Younger age for the oldest magnetic white dwarfs

TL;DR

Magnetic fields in white dwarfs can significantly slow their cooling by inhibiting convection, leading to younger apparent ages and impacting cosmic chronologies and magnetic field origins.

Contribution

This study implements a modified convective criterion in MESA to show how magnetic fields affect white dwarf cooling times, revealing a substantial impact even with relatively weak fields.

Findings

Magnetic fields ≥1 MG open a radiative window, decoupling core and envelope.

Magnetic inhibition can make white dwarfs appear up to a Gyr younger.

Magnetic field strength and frequency increase with white dwarf age.

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 $B\gtrsim 1\textrm{ MG}$ 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.