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

P. Woitke, Ch. Helling+6 more|arXiv (Cornell University)|Dec 4, 2017

Astro and Planetary Science104 references58 citations

TL;DR

The paper presents GGchem, a fast thermo-chemical equilibrium code (down to 100 K) that includes equilibrium gas-phase chemistry and condensation, and analyzes how silicate and phyllosilicate condensation alters the gas-phase C/O ratio and dust formation seeding (notably tungsten).

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.

Motivation & Objective

• Develop and validate a fast, versatile code (GGchem) to determine gas-phase abundances in thermo-chemical equilibrium down to 100 K, with or without equilibrium condensation.
• Review and benchmark molecular and condensed-phase thermo-chemical data from multiple sources (NIST-JANAF, SUPCRTBL, Barklem & Collet) for consistency and low-temperature reliability.
• Analyze how condensation sequences (especially Mg-silicates) modify element budgets and the resulting C/O ratios in the gas phase, including the role of phyllosilicates at T < 400 K.
• Investigate the first condensates (e.g., tungsten) and their potential as seed particles for astrophysical dust formation.
• Provide public access to the GGchem code and data for the community.

Proposed method

• Solve gas-phase chemical abundances by minimizing Gibbs free energy with or without condensation.
• Use Gibbs free energy data to derive equilibrium constants k_p(T) for molecular formation (k_p from ΔG_f°) with temperature-dependent fits (Stock et al. form chosen).
• Incorporate phase equilibrium via supersaturation ratios S_j and condensate vapor pressures p_vap(T); iteratively reduce gas-phase element abundances due to condensation.
• Employ a Newton-Raphson iteration with nested calls to equilibrium chemistry and quadruple-precision arithmetic for numerical stability at low T (down to 100 K).
• Utilize data from NIST-JANAF and SUPCRTBL for ΔG_f°(T) of condensates, and fit multiple datasets with robust functions to enable low-temperature extrapolation.
• Provide a public GGchem implementation (FORTRAN-90) with updated data and an added pre-iteration scheme.

Experimental results

Research questions

• RQ1 How does equilibrium condensation alter the gas-phase element abundances and the overall chemical composition at temperatures down to 100 K?
• RQ2 What is the condensation sequence of elements for solar abundances, and how does silicate and phyllosilicate formation affect the gas-phase C/O ratio?
• RQ3 Which condensates form first (e.g., Mg2SiO4, MgSiO3, tungsten) and under what conditions do they become thermodynamically stable?
• RQ4 Can tungsten act as an early seed particle for dust formation despite its low abundance?
• RQ5 How do different thermo-chemical data sources compare in predicting molecular and condensed-phase equilibria across 100–6000 K?

Key findings

• Condensation of major magnesium silicates (Mg2SiO4 and MgSiO3) triggers a dust/gas mass ratio jump to about 0.0045.
• Silicate condensation raises the gas-phase C/O from solar ~0.55 to ~0.71, and phyllosilicate formation below ~400 K can further raise C/O to ~0.83.
• Tungsten (W) is the first condensate to become thermodynamically stable around 1600–2200 K (pressure-dependent), preceding ZrO2 and Al2O3, and may seed astrophysical dust formation.
• The GGchem code, with quadruple precision and revised pre-iteration strategies, provides accurate results down to 100 K and is publicly available at the GGchem GitHub repository.
• A comprehensive comparison of k_p(T) data from multiple sources reveals notable deviations (up to ~10 kJ/mol) for some molecules, underscoring the importance of data choice and fit form for low-temperature extrapolations.
• The condensation-process integration significantly affects gas-phase abundances and spectral predictions, highlighting the need to model equilibrium condensation in cool/exoplanetary environments.