Magnetic dynamos powered by white dwarf superficial convection

Abstract

When the effective temperature of a cooling white dwarf $T_{\mathrm{eff}}$ drops below the ionization limit, it develops a surface convection zone that may generate a magnetic field $B$ through one of several dynamo mechanisms. We revisit this possibility systematically using detailed stellar evolution computations, as well as a simple analytical model that tracks the expansion of the convection zone. The magnetic field reaches a maximum of several kG (for a hydrogen atmosphere) shortly after a convection zone is established at a cooling time $t=t_{\mathrm{conv}}$. The field then declines as $B\propto T_{\mathrm{eff}}\propto t^{-7/20}$ until the convective envelope couples to the degenerate core at $t=t_{\mathrm{coup}}$. We compare the onset of convection $t_{\mathrm{conv}}\propto M^{25/21}$ to the crystallization of the white dwarf's core $t_{\mathrm{cryst}}\propto M^{-5/3}$, and find that in the mass range $0.5{\mathrm{M}}_{\odot }<M<0.9{\mathrm{M}}_{\odot }$ the order of events is $t_{\mathrm{conv}}<t_{\mathrm{cryst}}<t_{\mathrm{coup}}$. Specifically, surface dynamos are active for a period $\Delta t\approx t_{\mathrm{cryst}}-t_{\mathrm{conv}}$ of about a Gyr (shorter for higher masses), before the convection zone is overrun by a stronger magnetic field emanating from the crystallizing core. Our predicted magnetic fields are at the current detection limit, and we do not find any observed candidates that fit the theory. None the less, surface dynamos may be an inevitable outcome of white dwarf cooling, significantly affecting white dwarf accretion and seismology.

matches published version

References in corpus (53)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Giant Planets, Oscillations, Rotation, and Massive Stars
• Modules for Experiments in Stellar Astrophysics (MESA): Binaries, Pulsations, and Explosions
• Modules for Experiments in Stellar Astrophysics (MESA): Convective Boundaries, Element Diffusion, and Massive Star Explosions
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• Dynamo action by differential rotation in a stably stratified stellar interior
• Modules for Experiments in Stellar Astrophysics (MESA): Time-Dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure
• A fossil origin for the magnetic fields of A-stars and white dwarfs
• Pulsating White Dwarf Stars and Precision Asteroseismology
• Slowing the Spins of Stellar Cores
• Magnetic White Dwarfs
• A solar surface dynamo
• Binary Star Origin of High Field Magnetic White Dwarfs
• White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Linewidths: Kepler Observations of 27 Pulsating DA White Dwarfs Through K2 Campaign 8
• Double degenerate mergers as progenitors of high-field magnetic white dwarfs
• High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey. III. DA white dwarfs
• Magnetic activity of F stars observed by Kepler
• Global Models of Runaway Accretion in White Dwarf Debris Disks
• Skye: A Differentiable Equation of State
• Magnetic fields in isolated and interacting white dwarfs
• The origin and evolution of magnetic white dwarfs in close binary stars
• The formation of high-field magnetic white dwarfs from common envelopes
• A common origin of magnetism from planets to white dwarfs
• Discovery of kilogauss magnetic fields in three DA white dwarfs
• The Magnetic Furnace: Intense Core Dynamos in B-stars
• The 40 pc sample of white dwarfs from Gaia
• On The Evolution of Magnetic White Dwarfs
• Prospects for Measuring Differential Rotation in White Dwarfs Through Asteroseismology
• On the incidence of weak magnetic fields in DA white dwarfs
• The fraction of DA white dwarfs with kilo-Gauss magnetic fields
• Multiple channels for the onset of magnetism in isolated white dwarfs
• A study of high proper-motion white dwarfs I. Spectropolarimetry of a cool hydrogen-rich sample
• Universality of the Small-Scale Dynamo Mechanism
• A Spectro-photometric Analysis of Cool White Dwarfs in the Gaia and Pan-STARRS Footprint
• Rossby and Magnetic Prandtl Number Scaling of Stellar Dynamos
• Horizontal spreading of planetary debris accreted by white dwarfs
• Magnetism, X-rays, and Accretion Rates in WD 1145+017 and other Polluted White Dwarf Systems
• Searching for the weakest detectable magnetic fields in white dwarfs. Highly-sensitive measurements from first VLT and WHT surveys
• Enhanced mixing in magnetized fingering convection, and implications for RGB stars
• Heat transport and convective velocities in compositionally-driven convection in neutron star and white dwarf interiors
• A novel and sensitive method for measuring very weak magnetic fields of DA white dwarfs: A search for a magnetic field at the 250 G level in 40 Eri B
• Monitoring and modelling of white dwarfs with extremely weak magnetic fields
• Discovery of weak magnetic fields in four DZ white dwarfs in the local 20pc volume. Implications for the frequency of magnetic fields with cooling age
• Probing the internal rotation of pre-white dwarf stars with asteroseismology: the case of PG 122+200
• Magnetic field breakout from white dwarf crystallization dynamos
• A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs
• Main sequence dynamo magnetic fields emerging in the white dwarf phase
• Magnetic field evolution for crystallization-driven dynamos in C/O white dwarfs
• Fast and Slow Crystallization-driven Convection in White Dwarfs
• Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with {\it TESS}: III. Asteroseismology of the DBV star GD 358
• Analytic solution of an oscillatory migratory alpha^2 stellar dynamo
• Photometric White Dwarf Rotation
• Younger age for the oldest magnetic white dwarfs

Cited by in corpus (1)

• Seismology and diffusion of ultramassive white dwarf magnetic fields