EDP Sciences
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Volume 398, Number 1, January IV 2003
Page(s) 203 - 211
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20021577

A&A 398, 203-211 (2003)
DOI: 10.1051/0004-6361:20021577

High-resolution X-ray spectroscopy of $\tau$ Scorpii (B0.2V) with XMM-Newton

R. Mewe1, A. J. J. Raassen1, 2, J. P. Cassinelli3, K. A. van der Hucht1, N. A. Miller3 and M. Güdel4

1  SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2  Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3  Dept. of Astronomy, University of Wisconsin at Madison, 6251 Sterling Hall, North Charter Str., Madison, WI 53706, USA
4  Paul Scherrer Institut, Würenlingen & Villigen, 5232 Villigen PSI, Switzerland

(Received 24 July 2002 / Accepted 30 October 2002 )

We report the analysis of the first high-resolution X-ray spectrum of the B0.2V star $\tau$ Scorpii obtained with the Reflection Grating Spectrometers (RGS) and the EPIC-MOS CCD spectrometers on board XMM-Newton. The spectrum exhibits bright emission lines of the H- and He-like ions of C, N, O, Ne, Mg, and Si, as well as Fe XVII & Fe  XVIII lines. Line fluxes have been determined. Simultaneous fits to the RGS and EPIC spectra were used to obtain four plasma temperatures, emission measures, and the overall elemental abundances. This multi-temperature fitting yielded temperatures of 1.6, 5.2, 8.2, and $\ga$20 MK. These temperatures are confirmed by DEM modelling. The nitrogen lines are relatively strong: the N/O abundance ratio is about 3 $\times$ solar. No indication of a solar-type "FIP effect" was found for the other elements. According to the derived models the X-ray luminosity in the energy range 0.3-10 keV is $L_{\rm
x}$ = 3.2 $\times$ 10 31 erg s -1 at a distance of 132 pc. The sensitivity of the He-like forbidden and intercombination lines to a strong ultraviolet radiation field is used to derive upper limits to the radial distances at which lines of Mg XI, Ne IX, O VII, and N VI originate. The results suggest that the soft X-rays ( $\la$8 MK) originate from shocks low in the wind that are produced by the common mechanism of radiation line-driven instabilities. This is consistent with the observed emission line profiles that are much narrower ( $\la$500 km s -1) than the broad lines (up to 1500 km s -1) observed high up in the wind of $\zeta$ Puppis. The hot (~20-40 MK) component may be explained by a model involving dense clumps embedded in a wind which sweeps past them at high relative velocity (~1400-1700 km s -1). Such an interaction would produce the strong shocks required.

Key words: stars: individual: $\tau$ Scorpii -- stars: coronae -- stars: early-type -- stars: abundances -- X-rays: stars

Offprint request: R. Mewe, r.mewe@sron.nl

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