Iron as a building block material of the Earth naturally received significant attention, but the major effort in the past was put on the high-density and high-temperature region which are typical conditions for the Earth's core (126-360 GPa and 3500-6500 K). Much less is known about the thermodynamic and thermophysical properties of iron below 1 GPa and above 1850 K. Indeed, the regime of low densities and high temperatures is typical for the after-shock state of proto-planetary cores occurring in the aftermath of catastrophic events such as giant impacts, which is thought to generate the Moon. To fill in the knowledge gap of thermodynamic and thermophysical properties of iron in the low-density regime, in this study, we have performed ab initio molecular dynamics and ab initio Gibbs ensemble simulations to determine the position of the critical, and to characterize the fluid iron over a wide density and temperature range, with a special focus on the supercritical state. Based on our calculations, we predict the critical point of iron to be in the 9000-9350 K temperature range and 1.85-2.40 g/cm3 density range, corresponding to a pressures range of 4-7 kbars. The determination of the Hugoniot lines and our estimations of the amounts of entropy gained during the Giant Impact show that the core of Theia underwent partial vaporization, which would easily explain the recent W-isotope data which requires at least 30% core-mantle quilibration aftermath of the giant impact. The use of our results related to the position of the CP and the entropy during shock, together with better high-density EOS would definitely improve the reliability of such disk-scale simulations.
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