A celestial matryoshka: dynamical and spectroscopic analysis of the Albireo system

We present a spectroscopic characterization and a new orbital solution for the binary system beta CygAa/Ac (MCA 55), the primary component (beta CygA) of the well-known wide double star Albireo. By matching evolutionary tracks to the physical parameters of all three Albireo stars (beta CygAa, Ac, and B) as obtained from a spectroscopic analysis of TIGRE and IUE spectra, we confirm that they are likely coeval. Our final orbit solution is based on radial-velocity measurements taken over a baseline exceeding 120 yr, combined with relative astrometry from speckle interferometric observations and the absolute astrometry from the Hipparcos and Gaia missions. Our final orbit solution has a period of 121.65(-2.90)(+3.34) yr with an eccentricity of 0.20(-0.02)(+0.01). Thanks to the inclusion of the absolute astrometry, we find a mass ratio of q = 1.25(-0.17)(+0.19), and a total mass of 9.47(-3.24)(+5.88) M-circle dot, indicating that the secondary (Ac) is the more massive of the pair. These results strongly suggest the presence of a fourth, unseen, member of the Albireo system. Given the current photometric data, it is likely that beta CygA is itself a hierarchical triple. We also derive the systemic proper motion, line-of-sight velocity, and an orbital parallax of the beta CygA system, allowing us to quantitatively assess the hypothesis that Albireo A and B form a physically bound and genealogically connected system. Finally, we find four potential members of a common proper motion group with Albireo, though none anywhere as close by as the Albireo components A to B.

Automated summary

This study characterized the binary system beta CygAa/Ac and confirmed the likely coeval nature of the three Albireo stars. The presence of a fourth unseen member in the Albireo system was strongly suggested, with beta CygA possibly being a hierarchical triple star.