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

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Abstract
The ORFEUS telescope obtained far-ultraviolet spectra (ll912-1218) of three luminous cool stars: b Draconis (HD 159181) and two hybrid stars, a Aquarii (HD 209750) and a Trianguli Australis (HD 150798). Most striking is the detection of blueshifted O vi l1032, and C iii l977 emission relative to Si iii l1206, 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 b Dra. The observed Doppler shifts suggest that a warm wind of at least K occurs 5 3 # 10 in the hybrid stars with outflows ∼100-200 km s ; narrow lines of C iii and Si iii are consistent with signatures Ϫ1 of wind absorption. The asymmetric profile of C iii in b 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 b Dra and a TrA indicate values between 8.3 and 8.6 dex (cm Ϫ3), giving a pressure of ∼ cm Ϫ3 K, at K. Wind absorption and interstellar extinction must be 13 4 2 # 10 T ∼ 7 # 10 considered when applying emission-line diagnostics in this wavelength region.

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