Magnetic Field Generation in Stars

Abstract

Enormous progress has been made on observing stellar magnetism in stars from the main sequence (particularly thanks to the MiMeS, MAGORI and BOB surveys) through to compact objects. Recent data have thrown into sharper relief the vexed question of the origin of stellar magnetic fields, which remains one of the main unanswered questions in astrophysics. In this chapter we review recent work in this area of research. In particular, we look at the fossil field hypothesis which links magnetism in compact stars to magnetism in main sequence and pre-main sequence stars and we consider why its feasibility has now been questioned particularly in the context of highly magnetic white dwarfs. We also review the fossil versus dynamo debate in the context of neutron stars and the roles played by key physical processes such as buoyancy, helicity, and superfluid turbulence, in the generation and stability of neutron star fields.Independent information on the internal magnetic field of neutron stars will come from future gravitational wave detections. Coherent searches for the Crab pulsar with the Laser Interferometer Gravitational Wave Observatory (LIGO) have already constrained its gravitational wave luminosity to be 2% of the observed spin-down luminosity, thus placing a limit of 10 16 G on the internal field. Indirect spin-down limits inferred from recycled pulsars also yield interesting gravitational-wave-related constraints. Thus we may be at the dawn of a new era of exciting discoveries in compact star magnetism driven by the opening of a new, non-electromagnetic observational window.We also review recent advances in the theory and computation of magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo theory. These advances offer insight into the action of stellar dynamos as well as processes which control the diffusive magnetic flux transport in stars.

Asteroseismic Signatures of Evolving Internal Stellar Magnetic Fields

“…Equipartition fields of ∼2 × 10 5 G generated by a core dynamo would evolve into fields of ∼2 × 10 7 G if their flux is conserved from the MS to the WD phase. Thus, the field strengths inferred for core-dynamo-generated fields are squarely within the observed distribution of WD surface fields (Ferrario et al 2015a(Ferrario et al , 2015b. Figure 3 shows that the core convective region is restricted to mass coordinates below ≈0.5 M e for stars of M  2.5 M e .…”

Chiral magnetic effect in protoneutron stars and magnetic field spectral evolution

“…There are no simple equations for the conductivity for this case. In the degenerate limit the conductivity is given by the following expression [20], 4) with ρ p the proton density. In the relativistic non-degenerate high temperature limit the conductivity would be dominated by the pair plasma and be of the order of the temperature [21] which is lower than the obtained through (2.4).…”

“…The most popular explanations include adiabatically compressed fossil fields of the parent star and dynamo generated fields [4]. More recently it has been suggested that the magnetic field of magnetars is related to a chiral asymmetry of particles, produced during the core collapse of supernovae [5].…”

“…We summarize our results and conclude in Sect. 4. Throughout the paper we will use Gaussian natural units, c 0 = = k B = 1, and the electric permittivity and magnetic permeability of the vacuum are set to 0 = 1/(4π) and µ 0 = 4π, respectively.…”

Role of particle masses in the magnetic field generation driven by the parity violating interaction

“…Some models accounting for the generation of such magnetic fields based on, e.g., the turbulent dynamo and strong fossil fields, are reviewed in Ref. [1]. However none of these models adequately describes all the observed characteristics of magnetars.…”