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arXiv · 2512.15019

HOney-BeeS II. Be-X-ray binaries as testbeds for spectroscopic studies of Be stars

S. Janssens·H. Sana·T. Shenar·J. Bodensteiner·M. Abdul-Masih
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Abstract

The majority of massive classical Be stars are thought to be binary interactions products. Their rapid rotation and often strong, variable, and emission-line dominated spectrum, make spectroscopic analysis challenging. Hence, robust binary properties and statistical constraints are still lacking for the Be population. In this study, we use seven Be-X-ray binaries, and their orbital periods derived from the X-rays, to investigate the reliability of different spectral lines and numerical methods for the measuring of radial-velocities and orbital period determination of Be stars. We use multi-epoch high-resolution HERMES spectra and compare absorption- and emission-line radial velocities obtained with cross-correlation, line-profile fitting, and the bisector method. Line-profile variability affects the bisector method and line-profile fitting requires model templates that do not encompass the complexity of Be-star line profiles. Therefore, we recommend using cross-correlation: it is independent of models and easily compatible with line blends seen in Be-star spectra. The obtained statistical uncertainty on the radial-velocities from cross-correlation is 0.2-0.3km/s for H$\alpha$ (emission) and ~5km/s for absorption lines, excluding potential systematics. In general, whether the goal is to do binary statistics of a population or an in-depth study of a specific system, we suggest using emission lines, due to a higher precision and less scatter than absorption lines. Here, H$\beta$ is preferred over H$\alpha$ because of its lesser variability. However, large-scale variability may cause large shifts in emission-line radial velocities, resulting in spurious eccentricities. In this case, orbital solutions should ideally be compared to lower-signal absorption lines (if present). Finally, we highlight the need for understanding how companion-disc interactions alter emission-line appearance.

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S. Janssens, H. Sana, T. Shenar, J. Bodensteiner, M. Abdul-Masih. 2025-12-17. HOney-BeeS II. Be-X-ray binaries as testbeds for spectroscopic studies of Be stars. https://arxiv.org/abs/2512.15019

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