Chemical Evolution in Protoplanetary Disks.

Abstract

This dissertation presents a new model for calculating the chemistry of protoplanetary disks prior to and during the initial stage of planet formation. In this dissertation we analyze the effects of some important physical processes on determining that chemistry.

Chapter 2 describes the new model and analyzes the effects of including dust settling and Ly alpha radiation in the stellar UV field on the protoplanetary disk chemistry, two elements missing from most previous models. In agreement with earlier studies, the evolution of dust grains plays a large role in determining how deep the UV radiation penetrates into the disk. Significant grain settling at the midplane leads to much smaller freeze-out regions and correspondingly larger molecular layers. The inclusion of Ly alpha radiation impacts the disk chemistry through specific species that have large photodissociation cross sections at 1216 A. These include HCN, NH3 and CH4, for which the column densities are decreased by an order of magnitude or more.

Chapter 3 looks at the effects of including dust-dependent X-ray opacity on the chemistry and analyzes the location of the dead zone in these disks. This is the first work that calculates a dead zone from the ion-neutral collision rate combined with a detailed disk chemical model. In our most realistic model, where cosmic rays are deflected by a stellar wind, the active zone at 5 AU is only 3 g cm−2 with dust settling included, which is too low to match the observed mass accretion rates. The dust-dependent X-ray opacities affect the abundance of related species, such as N2H+ and HCO+, but rarely by more than a factor of two.

Chapter 4 analyzes the effects that central star spectral type has on the protoplanetary disk chemistry and specifically the chemistry of the midplane. The spectral type will change the temperature and density structure of the disk, which influences the overall chemical composition, as has been seen in recent observational disk surveys. We find that disks around G, F and A stars have warm CO-gas rich midplanes while less luminous stars have CO-gas poor midplanes.