Work

Full orbital solution and dynamical masses for a new shell-Be + sdOB binary AN Col

Year: 2026

Type: article

Abstract: Context. AN Col is a shell Be star that has shown transient spectral features caused by a hot, evolved companion on a months-long orbit. If confirmed, this would place it among the growing category of post-mass-transfer spun-up Be and stripped subdwarf OB-type (sdOB) binaries. Characterizing the binary orbit makes it possible to obtain dynamical masses and provides a rare anchor for models of binary evolution and mass transfer. Aims. We aim to present a full three-dimensional orbital solution, dynamical masses, and a dynamical parallax for the newly discovered Be + sdOB binary AN Col. As a prerequisite to obtain a reliable orbital solution, we need to characterize the disk-wide density oscillation cycle that gives rise to the dominant spectroscopic cycles in order to separate the weaker periodic orbital variations. Methods. We used near-infrared interferometry time series from GRAVITY to confirm the companion and to map the relative astrometric orbit. With the knowledge of the orbital period, we identified spectral lines that trace the orbit of each of the two stellar components and measured their radial velocities to obtain a double-lined spectroscopic orbit. Combining interferometry and spectroscopy, we derived the full three-dimensional orbital solution. Results. The disk has completed three full oscillation cycles since 2010, with a cycle length of ∼5.1 yr. The ∼87.7-day orbit is compatible with an edge-on orientation, but the lack of eclipses in TESS light curves limits the orbital inclination to 88.4° or less. The resulting masses are M Be = 8.39 ± 0.67 M ⊙ and M sdOB = 0.81 ± 0.22 M ⊙ . As in other Be + sdOB binaries, the estimated efficiency of the past mass transfer is unexpectedly high. Detections of a weak He II λ 4686 absorption and other He I lines from the companion indicate that its effective temperature is close to 30 kK and that it is a slow rotator. Conclusions. This work establishes AN Col as a new benchmark Be + sdOB system, whose fundamental properties can be used to constrain models of binary evolution, as well as the physics of binary mass transfer that probably lead to the formation of the majority of Be stars. AN Col is only the second Be + sdOB binary with a full orbital solution enabling the determination of parallax-independent dynamical masses for both the spun-up Be star and its stripped helium companion.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202662646