Ultraviolet observations of cool stars. III - Chromospheric and coronal lines in alpha Tauri, beta Geminorum, and alpha Bootis

The ultraviolet spectrometer of the Princeton Experiment Package aboard the Copernicus satellite has been used to obtain high-resolution measurements of La, the Mg ii A2800 doublet, and upper limits on the Si iii Al 206 line in the K giants a Tau and fl Gem. The intensities and line shapes are compared with earlier observations of a Boo. The La and Mg ii profiles for a Tau resemble those for a Boo, in that they are highly asymmetrical, while fl Gem shows much more symmetrical profiles. The asymmetries for all lines except for those of a Boo and the Mg ii lines of a Tau could be due to interstellar absorption. in the case of p Gem only, the 0 V intercombination line at 1218 A is observed, suggesting a well-developed corona substantially cooler than that of the Sun. The La profiles of a Tau and p Gem are consistent with the low interstellar hydrogen abundance in the solar neighborhood previously obtained from a similar observation of the a Boo La profile. The strength of the Mg ii A2796 line can be used to measure transition region and coronal pressures, and indicates a decrease in both with later spectral type andlor increasing luminosity. Subject headings: chromospheres, stellar - coronae, stellar - late-type stars - spectra, ultraviolet