2017A&A...606A..91S

Context. HD 54879 (O9.7 V) is one of a dozen O-stars for which an organized atmospheric magnetic field has been detected. Despite their importance, little is known about the winds and evolution of magnetized massive stars.
Aims. To gain insights into the interplay between atmospheres, winds, and magnetic fields of massive stars, we acquired UV and X-ray data of HD 54879 using the Hubble Space Telescope and the XMM-Newton satellite. In addition, 35 optical amateur spectra were secured to study the variability of HD 54879.
Methods. A multiwavelength (X-ray to optical) spectral analysis is performed using the Potsdam Wolf-Rayet (PoWR) model atmosphere code and theXSPEC software.
Results. The photospheric parameters (T_*=30.5kK, logg=4.0[cm/s²], logL=4.45[L_☉]) are typical for an O9.7 V star. The microturbulent, macroturbulent, and projected rotational velocities are lower than previously suggested (ξ_ph,v_mac,vsini≥4km/s). An initial mass of 16M_☉ and an age of 5Myr are inferred from evolutionary tracks. We derive a mean X-ray emitting temperature of log T_X=6.7[K] and an X-ray luminosity of L_X=1x10³²erg/s. Short- and long-scale variability is seen in the Hα line, but only a very long period of P~=5 yr could be estimated. Assessing the circumstellar density of HD 54879 using UV spectra, we can roughly estimate the mass-loss rate HD 54879 would have in the absence of a magnetic field as logM_B=0~=-9.0[M_☉/yr]. The magnetic field traps the stellar wind up to the Alfven radius r_A≥12R_*, implying that its true mass-loss rate is log M≥-10.2[M_☉/yr]. Hence, density enhancements around magnetic stars can be exploited to estimate mass-loss rates of non-magnetic stars of similar spectral types, essential for resolving the weak wind problem.
Conclusions. Our study confirms that strongly magnetized stars lose little or no mass, and supplies important constraints on the weak-wind problem of massive main sequence stars.