A combined HST and XMM-Newton campaign for the magnetic O9.7 V star HD 54879: constraining the weak-wind problem of massive stars

T. Shenar(1), L. M. Oskinova, S. P. Järvinen, P. Luckas, R. Hainich, H. Todt, S. Hubrig, A. A. C. Sander, I. Ilyin, W.-R. Hamann

1 - University of Potsdam, Germany

Context: HD 54879 (O9.7 V) is one of a dozen O-stars for which an organized atmospheric magnetic field has been detected. 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. A multiwavelength (X-ray to optical) spectral analysis is performed using the Potsdam Wolf-Rayet (PoWR) model atmosphere code and the xspec software.
Results: The photospheric parameters are typical for an O9.7 V star. The microturbulent, macroturbulent, and projected rotational velocities are lower than previously suggested (<4 km/s). An initial mass of 16 M⊙ and an age of 5 Myr are inferred from evolutionary tracks. We derive a mean X-ray emitting temperature of log TX = 6.7 [K] and an X-ray luminosity of log LX = 32 [erg/s]. Short- and long-scale variability is seen in the H-alpha line, but only a very long period of P ≈ 5yr 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 log Mdot(B=0) ≈ −9.0 [M⊙/yr]. The magnetic field traps the stellar wind up to the Alfv\'en radius > 12 R⊙, implying that its true mass-loss rate is log Mdot < −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.

Reference: A&A, in press
Status: Manuscript has been accepted

Weblink: https://arxiv.org/abs/1708.01261


Email: shtomer@astro.physik.uni-potsdam.de