Diffusion Coefficients of 56Fe in C–O and O–Ne White Dwarfs

Abstract

The diffusion coefficients of neutron rich nuclei in crystallizing white dwarf (WD) stars are essential microphysics input for modeling the evolution of the composition profile. Recently, molecular dynamics simulations have been used to compute diffusion coefficients for realistic mixtures of C–O and O–Ne WDs with many trace nuclides that could be important sedimentary heat sources such as 22Ne, 23Na, 25Mg, and 27Mg. In this brief note, I repeat these simulations but now include 56Fe. I find that for the large charge ratios involved in these mixtures the empirical law developed in our earlier work tends to under-predict diffusion coefficients in the moderately coupled regime by 30%–40%. As this formalism is presently implemented in the stellar evolution code MESA, it is important for authors studying mixtures containing heavy nuclides like 56Fe to be aware of these systematics. However, the impact on astrophysics is expected to be small.