Chromospheric two - component NLTE modelling of the binary system V 711 Tau = HR 1099

In this paper we present the first two-component, semi-empirical chromospheric model of the RS CVn binary system V 711 Tau based on detailed fits of Hα and Mg II h line profiles. Synthetic Hα and Mg II h line profiles are computed by means of NLTE radiative transfer calculations. Using the observed Emission Measure distribution to constrain the transition region structure and the photospheric Next-Generation model with log(g)=3.5 and Teff =4800 K, we have built a grid of chromospheric models for the K1 IV component of the binary system by means of spline interpolations between photospheric and transition region models. With such interpolations the free parameters are restricted to the temperature minimum and the position of an interpolation knot. We find that a mean singlecomponent model cannot satisfactorily reproduce the observed line profiles. Chromospheric Doppler Imaging is then used to consider the chromospheric emission as due to two different components of known surface coverage and emission. Extensive grids of semi-empirical chromospheric models are built separately for the two components and the weighted-combined synthetic Hα and Mg II h profiles are compared with observations. Two-component synthetic line profiles reproduce the observed Hα and Mg II h in detail, indicating that the broad and narrow components can be interpreted in terms of radiative transfer effects in different atmospheric structures for the active and quiet regions. From this analysis we infer information on physical properties of the active region, from which the broad component originates, with respect to the remaining chromosphere from which the narrow component originates. We show also that the variability outside flares can be reproduced considering slightly different models for the active region.