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

TL;DR

This paper explores an alternative magnetic braking mechanism involving Ap/Bp stars with strong magnetic fields, demonstrating it can lead to the formation of compact intermediate-mass binary pulsars with short orbital periods.

Contribution

It introduces a novel magnetic braking mechanism for Ap/Bp stars and shows its potential to produce short-period IMBPs, addressing a gap in previous models.

Findings

IMXBs with Ap/Bp stars can evolve into short-period IMBPs

The magnetic braking mechanism enables formation of binaries with periods less than 3 days

Discrepancy remains between simulated and observed pulsar spin periods

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 $\rm M_{\odot}$ in the canonical model. However, some intermediate-mass stars have anomalously strong magnetic fields (about 100 -- 10000 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 NS and an Ap/Bp star. For the spin evolution of the NS, we consider the accretion torque, the propeller torque, and the spin-down 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.