  (abridged) We study the optical spectrum of the exciting B[e] star Hen 2-90
based on new high-resolution observations that cover the innermost 2". Our
investigation is splitted in two parts, a qualitative study of the presence of
the numerous emission lines and the classification of their line profiles which
indicate a circumstellar environment of high complexity, and a quantitative
analysis of numerous forbidden lines, e.g. [OI], [OII], [OIII], [SII], [SIII],
[ArIII], [ClII], [ClIII] and [NII]. We find a correlation between the different
ionization states of the elements and the velocities derived from the line
profiles: the highly ionized atoms have the highest outflow velocity while the
neutral lines have the lowest outflow velocity. The recent HST image of Hen
2-90 reveals a bipolar, highly ionized region, a neutral disk-like structure
and an intermediate region of moderate ionization. It seems that a
non-spherical stellar wind model is a good option to explain the ionization and
spatial distribution of the circumstellar material. We modelled the forbidden
lines under the assumption of a non-spherically symmetric wind based on the HST
image. We find that in order to fit the observed line luminosities, the mass
flux, surface temperature, and terminal wind velocities need to be latitude
dependent, which might be explained in terms of a rapidly rotating central
star. A rotation speed of 75-80 % of the critical velocity has been derived.
The total mass loss rate of the star was determined to be of order 3 10^{-5}
M_sun/yr. Such a wind scenario and the fact that compared to solar abundances
C, O, and N seem to be underabundant while S, Ar and Cl have solar abundances,
might be explained in terms of a rapidly rotating post-AGB star.
