$m with a binning of 30 slices (0.01 $\mu$m ). The solid blue line represents the distance between the two nebulae and the dotted blue lines represent the limits over which we measured this distance.
• Figure 2: Left: Distance between the two nebulae as a function of wavelength in the edge-on disk model presented in Martinien_2025 for different water ice abundances. Right: Bumps heights relative to the continuum in the same models as a function of ice abundance.
• Figure 3: Distance between the two nebulae as a function of wavelength in JWST/NIRSpec observations. In Oph 163131, the gray points correspond to a wavelength range where the two nebulae are no longer clearly separated and the results of the gaussian fit are likely biased upwards. The horizontal bars correspond to the wavelength coverage of each imaging filter.