The Fraction of Distilled White Dwarfs with Long-Lived Habitable Zones

Abstract

After carbon and oxygen, ${}^{22}$Ne is the most abundant element in white dwarf interiors. As C/O white dwarfs (WDs) crystallize, they are predicted to go through a distillation process in the central layers if they have a sufficiently high ${}^{22}$Ne mass fraction of $\gtrsim 2.5$\%. Observational evidence for distillation comes from an over-density of WDs on the Q-branch in Gaia color-magnitude diagrams, which indicates that $\sim 6$\% of massive WDs are delayed in their cooling by as much as $\sim 10$ Gyr. However, it is unclear how these stars end up with such a high concentration of ${}^{22}$Ne and if a significant fraction of the more common average-mass WDs go through distillation. We argue that a significant metal-rich stellar population in the solar neighborhood should lead to distilled WDs, without requiring a binary merger. We use MESA along with the CNO abundances derived from high-resolution spectroscopy of stars included in the Hypatia catalog to predict the ${}^{22}$Ne mass fraction in their descendant WDs. We find that 0.6-2.5\% of the WDs in the solar neighborhood have sufficient ${}^{22}$Ne in their interiors to go through multi-Gyr cooling delays, which could significantly inflate their numbers in the observed samples. Hence, ${}^{22}$Ne distillation and long-lived habitable zones around WDs should be relatively common in the solar neighborhood. We also use a Galactic model to predict the fraction of WDs that go through distillation as a function of Galactocentric distance. The fraction of distilled WDs is $\sim 2$-8\% near the Galactic center, and declines steadily toward the outer disk.

Accepted for Publication in ApJL. 7 pages, 4 figures

References in corpus (11)

• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• Stellar Abundances in the Solar Neighborhood: The Hypatia Catalog
• The 100 pc White Dwarf Sample in the SDSS Footprint
• The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood
• ${}^{22}$Ne Phase Separation As A Solution To The Ultramassive White Dwarf Cooling Anomaly
• A Comparison of Stellar Elemental Abundance Techniques and Measurements
• Asteroseismology of ZZ Ceti stars with fully evolutionary white dwarf models: I. The impact of the uncertainties from prior evolution on the period spectrum
• A Catalog of Stellar Unified Properties (CATSUP) for 951 FGK-Stars Within 30 pc
• Buoyant crystals halt the cooling of white dwarf stars
• A Concise Treatise on Converting Stellar Mass Fractions to Abundances to Molar Ratios
• The impact of the uncertainties in the 12C(α,γ)16O reaction rate on the evolution of low- to intermediate-mass stars