2022MNRAS.515.2725G

Black widows (BWs) are a type of eclipsing millisecond pulsars (MSPs) with companion masses $M_2\lesssim 0.05\, \rm M_\odot$, which can be used to study the accretion history and the radiation of pulsars, as well as the origin of isolated MSPs. Recent observations indicate that there are two subtypes of BWs. One is the BWs with $M_2 \sim 0.01\!-\!0.05\, \rm M_\odot$, whereas another with $M_2 \lesssim 0.01\, \rm M_\odot$. However, the origin of the latter is still highly uncertain. In this paper, we investigated the formation of BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ through ultracompact X-ray binaries (UCXBs) with He star companions, in which a neutron star (NS) accretes material from a He star through Roche lobe overflow. By considering different He star masses and evaporation efficiencies with the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA ), we evolved a series of NS+He star systems that can undergo UCXB stage. This channel can explain the formation of the BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ within the Hubble time, especially three widely studied BWs, i.e. PSRs J1719-1438, J2322-2650, and J1311-3430. We found that X-ray irradiation feedback does not affect the evolutionary tracks of evaporation process. The simulations indicate that the UCXBs with He star companions are the potential progenitors of isolated MSPs, and that the origin of BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ is different with another subtype of BWs. In addition, this work suggests that the BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ may not be produced by redback systems.