Study of dust and ices properties in highly inclined protoplanetary disks

Thesis defense of Laurine Martinien - Wednesday, October 7th octobre at 10.00 am - Manuel Forestini room IPAG

Planets form in protoplanetary disks surrounding young stars. Composed of gas, dust, and ices, these disks are the location of grain growth, during which particles coagulate and grow from a few micrometers to bodies several thousand kilometers in size. However, the physical mechanisms driving this growth remain poorly constrained. The study of micrometer- and millimeter-sized dust provides valuable insights into the early stages of planet formation. In addition, ices (H2O, CO2, and CO) have been shown to be ubiquitous in protoplanetary disks and to play a key role in planetary formation. Characterizing their properties has therefore become a major challenge in protoplanetary disk research, especially timely in the new era opened by the James Webb Space Telescope.

Thanks to their particular viewing geometry, highly inclined disks are ideal targets for investigating the spatial distribution of ices and provide a unique view of the disk vertical structure, revealing signatures of vertical dust settling, a key process in grain growth. During this thesis, I investigated the properties of dust and ices in highly inclined protoplanetary disks by combining a multi-wavelength observational approach (VLT-HST-JWST-ALMA) with radiative transfer modeling using MCFOST.

First, I carried out the first systematic study of a sample of highly inclined protoplanetary disks using millimeter continuum and gas observations. Thanks to line-of-sight integration, these systems provide more robust constraints on the faint outer regions of the disks. I confirmed that the gas is more radially extended than the micrometer-sized dust, which itself is more extended than the millimeter-sized dust. However, highly inclined disks appear less extended in gas compared to the millimeter continuum than disks observed at lower inclinations, consistent with optical depth effects and/or radial drift. I also confirmed, from the measurements of vertical extents, that large grains undergo vertical settling toward the disk midplane. Nevertheless, a few exceptions suggest that small grains are not perfectly coupled to the gas.

In the second part of this work, I conducted a detailed study of ice absorption bands in protoplanetary disks. By combining a detailed study of the grazing-angle system PDS 453 with a more general investigation, I confirmed the trend that the commonly used properties of ice absorption bands (shape, minimum position, and depth) depend not only on the intrinsic ice properties but also on the system inclination. Furthermore, I showed that their interpretation is even more complex because the observed band properties also depend on the location within the disk from which the spectra are extracted. In addition, the absorption bands become saturated when the disks are too highly inclined, preventing a robust determination of ice abundances. To overcome this limitation, I developed a new method based on measuring the apparent disk thickness as a function of wavelength. This thickness is directly and quantitatively linked to the disk mass and opacity. As a result, the disk appears thicker, relative to the continuum, at the wavelengths corresponding to the water, CO2, and CO ice absorption bands, as seen both in my models as well as in JWST/NIRSpec observations of 4 edge-on disks. According to the models, this method is not subject to saturation unlike spectra. Ultimately, this method will enable robust measurements of ice abundances in highly inclined protoplanetary disks by constructing radiative transfer models that simultaneously reproduce both the observed disk thickness and the spectral features.

Thesis Direction

• François Ménard

The Jury will consist of

• Guillaume Laibe, Professor ENS Lyon, Examiner
• Myriam Benisty, Associate Astronomer MPIA Heidelberg, Referee
• Olivier Lagage, Senior Researcher CEA Saclay, Referee
• Sébastien Maret, Researcher CNRS IPAG, Examiner
• Cecilia Ceccarelli, Astronomer UGA IPAG, Examiner