Freezing and Cluster Formation in Dense White Dwarf Plasmas

Recent observational evidence shows that the cooling of white dwarf (WD) stars slows temporarily when the strongly coupled plasma in the star freezes. The energy maintaining the star’s temperature comes from latent heat and gravitational energy from the inward migration of heavier elements. For a small subpopulation of WDs, the cooling delay is anomalously long. The leading explanation for these extra-long delays is neon distillation, which requires Ne-depleted solids to form and float up through the heavier Ne-rich liquid. This explanation relies on the untested assumption that solid clusters form and are stable enough to diffuse through surrounding liquid. We are exploring the formation and properties of clusters in dense single-component Yukawa (screened Coulomb) fluids with different screening lengths. We performed brute-force and seeded molecular dynamics simulations of crystal nucleation in undercooled liquids. With a classical nucleation theory framework, we extract nucleation rates and cluster size distributions at a range of temperatures relevant to crystallizing WDs. This is a first step toward a full description of the size and stability of clusters in the multicomponent fluids that may drive Ne distillation.