Cooling Delays from Iron Sedimentation and Iron Inner Cores in 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.

8 pages, 5 figures, accepted for publication in ApJ Letters

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