Analyzing the Distribution and Chemical Evolution of the Major Nitrogen Carriers and Volatile Elemental Ratios in Theoretical Protoplanetary Disks

Abstract

The presence of nitrogen is crucial for the existence and survival of life as we know it today. And yet, nitrogen is depleted in Earth and unearthed meteorites, relative to solar abundances and other volatiles in the Solar System. The distribution of nitrogen in the bodies of the Solar System, and other extra-solar systems, is a product of the system’s earlier chemical evolution as a protoplanetary disk. These bodies develop their chemical composition based upon (1) where they form with respect to the disk’s snowsurfaces, and (2) the initial chemical conditions that guide the disk’s chemical evolution as a whole. In this thesis, we explore the distribution and evolution of nitrogen in chemically non-evolving and chemically evolving protoplanetary disk models. We analyze the effects of initial conditions, such as chemical abundance and ice morphology, as well as the effects of time-dependent chemistry, on the major carriers of nitrogen and the volatile elemental ratios (N/O and N/C) across the disk. For the chemically non-evolving model, we find that the midplane gas-phase N/O and N/C ratios are well over the solar N/O and N/C ratios by more than factors of 2.0 and 1.5 across the entire disk. The solid-phase ratios are lower than the solar ratios interior to the N2 midplane snowline, with maximum values of around 0.02 and 0.09. Ice morphology affects snowline location; when CO and N2 are bound in ices mixed with water, compared to when they are bound in pure ices, their snowlines are shifted inwards. In our model, the snowlines move from 28 AU to 12 AU for CO and from 37 AU to 21 AU for N2. With increased NH3 abundance, exterior to the NH3 snowline, the gas-phase N/O and N/C ratios decrease, while the solid-phase ratios increase. Even with the maximum NH3 abundance considered, the gas-phase ratios are higher than the solar ratios across the disk. For the chemically evolving model, we find that depletions in the major initial nitrogen carriers along the midplane lead to the formation of new significant nitrogen carriers by the end of the disk’s lifetime, including HNC, HNO, and N2H2. Much of the evolution of the disk’s gas-phase and solid-phase N/O and N/C ratios is concentrated in the warm molecular and lower atmospheric layers, as well as along the upper midplane. The gas-phase ratios in the warm molecular layer decrease over 1 million years due to the formation of HNO and N2O; over that same timespan, the solidphase ratios in the upper warm molecular layer increase above 2.0. Ratios in the atmospheric layer stay largely constant with time. When all nitrogen is split between atomic N and N2 at the start of the model, we find an overabundance of the gas-phase N/O and N/C column density ratios, interior to the NH3 snowline, specifically at the end of the disk’s lifetime. In comparing the two models, we find that the lower midplane N/O and N/C ratios of the two models are similar after 1 million years of evolution. We determine a comet-forming regime for the models that supports the high abundances of CO, relative to a lack of abundance of N2, measured spectroscopically in some comets.