Abstract
Planets form in disks of gas and dust surrounding young stars known as protoplanetary disks. The characteristics of the resulting planets depend largely on the environment in which they form. These formation environments consist of a young star, a rotating disk of material, and a variety of influencing factors—including radiation, turbulence, and winds—that all play a role in determining how planets form. The complexity of these environments makes it challenging to build a model of planet formation that accurately explains the population of planets observed in our Solar System and beyond. This dissertation aims to deepen our understanding of planet formation by constraining ionization, a key property that is central to the chemical and physical evolution of disks.
Ionization is driven by the star and surrounding environment and in turn drives many important processes in the disk. Ions speed up chemical reactions and drive complexity in the gas and on icy dust grains, which go on to become the building blocks of planets. Ion-driven reactions produce crucial ingredients for habitable worlds such as water and organic carbon-compounds. Ionization also influences the transport of dust and gas and sets the degree of turbulence in the gas, which determines how and where planets will form. The degree of ionization in protoplanetary disks can be estimated by observing the light emitted by molecular ions with radio telescopes such as the Atacama Large Millimeter/submillimeter Array (ALMA).
In this dissertation, I employ observations of molecular ions and astrochemical simulations of protoplanetary disks to study the chemical and physical processes that influence planet formation, from the star-disk systems to young planets themselves. In Chapter 1, I review our current understanding of protoplanetary disks and the roles that ionization plays in their evolution. In Chapter 2, I investigate the ionization environment of the turbulent DM Tau disk through a detailed forward-modeling analysis. This work yields evidence of an ionization gradient, possibly related to disk structure, and has important implications for the mechanism(s) driving disk accretion. In Chapter 3, I present a survey of high-resolution observations of ionization-tracing molecules in a diverse sample of disks. This work demonstrates the importance of surveying multiple ionization-tracers in protoplanetary disks and reveals a diversity of ionization environments across the sample. In Chapter 4, I test the reliability of two molecular ions, DCO+ and N2H+, as tracers of the CO snow surface—an important constraint for disk evolution and planet formation studies. Finally, in Chapter 5, I summarize the findings presented in this dissertation and comment on avenues for future work.
