Recently, an international research team, consisting of researchers from the National Astronomical Observatory of the Chinese Academy of Sciences, the University of Chinese Academy of Sciences, Yunnan Astronomical Observatory, and Xihua University, along with Hungarian astronomers, discovered a rare super-Chandrasekhar binary system composed of a massive white dwarf and a hot subdwarf star. This discovery was made using data from the LAMOST spectroscopic survey. The results were published on April 22, 2025, in SCIENCE CHINA Physics, Mechanics & Astronomy. This study supports the possibility of new evolutionary paths for binary systems involving white dwarfs and provides a new potential formation channel for neutron stars. It holds significant implications for theoretical and observational studies of neutron stars, type Ia supernovae, and other related fields.
The specific process of stellar death is a key topic in stellar physics and has far-reaching impacts on various fields of astrophysics, such as cosmic expansion and gravitational wave detection. In most cases, binary systems containing white dwarfs that exceed the Chandrasekhar limit in total mass will end their evolutionary process in a type Ia supernova. However, other possibilities theoretically exist. One such path is Accretion-Induced Collapse (AIC), where the binary system does not explode as a supernova but instead directly collapses to form a neutron star, a crucial formation pathway for neutron stars. Although AIC events have not yet been directly observed, discovering the progenitors of AIC systems could strongly constrain the probability of AIC occurrences and help further our understanding of the complex physical processes involved in binary star evolution endpoints.
In this study, the research team used spectroscopic data from the LAMOST survey and observational data from the 5-meter Hale Telescope at Palomar Observatory, the 2.16-meter telescope at the Xinglong Station of the National Astronomical Observatory of the Chinese Academy of Sciences, and other open-access observation databases to confirm the existence of a rare super-Chandrasekhar binary system consisting of a massive white dwarf and a hot subdwarf star—Lan11. By combining spectroscopic and photometric observation data covering the orbital period of this binary system, the team was able to describe the system in detail: it is composed of two stars, one of which is a hot subdwarf star with a mass between 0.52 and 0.65 solar masses. Observations revealed that this star is not spherical but elliptical, suggesting that it is accompanied by a hidden, small, but very massive compact object that is difficult to observe. Precise measurements show that this unseen object has a mass between 1.07 and 1.35 solar masses and a radius much smaller than that of normal stars. These characteristics indicate that the compact object is likely a rare high-mass white dwarf, with its core likely composed of oxygen and neon, known as an ONe white dwarf. The total mass of the binary system is between 1.67 and 1.92 solar masses, significantly exceeding the Chandrasekhar limit (around 1.4 solar masses). This is the largest hot subdwarf-white dwarf system ever observed optically. Theoretically, this system will merge in 500-540 million years due to the continuous emission of gravitational waves. Typically, if a binary system containing a white dwarf exceeds the Chandrasekhar limit, the merger will result in a supernova explosion. However, if the white dwarf is an ONe white dwarf, the energy released during the merger would be absorbed by the electron capture process, preventing an explosion and instead leading to direct collapse into a neutron star through the AIC process.
Further theoretical studies have revealed the system's history. It is predicted that its early evolution included a star with a mass of 5-8 solar masses, which quickly formed an ONe white dwarf through single-star evolution instead of the typical accretion-induced mass gain. This is the only known case of an intrinsic ONe white dwarf. Subsequently, the hot subdwarf star underwent a common-envelope evolution, where its outer layers were stripped away, exposing its hot core and forming the present-day hot subdwarf star. Renowned international expert Stephan Geier published a commentary titled The treasures in the backyard – a bright binary revealed as potential progenitor of a neutron star, where he highly praised the discovery. He emphasized the importance of discovering this rare progenitor of neutron stars formed via AIC and described the system as a "remarkable object." He further pointed out that this discovery shows that major breakthroughs in astrophysics may not only be found in distant galaxies but also in our solar neighborhood, providing important insights for detailed astronomical research near the Sun.
This study is a typical example of the efficient observational research path facilitated by the LAMOST survey, where initial screening is conducted using its massive spectral database, followed by confirmation through large and medium-sized telescopes. It highlights the large-sample capacity of LAMOST surveys and the irreplaceable role of large telescopes in precise measurement.
Figure 1, Artistic illustration of LAN11 Binary System
