Spin and Magnetism of White Dwarfs

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 $\sim 0.55M_{\odot }$ 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 $\lesssim 0.1M_{\odot }$ 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.

23 pages, 27 figures, accepted to the ApJ, typos and figures corrected

References in corpus (8)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Binary Star Origin of High Field Magnetic White Dwarfs
• Isolated vs. Common Envelope Dynamos in Planetary Nebula Progenitors
• The Magnetic Fields at the Surface of Active Single G-K Giants
• Magnetic Mixing in Red Giant and Asymptotic Giant Branch Stars
• A new detailed examination of white dwarfs in NGC3532 and NGC2287
• Rotation of Giant Stars
• Explaining the Observed Relation Between Stellar Activity and Rotation

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