Equilibrium chemistry down to 100 K - Impact of silicates and phyllosilicates on carbon/oxygen ratio

TL;DR

This paper introduces GGchem, a fast code for calculating gas compositions in thermo-chemical equilibrium down to 100 K, highlighting the impact of silicates and phyllosilicates on the C/O ratio and dust formation.

Contribution

The paper presents a new versatile code, GGchem, capable of modeling chemical equilibrium including condensation processes at very low temperatures, with detailed data benchmarking and implications for dust formation.

Findings

Silicate condensation raises the gas C/O ratio from 0.55 to 0.71.

Phyllosilicates further increase the C/O ratio to about 0.83.

Tungsten condenses before other refractory materials, potentially seeding dust formation.

Abstract

We introduce a fast and versatile computer code, GGchem, to determine the chemical composition of gases in thermo-chemical equilibrium down to 100 K, with or without equilibrium condensation. We review the data for molecular equilibrium constants, kp(T), from several sources and discuss which functional fits are most suitable for low temperatures. We benchmark our results against another chemical equilibrium code. We collect Gibbs free energies, dG(T), for about 200 solid and liquid species from the NIST-JANAF database and the geophysical database SUPCRTBL. We discuss the condensation sequence of the elements with solar abundances in phase equilibrium down to 100 K. Once the major magnesium silicates Mg2SiO4[s] and MgSiO3[s] have formed, the dust/gas mass ratio jumps to a value of about 0.0045 which is significantly lower than the often assumed value of 0.01. Silicate condensation is found to increase the carbon/oxygen ratio (C/O) in the gas from its solar value of ~0.55 up to ~0.71, and, by the additional intake of water and hydroxyl into the solid matrix, the formation of phyllosilicates at temperatures below ~400 K increases the gaseous C/O further to about 0.83. Metallic tungsten (W) is the first condensate found to become thermodynamically stable around 1600 - 2200 K (depending on pressure), several hundreds of Kelvin before subsequent materials like zirconium dioxide (ZrO2) or corundum (Al2O3) can condense. We briefly discuss whether tungsten, despite its low abundance of ~2.E-7 times the silicon abundance, could provide the first seed particles for astrophysical dust formation. The GGchem code is publicly available at https://github.com/pw31/GGchem.