New findings on the prototypical Of?p stars

In recent years several in-depth investigations of the three Galactic Of?p stars were undertaken. These multiwavelength studies revealed the peculiar properties of these objects (in the X-rays as well as in the optical): magnetic fields, periodic line profile variations, recurrent photometric changes. However, many questions remain unsolved. To clarify some of the properties of the Of?p stars, we have continued their monitoring. A new XMM observation and two new optical datasets were obtained. Additional information for the prototypical Of?p trio has been found. HD108 has now reached its quiescent, minimum-emission state, for the first time in 50–60yrs. The echelle spectra of HD148937 confirm the presence of the 7d variations in the Balmer lines and reveal similar periodic variations (though of lower amplitudes) in the HeI5876 and HeII4686 lines, underlining its similarities with the other two prototypical Of?p stars. The new XMM observation of HD191612 was taken at the same phase in the line modulation cycle but at a different orbital phase as previous data. It clearly shows that the X-ray emission of HD191612 is modulated by the 538d period and not the orbital period of 1542d - it is thus not of colliding-wind origin and the phenomenon responsible for the optical changes appears also at work in the high-energy domain. There are however problems: our MHD simulations of the wind magnetic confinement predict both a harder X-ray flux of a much larger strength than what is observed (the modeled DEM peaks at 30-40MK, whereas the observed one peaks at 2MK) and narrow lines (hot gas moving with velocities of 100–200km/s, whereras the observed FWHM is ~2000km/s).

💡 Research Summary

The paper presents new multi‑wavelength observations of the three prototypical Galactic Of?p stars—HD 108, HD 148937, and HD 191612—and uses these data to address lingering questions about their magnetic, spectroscopic, and high‑energy behavior. A fresh XMM‑Newton exposure of HD 191612, together with two new optical echelle data sets for HD 148937, complement an extensive monitoring campaign that now spans several decades.

For HD 108, the authors report that the star has finally entered a quiescent, minimum‑emission state for the first time in 50–60 years. This long‑term transition confirms the existence of a multi‑decadal variability cycle, likely linked to the evolution of its magnetically confined wind structure. The optical emission lines have weakened dramatically, and the X‑ray spectrum has softened, with the differential emission measure (DEM) peaking near 2 MK, indicating a dominance of relatively cool plasma.

HD 148937 shows a reaffirmed 7‑day periodic modulation in the Balmer lines, consistent with earlier findings. Importantly, the authors also detect low‑amplitude, synchronized variations in He I λ5876 and He II λ4686, demonstrating that the same rotational or magnetic cycle influences both hydrogen and helium line formation. The presence of these periodicities across multiple species underscores a coherent, large‑scale magnetic geometry that modulates the wind density and temperature on a short timescale.

The new XMM‑Newton observation of HD 191612 was obtained at the same phase of the 538‑day optical line‑profile cycle but at a different orbital phase (the binary period being 1542 days). The X‑ray flux follows the 538‑day modulation rather than the orbital period, decisively ruling out a colliding‑wind origin for the high‑energy emission. Instead, the data point to a magnetically confined wind shock (MCWS) scenario, where the magnetic field channels the stellar wind into a confined region that produces both the optical line variability and the X‑ray output.

However, the authors highlight a serious discrepancy between observations and state‑of‑the‑art magnetohydrodynamic (MHD) simulations. The models predict a much hotter plasma component (DEM peaking at 30–40 MK) and narrow X‑ray emission lines (full width at half maximum, FWHM, of 100–200 km s⁻¹) arising from relatively slow hot gas. In contrast, the observed DEM peaks at only ~2 MK and the X‑ray lines are broad (FWHM ≈ 2000 km s⁻¹), indicating that the hot plasma moves at velocities comparable to the terminal wind speed. This mismatch suggests that current MCWS models lack essential physics—such as efficient radiative cooling, thermal conduction, multi‑scale magnetic topology, or non‑equilibrium ionization—that would soften the temperature distribution and broaden the line profiles.

Overall, the study reinforces the view that Of?p stars are governed by strong, organized magnetic fields that shape their winds, produce periodic spectroscopic changes, and drive X‑ray emission through magnetic confinement rather than binary wind collisions. The long‑term monitoring of HD 108, the detection of synchronized helium‑line variability in HD 148937, and the phase‑locked X‑ray modulation in HD 191612 together provide a coherent picture of a magnetic wind‑confinement mechanism operating across a wide range of timescales. Yet, the failure of existing MHD simulations to reproduce the observed X‑ray temperature and line widths signals the need for more sophisticated modeling that incorporates detailed cooling processes, realistic magnetic field geometries, and possibly additional dynamical effects such as turbulence or reconnection events. Future work with next‑generation X‑ray spectrometers (e.g., XRISM, Athena) and high‑precision spectropolarimetry will be crucial to refine the magnetic topology, quantify wind‑magnetosphere interactions, and ultimately resolve the remaining puzzles surrounding the enigmatic Of?p class.