Diving into the magnetosphere of the Of?p star HD108

Abstract

We analyse optical and X-ray spectroscopy of the Of?p star HD108, known for its strong dipolar magnetic field and its optical line profile variability with a timescale of $54\pm 3$ yrs, interpreted as the stellar rotation period. Optical emission lines have now recovered from their minimum emission state reached in 2007 - 2008. The variations of the equivalent width of the H$\alpha$ emission provide constraints on the inclination of the rotation axis ($i$) and the obliquity of the magnetic axis ($\beta$). The best agreement between model and observations is found for ($i$, $\beta$) pairs with $i+\beta \simeq 85^{\circ }$ and $i\in [30^{\circ },55^{\circ }]$. The Balmer emission lines display stochastic variability at the $\sim 5$ % level on timescales of a few days. TESS photometry unveils transient modulations on similar timescales in addition to prominent red noise variations. A Chandra X-ray observation of December 2021, when the star was at a higher emission level, indicates a slight increase of the flux and a spectral hardening compared to the August 2002 XMM-Newton observation, taken near minimum emission state. Magnetohydrodynamic simulations are used to compute synthetic X-ray spectra. With our current best estimate of the ${\dot{M}}_{B=0}$ mass-loss rate, the simulated X-ray luminosity and spectral energy distribution agree very well with the observations. Finally, the radial velocities vary on a period of 8.5 years with a peak-to-peak amplitude of 10 - 11 km s${}^{-1}$, suggesting orbital motion with an unseen companion of at least 4 M${}_{\odot }$.

Accepted for publication in MNRAS

References in corpus (21)

• The Transiting Exoplanet Survey Satellite
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: II. The Effects of Field-Aligned Rotation
• Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: III. Angular Momentum Loss and Rotational Spindown
• Low-frequency gravity waves in blue supergiants revealed by high-precision space photometry
• The MiMeS Survey of Magnetism in Massive Stars: CNO surface abundances of Galactic O stars
• Photometric detection of internal gravity waves in upper main-sequence stars. II. Combined TESS photometry and high-resolution spectroscopy
• Identifying quiescent compact objects in massive Galactic single-lined spectroscopic binaries
• X-rays from Magnetically Confined Wind Shocks: Effect of Cooling-Regulated Shock Retreat
• BRITE-Constellation high-precision time-dependent photometry of the early-O-type supergiant $\zeta$ Puppis unveils the photospheric drivers of its small- and large-scale wind structures
• An `Analytic Dynamical Magnetosphere' formalism for X-ray and optical emission from slowly rotating magnetic massive stars
• Low-frequency variability in massive stars: Core generation or surface phenomenon?
• HD148937: a multiwavelength study of the third Galactic member of the Of?p class
• Towards an understanding of the Of?p star HD191612: Phase-resolved multiwavelength observations
• Red noise and pulsations in evolved massive stars
• TESS lightcurves of gamma-Cas stars
• MONOS.II. Orbit review and analysis for 35 single-lined spectroscopic binary systems and candidates
• Magnetic inhibition of convection in O star envelopes
• Modelling the photometric variability of magnetic massive stars with the Analytical Dynamical Magnetosphere model
• Similar but different: the varied landscape of Onfp/Oef stars variability
• The mysterious Of?p class and the magnetic O-star Theta Ori C

Cited by in corpus (1)

• A magnetic field detection in the massive O-type bright giant 63 Oph