The very massive X-ray bright binary system Wack2134 (= WR21a)

NIEMELA, V. S.; GAMEN, R. C.; BARBÁ, R.; FERNANDEZ LAJUS, E.; BENAGLIA, P.; SOLIVELLA, G.; REIG, P.; COE, M.J.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

Año: 2008 vol. 389 p. 1447 - 1452

0035-8711

From the radial velocities of the N IV λ4058 and He II λ4686 emission lines, and the N Vλ4604-20 absorption lines, determined in digital spectra, we report the discovery that theX-ray bright emission line star Wack 2134 (= WR 21a) is a spectroscopic binary systemwith an orbital period of 31.673 ± 0.002 d. With this period, the N IV and He II emission andN V absorption lines, which originate in the atmosphere of the primary component, definea rather eccentric binary orbit (e = 0.64 ± 0.03). The radial velocity variations of the N Vabsorptions have a lower amplitude than those of the He II emission. Such a behaviour ofthe emission line radial velocities could be due to distortions produced by a superimposedabsorption component from the companion. High-resolution echelle spectra observed duringthe quadrature phases of the binary show H and He II absorptions of both components with aradial velocity difference of about 541 km s−1 . From this difference, we infer quite high valuesof 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 theprimary. No He I absorption lines are observed in our spectra. Thus, the secondary componentin the Wack 2134 binary system appears to be an early O-type star. From the presence ofH, He II and N V absorptions, and N IV and C IV emissions, in the spectrum of the primarycomponent, it most clearly resembles those of Of/WNLha-type stars.