Study of dust properties in protoplanetary disks

Thesis defense of Maxime Roumesy - Thursday, November 12th at 2.00 pm - Manuel Forestini room IPAG

Since the discovery of the first exoplanet, their number has continued to rise, exceeding 8,000 exoplanets catalogued today. However, the question of their formation remains a mystery. Everything starts in protoplanetary disks, gas- and dust-rich environments that constitute the birthplace of planets. In these young disks, dust grains evolve through processes of coagulation, fragmentation, and dynamical interactions, forming increasingly massive bodies. Understanding how these grains grow from submicrometer sizes to planetesimals of several kilometers, which are large enough to initiate planetary accretion, remains a major challenge of modern research.

The aim of my work is to better characterize the dust within these discs. Indeed, the size, structure, and composition of the dust directly influence its evolution, and through observations in the near-infrared, the received light is primarily scattered by the dust and carries all the information about its nature. The challenge is to interpret this information correctly. Among the diagnostic measures, the scattering phase function describes the angular distribution of light scattered by the dust. However, extracting this information from protoplanetary disks remains difficult, particularly due to their complex geometry.

Within this context, I developed DRAGyS, a tool designed to estimate the geometry of protoplanetary disks and extract their phase function directly from the disk image. DRAGyS uses the ring structure of the disks to identify their geometry and then measures the phase function while correcting for a geometric effect that is often overlooked: limb brightening. Thanks to its graphical user interface, the extraction process is fast and consistent, paving the way for a statistical study of dust in protoplanetary disks.

Then I applied this tool to about thirty disks to identify the major trends. This statistical analysis reveals two main families of dust: one consistent with highly porous fractal aggregates and the other with smaller and more compact grains. These results suggest that a diversity of dust populations exists within the disks, perhaps linked to a form of evolution, but the study also shows that the analysis of an individual disk is highly degenerate. In other words, the phase function is rich in information, but it is not sufficient by itself to precisely constrain the nature of the grains.

To overcome this constraint, I studied the color of the disks, which is the variation in scattered intensity across multiple wavelengths. Using radiative transfer simulations, I explored the impact of a wide variety of particles: spherical, irregular, compact, and fractal. The results show that color becomes less sensitive to the overall size of the particles as their structure becomes more complex. Thus, color depends strongly on the size of the monomers that make up the particle, as well as on the composition and morphology of the aggregates. Therefore, combining the phase function and color represents a promising approach for more effectively constraining the properties of dust.

This work helps improve our understanding of dust properties within protoplanetary disks. It introduces a powerful analytical tool, DRAGyS, as well as an approach based on the combined use of phase function and color to connect observations of the disks to the physical properties of the grains within them. Ultimately, this combination promises to answer the following question: What is the nature of the dust within these protoplanetary disks, designed to form planets?

Thesis direction

  • François Ménard & Gaspard Duchêne

Examination committee:

  • Olga Muñoz, Senior researcher, Instituto de Astrofisica de Andalucia, Referee
  • Jean-François Gonzalez, Full Professor, UCB Lyon 1, Referee
  • Nathalie Ysard, Senior researcher IRAP CNRS, Examiner
  • Jean-Charles Augereau, Astronomer, IPAG UGA, Examiner
  • Julien Milly, Associate Astronomer, IPAG UGA, Examiner
  • Gaspard Duchêne, Associate Astronomer, IPAG UGA, Guest