Spin and Magnetism of White Dwarfs

TL;DR

This paper explores how magnetic fields and rotation in white dwarfs originate from dynamo processes during their progenitors' shell burning phases, linking core-envelope interactions to observed magnetic and rotational properties.

Contribution

It proposes a new model connecting progenitor dynamo activity and core-envelope magnetic flux transfer to the magnetic fields and spin periods of white dwarfs.

Findings

Magnetic fields in white dwarfs can reach up to 10 MG.

The outer shell of the white dwarf contains most of the magnetic flux.

White dwarf spin periods typically range from hours to days.

Abstract

The magnetism and rotation of white dwarf (WD) stars are investigated in relation to a hydromagnetic dynamo operating in the progenitor during shell burning phases. The downward pumping of angular momentum in the convective envelope, in combination with the absorption of a planet or tidal spin-up from a binary companion, can trigger strong dynamo action near the core-envelope boundary. Several arguments point to the outer core as the source for a magnetic field in the WD remnant: the outer third of a $\sim 0.55\,M_\odot$ WD is processed during the shell burning phase(s) of the progenitor; the escape of magnetic helicity through the envelope mediates the growth of (compensating) helicity in the core, as is needed to maintain a stable magnetic field in the remnant; and the intense radiation flux at the core boundary facilitates magnetic buoyancy within a relatively thick tachocline layer. The helicity flux into the growing core is driven by a dynamical imbalance with a latitude-dependent rotational stress. The magnetic field deposited in an isolated massive WD is concentrated in an outer shell of mass $\lesssim 0.1\,M_\odot$ and can reach $\sim 10\,$MG. A buried toroidal field experiences moderate ohmic decay above an age $\sim 0.3$ Gyr, which may lead to growth or decay of the external magnetic field. The final WD spin period is related to a critical spin rate below which magnetic activity shuts off, and core and envelope decouple; it generally sits in the range of hours to days. WD periods ranging up to a year are possible if the envelope re-expands following a late thermal pulse.