Optical Spectroscopy of EZ Canis Majoris: Indication for Large‐Scale Structures in a Wolf‐Rayet Wind

We have carried out optical spectroscopy of the Wolf-Rayet star EZ CMa during 20 consecutive nights in 1995 January in support of the IUE Mega-project. In parallel with this optical spectroscopy, we also monitored EZ CMa using narrowband photometry. The light curve was found to be remarkably stable when folded with the P \ 3.77 day period, and it had a peak-to-valley amplitude of 0.1 mag. The P Cygni absorption components of He I j3889 and He I j5876 display a similar global pattern of variability as was found for the simultaneously acquired UV pro-les. The strengthening of the P Cygni absorption component of these transitions is associated with the maximum of the continuum Nux. Con- versely, the absorption trough of N V j4604 gradually disappears as the star brightens. Although the emission parts of the lines are variable at di†erent levels, they all show the same pattern of variability, which consists of phase-dependent shifts of extra emission components superposed on the pro-les. A strong correlation is found between the continuum-light level and the equivalent width of most transitions. The line skewness and the full-width at half-maximum show a daily recurrence timescale, reNecting the light curve changes. We have addressed in a rigorous statistical way the signi-cance of the variations by calculating the "" temporal variance spectrum.II For any given line, we found enhanced variability at some velocities, although the whole pro-le displays a statistically signi-cant level of varia- bility. Arguments against a compact companion as the cause of the observed periodic variability are present- ed. Instead, our observations strongly support the suggestion in the IUE Mega analysis that the atypical level of variability results from the rotation of a structured wind. We propose that the wind variability of EZ CMa is triggered by photospheric activity, or that the wind is controlled by a large-scale magnetic -eld.