Evolution of neutron star + He star binaries: an alternative evolutionary channel to intermediate-mass binary pulsars

TL;DR

This study investigates how neutron star + helium star binaries can form short-period intermediate-mass binary pulsars, providing an alternative evolutionary pathway supported by simulations matching some observed systems.

Contribution

It demonstrates that neutron star + helium star binaries can produce short-period IMBPs, offering a new formation channel supported by detailed stellar evolution calculations.

Findings

Can produce IMBPs with 0.5-1.1 solar mass white dwarfs and 0.03-20 day orbital periods

Simulations match 4 out of 9 observed compact IMBPs

Supports NS + He star binaries as an alternative formation channel

Abstract

It is difficult for intermediate-mass X-ray binaries to form compact intermediate-mass binary pulsars (IMBPs) with a short orbital-period ($\la 3 \rm d$), which have a heavy ($\ga 0.4 M_{\odot}$) CO or ONeMg white dwarf companions. Since neutron star + He star binaries may experience common-envelope evolution, they have some advantage to account for the formation of short orbital-period IMBPs. In this work, we explore the probability of IMBPs formed by this evolutionary channel. Using Eggleton's stellar evolution code, considering that the dead pulsars were spun up by the accreting material and angular momentum from the He star companions, we have calculated the evolution of a large number of neutron star + He star binaries. Our simulated results indicate that, the NS + He star evolutionary channel can produce IMBPs with a WD of $\sim0.5 - 1.1 M_{\odot}$ and an orbital period of $0.03 - 20$ d, in which pulsars have a spin-period of $1.4 - 200$ ms. Comparing the calculated results with the observational parameters (spin period and orbital period) of 9 compact IMBPs, the NS + He star evolutionary channel can account for the formation of 4 sources. Therefore, NS + He star binaries offer an alternative evolutionary channel to compact IMBPs.