  We analyze HST/GHRS spectra of AB Doradus, the prototypical ultra-rapidly
rotating K dwarf. We observed chromospheric (Mg II) and transition region (C
II, Si IV, C IV, and N V) lines periodically throughout the stellar rotation
period, and provide a low dispersion stellar atlas of 78 emission lines. The
quiescent line profiles of the chromospheric and transition region lines show
narrow cores superposed on very broad wings. The broad wings of the Mg II k & h
lines and of the transition region lines can be explained by emission from gas
co-rotating with the star and extending out to near the Keplerian co-rotation
radius (2.8 stellar radii). While this is not a unique solution, it is
consistent with previous studies of H-alpha emission that are naturally
explained by large co-rotating prominences. We find no evidence for rotational
modulation of the emission line fluxes. The density diagnostics suggest that
the transition region is formed at constant pressure, with an electron density
2-3 E12 /cm^3 at a temperature of 30,000 K. The electron pressure is about 100
times larger than that for the quiet Sun. The emission measure distribution
shows a minimum between log(T) = 5 and 5.5. The Mg II line exhibits three
interstellar absorption components along the 15 pc line of sight. We identify
the lowest velocity component with the G cloud, but the other components are
not identified with any interstellar clouds previously detected from other
lines of sight.
