Constraining the weak-wind problem: an XMM-HST campaign for the magnetic star HD54879

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

Mass-loss rates of massive, late type main sequence stars are much weaker than currently predicted, but their true values are very difficult to measure. We suggest that conned stellar winds of magnetic stars can be exploited to constrain the true mass-loss rates of massive main sequence stars. We acquired UV, X-ray, and optical amateur data of HD54879 (O9.7 V), one of a few O-type stars with a detected atmospheric magnetic eld (Bd > 2 kG). We analyze these data with the Potsdam Wolf-Rayet (PoWR) and xspec codes. We can roughly estimate the mass-loss rate the star would have in the absence of a magnetic field as log M-dot less than about -9.0 [M_sun/yr]. Since the wind is partially trapped within the Alfven radius rA > 12 R*, the true mass-loss rate of HD54879 is log M-dot ~ -10.2 [M_sun/yr]. Moreover, we find that the microturbulent, macroturbulent, and projected rotational velocities are lower than previously suggested (< 4km/s). An initial mass of 16 M_sun and an age of 5 Myr are inferred. 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]. The latter implies a significant X-ray excess (log LX / LBol ~ 6.0), most likely stemming from collisions at the magnetic equator. A tentative period of P ~ 5 yr is derived from variability of the Hα line. 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.

Preprint (Shenar_HD54879_Brno.pdf, 0.5MB)

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