Slow convection and fast rotation in crystallization-driven white dwarf dynamos

Abstract

It has been recently suggested that white dwarfs generate magnetic fields in a process analogous to the Earth. The crystallization of the core creates a compositional inversion that drives convection, and combined with rotation, this can sustain a magnetic dynamo. We reanalyse the dynamo mechanism, arising from the slow crystallization of the core, and find convective turnover times $t_{\mathrm{conv}}$ of weeks to months - longer by orders of magnitude than previously thought. With white dwarf spin periods $P\ll t_{\mathrm{conv}}$, crystallization-driven dynamos are almost always in the fast rotating regime, where the magnetic field $B$ is at least in equipartition with the convective motion and is possibly further enhanced by a factor of $B\propto (t_{\mathrm{conv}}/P{)}^{1/2}$, depending on the assumed dynamo scaling law. We track the growth of the crystallized core using MESA and compute the magnetic field $B(T_{\mathrm{eff}})$ as a function of the white dwarf's effective temperature $T_{\mathrm{eff}}$. We compare this prediction with observations and show that crystallization-driven dynamos can explain some - but not all - of the $\sim$MG magnetic fields measured for single white dwarfs, as well as the stronger fields measured for white dwarfs in cataclysmic variables, which were spun up by mass accretion to short $P$. Our $B(T_{\mathrm{eff}})$ curves might also explain the clustering of white dwarfs with Balmer emission lines around $T_{\mathrm{eff}}\approx 7500\text{ K}$.

Accepted version (MNRAS): improved calculation of the convection zone and the magnetic diffusion time, and a larger observational sample

References in corpus (15)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• Magnetic White Dwarfs
• Binary Star Origin of High Field Magnetic White Dwarfs
• Magnetic fields in isolated and interacting white dwarfs
• A common origin of magnetism from planets to white dwarfs
• Powering Stellar Magnetism: Energy Transfers in Cyclic Dynamos of Sun-like Stars
• Theory and simulations of rotating convection
• The incidence of magnetic fields in cool DZ white dwarfs
• Enigmas from the Sloan Digital Sky Survey DR7 Kleinman White Dwarf Catalog
• Single magnetic white dwarfs with Balmer emission lines: A small class with consistent physical characteristics as possible signposts for close-in planetary companions
• Are white dwarf magnetic fields in close binaries generated during common-envelope evolution?
• Magnetic dynamos in white dwarfs -- II. Relating magnetism and pollution
• Magnetic dynamos in white dwarfs -- I. Explaining the dearth of bright intermediate polars in globular clusters
• A test of the planet-star unipolar inductor for magnetic white dwarfs