Cooling Delays from Iron Sedimentation and Iron Inner Cores in White Dwarfs

TL;DR

This paper investigates the possibility of iron sedimentation forming inner cores in white dwarfs, using phase diagrams and simulations, and explores its effects on cooling delays and asteroseismology detection.

Contribution

It introduces new phase diagrams and molecular dynamics simulations showing iron crystallization and core formation in white dwarfs, a novel insight into their internal structure.

Findings

Iron crystallites form and precipitate rapidly in white dwarfs.

Inner iron cores of about 100 km could be detectable via asteroseismology.

Cooling delays due to iron sedimentation could reach up to a Gyr in low-mass white dwarfs.

Abstract

Do white dwarfs have inner cores made of iron? Neutron rich nuclei like $^{56}$Fe experience a net gravitational force and sediment toward the core. Using new phase diagrams and molecular dynamics simulations, we show that $^{56}$Fe should separate into mesoscopic Fe-rich crystallites due to its large charge relative to the background. At solar abundances, these crystallites rapidly precipitate and form an inner core of order 100 km and $10^{-3} M_\odot$ that may be detectable with asteroseismology. Associated cooling delays could be up to a Gyr for low mass white dwarfs but are only $\sim$0.1 Gyr for massive white dwarfs, so while this mechanism may contribute to the Q-branch the heating is insufficient to fully explain it.