Stars of spectral types O and B produce neutron stars (NSs) after supernova explosions. Most of NSs are strongly magnetized including normal radio pulsars with B ∝ 1012 G and magnetars with B ∝ 1014 G. A fraction of 7–12 per cent of massive stars are also magnetized with B ∝ 103 G and some are weakly magnetized with B ∝ 1 G. It was suggested that magnetic fields of NSs could be the fossil remnants of magnetic fields of their progenitors. This work is dedicated to study this hypothesis. First, we gather all modern precise measurements of surface magnetic fields in O, B, and A stars. Secondly, we estimate parameters for lognormal distribution of magnetic fields in B stars and found μB = 2.83 ± 0.1 log10 (G), σB = 0.65 ± 0.09 for strongly magnetized and μB = 0.14 ± 0.5 log10 (G), $\sigma =0.7_{-0.27}^{+0.57}$ for weakly magnetized. Third, we assume that the magnetic field of pulsars and magnetars have 2.7-dex difference in magnetic fields and magnetars represent 10 per cent of all young NSs and run population synthesis. We found that it is impossible to simultaneously reproduce pulsars and magnetars populations if the difference in their magnetic fields is 2.7 dex. Therefore, we conclude that the simple fossil origin of the magnetic field is not viable for NSs.
A longitudinal magnetic field with a strength of 5.2 kG was recently detected in CPD −62 • 2124, which has a fractional main-sequence lifetime of about 60%. Strongly magnetic early-B type chemically peculiar stars in an advanced evolutionary state are of special interest to understand the evolution of the angular momentum and spin-down timescales in the presence of a global magnetic field. We exploited 17 FORS 2 low-resolution spectropolarimetric observations and 844 ASAS3 photometric measurements for the determination of the rotation period, pulsations, and the magnetic field geometry of the star. We calculated periodograms and applied phenomenological models of photometric, spectral and spectropolarimetric variability. We found that all quantities studied, specifically equivalent widths, the mean longitudinal magnetic field B z , and the flux in the V filter, vary with the same period P = 2.628 d, which was identified as the rotation period. The observed variations can be fully explained by a rigidly rotating main-sequence star with an uneven distribution of chemical elements, photometric spots, and a stable, nearly dipolar magnetic field with a polar field strength of about 21 kG, frozen into the body of the star. The magnetic field of CPD −62 • 2124 is tilted to the rotation axis by β = 28 • ± 7 • , while the inclination of the rotation axis towards the line of sight is only i = 20 • ± 5 • . In the acquired FORS 2 spectra, we detect short-term line profile variations indicating the presence of β Cephei type pulsations. As of today, no other pulsating star of this type is known to possess such a strong magnetic field.
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