We use chemical thermodynamics and where relevant chemical kinetics to model chemistry in different astrophysical environments including protoplanetary accretion disks, hot silicate vapor atmospheres on accreting rocky planets, and cool stars with molecules and dust in their visible atmospheres (photospheres) and in circumstellar shells of evolved stars. Necessary inputs for thermochemical computations are elemental abundances, complete thermodynamic functions, and temperature and total pressure.

Elemental abundances for solar composition systems come from chemical analyses of stony meteorites called CI-chondrites, photospheric spectra, and other datasets as described in our 2020 review of elemental and isotopic abundances of the 83 elements that occur in nature. Solar elemental abundances are the baseline for abundances in astronomical environments with variable metal/hydrogen ratios, i.e., variable metallicity [M/H]. We described the historical approaches that were made to determine the overall elemental composition of the solar system and give many data tables. Our 2020 paper is at the Oxford Research Encyclopedia of Planetary Sciences (https://doi.org/10.1093/acrefore/9780190647926.013.145) Our website (The Planetary Chemistry Laboratory, solarsystem.wustl.edu) also has our papers and data tables.

It is not as simple as it seems to determine the correct solar system abundance of an element. For example, results from other researchers in 2017 suggested that chlorine, bromine, and iodine abundances in CI-chondrites are up to ten times smaller than previously thought. However, this is not plausible because those Cl, Br, I abundances are out of trend with other elements and plot too low on the smooth curve when odd-numbered nuclide abundances are plotted versus mass numbers. Those smaller values also disagree with the chlorine abundances in the Sun, other dwarf stars, and other astronomical environments. We recommend a set of halogen abundances (F, Cl, Br, I) from careful analysis of a large dataset of astronomical and geochemical measurements and used them to redo condensation temperatures for the halogens. Lodders (2020) and another paper by Fegley et al (2020) report this work.

Although many stars have the same composition as our Sun, some contain more metals while others contain less metals. We studied the effects of these metallicity variations ([M/H]) on our chemical equilibrium condensation temperatures and present the results in a new paper currently being prepared for submission to a scientific journal. One important finding is that functions such as

1/Tcond = a + b log Ptotal + c [M/H] +{g(P, [M/H]}

give condensation temperatures for many elements within 1-5 K. We did condensation calculations over wide total pressure, temperature and metallicity ranges, e.g., 10^-6 to 100 bar, and 0.1 - 10 times the solar metallicity. We describe the calculations and results for many of the 83 natural elements and give the necessary equations for easy computations of condensation temperatures as a function of pressure and metallicity. The pressure and metallicity ranges correspond to appropriate ranges given by independent physical modeling, e.g., of protoplanetary disks, but the physical models are not critical for the computations because thermodynamic equilibrium is path independent and can be calculated for any given temperature, total pressure, and composition. Thus in turn, the results here could be used to constrain total pressure and metallicity ranges when they can be matched with observations of elemental abundances that show characteristics of chemical gas and condensate fractionations.

 We have updated and expanded our thermodynamic data base as part of this project. Thermodynamic (and where relevant kinetic) data, temperature, pressure, and elemental abundances are the necessary inputs for our modeling of chemistry in diverse astronomical environments. Thermodynamic data come from standard compilations such as the JANAF Tables and its Russian counterpart the IVTAN Tables and numerous individual research papers in the literature. Kinetic data come from standard compilations such as the NIST kinetic databases. In addition to the enthalpies of formation, it is important to include the temperature-dependent entropies and heat capacities as a function of temperature for all gases, solids, and liquids to have complete thermodynamic functions for the computations. The thermodynamic data for all substances need to be self-consistent and be referenced to the same set of standard state for each element. Not all compilations use the same reference data for the elements, but this is ensured in our data base.

Last Modified: 06/23/2021
Modified by: Katharina Lodders