The Effects of Initial Abundances on Nitrogen in Protoplanetary Disks

TL;DR

This study uses a detailed chemical model to examine how initial nitrogen abundances in protoplanetary disks influence the chemical evolution and dominant nitrogen-bearing species, with implications for observations of comets and disks.

Contribution

It introduces a comprehensive model analyzing the impact of initial nitrogen states on disk chemistry and compares predictions with observed cometary and disk nitrogen species.

Findings

Initial nitrogen form affects dominant nitrogen species over time.

N2H+ emission indicates N2 abundance greater than 10^-6.

Initial atomic N best matches cometary ammonia observations.

Abstract

The dominant form of nitrogen provided to most solar system bodies is currently unknown, though available measurements show that the detected nitrogen in solar system rocks and ices is depleted with respect to solar abundances and the interstellar medium. We use a detailed chemical/physical model of the chemical evolution of a protoplanetary disk to explore the evolution and abundance of nitrogen-bearing molecules. Based on this model we analyze how initial chemical abundances, provided as either gas or ice during the early stages of disk formation, influence which species become the dominant nitrogen bearers at later stages. We find that a disk with the majority of its initial nitrogen in either atomic or molecular nitrogen is later dominated by atomic and molecular nitrogen as well as NH$_{3}$ and HCN ices, where the dominant species varies with disk radius. When nitrogen is initially in gaseous ammonia, it later becomes trapped in ammonia ice except in the outer disk where atomic nitrogen dominates. For a disk with the initial nitrogen in the form of ammonia ice the nitrogen remains trapped in the ice as NH$_{3}$ at later stages. The model in which most of the initial nitrogen is placed in atomic N best matches the ammonia abundances observed in comets. Furthermore the initial state of nitrogen influences the abundance of N$_{2}$H$^{+}$, which has been detected in protoplanetary disks. Strong N$_{2}$H$^{+}$ emission is found to be indicative of an N$_{2}$ abundance greater than $n_{\mathrm{N_{2}}}/n_{\mathrm{H_{2}}}>10^{-6}$, in addition to tracing the CO snow line. Our models also indicate that NO is potentially detectable, with lower N gas abundances leading to higher NO abundances.