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 (hereafter SPB) stars, along with a small number of normal B-type stars. A weak mean longitudinal magnetic field of the order of a few hundred Gauss was detected in three beta Cephei stars and two stars suspected to be beta Cephei stars, in five SPB stars and eight stars suspected to be SPB stars. Additionally, a longitudinal magnetic field at a level larger than 3sigma has been diagnosed in two normal B-type stars, the nitrogen-rich early B-type star HD52089 and in the B5 IV star HD153716. Roughly one third of beta Cephei stars have detected magnetic fields: Out of 13 beta Cephei stars studied to date with FORS1, four stars possess weak magnetic fields, and out of the sample of six suspected beta Cephei stars two show a weak magnetic field. The fraction of magnetic SPBs and candidate SPBs is found to be higher: roughly half of the 34 SPB stars have been found to be magnetic and among the 16 candidate SPBs eight stars possess magnetic fields. In an attempt to understand why only a fraction of pulsating stars exhibit magnetic fields, we studied the position of magnetic and non-magnetic pulsating stars in the H-R diagram. We find that their domains 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. It is possible that stronger fields tend to be found in stars with lower pulsating frequencies and smaller pulsating amplitudes. A somewhat similar trend is found if we consider a correlation between the field strength and the v sin i-values, i.e. stronger magnetic fields tend to be found in more slowly rotating stars.

💡 Research Summary

The paper reports the results of an extensive magnetic survey of B‑type pulsators carried out with the FORS1 spectropolarimeter on the Very Large Telescope. The sample comprises 13 confirmed β Cephei stars, six β Cephei candidates, 34 confirmed Slowly Pulsating B (SPB) stars, 16 SPB candidates, and a handful of apparently normal B‑type stars. Using low‑resolution spectropolarimetry the authors measured the mean longitudinal magnetic field ⟨Bz⟩ from Stokes V profiles and considered a detection significant when it exceeded the 3σ level.

A weak field of a few hundred gauss was found in three β Cephei stars and two β Cephei candidates, giving a detection rate of roughly 30 % among confirmed β Cephei objects (four out of 13) and about one‑third among the candidates (two out of six). In the SPB group the incidence is markedly higher: about half of the confirmed SPB stars (17 of 34) and half of the candidates (8 of 16) show a measurable field. Two apparently normal B‑type stars, the nitrogen‑rich early B star HD 52089 and the B5 IV star HD 153716, also display longitudinal fields above the 3σ threshold.

The authors placed all magnetic and non‑magnetic objects on the Hertzsprung–Russell diagram to search for evolutionary trends. The magnetic and non‑magnetic pulsators occupy largely overlapping regions of the main‑sequence band, indicating that simple age or evolutionary stage does not dictate the presence of a field. However, a tentative correlation emerges when the pulsation characteristics are examined: stars with lower pulsation frequencies and smaller amplitudes tend to host relatively stronger magnetic fields. This suggests that magnetic fields may influence mode selection or energy transport within the stellar interior.

A second notable trend concerns stellar rotation. The data reveal an inverse relationship between v sin i and field strength: more slowly rotating stars generally exhibit larger ⟨Bz⟩ values. This is consistent with magnetic braking scenarios, where a magnetic torque extracts angular momentum, leading to slower rotation and, possibly, a more detectable field.

The detection of magnetic fields in a substantial fraction of β Cephei and SPB stars challenges the traditional view that these pulsators are predominantly non‑magnetic. It implies that magnetic effects must be incorporated into models of their internal structure and pulsation driving mechanisms. The overlap in the HR diagram further indicates that magnetic fields are not confined to a narrow evolutionary window but can persist throughout a significant portion of the main‑sequence lifetime.

The paper concludes by emphasizing the need for follow‑up investigations. Long‑term monitoring of magnetic variability, high‑resolution spectropolarimetry to resolve field geometry, and three‑dimensional magnetohydrodynamic simulations are essential to quantify how magnetic fields interact with pulsation modes, affect chemical mixing (as hinted by the nitrogen enrichment in HD 52089), and evolve as the star ages. Such studies will refine our understanding of the complex interplay between magnetism, rotation, and stellar oscillations in massive B‑type stars.