Identify the physical nature of the Type III companions

Determine whether the unseen companions in the 12 Type III long-period binary candidates are compact objects or faint stellar companions by obtaining high-resolution ground-based spectroscopy and space-based ultraviolet imaging.

Background

The Type III systems have low expected companion-to-primary luminosity ratios, with an average theoretical ratio of approximately 0.073. Their companions therefore contribute little detectable optical or infrared flux, preventing the current photometric data from distinguishing degenerate compact objects from faint ordinary stars.

The proposed observations have complementary purposes: high-resolution spectroscopy can search for spectral-line contamination from an unseen stellar companion, while ultraviolet imaging can test for emission from a hot compact object.

References

At such low luminosity ratios, the expected companion flux is small in the optical and infrared bands, making it difficult to distinguish compact objects from faint stellar companions using the current photometric data alone. These systems are therefore lower-priority but still astrophysically interesting follow-up targets. Determining the physical nature of their unseen companions will require additional observations, including ground-based high-resolution spectroscopy to search for spectral-line contamination and space-based ultraviolet imaging to test for hot compact companions.

— A Sample of Long-Period Binaries with Dormant Compact-Object Candidates from LAMOST DR13 MRS and Gaia DR3  (2609.30764 - Zhou et al., 25 Sep 2026) in Section 4.2, Nature and Follow-Up Requirements of Type II and Type III Sources