Abstract

We consider a simple model for the evolution of a poloidal magnetic field initally trapped in a region containing normal npe matter within the outer liquid core of a neutron star. We have performed numerical computations for neutron stars with masses of 1.4, 1.6, and 1.7 M⊙ that undergo very rapid cooling due to the direct Urea process. Because the timescale for the magnetic field decay is directly proportional to T2, such a cooling history produces a rapid decline in the magnetic-field strength B, even for B as low as ∼ 1012 G. In particular, we show that an initially quasi-homogeneous magnetic field of strength B = 1012 G declines during the first ∼ 1 Myr.

Introduction

The calculations of Baym, Pethick, and Pines (1969a) have shown that the electrical conductivity of matter in the core of a neutron star is too large to permit ohmic decay of the magnetic field within the age of the Universe. Recently, Haensel, Urpin, and Yakovlev (1990; hereafter HUY) have pointed out that the magnetic-field strength |B| ∼ 1012 G typical of pulsars is sufficiently strong that the anisotropy of the transport coefficients cannot be neglected and that the “resistivity” for current flow perpendicular to B is many orders of magnitude larger than that for current flow parallel to B. Using a simple “toy” model, they found that internal fields B ≥ 1013 G can decline to ∼ 1012 G in times ∼ 107 years, but that fields ≤ 1012 G remain practically unchanged on this timescale.