Journal Article

The very massive X-ray bright binary system Wack 2134 (= WR 21a)*

V. S. Niemela, R. C. Gamen, R. H. Barbá, E. Fernández Lajús, P. Benaglia, G. R. Solivella, P. Reig and M. J. Coe

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 389, issue 3, pages 1447-1452
Published in print September 2008 | ISSN: 0035-8711
Published online September 2008 | e-ISSN: 1365-2966 | DOI:
The very massive X-ray bright binary system Wack 2134 (= WR 21a)*

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From the radial velocities of the N ivλ4058 and He iiλ4686 emission lines, and the N vλ4604-20 absorption lines, determined in digital spectra, we report the discovery that the X-ray bright emission line star Wack 2134 (= WR 21a) is a spectroscopic binary system with an orbital period of 31.673 ± 0.002 d. With this period, the N iv and He ii emission and N v absorption lines, which originate in the atmosphere of the primary component, define a rather eccentric binary orbit (e= 0.64 ± 0.03). The radial velocity variations of the N v absorptions have a lower amplitude than those of the He ii emission. Such a behaviour of the emission line radial velocities could be due to distortions produced by a superimposed absorption component from the companion. High-resolution echelle spectra observed during the quadrature phases of the binary show H and He ii absorptions of both components with a radial velocity difference of about 541 km s−1. From this difference, we infer quite high values of the minimum masses, of about 87 and 53 M for the primary and secondary components, respectively, if the radial velocity variations of the He ii emission represent the true orbit of the primary. No He i absorption lines are observed in our spectra. Thus, the secondary component in the Wack 2134 binary system appears to be an early O-type star. From the presence of H, He ii and N v absorptions, and N iv and C iv emissions, in the spectrum of the primary component, it most clearly resembles those of Of/WNLha-type stars.

Keywords: binaries: spectroscopic; stars: Wolf-Rayet; stars: individual: Wack 2134; stars: individual: WR 21a

Journal Article.  3044 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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