Magnetic braking of Ap/Bp stars: An alternative formation mechanism of compact intermediate-mass binary pulsars

Abstract

It is difficult for the intermediate-mass X-ray binaries (IMXBs) evolutionary channel to form intermediate-mass binary pulsars (IMBPs) with a short orbital period (less than 3 d) via stable mass transfer. The main reason is that the magnetic braking mechanisms are generally thought not to work for donor stars with a mass of greater than 1.5 M⊙ in the canonical model. However, some intermediate-mass stars have anomalously strong magnetic fields (about 100- 10 000 G), i.e. so-called Ap or Bp stars. With the coupling between the magnetic field and the irradiation-driven wind from the surface of Ap/Bp stars, a plausible magnetic braking mechanism should be expected. In this work, we attempt to investigate if IMXBs with Ap/Bp stars can produce IMBPs with a short orbital period (less than 3 d) by such an anomalous magnetic braking mechanism. Using a stellar evolution code, we have simulated the evolution of a large number of IMXBs consisting of a neutron star (NS) and an Ap/Bp star. For the spin evolution of the NS, we consider the accretion torque, the propeller torque and the spindown torque caused by the interaction between the magnetic field and the accretion disc. The calculated results show that, employing anomalous magnetic braking of Ap/Bp stars, IMXBs can evolve into compact IMBPs with short orbital periods of less than 3 d. However, there exists significant discrepancy between the spin periods of IMBPs in our simulated results and those observed. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.