4686
map which was used to identify
Wolf-Rayet (WR) stars recently found by ground-based spectroscopy and
to locate diffuse nebular emission. Most of the
HeII 4686 emission
is associated with the NW
stellar cluster, clearly displaced from the surrounding shell-like
[OIII] and
H
emission. We found evidence for
HeII 4686
sources, compatible with 5-9 WNL
stars and/or compact nebular
HeII 4686 emission, as well as residual
diffuse emission. Only one of them is outside the NW cluster. We have
done an extensive comparison between our results and the recent
ground-based data used by Izotov et al. (1997) and
Legrand et al. (1997) to identify WN and WC stars in
I Zw 18. The differences between the various data
may be understood in terms of varying slit locations, continuum fits,
and contamination by nebular lines. We have calculated evolutionary
tracks for massive stars and synthesis models at the appropriate
metallicity (Z
0.02 Z
). These single star models predict a mass
limit MWR90 M for WR stars to become WN and WC/WO.
For an instantaneous burst model with a Salpeter IMF extending up to
Mup120-150 Mour model predictions are in
reasonable agreement with the observed equivalent widths. Our model is
also able to fully reproduce the observed equivalent widths of nebular
HeII 4686 emission due to the presence
of WC/WO stars. This quantitative agreement and the spatial
correlation of nebular HeII 4686 with
the stellar cluster and the position of WR stars shown from the
ground-based spectra further supports the hypothesis that WR stars are
responsible for nebular HeII 4686 emission in extra-galactic HII regions.