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

TL;DR

This study reveals that Wack 2134 (WR 21a) is a highly eccentric spectroscopic binary with a 31.67-day period, comprising a primary of Of/WNLha type and a secondary likely an early O star, with high minimum masses.

Contribution

First spectroscopic binary orbit determination of Wack 2134, revealing its eccentricity, period, and component types, with high minimum mass estimates for both stars.

Findings

Binary orbit period of 31.673 days

High eccentricity of 0.64

Minimum masses of about 87 and 53 solar masses

Abstract

From the radial velocities of the N IV 4058 and He II 4686 emission lines, and the N V 4604-20 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 $31.673\pm0.002$ days. 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$\pm$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. From this difference, we infer quite high values of the minimum masses, of about 87Mo and 53Mo 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 Wack2134 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.