Evolving ONe WD+He WD systems to ultra-compact X-ray binaries

ABSTRACTIt has been proposed that accretion-induced collapse (AIC) of massive white-dwarfs (WDs) is an indispensable path for the formation of neutron star (NS) binaries. Although there are still no direct evidence for the existence of AIC events, several kinds of NS systems are suggested to originate from the AIC processes. One of the representative evidence is the detection of the strong magnetic field and slow spin NSs with ultra-light companions (${\le}0.1\, \rm M_{\odot }$) in close orbits. However, previous studies cannot explain the formation of AIC events with such low-mass companions. In the present work, we evolve a series of ONe WD+He WD systems to the formation of AIC events (named as the He WD donor channel), and the NS binaries behave as ultra-compact X-ray binaries when the He WDs refill their Roche lobes. We found that the ONe WD+He WD systems a possible channel for the formation of the newly formed NS+ultra-light companion systems just after the AIC event. Although there are some other inconsistent properties, the detected companion mass and orbital period of 4U 1626–67 (one of the newly formed NS binaries with ultra-light companions) can be reproduced by the He WD donor channel. In addition, combined with previous asteroseismology results, we speculate that an UCXB source (XTE J1751−305) may originate from the He WD donor channel.