The effect of stellar composition on nebular condensation

The volatility of an element is defined by its 50% condensation temperature (T_c⁵⁰) from a nebular gas of Solar composition at 10^-4 bar. However, the variability in compositions of extrasolar systems inferred from the spectroscopic measurements of their parent stars implies that the identity, abundance and sequence of condensing phases should deviate from that of our Solar System. Here we perform Gibbs free energy minimisation with FactSage over temperatures from 1723 K to 473 K and 10^-4 bar total pressure to calculate the condensation sequence of 39 stellar compositions in the system H-C-O-Na-Mg-Al-Si-S-Ca-Ti-Fe-Ni that span the range from -0.4 to +0.4 dex. In accord with previous work, the C/O ratio of the gas plays a profound role in determining the mineralogy and order of condensing phases. Nebulae with 0.3 < C/O < 0.75 display broadly solar condensation sequences. Condensing nebulae with C/O > 0.75 exhibit strongly decreasing T_c⁵⁰ of lithophile elements whose condensation reactions depend upon the availability of oxygen (Ca, Al, Si, Ti, Mg) relative to those whose condensation does not (Fe, Ni). This trend is progressively reversed at very high C/O ratios for elements that form stable carbides, namely, TiC (C/O > 0.9) and SiC (C/O > 0.95), causing a dramatic increase in the T_c⁵⁰ of Ti and Si. The T_c⁵⁰ of S jumps from ~650 K below C/O = 0.75 to ~1100 K above C/O = 0.75 due to the condensation of oldhamite (CaS) and plateaus at higher C/O. The T_c⁵⁰ of Fe and Ni increase monotonically with Fe/H, and are fit to equations T_c⁵⁰(Fe) = 87.3(Fe/H) + 682 (r² = 0.999) and T_c⁵⁰(Ni) = 88.7(Fe/H) + 696 (r² = 0.999) valid for 7.08 < Fe/H < 7.95 and 0.3 < C/O < 0.95. As the C/O and Fe/H ratios are broadly correlated, only high metallicity stars are expected to produce nebula condensates whose compositions diverge drastically from those of our Solar System. Planets formed from these systems should be richer in Fe, Ni and S at the expense of Al and Ca.