L. Mestel

An idealized pulsar magnetosphere: the relativistic force-free approximation

The non-dissipative relativistic force-free condition should be a good approximation to describe the electromagnetic field in much of the pulsar magnetosphere, but we may plausibly expect it to break down in singular domains. Self-consistent magnetospheric solutions are found with field lines closing both at and within the light-cylinder. In general, the detailed properties of the solutions may be affected critically by the physics determining the appropriate choice of equatorial boundary condition beyond the light-cylinder.

Magnetic braking during star formation I

We study the transport of angular momentum from a cool massive gas cloud by Alfvén waves travelling along the distorted magnetic field linking the cloud with the hot galactic background. The efficiency of braking is never so great as to keep the cloud even roughly corotating with the back-ground. The mathematical approximations are not quite sharp enough to give a definitive answer to the question: is the braking ever so efficient that the centrifugal forces are kept well below gravity, so that the cloud contracts at the (magnetically-diluted) free-fall rate?; or is the cloud maintained in approximate centrifugal balance, with the rate of contractiondetermined by the loss of angular momentum?

The interaction of rotation and magnetic field in the Solar System

During the early phases of star formation, torsional Alfven waves propagating along the locally distorted galactic magnetic field can transport away the bulk of the initial angular momentum of a condensation, so enabling it to contract to solar nebula dimensions. Enough primeval magnetic flux may be retained for magnetic redistribution of angular momentum to continue until the proto-Sun reaches the pre-Main Sequence Hayashi phase. A rotating star with a convective envelope has a dynamomaintained field which brakes the star through coupling to the stellar wind. Observational evidence from young star clusters and from T Tauri stars suggests that the zero-age Main Sequence Sun had about ten times its present angular momentum. The same braking process, scaled up because of the much more powerful T Tauri winds, can explain why the zero-age Sun had lost at least nine-tenths of the centrifugal upper limit, and is more acceptable than the suggestion that the ‘missing’ solar angular momentum has been magnetically fed into the planetary system.

The early days of stellar structure theory

Abstract Eddington’s Internal Constitution of the Stars remains both a classic and in parts a tour de force . As a paradigm for a homogeneous star in thermal equilibrium, Eddington’s ‘standard model’ is better replaced by Cowling’s point-convective model. Jeans ultimately accepted the essence of Eddington’s approach, unlike Milne, whose lasting contributions were to the theory of stellar atmospheres. Some of Milne’s arguments turn out be relevant not to main sequence stars, but to pre-main sequence or to evolved, post-main sequence stars.

Magnetic Fields and Rotation in Late-Type Stars

I wish to discuss three separate but related problems concerning the properties of magnetic fields in late type stars. It is generally accepted that the surface magnetic fields of late type main sequence stars are being maintained by a dynamo process in the outer convection zone which probably has its seat near the bottom of the zone. It is, however, uncertain whether or not there is a significant field in the radiative interior and, if so, whether the field is a relic of a dynamo-produced field from a pre-main-sequence phase or a genuine “fossil” field which was present when the star was formed. Two of the topics which I discuss are concerned with the properties of “fossil” fields, while the third is principally concerned with the influence of dynamo fields on convection and hence on the observed properties of stars. The second and third topics concern work which has already been published, while the first concerns work which is at present in progress, where I shall therefore stress the idea and not the details.

The Relativistic Force-free Equation

This paper is a preliminary report on ongoing work, in collaboration with Drs S. P. Goodwyn, A. J. Mestel, and G. A. E. Wright. The non-dissipative force-free condition should be a good approximation to describe the electromagnetic field in much of the pulsar magnetosphere, but we may plausibly expect it to break down in singular domains. The detailed properties of the solutions will be affected critically by the choice of equatorial boundary condition beyond the light-cylinder.