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Researchers analyzed spectroscopic and photometric data of the massive O-type star xi Persei spanning over three decades and determined a precise rotation period of 2.04 days. The study reveals periodic variations in ultraviolet wind lines and other spectral features that consistently align across different wavelengths and instruments, suggesting the star possesses a weak, previously undetected global magnetic field. The team proposes a model where this weak magnetic field creates a small magnetospheric disc near the stellar surface while allowing corotating interaction regions at higher latitudes, with observed variations resulting from the changing view of this disc as the star rotates.
Why it matters
This discovery adds xi Persei to the rare class of magnetic massive stars and demonstrates that weak magnetic fields can significantly affect stellar winds and mass loss in hot luminous stars. The findings provide new constraints for understanding how magnetic fields influence the evolution and observable properties of massive stars, which play crucial roles in galactic chemical enrichment and stellar population studies.
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⚠️ Preprint – Noch nicht peer-reviewed
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Abstract: OB stars exhibit line profile variability, often associated with rotation; notably, in their UV wind-sensitive lines where discrete absorption components tend to recur cyclically. We searched for periodicity in spectral regions of the O7.5III(n)((f)) star xi Persei formed very close to the star. The NIV 1718 A line was identified as the most uncontaminated spectral with a large and homogeneous available dataset is available: 307 IUE spectra over 12 years and 11 STIS spectra taken 21 years later. We also studied 322 time-resolved HeII 4686 A spectra and MOST, BRITE, and TESS space photometry, covering a time span of 13 years. We also reconsidered X-ray studies with Chandra. A CLEAN analysis and subsequent weighted least-squares fit of the flux in this region resulted in a unique period of 2.040514(20) d, attributed to rotation. The phase of maximum flux in the NIV, SiIV, Ha, and HeII line profiles coincide with the results of X-ray variability studies. This period was also found in the photometric data. Given the coherent periodic behaviour probing the region of the wind nearest the stellar surface, we propose that xi Per has an as-yet undetected weak global surface magnetic field. The sinusoidal behaviour suggested that only one magnetic pole is visible, implying an inclination of i approximately 51 deg, and therefore, 39 deg for the magnetic obliquity. We present a conceptual framework within which weak wind confinement by a global magnetic field can create a small magnetospheric disc, while allowing for the formation of more classic corotating interaction regions at higher magnetic latitudes. The periodic variations are then understood to result from the changing projected area of the magnetospheric disc as a function of rotational phase, both along the line of sight and off the stellar limb.