Early magnetic B-type stars: X-ray emission and wind properties
L.M. Oskinova1, H. Todt1, R. Ignace2, J.C. Brown3, J.P. Cassinelli4, W.-R. Hamann1
1 Institute for Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany
2 Department of Physics and Astronomy, East Tennessee State University, Johnson City, TN 37614, USA
3 School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
4 Department of Astronomy, University of Wisconsin-Madison, Madison, WI 53711, USA
We present a comprehensive study of X-ray emission and wind properties
of massive magnetic early B-type stars. Dedicated XMM-Newton
observations were obtained for three stars xi1 CMa, V2052 Oph, and zeta
Cas. We report the first detection of X-ray emission from V2052 Oph and
zeta Cas. The observations show that the X-ray spectra of our program
stars are quite soft. We compile the complete sample of early B-type
stars with detected magnetic fields to date and existing X-ray
measurements, in order to study whether the X-ray emission can be used
as a general proxy for stellar magnetism. We find that hard and strong
X-ray emission does not necessarily correlate with the presence of a
magnetic field. We analyze the UV spectra of five non-supergiant B stars
with magnetic fields by means of non-LTE iron-blanketed model
atmospheres. The latter are calculated with the PoWR code, which treats
the photosphere as well as the the wind, and also accounts for X-rays.
Our models accurately fit the stellar photospheric spectra in the
optical and the UV. The parameters of X-ray emission, temperature and
flux are included in the model in accordance with observations. We
confirm the earlier findings that the filling factors of X-ray emitting
material are very high. Our analysis reveals that the magnetic early
type B stars studied here have weak winds. The mass-loss rates are
significantly lower than predicted by hydrodynamically consistent
models. We find that, although the X-rays strongly affect the ionization
structure of the wind, this effect is not sufficient in reducing the
total radiative acceleration. When the X-rays are accounted for at the
intensity and temperatures observed, there is still sufficient radiative
acceleration to drive stronger mass-loss than we empirically infer from
the UV spectral lines. (abridged)
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