EDP Sciences
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Volume 478, Number 2, February I 2008
Page(s) 513 - 520
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20077891

A&A 478, 513-520 (2008)
DOI: 10.1051/0004-6361:20077891

XMM-Newton observations of $\zeta$ Orionis (O9.7 Ib): a collisional ionization equilibrium model

A. J. J. Raassen1, 2, K. A. van der Hucht1, 2, N. A. Miller3, and J. P. Cassinelli4

1  SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
    e-mail: a.j.j.raassen@sron.nl
2  Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3  Dept. of Physics & Astronomy, University of Wisconsin-Eau Claire, 105 Garfield Avenue, Eau Claire WI 54702, USA
4  Dept. of Astronomy, University of Wisconsin at Madison, 6251 Sterling Hall, North Charter Str., Madison, WI 53706, USA

(Received 15 May 2007 / Accepted 18 November 2007 )

We present XMM-Newton observations of the O supergiant $\zeta$ Orionis (O9.7 Ib). The spectra are rich in emission lines over a wide range of ionization stages. The RGS spectra show for the first time lines of low ion stages such as $\ion{C}{\sc vi}$, $\ion{N}{\sc vi}$, $\ion{N}{\sc vii}$, and $\ion{O}{\sc vii}$. The line profiles are symmetric and rather broad ( ${\it {\it FWHM}}\approx 1500$ km s-1) and show only a slight blue shift. With the XMM-EPIC spectrometer several high ions are detected in this star for the first time including $\ion{Ar}{\sc xvii}$ and $\ion{S}{\sc xv}$. Simultaneous multi-temperature fits and DEM-modeling were applied to the RGS and EPIC spectra to obtain emission measures, elemental abundances and plasma temperatures. The calculations show temperatures in the range $\approx$0.07-0.6 keV. According to the derived models the intrinsic source X-ray luminosity at a distance of 251 pc is $L_{\rm x}$ = 1.37(.03) $\times$ 1032 erg$\,$s-1 in the energy range 0.3-10 keV. In the best multi-temperature model fit, the abundances of C, N, O, and Fe are near their solar values, while the abundances of Ne, Mg, and Si appear somewhat enhanced. The sensitivity of the He-like forbidden and intercombination lines to $\zeta$ Ori's strong radiation field is used to derive the radial distances at which lines are formed. Values of 34 R* for $\ion{N}{\sc vi}$, 12.5 R* for $\ion{O}{\sc vii}$, 4.8 R* for $\ion{Ne}{\sc ix}$, and 3.9 R* for $\ion{Mg}{\sc xi}$ are obtained.

Key words: stars: early-type -- stars: winds, outflows -- X-rays: individual: $\zeta$ Orionis -- X-rays: stars

© ESO 2008