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 \times 10 ^ { 12 } cm ^ { -3 } at a temperature of 3 \times 10 ^ { 4 } 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 .