Looking for nebular He II emission south of the multiple-massive star system, HD 5980

Abstract

The nebular He II {\lambda}1640 emission line is observed in star forming galaxies out to large distances and can be used to constrain the properties of sources of He+ ionizing photons. For this purpose, it is crucial to understand which are the main stellar sources of these photons. In some nearby metal poor starburst galaxies, nebular He II {\lambda}4686 (optical equivalent) is accompanied by a broad underlying component, which is generally attributed to formation in the winds of classical (He burning) Wolf Rayet stars, primarily of the WN subtype. In such cases, the origin of the nebular component has been proposed to be the escape of He+ ionizing photons from the winds of the WN stars, at least partially. We use archival long slit observations obtained with Focal Reducer Low Dispersion Spectrograph (FORS1) on the Very Large Telescope to look for nebular He II {\lambda}4686 emission south of the WN6h + WN6(7) close binary in HD 5980. We only find broad He II {\lambda}4686 emission, as far as aprox. 7.6 pc from the binary. A comparison with observations obtained with Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope, at a similar orbital phase, shows that the FORS1 broad He II emission is likely contamination from the multiple star system HD 5980. We use models to show that no significant He+ ionizing flux is expected from the WN stars in HD 5980 and that when similar stars are present in a coeval stellar population, the O stars can be far greater emitters of He+ ionizing radiation.