The astrometry team at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS) has recently developed a new method that uses Gaia DR3 astrometric "excess noise" to constrain the orbital inclinations of binary systems and, in turn, improve mass estimates for unseen companions.

The study was published in the international journal Astronomy & Astrophysics on July 1.

Binary systems are vital for understanding stellar physics, including stellar evolution and the nature of compact objects. While astronomers often use radial velocity measurements to identify companions, accurately determining orbital inclination remains a challenge. Without a precise inclination, the mass of a dark companion can remain ambiguous, as different combinations of velocity and inclination can produce the same observational data.

To address this challenge, the SHAO team leveraged the high-precision measurements of the Gaia satellite. When a star is part of a binary system, it undergoes a subtle "wobble" due to the gravitational pull of its companion. If this orbital motion cannot be fully explained by standard single-star models, it appears in the Gaia catalog as "astrometric excess noise".

By building a sophisticated simulation framework, the SHAO team calculated expected excess noise values for various orbital inclinations and compared them with observed values from the Gaia DR3 catalog, thereby identifying the most likely range of orbital inclinations.

Schematic representation of constraining binary orbital inclination using Gaia DR3 astrometric excess noise. The blue region represents the simulated range of excess noise corresponding to different orbital inclinations, while the red line indicates the excess noise provided by the Gaia DR3 catalog. (Image by SHAO)

The team validated this method using 221 binary systems with established orbital solutions. The results demonstrated that, for 83.7% of the samples, the Gaia DR3 orbital inclination was consistent with the range estimated by their new method.

For systems with more pronounced astrometric signals, this accuracy increased to approximately 92.1%. Furthermore, the method was successfully applied to candidate systems harboring dark companions—such as Gaia BH1, Gaia BH2, and LB-1—effectively reducing uncertainties in both orbital inclination and companion mass.

"Gaia catalog excess noise was traditionally viewed as a measure of astrometric solution quality," said Dr. LIAO Shilong from SHAO, lead and corresponding author of the study. "Our work demonstrates that in binary systems, it also serves as a crucial clue for revealing orbital motion."

"This method fully exploits the potential of the existing data in the Gaia DR3 catalog," added co-corresponding author Dr. QI Zhaoxiang from SHAO. "Looking ahead, the release of more abundant epoch astrometric data in Gaia DR4 is expected to further enhance the precision of determining binary orbits and the masses of dark companions".

The research team also included SHAO doctoral students DING Ye and WEN Shangyu and postdoctoral researcher WU Qiqi. The project received support from the National Key R&D Program of China, the Youth Innovation Promotion Association of the CAS, and other funding initiatives.