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.

matches published version

References in corpus (18)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• Pulsating White Dwarf Stars and Precision Asteroseismology
• 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
• The 40 pc sample of white dwarfs from Gaia
• ${}^{22}$Ne Phase Separation As A Solution To The Ultramassive White Dwarf Cooling Anomaly
• Multiple channels for the onset of magnetism in isolated white dwarfs
• Horizontal spreading of planetary debris accreted by white dwarfs
• Spectrophotometric analysis of magnetic white dwarf I: Hydrogen-rich compositions
• Buoyant crystals halt the cooling of white dwarf stars
• The spectral evolution of white dwarfs: where do we stand?
• A Short Intense Dynamo at the Onset of Crystallization 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 (1)

• Identifying highly magnetized white dwarfs: A dimensionality reduction framework for estimating magnetic fields