Professor Li Kai’s team from the School of Space Science and Technology at Shandong University recently discovered the lowest mass ratio contact binary in the universe using several domestic telescopes. The related result has been published in Astronomy & Astrophysics (2024, A&A, 692, L4) with the title "Detection of the lowest mass ratio contact binary in the universe: TYC 3801-1529-1". Professor Li Kai is the first author and corresponding author of the paper, and the School of Space Science and Technology at Shandong University is the first completed unit and the corresponding author affiliation.

Fig 1 Schematic diagram of a contact binary star (from the internet)

Contact binary stars are interacting binary stars with a common envelope, where the two components are very close to each other. Due to their unique physical properties, they provide a natural laboratory for studying the structure and evolution of stars under extreme physical conditions. Theoretical research suggests that there is a lower limit to the mass ratio of contact binaries, which is approximately between q_min~0.04-0.07. When the mass ratio falls below this theoretical limit, the contact binary will be merged and produce a bright red nova outburst. However, to date, only one system, V1309 Sco, has been confirmed as a contact binary merger. Therefore, the study of contact binaries with extremely low mass ratios is crucial. It helps to accurately determine the lower limit of the mass ratio for contact binaries and plays an irreplaceable role in searching for potential contact binary merger candidates and understanding the origin of luminous red novae.

Fig 2 The left panel shows a comparison between the light curves observed by the Weihai 1m telescope and the theoretical curves, while the right panel displays the fit of the radial velocity curve obtained by the 2.16m telescope.

Li Kai’s team utilized the 1m and 50cm telescopes at Shandong University's Weihai Observatory, as well as the 2.16m and 85cm telescopes at the National Astronomical Observatories of the Chinese Academy of Sciences, and the 2.4m telescope at Yunnan Observatories of the Chinese Academy of Sciences to conduct photometric and spectroscopic observations of the contact binary star TYC 3801-1529-1. Analysis results indicate that the mass ratio of TYC 3801-1529-1 is only 0.035, making it the lowest mass ratio contact binary in the universe to date, surpassing the theoretical lower limit. Due to its spin angular momentum to orbital angular momentum ratio and mass ratio both exceeding the boundary of instability, TYC 3801-1529-1 is a potential candidate for contact binary mergers. Therefore, long-term monitoring of this target is crucial for studying its future evolution. This work not only challenges the existing theories on the stability and minimum mass ratio of contact binaries but also poses new questions regarding the internal structure of stars. At the same time, this work provides significant clues for exploring the merging conditions of contact binaries and the origin of luminous red novae.

This work was supported by the National Natural Science Foundation of China, the Qilu Young Scholars Project of Shandong University, and the Young Data Scientist Project of the National Astronomical Data Center.