Actinide Crystallization and Fission Reactions in Cooling White Dwarf Stars

The first solids that form as a cooling white dwarf (WD) starts to crystallize are expected to be greatly enriched in actinides. This is because the melting points of WD matter scale as $Z^{5/3}$ and actinides have the largest charge $Z$. We estimate that the solids may be so enriched in actinides that they could support a fission chain reaction. This reaction could ignite carbon burning and lead to the explosion of an isolated WD in a thermonuclear supernova (SN Ia). Our mechanism could potentially explain SN Ia with sub-Chandrasekhar ejecta masses and short delay times.

Figure 1

Phase diagram for a mixture of N and U. The $y$ axis is the temperature in units of the melting temperature of pure N (about 0.5 keV) and the $x$ axis is the number fraction of U. The liquid phase, with composition given by the solid black line, is in equilibrium with the solid phase (dashed red line).

Figure 2

Composition of the equilibrium solid phase (solid red line) and liquid phase (dashed black line) for all chemical elements $Z$. This assumes a linear mixing approximation for the free energies, see text. Numerical values are listed in the Supplemental Material [25].