  We present observations of the UV absorption lines in the Seyfert 1 galaxy
NGC 3783, obtained with the medium resolution echelle gratings of the Space
Telescope Imaging Spectrograph(STIS) on the Hubble Space Telescope (HST). The
spectra reveal the presence of three kinematic components of absorption in
Ly-alpha, C IV, and N V, at radial velocities of -1365, -548, and -724 km/s
with respect to the systemic velocity of the host galaxy (Components 1, 2 and
3, respectively); Component 1 also shows absorption by Si IV. Component 3 was
not detected in any of the earlier Goddard High Resolution Spectrograph (GHRS)
spectra, and the C IV absorption in the other components has changed since the
most recent GHRS observation obtained 5 yr earlier. Somewhat unexpectedly, each
component has a covering factor (of the continuum source + broad emission line
region) of about 0.6. We have calculated photoionization models to match the UV
column densities in each of the three components. Based on the model results,
there should be strong absorption in the bandpass of the Far Ultraviolet
Spectroscopic Explorer (FUSE), 912 - 1200 A, including saturated O VI lines at
each component velocity. The models also predict large O VII and O VIII column
densities but suggest that the UV absorbers cannot account for all of the X-ray
absorption detected in recent Chandra spectra. Finally, there is no evidence
for a correlation between the characteristics of the UV absorbers and the UV
continuum flux, which suggests that the variations observed in the GHRS and
STIS spectra are due in large part to changes in the column densities of the
absorbers as the result of transverse motion.
