Younger age for the oldest magnetic white dwarfs

Abstract

Sufficiently old white dwarfs cool down through a convective envelope that directly couples their degenerate cores to the surface. Magnetic fields may inhibit this convection by stiffening the criterion for convective instability. We consistently implemented the modified criterion in the stellar evolution code MESA, and computed the cooling of white dwarfs as a function of their mass and magnetic field $B$. In contrast to previous estimates, we find that magnetic fields can significantly change the cooling time $t$ even if they are relatively weak $B^2\ll 8\pi P$, where $P$ is the pressure at the edge of the degenerate core. Fields $B\gtrsim 1\text{ MG}$ open a radiative window that decouples the core from the convective envelope, effectively lowering the luminosity to that of a fully radiative white dwarf. We identified a population of observed white dwarfs that are younger by $\Delta t\sim$ Gyr than currently thought due to this magnetic inhibition of convective energy transfer - comparable to the cooling delay due to carbon-oxygen phase separation. In volume-limited samples, the frequency and strength of magnetic fields increase with age. Accounting for magnetic inhibition is therefore essential for accurate cooling models for cosmic chronology and for determining the origin of the magnetic fields.

arXiv commentmatches published version

References in corpus (40)

• Gaia Data Release 2. Summary of the contents and survey properties
• 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
• 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
• Evolutionary and pulsational properties of white dwarf stars
• Pulsating White Dwarf Stars and Precision Asteroseismology
• Magnetic White Dwarfs
• New cooling sequences for old white dwarfs
• Binary Star Origin of High Field Magnetic White Dwarfs
• Double degenerate mergers as progenitors of high-field magnetic white dwarfs
• Core crystallization and pile-up in the cooling sequence of evolving white dwarfs
• A cooling anomaly of high-mass white dwarfs
• HST Observations of the White Dwarf Cooling Sequence of M4
• 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
• The 40 pc sample of white dwarfs from Gaia
• On The Evolution of Magnetic White Dwarfs
• ${}^{22}$Ne Phase Separation As A Solution To The Ultramassive White Dwarf Cooling Anomaly
• Multiple channels for the onset of magnetism in isolated white dwarfs
• Multi-Gigayear White Dwarf Cooling Delays from Clustering-Enhanced Gravitational Sedimentation
• Horizontal spreading of planetary debris accreted by white dwarfs
• Forever young white dwarfs: when stellar ageing stops
• Spectrophotometric analysis of magnetic white dwarf I: Hydrogen-rich compositions
• Buoyant crystals halt the cooling of white dwarf stars
• Carbon-Oxygen Phase Separation in MESA White Dwarf Models
• The spectral evolution of white dwarfs: where do we stand?
• Heat transport and convective velocities in compositionally-driven convection in neutron star and white dwarf interiors
• Luminosity and cooling of highly magnetized white dwarfs: suppression of luminosity by strong magnetic fields
• Magnetic field breakout from white dwarf crystallization dynamos
• A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs
• Fast and Slow Crystallization-driven Convection in White Dwarfs
• Discovery of magnetic fields in five DC white dwarfs
• Stochastic excitation of waves in magnetic stars -- I. Scaling laws for the modes amplitudes
• Searching for magnetic fields in featureless white dwarfs with the DIPOL-UF polarimeter at the Nordic Optical Telescope

Cited by in corpus (3)

• Magnetic dynamos powered by white dwarf superficial convection
• Identifying highly magnetized white dwarfs: A dimensionality reduction framework for estimating magnetic fields
• Double-faced white dwarfs and the magnetic inhibition of convection