Phase-resolved Simultaneous [ITAL]ORFEUS[/ITAL] Far-Ultraviolet and [ITAL]ROSAT[/ITAL] X-Ray Observations of the Active Star AB Doradus

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Abstract
We report phase-resolved simultaneous ORFEUS far-ultraviolet (FUV) and ROSAT soft X-ray observations of the rapidly rotating young star AB Doradus (HD 36705), obtained with the FUV spectrometer flown on board ORFEUS II and the High Resolution Imager (HRI) on board ROSAT. The lines of C III at 977 and 1176 Å , O V at 1218 Å, and O VI at 1032 and 1038 Å, as well as N II at 1085 Å and Si III at 1206 Å, are clearly detected in the ORFEUS spectra. The X-ray flux intrinsically varied during the observation and FUV and X-ray fluxes are found to correlate. No changes in the line profiles are detectable, but the line profiles of C III λ977 and O VI λ1032 are definitely broadened with respect to the instrumental line profile. The observed broadening does not exceed the photospheric v sin i value, suggesting that the observed C III and O VI emission, formed at a characteristic temperature of ≈ 80,000-300,000 K, is produced close to the star's surface. The C III λ1176/λ977 line ratio is found to be larger than that in the Sun, indicating electron densities (at ≈ 80,000 K) of ne ~ 1011 cm−3 or higher.