H. C. Spruit

Dynamo action by differential rotation in a stably stratified stellar interior

Magnetic fields can be created in stably stratified (non-convective) layers in a differentially rotating star. A magnetic instability in the toroidal field (wound up by differential rotation) replaces the role of convection in closing the field amplification loop. Tayler instability is likely to be the most relevant magnetic instability. A dynamo model is developed from these ingredients, and applied to the problem of angular momentum transport in stellar interiors. It produces a predominantly horizontal field. This dynamo process is found to be more effective in transporting angular momentum than the known hydrodynamic mechanisms. It might account for the observed pattern of rotation in the solar core.

CIRCUMBINARY DISKS AND CATACLYSMIC VARIABLE EVOLUTION

The influence of a circumbinary (CB) disk on the evolution of cataclysmic variable (CV) binary systems is investigated. We show that CB mass surface densities sufficient to influence the evolution rate are plausibly provided by the outflows observed in CVs, if the net effect of these winds is to deliver 10-4 to 10-3 of the mass transfer rate to the CB disk. The torque exerted by the CB disk provides a positive feedback between mass transfer rate and CB disk mass that can lead to mass transfer rates of ~10-8 to 10-7 M☉ yr-1. This mechanism may be responsible for causing the range of variation of mass transfer rates in CVs. In particular, it may explain rates inferred for the nova-like variables and the supersoft X-ray binary systems observed near the upper edge of the period gap (P ~ 3-4 hr), as well as the spread in mass transfer rates above and below the period gap. Consequences and the possible observability of such disks are discussed.

Stability of accretion discs threaded by a strong magnetic field

We study the stability of poloidal magnetic fields anchored in a thin accretion disc. The two-dimensional hydrodynamics in the disc plane is followed by a grid-based numerical simulation including the vertically integrated magnetic forces. The three-dimensional magnetic field outside the disc is calculated in a potential field approximation from the magnetic flux density distribution in the disc. For uniformly rotating discs we confirm numerically the existence of the interchange instability as predicted by Spruit, Stehle & Papaloizou. In agreement with predictions from the shearing sheet model, discs with Keplerian rotation are found to be stabilized by the shear, as long as the contribution of magnetic forces to support against gravity is small. When this support becomes significant, we find a global instability which transports angular momentum outwardly and allows mass to accrete inwardly. The instability takes the form of a m=1 rotating ‘crescent’, reminiscent of the purely hydrodynamic non-linear instability previously found in pressure-supported discs. A model where the initial surface mass density Σ(r) and Bz(r) decrease with radius as power laws shows transient mass accretion during about six orbital periods, and settles into a state with surface density and field strength decreasing approximately exponentially with radius. We argue that this instability is likely to be the main angular momentum transport mechanism in discs with a poloidal magnetic field sufficiently strong to suppress magnetic turbulence. It may be especially relevant in jet-producing discs.

Gamma-ray bursts from x-ray binaries

A weakly magnetized ( 10 7 G) neutron star, slowly spun up by accretion in an X-ray binary, crosses the instability boundary for r-mode instability at P=1-2 msec. The amplitude of the oscillation, which initially increases only at the spinup time scale, is secularly unstable due to the negative temperature dependence of the viscosity in neutron star matter, and diverges after a few hundred years. Angular momentum loss by the grav- itational wave causes strong differential rotation, in which the magnetic field is wound up to 10 17 G on a time scale of a few months. When this field becomes unstable to buoyancy insta- bility, a surface field strength of a few 10 16 G is produced on a time scale of seconds, which then powers a GRB with energies of 10 51 -10 52 and duration of 1-100 sec.