Magnetic field evolution for crystallization-driven dynamos in C/O white dwarfs

Abstract

We investigate the evolution of magnetic fields generated by the crystallization-driven dynamo in carbon-oxygen white dwarfs (WDs) with masses $\lesssim 1.05M_{\odot }$. We use scalings for the dynamo to demonstrate that the initial magnetic field strength ($B_0$) has an upper limit that depends on the initial convection zone size ($R_{\mathrm{out},0}$) and the WD mass. We solve the induction equation to follow the magnetic field evolution after the dynamo phase ends. We show that the predicted surface magnetic field strength ($B_{\mathrm{surf}}$) differs from $B_0$ by at least a factor of $\sim$0.3. This reduction depends on $R_{\mathrm{out},0}$, where values smaller than half of the star radius give $B_{\mathrm{surf}}\lesssim 0.01B_0$. We implement electrical conductivities that account for the solid phase effect on the Ohmic diffusion. We observe that the conductivity increases as the solid core grows, freezing in the magnetic field at a certain point of the evolution and slowing its outwards transport. We study the effect of turbulent magnetic diffusivity induced by the convection and find that for a small $R_{\mathrm{out},0}$, $B_{\mathrm{surf}}$ is stronger than the non-turbulent diffusion cases because of the more rapid transport, but still orders of magnitude smaller than $B_0$. Given these limitations, the crystallization-driven dynamo theory could explain only magnetic C/O WDs with field strengths less than a few MG for the mass range 0.45-1.05 $M_{\odot }$. Our results also suggest that a buried fossil field must be at least 100 times stronger than observed surface fields if crystallization-driven convection is responsible for its transport to the surface.

Published in ApJ

References in corpus (28)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• Modules for Experiments in Stellar Astrophysics (MESA): Time-Dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure
• How mergers magnetise massive stars
• The 12C(a,g)16O reaction and its implications for stellar helium burning
• 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
• New insight into the magnetism of degenerate stars from the analysis of a volume limited sample of white dwarfs
• Magnetic Field Generation in Stars
• Merging Binary Stars and the magnetic white dwarfs
• Multiple channels for the onset of magnetism in isolated white dwarfs
• Toward Precision Cosmochronology: A New C/O Phase Diagram for White Dwarfs
• A Spectro-photometric Analysis of Cool White Dwarfs in the Gaia and Pan-STARRS Footprint
• Spectrophotometric analysis of magnetic white dwarf I: Hydrogen-rich compositions
• Carbon-Oxygen Phase Separation in MESA White Dwarf Models
• Enhanced mixing in magnetized fingering convection, and implications for RGB stars
• Direct Evaluation of the Phase Diagrams of Dense Multicomponent Plasmas by Integration of the Clapeyron Equations
• Heat transport and convective velocities in compositionally-driven convection in neutron star and white dwarf interiors
• The evolution of ultra-massive carbon oxygen white dwarfs
• Magnetic dynamos in white dwarfs -- II. Relating magnetism and pollution
• Magnetic field breakout from white dwarf crystallization dynamos
• On Trapped Modes In Variable White Dwarfs As Probes Of The ${}^{12}$C($\alpha ,\gamma$)${}^{16}$O Reaction Rate
• A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs
• Magnetic dynamos in white dwarfs -- I. Explaining the dearth of bright intermediate polars in globular clusters
• Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?
• Fast and Slow Crystallization-driven Convection in White Dwarfs
• The formation of the magnetic symbiotic star FN Sgr

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