[ITAL]ORFEUS[/ITAL] Observations of Luminous Cool Stars

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Abstract
The ORFEUS telescope obtained far-ultraviolet spectra (λλ912-1218) of three luminous cool stars: β Draconis (HD 159181) and two hybrid stars, α Aquarii (HD 209750) and α Trianguli Australis (HD 150798). Most striking is the detection of blueshifted O VI λ1032, and C III λ977 emission relative to Si III λ1206, indicating outward motion, and the markedly narrow resonance line profiles of C III and Si III in the two hybrid objects as compared with the coronal star β Dra. The observed Doppler shifts suggest that a warm wind of at least 3×105 K occurs in the hybrid stars with outflows ~100-200 km s−1; narrow lines of C III and Si III are consistent with signatures of wind absorption. The asymmetric profile of C III in β Dra might also result from wind absorption, although the acceleration would have to begin at higher temperatures (T ~ 70,000 K) in this coronal star as compared with the hybrids, since its chromospheric Mg II profiles show no asymmetry due to wind absorption. Electron densities determined from C III multiplets in β Dra and α TrA indicate values between 8.3 and 8.6 dex (cm-3), giving a pressure of ~2×1013 cm-3 K, at T~7×104 K. Wind absorption and interstellar extinction must be considered when applying emission-line diagnostics in this wavelength region.