Magellanic Cloud WC/WO Wolf-Rayet stars - I. Binary frequency and Roche lobe overflow formation

A nearly complete sample of 24 Magellanic Cloud WC/WO subclass Wolf-Rayet stars is studied spectroscopically and photometrically to determine its binary frequency. Theory predicts the Roche lobe overflow produced Wolf-Rayet binary frequency to be 52 +/- 14 per cent in the Large Magellanic Cloud and 100 per cent in the Small Magellanic Cloud, not counting non-Roche lobe overflow Wolf-Rayet binaries. Lower ambient metallicity (Z) leads to lower opacity, preventing all but the most massive (hence luminous) single stars from reaching the Wolf-Rayet stage. However, theory predicts that Roche lobe overflow even in binaries of modest mass will lead to Wolf-Rayet stars in binaries with periods below approximately 200 d, for initial periods below approximately 1000 d, independent of Z, By examining their absolute continuum magnitudes, radial velocity variations, emission-line equivalent widths and full widths at half-maximum, a WC/WO binary frequency of only 13 per cent, significantly lower than the prediction, is found in the Large Magellanic Cloud. In the unlikely event that all of the cases with a less certain binary status actually turn out to be binary, current theory and observation would agree. (The Small Magellanic Cloud contains only one WC/WO star, which happens to be a binary.) The three WC + O binaries in the Large Magellanic Cloud all have periods well below 1000 d, The large majority of WC/WO stars in such environments apparently can form without the aid of a binary companion. Current evolutionary scenarios appear to have difficulty explaining either the relatively large number of Wolf-Rayet stars in the Magellanic Clouds, or the formation of Wolf-Rayet stars in general.