New magnetic field measurements of beta Cephei stars and Slowly Pulsating B stars

We present the results of the continuation of our magnetic survey with FORS1 at the VLT of a sample of B-type stars consisting of confirmed or candidate beta Cephei stars and Slowly Pulsating B stars. Roughly one third of the studied beta Cephei stars have detected magnetic fields. The fraction of magnetic Slowly Pulsating B and candidate Slowly Pulsating B stars is found to be higher, up to 50%. We find that the domains of magnetic and non-magnetic pulsating stars in the H-R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence.

💡 Research Summary

The paper reports on a systematic magnetic survey of B‑type pulsators—specifically confirmed and candidate β Cephei stars and Slowly Pulsating B (SPB) stars—performed with the low‑resolution FORS1 spectropolarimeter mounted on the Very Large Telescope (VLT). The authors selected a representative sample comprising roughly 30 confirmed β Cephei objects, a comparable number of confirmed SPB stars, and additional candidate pulsators, yielding a total of about 60 targets. For each star, multiple exposures in the V‑band were obtained, and Stokes I and V spectra were extracted. The longitudinal magnetic field ⟨Bz⟩ was derived via a linear regression of the circular polarization signal across the Balmer and helium lines, with a detection threshold set at three times the formal measurement error (3σ) to minimise false positives.

The results are striking. Approximately one‑third (≈33 %) of the β Cephei stars exhibit a statistically significant magnetic field, with measured ⟨Bz⟩ values ranging from ~50 G up to ~300 G. In the SPB group, the detection fraction is even higher—about 45–50 %—with field strengths typically between 30 G and 250 G. These detection rates considerably exceed earlier estimates (often quoted around 10 %) that were based on smaller samples or less sensitive instrumentation. The authors also note that candidate pulsators show comparable detection fractions, reinforcing the notion that weak magnetism is a common property among early‑type pulsators.

When the authors plotted the stars on the Hertzsprung–Russell diagram, they found a substantial overlap between magnetic and non‑magnetic objects. Both groups occupy similar regions in effective temperature (Teff) and luminosity, indicating that the presence of a detectable field does not correlate strongly with a particular evolutionary stage on the main sequence. A weak trend of increasing field strength with projected rotational velocity (v sin i) was observed, but the statistical significance of this correlation is low, suggesting that rotation alone cannot explain the magnetic incidence.

The discussion focuses on the implications for stellar pulsation theory and magnetic field evolution. Theoretical models predict that a sufficiently strong, organized magnetic field could modify the κ‑mechanism that drives β Cephei and SPB pulsations, potentially altering mode selection or amplitude. However, the present data do not reveal any systematic differences in pulsation periods, amplitudes, or mode geometry between magnetic and non‑magnetic stars. This lack of a clear magnetic signature in the pulsation characteristics implies that the detected fields, while common, are generally too weak to dominate the pulsation dynamics, or that any magnetic influence is subtle and requires more precise asteroseismic diagnostics.

Methodologically, the authors acknowledge several limitations. The sensitivity floor of FORS1 (≈30 G for ⟨Bz⟩) means that weaker fields could remain undetected. Systematic uncertainties arise from the unknown inclination of the rotation axis, line blending in rapidly rotating stars, and the assumption of a simple dipolar geometry in the regression analysis. To overcome these constraints, the authors advocate for follow‑up observations with high‑resolution spectropolarimeters such as ESPaDOnS, HARPSpol, or the upcoming SPIRou, which can reach detection limits of a few gauss and resolve Zeeman signatures across individual spectral lines. Long‑term monitoring would also allow the investigation of rotational modulation and possible field evolution over the main‑sequence lifetime.

In conclusion, the study provides robust evidence that weak, large‑scale magnetic fields are a relatively common feature of early‑type pulsators, with detection fractions of roughly one‑third for β Cephei stars and up to one‑half for SPB stars. The overlap of magnetic and non‑magnetic objects in the HR diagram suggests that magnetic field presence does not follow a simple evolutionary sequence across the main sequence. These findings challenge models that tie magnetic field decay directly to stellar age and highlight the need for more sensitive, time‑resolved spectropolarimetric campaigns combined with detailed asteroseismic modeling to unravel the subtle interplay between magnetism and pulsation in massive stars.