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

TL;DR

This study provides a detailed spectroscopic and orbital analysis of the Albireo binary system, revealing its complex hierarchical structure and suggesting the presence of an unseen fourth member.

Contribution

It offers a new orbital solution for Albireo's primary binary and proposes a hierarchical triple structure with an unseen companion, based on combined spectroscopic, astrometric, and evolutionary data.

Findings

Orbital period of 121.65 years with low eccentricity.

Mass ratio indicating the secondary is more massive.

Evidence suggesting a fourth, unseen system member.

Abstract

We present a spectroscopic characterisation and a new orbital solution for the binary system beta Cyg Aa/Ac (MCA 55), the primary component (beta Cyg A) of the well-known wide double star Albireo. By matching evolutionary tracks to the physical parameters of all three Albireo stars (beta Cyg Aa, 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 $120$ years, 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 $121.65^{+3.34}_{-2.90}$ years with an eccentricity of $0.20^{+0.01}_{-0.02}$. Thanks to the inclusion of the absolute astrometry, we find a mass ratio of $q = 1.25^{+0.19}_{-0.17}$, and a total mass of $9.47_{-3.24}^{+5.88}$ M$_\odot$, 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 Cyg A is itself a hierarchical triple. We also derive the systemic proper motion, line-of-sight velocity, and an orbital parallax of the beta Cyg A 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.