1999A&A...350..571B

Spectroscopic observations of the bright RS CVn-type system V711 Tau = HR 1099 (K1 IV + G5 V) were obtained by IUE during the same period of a Multi-Site Continuous Spectroscopy (MUSICOS) campaign between 12-18 December 1992 (Huang et al., 1995, "MUSICOS: Multi-Site Continuous Spectroscopy", Proc. of the 4th MUSICOS Workshop, p. 131). We report on the results of a ``Doppler Imaging" analysis of the MgII h line. Broad, variable and extended wings have been detected and successfully fitted using a broad Gaussian, which accounts for a large fraction of flux from the global stellar Mg II h emission. This broad component is present in all spectra and presents velocity shifts with respect to the K1 star rest-frame that are variable in the range between -19 and +44km/s. Similar results have been obtained from Doppler Imaging of the other relevant chromospheric line (H_α) in other RSCVn stars (c.f. Hatzes, 1995AJ....109..350H and Hatzes, 1998A&A...330..541H). Furthermore, our analysis suggests that these shifts could be due to rotational modulation produced by an active region, that essentially straddles the pole of the K1 star in which down-flows dominate on up-flows. Finally, emitting matter between the two stars has been detected, indicating that mass-exchange is present in the binary system. Five flare episodes with strong flux enhancements in several transition region lines were observed. These flares were preliminarily reported by Neff et al. (1995, "MUSICOS: Multi-Site Continuous Spectroscopy", Proc. of the 4th MUSICOS Workshop, p. 157). We analyse in this paper the MgII h and k line emission during the major of these five flare (1992 December 14), since only a weak enhancement was observed in the Mg II h and k lines during the other four flares. Spectral imaging of the MgII lines during this flare indicates a flaring site on the visible K1 star hemisphere with mass ejection from the K1 star towards the G5 companion. The MgII emission shows a large broadening with FWHM reaching 1000km/s at flare peak and decreasing to 700km/s two hours after the peak. Our analysis have been carried out using both IUESIPS and NEWSIPS processed spectra. We find that the shapes of the spectral profiles are quite different when spectra are processed by IUESIPS or NEWSIPS procedures, the IUESIPS processed spectra showing a spurious excess of flux on the red wing of the lines. A comparison with results from GHRS-HST MgII spectra of V 711 Tau (Dempsey et al., 1996ApJ...470.1172D, Wood et al., 1996ApJ...458..761W) shows that, thanks to the complete phase coverage together with the improved quality obtained by the NEWSIPS reduction procedures, the bulk of information derived from GHRS data can be extracted also from the IUE spectra, which continues to be the most complete UV data set that is nowadays available on this source.