Magnetic fields and rotation in white dwarfs and neutron stars

Abstract

Recent spectropolarimetric observations of Ap and Bp stars with improved sensitivity have suggested that most Ap and Bp stars are magnetic with dipolar fields of at least a few hundred gauss. These new estimates suggest that the range of magnetic fluxes found for the majority of magnetic white dwarfs is similar to that of main-sequence Ap-Bp stars, thus strengthening the empirical evidence for an evolutionary link between magnetism on the main sequence and magnetism in white dwarfs. We draw parallels between the magnetic white dwarfs and the magnetic neutron stars and argue that the observed range of magnetic fields in isolated neutron stars (Bp∼ ∼ 10 11-1015 G) could also be explained if their mainly O-type progenitors have effective dipolar fields in the range of a few gauss to a few kilogauss, assuming approximate magnetic flux conservation with the upper limit being consistent with the recent measurement of a field of Bp ∼ 1100 G for θ Orion C. In the magnetic field-rotation diagram, the magnetic white dwarfs can be divided into three groups of different origin: a significant group of strongly magnetized slow rotators (Prot ∼ 50-100 yr) that have originated from single-star evolution, a group of strongly magnetized fast rotators (Prot ∼ 700 s), typified by EUVE J0317-853, that have originated from a merger, and a group of modest rotators (Prot ∼ hours-days) of mixed origin (single-star and CV-type binary evolution). We propose that the neutron stars may similarly divide into distinct classes at birth, and suggest that the magnetars may be the counterparts of the slowly rotating high-field magnetic white dwarfs.