Recently, Professor Wang Xiaofeng's research group and collaborators from the Department of Physics of Tsinghua University used the Tsinghua University-Ma Huateng Survey Telescope (TMTS) to discover a space about 2760 light-years away from the Earth with an orbital period of only 205 minutes of compact binary star system (i.e., TMTS J0526) and decryption of its physical origin. The binary star system consists of a star with a mass of about 0A carbon-oxygen white dwarf with a mass of 74 times the mass of the Sun and a white dwarf with a diameter of only 7 times the size of the Earth and a mass of about 033 times hot subdwarf stars (most of the hydrogen shell material stripped away). Such a pair of binary stars with extremely short orbital periods can produce strong gravitational-wave radiation in the millihertz band, which is expected to be significantly detected by space gravitational-wave observatories such as Lisa, Tianqin and Taiji in the future. It should be pointed out that the discovery and research of this binary star system have well verified the theoretical limit value of the formation of low-mass, short-orbit periodic hot subdwarf binary stars through the evolution of two common envelope ejections, and the evolution channel (proposed by the team of academician Han Zhanwen of Yunnan Astronomical Observatory of the Chinese Academy of Sciences in 2003).
Binary family synthesis studies based on the formation of hot subdwarf stars have shown that there are a small number of hot subdwarf stars formed by two common envelope ejection channels with masses as low as 032-0.36 times the mass of the sun. They are formed by igniting non-degenerate helium nuclei, and are similar to the widely recognized hot subdwarf stars formed by helium flash ignition of degenerate helium nuclei (about 045 times the mass of the sun) with a very different mass distribution. After the second co-envelope mass ejection, these hot subdwarf binary star systems containing a white dwarf star can form extremely short-period binary star systems with orbital periods as short as 20 minutes through gravitational wave radiation. However, before the discovery of the TMTS survey, there was no white dwarf-hot subdwarf binary star system that fully conformed to the above observational properties.
Figure 1TMTS J0526 binary star system drawn by the artist (Yu Jingchuan, Beijing Planetarium). The larger ones in the image are the hot subdwarf stars, and the smaller ones in the distance are their white dwarf companions.
The TMTS observation system is a uniquely designed multi-lens tube optical survey equipment built by Professor Wang Xiaofeng's team with the support of Ma Huateng** and Tsinghua University Xuetang. Since its official operation in 2020, the system has gazed at the cosmic starry sky in the Northern Hemisphere at a 1-minute observation frequency. By the end of 2023, TMTS has obtained densely sampled optical variation data of more than 27 million stars, including a large number of short-period variable sources with ** values, among which TMTS J0526 is one of the systems with the shortest optical variation period. Following the discovery of the source, the team used the 10-meter Aperture Keck telescope in Hawaii, USA, and the 10The 4-meter aperture GTC telescope made a high-time-resolution continuous spectral observation of the source, and Lijiang 2A 4-meter telescope was used for time-series photometric observations.
Combined with the optical curves and spectral radial velocity changes obtained from high-frequency sampling observations, the research team analyzed that TMTS J0526 is an orbital period of only 20A five-minute elliptical binary star in which a larger, brighter substar (i.e., a visible star) is deformed by the tidal gravity of another, denser white dwarf (invisible) (Figure 1). Analysis of brightness, surface gravity, effective temperature, and mass radius shows that the visible star is a low-mass, thin-enveloped hot subdwarf (Fig. 2). The binary star system contains low-mass hot subdwarfs, white dwarf companions, and extremely short orbital periods, all of which are consistent with the theoretical prediction of the formation of low-mass hot subdwarfs (binary stars) by two common envelope ejection channels.
Figure 2Mass-radius diagrams of different types of stars. Main-sequence stars, white dwarfs, and hot sub-dwarfs are distributed in three distinguishable regions of the diagram. The purple region represents the region where hot subdwarf stars are likely to be distributed, while the visible substar J0526b of the TMTS J0526 binary star system falls exactly at the very bottom of this region. This visible star is smaller than the smallest star ever discovered (2MASS-J0523) and represents the smallest non-degenerate star ever discovered.
The results of the above research are summarized in "20A seven-earth-radius helium-burning star inside a 205-min detached binary", published in Nature Astronomy on February 9. This is the TMTS project following the discovery of 18After 9 minutes of blue large-amplitude pulsating variable star, the research results were published in Nature Astronomy for the second time. Wang Xiaofeng is the corresponding author of this paper, Lin Jie, former postdoctoral fellow of Tsinghua University (currently a special associate researcher at the University of Science and Technology of China) is the first author of this paper, and Wu Chengyuan, an associate researcher at the Yunnan Astronomical Observatory of the Chinese Academy of Sciences (formerly a postdoctoral fellow in the Department of Physics of Tsinghua University), and Xiong Heran, a doctoral student at the Australian National University, are the co-first authors.
*The main collaborators also include Pitt, a researcher at the Institute of Astronomy of the Czech Academy of Sciences, a team of Academician Han Zhanwen and Chen Xuefei from the Yunnan Astronomical Observatory of the Chinese Academy of Sciences, a team of Dr. Ilyas Rosa from the Padua Observatory in Italy, and a team of Professor Filippenko from the University of California, Berkeley. This project is supported by the National Natural Science Center of China, key projects, Ma Huateng, China Manned Space Project, Beijing Academy of Sciences Scholars Project, and the Science Exploration Award.
*Link:* Department of Physics, Tsinghua University.