Recently, Ph.D. candidate GUO Yunlang and Prof. WANG Bo, from Yunnan Observatories of the Chinese Academy of Sciences, investigated the formation of black widows through ultracompact X-ray binaries (UCXBs) with He star companions. This work was published in Monthly Notices of the Royal Astronomical Society.

Millisecond pulsars (MSPs) are old radio pulsars with spin periods < 30 ms. They are proposed to be formed from the evolution of intermediate-mass X-ray binaries and low-mass X-ray binaries. The eclipsing MSPs have orbital periods ranging from ～ 0.1 to 1.0 days, which can be mainly divided into two types, i.e. black widows (BWs) and redbacks (RBs). BWs have companions with masses < 0.05 M_☉, while RBs have companions with masses in the range of 0.1—1.0 M_☉. It has been suggested that BWs may be the evolutionary link between the accreting X-ray pulsars and the isolated MSPs, in which the companion is ablated by the γ-ray and the energetic particles emitted by MSPs.

Recent observations indicate that BWs can be divided into two sub-types: one is the BWs with companion mass M₂ ～ 0.01—0.05 M_☉, while another sub-type of BWs have M₂ ≤ 0.01 M_☉. The BWs with M₂ ～ 0.01—0.05 M_☉ can be explained by the pulsar radiation evaporating normal stars. However, the BWs with M₂ ≤ 0.01 M_☉ are hardly reproduced by previous studies. Wang et al. (2021) studied the formation and evolution of UCXBs with He star companions systematically. They found that this channel can explain the formation of five transient sources with relatively long orbital periods. Following the work of Wang et al. (2021), the purpose of this paper is to explore the origin of BWs with M₂ ≤ 0.01 M_☉ by considering the evaporation process based on the UCXB channel.

In this study, the researchers calculated the long-term evolution of NS+He star systems by considering different He star masses and evaporation efficiencies. The evaporation efficiencies will affect the evolutionary track of the binary system when the evaporation process begins. The He star mass will decrease faster and the binary orbits widen more quickly as the evaporation efficiencies increases, owing to the more efficient evaporation. They found that the BWs with M₂ ≤ 0.01 M_☉ can be explained by the UCXB channel with He star companions. To further reproduce the properties of BWs, they considered different input parameters for initial models. The BW properties can be well reproduced by using appropriate evaporation efficiencies and initial NS spin periods.

In addition, they found that the mass of He star companions can decrease to < 0.001 M_☉ or even < 10^-5 M_☉ within the Hubble time. Previous studies suggested that the He degenerate companion will undergo the tidal disruption before the companion mass decrease to 0.004 M_☉. Therefore, the BWs through the UCXB channel with He star companions are the potential progenitors of isolated MSPs. Moreover, they found that the number of isolated MSPs through this channel is in the range of 10—80.

This work explains the formation of BWs with M₂ ≤ 0.01 M_☉, and proposed an alternative formation channel for producing isolated MSPs.

Contact:
GUO Yunlang
Yunnan Observatories, CAS
Email: yunlang@ynao.ac.cn