Miscibility of Hydrogen and Helium under Astrophysical Conditions

We investigate the stability and structural properties of dense hydrogen-helium mixtures under planetary interior conditions (Jupiter and Saturn). The excess free energy of mixing ΔG(x) is determined as a function of the helium atomic fraction x by ab initio molecular dynamics simulations in the pressure range 4≤P≤24Mbar at temperatures T≤3000K. Energy, volume, and entropic contributions are evaluated. Demixing temperatures at x0ex0ex=0ex0ex0.1 (the solar abundance) are 4000--9000 K (dependent on P), close to what planetary interior models predict for Saturn but well below the corresponding interior temperatures in Jupiter.