Reduction of chemical networks. II. Analysis of the fractional ionisation in protoplanetary discs

TL;DR

This paper investigates the fractional ionisation in protoplanetary discs, considering various ionisation sources and chemical reactions, and introduces reduction schemes to simplify the complex chemical networks while maintaining accuracy.

Contribution

It presents reduction schemes that identify small reaction sets capable of accurately modeling ionisation in protoplanetary discs.

Findings

Reduction schemes reproduce ionisation evolution with 50-100% accuracy.

Column densities of key molecules are consistent with observational data.

Analysis highlights the dominant chemical processes affecting ionisation.

Abstract

(abridged) We analyse the evolution of the fractional ionisation in a steady-state protoplanetary disc with a vertical temperature gradient and with gas-grain chemistry including surface reactions. The ionisation due to stellar X-rays, stellar and interstellar UV radiation, cosmic rays and radionuclide decay is taken into account. Using our reduction schemes as a tool for the analysis, we isolate small sets of chemical reactions that reproduce the evolution of the ionisation degree at representative disc locations with an accuracy of 50%-100%. Column densities of key molecules are calculated and compared to the results of other recent studies and observational data.