Recently, a research team led by Prof. WANG Bo from Yunnan Observatories of the Chinese Academy of Sciences explains the formation of millisecond pulsars (MSPs) with long orbital periods by accretion-induced collapse of white-dwarfs. This work was recently published in Monthly Notices of the Royal Astronomical Society.

Neutron star (NS) systems are generally believed to be formed via three ways, i.e., core-collapse supernovae of massive stars, electron-capture supernovae of intermediate-mass stars, and accretion-induced collapse (AIC) of massive white-dwarfs (WDs). Oxygen-neon (ONe) WDs are suggested to collapse into neutron stars via electron-capture reactions once growing in mass approach the Chandrasekhar limit, called the AIC process. It has been predicted that AIC events may be the most likely short-lived and faint optical transients, and that a small ejecta mass is expected during the collapse.

There exists a lot of indirect evidence that has been proposed to support the AIC process. This process can be used to resolve a variety of troublesome NS systems in observations. Especially, the AIC process could lead to the formation of newborn NSs with small kicks, and thus it can be used to reproduce obviously young NSs in some globular clusters. These young NSs cannot be produced through the classic core-collapse supernova channel.

Meanwhile, the AIC process may help to explain the observed discrepancy between the large rate of millisecond pulsars (MSPs) and the small rate of their progenitor systems (i.e. low-mass X-ray binaries; LMXBs) in the Galaxy.

An ONe WD that accretes H-rich material from a red-giant (RG) star may experience the AIC process, eventually producing MSPs, known as the RG donor channel. It is worth noting that more and more MSPs with wide orbits more than 500 days (>500 d) have been detected by recent observations, but their origin is still highly uncertain. In the present work, the authors studied the formation of binary MSPs through the RG donor channel systematically using an integrated mass-transfer prescription for RG donors. They found that the RG donor channel can form binary MSPs with orbital periods ranging from 50 d to 1200 d, in which the final NS masses are in the range of ~1.26-1.55Msun and the masses of the WD companions are in the range of 0.30-0.55Msun.

The authors also found that the formed MSPs through the RG donor channel follow the correlation between the companion mass and the orbital period, and that there exists an anti-correlation between the final NS mass and the final orbital period. The pre-AIC systems with RG donors will show as symbiotics in the observations, whereas the post-AIC systems can be identified as LMXBs, finally evolving into young MSPs with wide orbits (>50d).

The authors emphasize that the RG donor channel provides a viable way to account for the observed MSPs with long orbital periods. More theoretical and observational studies on MSPs with wide separations would be helpful for our understanding of this kind of pulsar systems.

Contact:
WANG Bo, Yunnan Observatories, CAS
wangbo@ynao.ac.cn