A born ultramassive white dwarf-hot subdwarf super-Chandrasekhar candidate

Although supernovae are well-known endpoints of accreting white dwarfs, alternative theoretical possibilities have been widely discussed, such as the accretion-induced collapse (AIC) event as the endpoint of oxygen-neon (ONe) white dwarfs, either accreting up to or merging to exceed the Chandrasekhar limit (the maximum mass of a stable white dwarf). AIC is an important channel to form neutron stars, especially for those unusual systems that are unlikely produced by core-collapse supernovae. However, the observational evidence for this theoretically predicted event and its progenitor is very limited. In all of the known progenitor systems, white dwarfs increase in mass through accretion. Here, we report the discovery of an intriguing binary system Lan 11, composed of a stripped core-helium-burning hot subdwarf and an unseen compact object with a mass of 1.08M_⊙ to 1.35M_⊙. Our binary population synthesis calculations suggest that the latter is most likely to be an ONe white dwarf. Furthermore, the non-detection in deep radio observations by the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) does not exclude this interpretation. The total mass of this binary ranges from 1.67M_⊙ to 1.92M_⊙, significantly exceeding the Chandrasekhar limit. The reproduction of its evolutionary history indicates that the unique system has undergone two phases of common envelope ejection, implying a born nature of this massive ONe white dwarf rather than an accretion growth from its companion. These results, together with short orbital period of this binary (3.65 h), suggest that this system will merge in 500–540Myr, largely triggering an AIC event, although the possibility of type Ia supernova cannot be entirely ruled out. This finding greatly provides valuable constraints on our understanding of stellar endpoints, whatever leading to an AIC or a supernova.