Origin of neutron star magnetic fields

Abstract

PULSARS are generally thought to be rapidly rotating neutron stars which have poloidal magnetic fields with intensities consistently in the range 10¹²–10¹³ gauss (ref. 1). Most of them are believed to evolve from main sequence stars with original masses of >1.4–∼8 M_⊙. Evolutionary computations^2,3 indicate that stars in this mass range develop a dense core of carbon and oxygen in which carbon begins to burn when the temperature reaches about 3 × 10⁸ K and the density about 3 × 10⁹ g cm⁻³. The rapid production of energy during this carbon-burning stage drives almost the entire core into convection for an extended time of the order of 10¹¹ s (ref. 4), with convective velocities of the order of 10⁴ cm s⁻¹ (ref. 5), provided that the rate of cooling through the convectively driven ‘Urca process’ is sufficiently high to stabilise carbon burning and to prevent the core from detonating and dispersing. If this sequence is a reasonable approximation to reality then the existence of neutron star pulsars suggests that carbon burning is, in fact, stabilised until electron capture, or some other process, causes part of the core to collapse to a neutron star^2–7.