Formation of millisecond pulsar-helium star binaries

¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China
² Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, China
³ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
⁴ School of Science, Qingdao University of Technology, Qingdao 266525, China

^⋆ Corresponding author: lixd@nju.edu.cn

Received: 9 June 2025
Accepted: 15 October 2025

Abstract

Context. PSR J1928+1815, the first recycled pulsar-helium (He) star binary discovered by the Five-hundred-meter Aperture Spherical radio Telescope, consists of a 10.55 ms pulsar and a companion star with mass 1 − 1.6 M_⊙ in a 0.15-day orbit. Theoretical studies suggest that this system originated from a neutron star (NS) intermediate-mass or high-mass X-ray binary that underwent common envelope (CE) evolution, leading to the successful ejection of the giant envelope. The traditional view is that hypercritical accretion during the CE phase may have recycled the NS. However, the specific mechanism responsible for accelerating its spin period remains uncertain due to the complex processes involved in CE evolution.

Aims. In this study, we investigate the influence of Roche lobe overflow (RLO) accretion that takes place prior to the CE phase on the spin evolution of NSs. Our primary objective is to clarify how this process affects the spin characteristics of pulsars.

Methods. We utilized the stellar evolution code MESA and the binary population synthesis code BSE to model the formation and evolution of NS-He star binaries. We calculated the distributions of the orbital period, He star mass, NS spin period, and magnetic field for NS + He star systems in the Galaxy.

Results. Our results indicate that RLO accretion preceding the CE phase could spin up NSs to millisecond periods through super-Eddington accretion. Considering a range of CE efficiencies α_CE from 0.3 to 3, we estimate the birthrate (total number) of NS + He star systems in our Galaxy to be 9.0 × 10⁻⁵ yr⁻¹ (626 systems) to 1.9 × 10⁻⁴ yr⁻¹ (2684 systems).