A new method to determine ices abundances in protoplanetary disks

Protoplanetary disks are composed of a mixture of gas, dust, and ices, and constitue the birth place of planets. Ices play a key role in planetary formation, in delivering water to planets, and in composition of planetary atmospheres. It is therefore important to be able to characterize and quantify the abundance of ice within protoplanetary disks.

Previous studies, as well as programs using the James Webb Space Telescope, have attempted to quantify ice abundance through the study of ice absorption bands using infrared spectroscopy. Only highly ionclined systems without envelope allow to trace ices in disks because they absorb and scatter the stellar light. However, several observations and models have shown that ice bands become saturated when the disk inclination is too high, precluding the quantification of ice abundance in highly inclined disks.

A recent study lead by Laurine Martinien, a PhD student at IPAG, proposes a new method to overcome this problem of ice band saturation in highly inclined disks. This method is based on measuring the change in disk thickness (defined as dneb in the figure below) as a function of wavelength, which is directly related to grain opacity and disk mass.

Left panel : JWST/NIRSpec image of Tau 042021 at 3.1 μm. The two surfaces of the disk are clearly separated, allowing the measurement of the disk thickness, dneb. Right panel : Disk thickness as a function of wavelength in JWST/NIRSpec observations. The “bumps” associated with the most common ice species are indicated.

James Webb observations of four edge-on protoplanetary disks (highly inclined, close to 90°) have been obtained, allowing the measurement of their thickness as a function of inclination.

The disk thickness decreases with the increase of wavelength, except at wavelengths corresponding to the presence of ice, where a “bump” is present. This confirms the presence of water, CO2, and CO ices in all four disks. The increase in disk thickness indicated by the “bump” is due to the presence of ice, which adds opacity on top of the continuum. According to our models, the height of this “bump” is quantitatively correlated with the ice abundance within the disk.

Thanks to this robust method, which is not subject to saturation, it will be possible to constrain the ice abundance in highly inclined protoplanetary disks.