EDP Sciences
Free access
Issue A&A
Volume 490, Number 1, October IV 2008
Page(s) 259 - 264
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20079302
Published online 11 September 2008



A&A 490, 259-264 (2008)
DOI: 10.1051/0004-6361:20079302

Redshifted emission lines and radiative recombination continuum from the Wolf-Rayet binary $\theta$ Muscae: evidence for a triplet system?

Y. Sugawara1, Y. Tsuboi1, and Y. Maeda2

1  Department of Physics, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
    e-mail: sugawara@phys.chuo-u.ac.jp
2  Department of High Energy Astrophysics, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan

Received 21 December 2007 / Accepted 30 July 2008

Abstract
We present XMM-Newton observations of the WC binary $\theta$ Muscae (WR 48), the second brightest Wolf-Rayet binary in optical wavelengths. The system consists of a short-period (19.1375 days) WC5/WC6 + O6/O7V binary and possibly has an additional O supergiant companion (O9.5/B0Iab) which is optically identified at a separation of ~46 mas. Strong emission lines from highly ionized ions of C, O, Ne, Mg, Si, S, Ar, Ca and Fe are detected. The spectra are fitted by a multi-temperature thin-thermal plasma model with an interstellar absorption NH = 2– $3 \times 10^{21}$ cm-2. Lack of nitrogen line indicates that the abundance of carbon is at least an order of magnitude larger than that of nitrogen. A Doppler shift of ~630 km s-1 is detected for the O V I I I line, while similar shifts are obtained from the other lines. The reddening strongly suggests that the emission lines originated from the wind-wind shock zone, where the average velocity is ~600 km s-1. The red-shift motion is inconsistent with a scenario in which the X-rays originate from the wind-wind collision zone in the short-period binary, and would be evidence supporting the widely separated O supergiant as a companion. This may make up the collision zone be lying behind the short-period binary. In addition to the emission lines, we also detected the RRC (radiative recombination continuum) structure from carbon around 0.49 keV. This implies the existence of additional cooler plasma.


Key words: X-rays: stars -- stars: Wolf-Rayet -- binaries: spectroscopic -- stars: winds, outflows -- stars: individual: HD113904



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Editor-in-Chief: T. Forveille
Letters Editor-in-Chief: J. Alves
Managing Editor: C. Bertout

ISSN: 0004-6361 ; e-ISSN: 1432-0746
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Published by: EDP Sciences

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