Caractérisation Des Processus D'évolution Des Disques Protoplanétaires

Résumé du livre

The diversity observed in the exoplanet population likely originates in the variety of physical structures of protoplanetary disks, their progenitors. Considering the timescales for disk dissipation, it is clear that giant planets must form fast, within a few million years. In the standard core accretion model, this implies that micron sized particles have to grow within this short timescale to larger sizes. This PhD thesis is dedicated to adding observational constraints on several aspects of protoplanetary disk evolution in order to understand better the processes of planet formation. In particular, I worked on a statistical study of the dust mass distribution in protoplanetary disks located in the Chamaeleon II star-forming region, on the presence of dust depleted cavities in transition disks and on observational constraints on dust evolution mechanisms such as radial drift and vertical settling. I used mostly ALMA millimeter observations, which I combined with complementary disk tracers such as optical/infrared scattered light images. On several occasions, I modeled these observations with radiative transfer to constrain the structure of these protoplanetary disks.During this thesis, I have obtained important constraints on the vertical structure of protoplanetary disks, notably by studying a sample of 12 highly inclined disks. I find that the most inclined systems of this survey, where the vertical extent is best seen, are extremely thin at millimeter wavelengths, all the more when compared to their vertical extent seen with scattered light observations (probing smaller grains). This indicates that vertical settling is extremely efficient in these disks, which is favorable for fast grain growth in the disk midplane. Additionally, my study of the Chamaeleon~II star-forming region showed that the distribution of the disk dust mass statistically decreases with time. This is consistent with predictions from viscous evolution. Finally, with the study of a sample of 22 transition disks, I showed that their cavities can mostly be explained by the presence of planets. These results suggest that planets might already have formed in these disks, implying that the timescale available for planet formation is short. A possible mechanism allowing to boost planet formation is vertical settling. Indeed, as detailed previously, my studies showed that the vertical extent of protoplanetary disks is small at millimeter wavelengths, which implies that millimeter sized grains (and larger) are concentrated in a vertically thin midplane where the dust density is increased. Depending on when this vertical settling takes place, this mechanism is a good candidate for enhancing grain growth efficiency. Combined to other processes, vertical settling might allow to form planets in the earliest stages of disk evolution.