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Astron. Astrophys. 350, 571-581 (1999)

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4. Conclusion

We have carried out Doppler Imaging analysis of IUE spectra of V 711 Tau as part of a MUSICOS campaign between 12-18 December 1992. Broad and variable extended wings were detected on the Mg II h line profiles at all phases and in both orbits we observed. The broad wings were successfully fitted using a broad Gaussian, which is present in all spectra and accounts for a large fraction of the flux from the global stellar Mg II h emission, the broad/total flux ratio being 0.36 for the K1 star. This ratio is typical for the most active stars studied by Wood et al. (1997). Our results are in general agreement with Wood et al. (1996) and Dempsey et al. (1996) analysis of GHRS spectra of V 711 Tau, but our complete phase coverage allows us to ascertain that the broad components have velocity shifts with respect to the K1 star that are variable in the range from -19 to +44 km s-1.

Furthermore, complete phase coverage allows us to infer information of the surface structure. In particular, our analysis suggests that the broad component could represent emission that arises from a large active region close to the pole in which down-flows dominate on up-flows. If this is true the controversial physical conditions that generate the broad wings of many active stars (see, e.g., Linsky et al. 1995, Wood et al. 1996), can be ascribed to localised regions rather than to the whole star.

An intra-system active region located between the two components has been revealed. This may be indicative of phenomena of mass-exchange from the K1 towards the G5 component.

The quiescent emission flux is found to be [FORMULA] erg cm-2 s-1 for the K1 and [FORMULA] erg cm-2 s-1 for the G5 star. The G5 Mg II h line flux is [FORMULA] of the total Mg II h flux emitted from the binary system, in satisfactory agreement with the analysis of Wood et al. (1996), who found a contribution of [FORMULA].

We estimate that the ISM in the direction of V 711 Tau has a radial velocity of [FORMULA] km s-1. The equivalent width of the IS absorption line is found to be 62.3[FORMULA]14.0 mÅ.

From the spectral decomposition of spectra acquired during a major flare on December 14 we infer information on the kinematics and energetics of the flaring plasma. We find that the flare dynamics is compatible with a flaring site on the visible K1 star hemisphere with mass ejection towards the G5 companion. The emitted flux in the Mg II h and k lines was of the order of 7.7[FORMULA]1034 erg, and the emission is characterised by a large broadening with FWHM [FORMULA] km s-1 at flare peak and FWHM [FORMULA] km s-1 two hours later.

The IUESIPS MgII line profiles are affected by spurious asymmetries, that disappear in the NEWSIPS reduced spectra. A comparison between Doppler Imaging results from NEWSIPS and IUESIPS processed data shows a marked reduction of the uncertainties when using the NEWSIPS and it indicates possibly systematic errors in the wavelength calibration in the IUESIPS.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999