A chemical perspective on planet formation in reduced systems

Abstract

Relative abundances of refractory elements in planets are commonly assumed to reflect those of their host stars. However, because elements are classified according to their behaviour in the solar nebula, this implicitly assumes condensation is independent of nebular chemistry, despite evidence to the contrary in chemically reduced systems with high molar carbon-to-oxygen (C/O) ratios. We investigate how variations in stellar C/O ratio and disk pressure modify condensation chemistry and assess the reliability of mapping stellar compositions to planetary building blocks in reduced environments. For a sample of FGK stars with C/O ratios spanning 0.65-0.95 (solar = 0.50), we compute equilibrium phase stability using FactSage over 1900-400 K at total pressures of 1e-2, 1e-4, and 1e-6 bar. Bulk planetesimal compositions are derived using a stochastic accretion framework aggregating condensates from temperature-dependent feeding zones. We identify three distinct condensation regimes: (i) solar-like (C/O < 0.7), (ii) transitional (C/O ~0.7-0.91), and (iii) reduced (C/O > 0.92). Relative to solar sequences, oxygen-bearing silicates condense at lower temperatures in transitional and reduced regimes, while carbides, silicides, and sulfides appear. Bulk planetesimal Fe/Mg, Fe/Si, and Fe/O ratios deviate substantially from host stellar values, producing more diverse rocky building blocks within a single disk. Condensation sequences are not universal across stellar compositions. In reduced disks, elemental ratios commonly treated as refractory may not reliably trace planetary bulk composition, providing potential formation pathways for metal-enriched super-Mercury analogues and C- and S-rich rocky planets.

Figures (25)

• Figure 1: Distribution of our adopted dataset of elemental abundances of Sun-like stars (FGK dwarfs) on the [Fe/H]-C/O diagram. Sample of stars selected for this study is highlighted in red. The solar reference Lodders2003SolarElements is shown as a star sign. The typical error bars of the dataset in the [Fe/H]-C/O space are shown in the upper right corner.
• Figure 2: The variation of oxygen fugacity with respect to the Iron-Wüstite buffer ($\Delta$IW) as a function of disk mid-plane temperature for three representative cases: Lodders03 ('solar-like sequence'), HD94151 ('transitional sequence'), and HD24633 systems ('reduced sequence').
• Figure 3: Instantaneous fraction condensed ($f_c$) of each element as a function of temperature in solar-like (Lodders03; C/O$= 0.50$), transitional (HD94151; C/O$= 0.89$), and reduced (HD24633; C/O$= 0.95$) systems at $10^{-4}$ bar total pressure. For relevant reactions corresponding to the condensation peaks, see subsections \ref{['sec:Refractory']}--\ref{['sec:sulfide']}
• Figure 4: Condensation of (refractory) oxides (Al, Ca, Ti, O $\pm$ Mg, Fe, Cr, Ni) in solar-like (Lodders03; C/O$= 0.50$), transitional (HD94151; C/O$= 0.89$), and reduced (HD24633; C/O$= 0.95$) systems at a disk pressure of $10^{-4}$ bar. Species marked with an asterisk (*) are treated as solid solution phases in the calculations.
• Figure 5: Same as \ref{['fig:refractory_comp_linear']}, but for condensation of major silicate phases.