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Abstract From transmission electron microscopy and other laboratory studies of presolar grains, the implicit condensation sequence of carbon-bearing condensates in circumstellar envelopes of carbon stars is (from first to last) TiC-graphite-SiC. We use thermochemical equilibrium condensation calculations and show that the condensation sequence of titanium carbide (TiC), graphite (C(Gr)), and silicon carbide (SiC) depends on metallicity in addition to C/O ratio and total pressure. Calculations were performed for a characteristic carbon star ratio of C/O = 1.2 from 10⁻¹⁰to 10⁻⁴bars total pressure and for uniform metallicity variations ranging from 0.01 to 100 times solar elemental abundances. TiC always condenses at higher temperatures than SiC, and the carbide condensation temperatures increase with both increasing metallicity and increasing total pressure. Graphite, however, can condense in a cooling circumstellar envelope before TiC, between TiC and SiC, or after SiC, depending on the carbon-bearing gas chemistry, which is dependent on metallicity and total pressure. Analytical expressions for the graphite, TiC, and SiC condensation temperatures as functions of metallicity and total pressure are presented. The inferred sequence from laboratory presolar grain studies, TiC-graphite-SiC, is favored under equilibrium conditions at solar and subsolar metallicities between ∼10⁻⁵and 10⁻⁸bar total pressure within circumstellar envelopes of carbon stars with nominal C/O = 1.2. We also explored the dependence of the sequence at C/O ratios of 1.1 and 3.0, and found that as the C/O ratio increases, the TiC-graphite-SiC condensation sequence region occurs toward higher total pressures and lower metallicities. 

Abstract This article describes condensation of the elements and use of condensation temperatures to plot and interpret Earth’s apparent volatility trend. Major points covered include the following. (1) Updated 50% condensation temperatures (T₅₀) for all naturally occurring elements, Tc, and Pu are tabulated at 10⁻²to 10⁻⁸bar total pressure for solar composition material. (2) Condensation temperatures are mainly controlled by the Gibbs energy of condensation reactions and also by the Gibbs energy of ideal mixing if elements (compounds) condense in a solution. The additional Gibbs energy change due to non-ideal solution, i.e., activity coefficients ≠1, is a secondary effect. (3) The theoretically correct relationship between condensation temperature and fraction condensed ($$\alpha _{\mathrm{M}}$$ ${\alpha }_M$) is derived from mass balance and chemical thermodynamic considerations. For major elements the condensation temperature is inversely proportional to log (1-$$\alpha _{\mathrm{M}}$$ ${\alpha }_M$). For trace elements dissolving in solid solution the condensation temperature is inversely proportional to log [(1-$$\alpha _{\mathrm{M}}$$ ${\alpha }_M$)/$$\alpha _{\mathrm{M}}$$ ${\alpha }_M$]. (4) The maximum amount of element condensed per K⁻¹, i.e., the maximum in [d$$\alpha _{\mathrm{M}}$$ ${\alpha }_M$/d(1/T)] is at the inflection point in the logistic (sigmoid) curve for an element, which is also at (or close to) the 50% condensation temperature. (5) Plots of normalized elemental abundances versus 50% condensation temperatures (volatility trends) are qualitative indicators of elemental fractionations due to volatility because they do not use the theoretically correct and quantitative relationship between condensation temperature and fraction condensed. (6) Volatility trend plots for average elemental abundances in CM, CO, CV, CR, H, L, LL, EH, EL chondrites show different “trends” for moderately and highly volatile elements, which may be linear, curved, a step function, or plateau. A comparison of three abundance sets for CM and CV chondrites shows trends depend on which elements are plotted, which data sources are used, and which temperature range is considered. (7) Proposed mechanisms for volatile element depletion in carbonaceous chondrites and the Earth are reviewed. (8) Some possible implications of volatile element abundances in the bulk silicate Earth are discussed. 

Abstract Solar photospheric abundances and CI-chondrite compositions are reviewed and updated to obtain representative solar system abundances of the elements and their isotopes. The new photospheric abundances obtained here lead to higher solar metallicity. Full 3D NLTE photospheric analyses are only available for 11 elements. A quality index for analyses is introduced. For several elements, uncertainties remain large. Protosolar mass fractions are H (X = 0.7060), He (Y = 0.2753), and for metals Li to U (Z = 0.0187). The protosolar (C+N)/H agrees within 13% with the ratio for the solar core from the Borexino experiment. Elemental abundances in CI-chondrites were screened by analytical methods, sample sizes, and evaluated using concentration frequency distributions. Aqueously mobile elements (e.g., alkalis, alkaline earths, etc.) often deviate from normal distributions indicating mobilization and/or sequestration into carbonates, phosphates, and sulfates. Revised CI-chondrite abundances of non-volatile elements are similar to earlier estimates. The moderately volatile elements F and Sb are higher than before, as are C, Br and I, whereas the CI-abundances of Hg and N are now significantly lower. The solar system nuclide distribution curves of s-process elements agree within 4% with s-process predictions of Galactic chemical evolution models. P-process nuclide distributions are assessed. No obvious correlation of CI-chondritic to solar elemental abundance ratios with condensation temperatures is observed, nor is there one for ratios of CI-chondrites/solar wind abundances. 

Discussion of dust mineralogy and condensation temperatures of presolar grains forming in asymptotic giant branch (AGB) stars and in supernovae. Condensation temperatures as a function of total pressure and metallicity are listed for solar-like composition system. Reduced condensates at high C/O ratios are described. 

{"Abstract":["Machine-readable tables accompany the book chapter "Chemical Composition of the Sun", authors Maria Bergemann, Katharina Lodders, Herbert Palme,  Encyclopedia of Astrophysics 1st Edition (edited by I. Mandel, section editor F.R.N. Schneider) to be published by Elsevier as a Reference Module, 2025"]}