  Emission line studies from Hii regions in galaxies require tools for the
inversion of line ratios into desired physical properties. These tools
generally come in form of diagnostic ratios/diagrams that are based on grids of
photoionisation models. An important input to the photoionisation models is the
stellar atmosphere spectrum of the ionising sources. The current omission of
shocks in the calculation of the former set of models could threaten the
accuracy of the physical interpretation of emission line ratios from Hii
regions. Current stellar atmosphere models that are crucial inputs to the grid
of photoionisation models used to generate nebular emission line diagnostic
diagrams might produce significant biases due to the omission of shocks. We
investigate whether a new generation of photoionisation model grids, taking
shocks into account, is required to compensate for the biases. We make use of
the WM-Basic stellar atmosphere code, which can account for the extra energetic
emission in the stellar spectral energy distribution produced by shocks, in
conjunction with the photoionisation code MOCASSIN to determine whether shocks
produce significant biases in the determination of the physical parameters of
the interstellar medium and/or ionising stellar parameters. We conclude that
these effects are only important for stellar sources with effective
temperatures lower than 30kK and in this case they yield artificially high
stellar temperatures, electron temperatures and nebular ionisation parameters.
The magnitude of the effect is also obviously dependent on the strength of the
shock and is likely to be unimportant for the majority of stellar sources.
Nevertheless, we find our 20kK and 30kK shock models to strongly enhance the He
ii 4686 nebular emission line. This result is however not strong enough to
explain previously observed He ii 4686 line emission in the spectra of Hii
galaxies.
