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

Abstract

From the radial velocities of the NIV λ4058 and HeII λ4686 emission lines, and the NV λ4604-20 absorption lines, determined in digital spectra, we report the discovery that the X-ray bright emission line star Wack2134 (= WR21a) is a spectroscopic binary system with an orbital period of 31.673 +/- 0.002d. With this period, the NIV and HeII emission and NV 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 NV absorptions have a lower amplitude than those of the HeII 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 HeII absorptions of both components with a radial velocity difference of about 541kms-1. From this difference, we infer quite high values of the minimum masses, of about 87 and 53Msolar for the primary and secondary components, respectively, if the radial velocity variations of the HeII emission represent the true orbit of the primary. No HeI absorption lines are observed in our spectra. Thus, the secondary component in the Wack2134 binary system appears to be an early O-type star. From the presence of H, HeII and NV absorptions, and NIV and CIV emissions, in the spectrum of the primary component, it most clearly resembles those of Of/WNLha-type stars. Based partially on data collected at SAAO, CASLEO, LCO, CTIO and La Silla (under programme ID 68.D-0073) Observatories.