Constraining the weak-wind problem: an XMM-HST campaign for the magnetic O9.7 V star HD 54879

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

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 confined stellar winds of magnetic stars can be exploited to constrain the true mass-loss rates \dot{M} of massive main sequence stars. We acquired UV, X-ray, and optical amateur data of HD 54879 (O9.7 V), one of a few O-type stars with a detected atmospheric magnetic field (B_d higher than about 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_B=0 about -9.0 M_sun yr^-1. Since the wind is partially trapped within the Alfven radius r_A about 12 R_*, the true mass-loss rate of HD 54879 is log M-dot less than -10.2 M_sun yr^-1. Moreover, we find that the microturbulent, macroturbulent, and projected rotational velocities are lower than previously suggested (< 4 km/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 T_X = 6.7 K and an X-ray luminosity of log L_X = 32 erg/s. The latter implies a significant X-ray excess (log L_X/ L_Bol about -6.0), most likely stemming from collisions at the magnetic equator. A tentative period of about 5yr 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-2018-CoSka.pdf, 1.2MB)

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