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 chromosphe ric model of the RS CVn binary system V 711 Tau based on detailed fits of H a and Mg II h line profiles. Synthetic H alpha and ME LT h line profiles are computed by means of NLTE r adiative transfer calculations. Using the observed Emission Measure distrib ution to constrain the transition region structure and the photospheric Nex t-Generation model with log(g)=3.5 and T-eff =4800 K, we have built a grid of chromospheric models for the K1 IV component of the binary system by mea ns of spline interpolations between photospheric and transition region mode ls. With such interpolations the free parameters are restricted to the temp erature minimum and the position of an interpolation knot. We find that a m ean single component model cannot satisfactorily reproduce the observed lin e profiles. Chromospheric Doppler Imaging is then used to consider the chro mospheric emission as due to two different components of known surface cove rage and emission. Extensive grids of semi-empirical chromospheric models a re built separately for the two components and the weighted-combined synthe tic H alpha and Mg II it profiles are compared with observations. Two-compo nent synthetic line profiles reproduce the observed H alpha 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 o n physical properties of the active region, from which the broad component originates, with respect to the remaining chromosphere from which the narro w component originates. We show also that the variability outside flares ca n be reproduced considering slightly different models for the active region .