DECODING PROTOPLANETARY DISK EVOLUTION

Planets form within protoplanetary disks, rotating structures of gas and dust that surround young stars during the first few million years of their evolution. Understanding how these disks evolve is therefore essential to understanding how planetary systems, including our own Solar System, are assembled. Despite major observational and theoretical advances over the last two decades, the physical processes that govern the evolution and dispersal of protoplanetary disks remain poorly understood. One of the central questions in the field is how disks transport angular momentum while accreting material onto their host stars. In the classical picture, turbulent motions within the disk redistribute angular momentum radially, causing disks to spread over time. An alternative scenario invokes magnetically driven winds that remove angular momentum from the disk surface, allowing accretion while keeping disks compact, or even causing them to shrink. Discriminating between these two mechanisms is one of the major goals of modern studies of star and planet formation, as they predict different evolutionary pathways for disks and consequently different environments for planet formation. Among the most powerful observational diagnostics of these scenarios are gas-disk sizes: while viscous evolution predicts that disks become larger as they evolve, the evolution of disk sizes in the MHD wind-driven accretion scenario remains less clear. In simplified parameterized prescriptions for wind-driven accretion, disks remain compact or progressively shrink, although this conclusion may need to be reassessed in more complex models. Observationally testing the evolution of gas-disk sizes can therefore provide important constraints on the mechanisms governing disk evolution. However, measuring gas-disk sizes is observationally challenging because molecular-line emission is intrinsically faint and spatially resolved observations require high angular resolution and sensitivity. As a result, reliable gas-disk size measurements are currently available for only ~100 protoplanetary disks, most of which belong to just two nearby and well-characterized star-forming regions. Consequently, the gas properties of the majority of protoplanetary disks remain largely unexplored. This thesis was motivated by the need to overcome these observational limitations. During my PhD, I contributed to the development of the software csalt, a new framework for modeling molecular-line observations from the Atacama Large Millimeter/submillimeter Array (ALMA) directly in the interferometric domain (i.e., modeling visibilities, the raw products of an interferometer). By exploiting the full information content of the interferometric observations, csalt enables precise measurements of gas-disk properties even for relatively compact and faint disks. As part of the ALMA Disk-Exoplanet C/Onnection (DEC/O) Large Program, I applied this methodology to the largest homogeneous sample of protoplanetary disks observed in multiple molecular tracers, deriving gas-disk radii, stellar masses, and stellar ages in a homogeneous way. Together, these measurements provide new observational constraints on the evolution of protoplanetary disks and enable a direct test of whether turbulent transport or magnetically driven winds dominate the evolution of protoplanetary disks.