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

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$α$ (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$β$ is preferred over H$α$ 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.

💡 Research Summary

The research paper “HOney-BeeS II” presents a rigorous methodological evaluation of spectroscopic techniques used to study Be stars, specifically utilizing Be-X-ray binaries (BeXRBs) as controlled testbeds. Be stars are characterized by rapid rotation and circumstellar disks, which produce complex, highly variable emission lines. These features make it notoriously difficult to derive accurate radial velocities (RV) and orbital parameters, hindering our statistical understanding of the Be star population and their evolutionary links to binary interactions.

To address this, the authors utilized seven BeXRBs, where orbital periods are already precisely known from X-ray observations, to benchmark different spectroscopic analysis methods. Using high-resolution HERMS spectra collected over multiple epochs, the study compared three primary techniques: Cross-correlation (CCF), Line-profile fitting, and the Bisector method. The researchers specifically investigated the performance of both absorption and emission lines in determining orbital motion.

The findings reveal that Line-profile variability (LPV), a hallmark of Be stars, significantly degrades the accuracy of the Bisector method and Line-profile fitting. The latter suffers from the inadequacy of standard model templates, which fail to capture the intricate complexity of Be-star spectral profiles. In contrast, the Cross-correlation method emerged as the most robust and recommended approach. Its strength lies in being model-independent and its ability to handle spectral line blending, which is frequent in Be-star spectra.

Quantitatively, the study demonstrates a significant disparity in precision between different spectral features. The statistical uncertainty in radial velocities using H$\alpha$ (emission) was remarkably low, ranging from 0.2 to 0.3 km/s, whereas absorption lines exhibited much higher uncertainty, around 5 km/s. Consequently, the study advocates for the use of emission lines for both population statistics and in-depth single-system studies due to their superior precision and reduced scatter.

However, the authors provide a crucial caveat regarding the use of H$\alpha$: its extreme variability can induce “spurious eccentricities” in orbital solutions, potentially leading to incorrect physical interpretations of the binary orbit. Therefore, the study suggests that H$\beta$ is a preferred alternative due to its lower variability. For the most reliable results, orbital solutions derived from emission lines should ideally be cross-referenced with lower-signal absorption lines when available. Finally, the paper concludes by highlighting the necessity of further investigating how interactions between the companion star and the circumstellar disk alter the appearance of emission lines, a key step toward mastering the spectroscopy of these complex stellar systems.