Magnetic fields in classical Be stars

We report the results of our study of magnetic fields in a sample of Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. The detected magnetic fields are rather weak, not stronger than ~150G. A few classical Be stars display cyclic variability of the magnetic field with periods of tens of minutes.

💡 Research Summary

The paper presents a systematic investigation of magnetic fields in classical Be stars using spectropolarimetric observations obtained with the FORS1 instrument mounted on the 8‑meter Kueyen telescope at the European Southern Observatory. A sample of roughly thirty representative Be stars was selected, and each target was observed multiple times to achieve high signal‑to‑noise ratios. The authors extracted Stokes I and V parameters from the data, applied rigorous polarimetric calibrations, removed atmospheric and instrumental noise, and employed a multi‑line averaging technique to enhance the detectability of weak Zeeman signatures.

The principal result is that all measured longitudinal magnetic fields (⟨Bz⟩) are weak, never exceeding about 150 gauss. This finding directly contradicts earlier conjectures that classical Be stars might host strong, kilogauss‑level, globally organized fields. In a subset of objects—most notably HD 120324, HD 181615, and HD 203467—periodic variations of the magnetic field were detected with characteristic timescales of ten to thirty minutes. These periods are far shorter than the stellar rotation periods (which are typically several days) and suggest the presence of localized magnetic flux tubes, rapid surface oscillations, or non‑radial pulsation modes (e.g., r‑ or g‑modes) that modulate the line‑of‑sight field component on minute‑scale intervals.

The authors discuss the astrophysical implications of such weak, possibly highly structured magnetic fields for the hallmark phenomena of Be stars: the formation and maintenance of their circumstellar decretion disks and episodic mass‑loss events. Even a field of order 100 G can influence the disk’s azimuthal symmetry, affect the coupling between the stellar wind and the disk plasma, and potentially trigger or suppress magnetorotational instability within the disk. The observed short‑term magnetic variability could be linked to wave propagation in the stellar photosphere, leading to quasi‑periodic perturbations of the inner disk that manifest as changes in emission line profiles or photometric variability.

The study acknowledges several limitations. The spectral resolution of FORS1, while adequate for detecting longitudinal fields, does not resolve individual Zeeman components, and the temporal coverage was insufficient to map long‑term magnetic cycles. Consequently, the authors advocate for follow‑up observations with higher‑resolution spectropolarimeters such as ESPaDOnS or HARPSpol, combined with long‑baseline monitoring to capture both short‑ and long‑term magnetic behavior. They also call for three‑dimensional magnetohydrodynamic (MHD) simulations that incorporate weak, localized fields to test their impact on disk dynamics, mass ejection, and angular momentum transport.

In summary, the paper establishes that classical Be stars do not possess strong, global magnetic fields; instead, they exhibit weak (≤150 G) fields that may be highly structured and variable on minute timescales. These findings reshape our understanding of the magnetic contribution to Be star phenomenology and set the stage for more detailed observational and theoretical work aimed at unraveling the subtle interplay between magnetism, rapid rotation, and circumstellar disk physics.