Outer atmospheres of cool stars. XI - High-dispersion IUE spectra of five late-type dwarfs and giants

We present high-dispersion, far-ultraviolet (1150-2000 Å) spectra of five late-type dwarfs and giants obtained with the International Ultraviolet Explorer. The chromospheric (T ≲104K) emission lines in the giants tend to be about twice as broad as the corresponding features of the dwarf star spectra, suggesting a width-luminosity relation similar to the Wilson-Bappu effect for Ca II H and K. The Si III λ1892 and C III λ1909 intercombination lines formed in hotter layers (T ≈ 5 × 104K) also broaden by a factor of 2 from the main-sequence stars to the evolved stars, and the permitted resonance doublets of C II(3 × 104 K), Si IV (6 × 104 K), and C IV (105 K) are as much as a factor of 4 broader in the giants than in the dwarfs. However, we find no evidence for asymmetric or shifted emission profiles that might indicate the presence of warm (T≪105K) stellar winds. We conclude that broad C iv profiles, in particular, are typical of active chromosphere giant stars and are unlikely to be a unique signature of an extended, expanding warm wind. Since the resonance lines tend to be wider than the intersystem lines formed at similar temperatures in the chromosphere and in hotter layers, we conclude that opacity must be an important broadening enhancement mechanism in active chromosphere giant stars. Nevertheless, the intercombination line widths do indicate a general increase in the outer atmosphere Doppler motions from the dwarfs to the giants. Application of the density sensitive line ratio C III λ1909/Si III λ1892 suggests that the outer atmosphere pressures (T ≈ 5 × 104K) are similar in the active chromosphere subgiant λ And and the quiet chromosphere dwarfs, α Cen A and B. However, the pressures derived for the Capella secondary and β Dra are factors of 3 or more lower than the dwarfs, suggesting geometrically extended, low-density outer atmosphere structures qualitatively different from the high-pressure, compact structures typical of solar magnetic active regions. Finally, we have isolated the He II λ1640 emission component from contaminant blends, and we find that the line strength is well correlated with soft X-ray fluxes of the sample stars, as predicted by photoionization-recombination models of the He II Bα formation.