Variation in the disk thickness across ice bands: A method for determining ice abundances in highly inclined protoplanetary disks

Context. The James Webb Space Telescope provides unprecedented information about ices in protoplanetary disks. However, the saturation of ice bands in highly inclined disks hinders the measurement of ice abundances via classical spectroscopy. This is unfortunate as the presence and, more importantly, the abundance of ices play a key role in, for example, the evolution of dust (because it modifies the sticking properties) and the composition of planetesimals and exoplanetary atmospheres. Aims. 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 results in a thicker disk than in the adjacent continuum, and (ii) the thickness variations to be proportional to the abundance of ice. Methods. We extracted the disk thickness from model images of edge-on disks containing different abundances of water ice as well as from James Webb Space Telescope spectral imaging of four edge-on disks. Results. For both models and observations, the disk thickness decreases toward longer wavelengths except at the positions of ice absorption features, where the thickness is greater across the band. In the model images, we demonstrate that this effect increases with ice abundance without any hint of saturation. This demonstrates that ice species are present within each disk and confirms our expectation that this method can be applied to estimate ice abundances. Conclusions. Thanks to this method, it will thus be possible to constrain the abundance of ice in highly inclined disks with disk model fitting. Unlike spectroscopic analyses, 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.