Connecting models and observations of disc evolution and planet formation

Abstract

Planets are assembled in and inherit their properties and composition from gas- and dust-rich discs orbiting young stars. Over the last decade, the Atacama Large Millimeter/submillimeter Array revolutionised our understanding of planet formation, opening up the possibility of determining the physical-chemical properties of protoplanetary discs and their secular evolution. Both are essential ingredients to understand the disc's potential to form planets and interpret the enormous diversity of features among the fully-formed exoplanets around main-sequence stars observed today. The first part of my thesis deals with disc evolution and aims to assess what mechanism, turbulent angular momentum transport or angular momentum removal by magneto-hydrodynamic winds, drives disc evolution. This is a long-standing issue with crucial consequences for many aspects of planet formation, from dust growth, pebble and gas accretion on planetary embryos, to planet migration. Forward modelling the available data, I showed that, although dust disc fluxes and sizes from ALMA snapshot surveys in nearby star-formation regions, and mass accretion rates from complementary VLT/X-Shooter spectroscopic surveys, cannot tell these two evolutionary scenarios apart, CO fluxes are both easy to measure in large disc samples with ALMA and a promising tracer of disc evolution. I also investigated the impact of stellar multiplicity on disc evolution, showing that, other than making discs fainter and smaller, it affects the correlation between disc dust masses and mass accretion rates, substantially reducing the disc lifetime. The second part of my thesis deals with combining ALMA and VLA broadband multi-frequency continuum observations to determine the temperature, size, density, and potentially internal properties, such as porosity and composition, of the large grains in the disc mid-plane. In the case of CI Tau, where high angular resolution observations are available, I showed that mm-sized compact amorphous carbonaceous grains are present in the system, and constrained radial variations of the density and size of dust on the scale of disc substructures. Instead, for the moderate-resolution observations of tens of discs in the Lupus star-formation region, my analysis provided global constraints on dust properties and the impact of dust self-scattering on continuum emission. Finally, in the case of HD 163296, one of the closest and best studied discs, I combined literature estimates of dust properties and the amount of dust trapping in the disc substructures, to show that favourable conditions to kickstart the formation of planetesimal under streaming instability are met in one of the rings.