Researchers from the Yunnan Observatories (Chinese Academy of Sciences), working with the Ulugh Beg Astronomical Institute of Uzbekistan, have identified a compact companion orbiting the RR Lyrae star IY Lyr. By combining photometric, spectroscopic, and astrometric data—including LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) spectroscopy, ZTF (Zwicky Transient Facility), TESS (Transiting Exoplanet Survey Satellite), ASAS-SN (All-Sky Automated Survey for Supernovae), and dedicated BVRI observations from the Maidanak 60-cm telescope—they provided the first robust evidence for a companion with a mass of 1.37 solar masses around this RRc-type variable star.
The team collected 180 times of light maximum from multi-band photometry and constructed a high-precision O–C (observed-minus-calculated) diagram. The residuals revealed a long-term parabolic period decrease, superimposed by a periodic modulation with an orbital period of 3.94 years. This cyclic signal was naturally explained by the light-travel time effect induced by the binary companion.
To independently verify this interpretation, the authors analysed radial velocities (RVs) of IY Lyr from LAMOST low- and medium-resolution spectra. After carefully subtracting the pulsational RV contribution using multiple templates, the residual RVs closely matched the orbital curve predicted by the O–C solution. Furthermore, by comparing proper motions from Gaia Data Release 2 (DR2) and Data Release 3 (DR3), the team developed a method to constrain the orbital inclination and the longitude of the ascending node, yielding an inclination of 94.2° and a companion mass of 1.37 solar masses.
The derived companion mass lies between the Chandrasekhar limit and the typical neutron-star mass peak. The authors argued that a neutron-star interpretation is more plausible, as it does not require several gigayears of accretion growth. In contrast, a massive white-dwarf scenario would necessitate mass transfer, which is unlikely given the approximately 2–5 au orbital separation and the low mass (about 0.61 solar masses) of the pulsating primary. Dynamical orbital integration, combined with metallicity and α-enhancement, identified IY Lyr as an old (about 13 gigayears), high-α, thick-disk star—providing observational support for binary evolution channels capable of producing metal-rich RR Lyrae stars in the Galactic thick disk.
This work highlights future trends in the search for companions to RR Lyrae stars. Multi-method cross-validation is essential because the O–C method alone can be affected by intrinsic period fluctuations, such as the Blazhko effect. Combining this method with RVs and Gaia astrometry significantly enhances reliability. The upcoming Gaia Data Release 4 (DR4) time-series astrometry will enable direct orbital mapping with even higher precision, potentially revealing low-mass companions that are currently undetectable. The study also compared IY Lyr with a halo RRab binary candidate, OGLE-BLG-RRLYR-20376, which shares remarkably similar orbital parameters but belongs to a different Galactic population. This comparison offers a valuable opportunity to investigate binary formation and evolution across distinct stellar components of the Milky Way.
Contact:
LI Linjia
Yunnan Observatories, CAS
e-mail:lilinjia@ynao.ac.cn
