20 Novembre – Thesis defense - Sacha Gavino

13 h Room Univers - Building B18N (Pessac)

Observation and modeling of protoplanetary disks around young stars with ALMA: implication for planetary formation.

The star formation process usually proceeds with protoplanetary disks. These disks contain a mixture of gas, accounting for 99 % of the disk mass, and of solid particles called dust grains (1 % of the disk mass). These grains, initially at sub-micro metric sizes, gradually coagulate, grow, and potentially allow for the formation of planets about the star.
The study of the dust and molecular composition of young disks is fundamental to constraint the physical and chemical initial conditions of planetary formation and the origins of the chemical composition of the planetary cores.
The goal of this thesis was to build state-of-the-art models of typical young disks consisting of gas and of a population of grains of multiple sizes, then, in a new approach, to test with the use of numerical simulations the implication of the size and temperature distributions on the chemical evolution of disks.
To achieve this, we have coupled the 3D Monte-Carlo radiative transfer code POLARIS to the time-dependent gas-grain code NAUTILUS. The radiative transfer code allowed us to finely compute the grain temperature as a function of the size and location as well as the UV flux within the disk. The gas-grain code was able to simulate the evolution of the chemical abundances in our disk models. Moreover, the computation of the UV flux by POLARIS coupled to a set of molecular cross-sections extracted from a comprehensive database allowed us to compute as a function of the frequency the rates of molecular photoabsorption, photodissociation, and photoionization.

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