AB Dor in '94. I. Hubble Space Telescope Goddard High Resolution Spectrograph observations of the quiescent chromosphere of an active star

We analyze Hubble Space Telescope/Goddard High Resolution Spectrograph spec tra of AB Doradus, the prototypical, ultrarapidly rotating K dwarf. We obse rved 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 profi les of the chromospheric and transition-region lines show narrow cores supe rposed on very broad wings. The broad wings of the Mg II k and h lines and of the transition-region lines can be explained by emission from gas corota ting with the star and extending out to near the Keplerian corotation radiu s (2.8 stellar radii). While this is not a unique solution, it is consisten t with previous studies of Ha emission, which are naturally explained by la rge corotating prominences. We find no evidence for rotational modulation o f the emission-line fluxes. The density diagnostics suggest that the transi tion region is formed at constant pressure, with an electron density of 2-3 x 10(12) cm(-3) at a temperature of 3 x 10(4) K. The electron pressure is about 100 times larger than that for the quiet Sun. The emission-measure di stribution shows a minimum between log T = 5 and 5.5. The Mg II line exhibi ts three interstellar absorption components along the 15 pc line of sight. We identify the lowest velocity component with the G Cloud, but the other c omponents are not identified with any interstellar clouds previously detect ed from other lines of sight.