Loop Modelling of the RS CVn Stars V711 Tau, AR Lac and II Peg

We present energy balance loop models for the transition regions of three RS CVn late-type stars. A variety of archival data have been analysed to determine the transition region emission measure distributions. The lower transition region (3.8<log T_e<5.3) has been studied using emission line fluxes from IUE and the GHRS. Emission measure distributions for the upper transition region and corona (6.1<log T_e<7.4) have been determined using line fluxes from the Extreme Ultraviolet Explorer (EUVE). Electron densities of ~ 10(11.6) cm(-3) and ~ 10(11.2) cm(-3) have been determined at log T_e=4.7 and log T_e=7.0 respectively, using the density sensitive Si III] 1892{ Angstroms} and Fe XXI 128.7{ Angstroms} lines. The emission measure distributions from the EUVE spectra are compared with those resulting from a variety of low resolution X-ray spectra, and the reality of local emission measure maxima at log T_e ~ 6.9 is established. The observed emission measure distributions are investigated using theoretical models that assume an energy balance between radiation and conduction within magnetic loop geometries. Using two families of loops these models can simultaneously reproduce both the observed emission measure distributions and the measured electron densities. The hotter loops have peak temperatures of log T_e ~ 7.35 and a height ~ 0.05R_⋆, whereas the cooler loops have peak temperatures of log T_e ~ 6.9 and a height ~ 0.15R_⋆. The loop models are discussed in terms of possible heating mechanisms, and the inferred magnetic fields are consistent with those determined by Zeeman-Doppler Imaging techniques. The area coverage of the loops on the surface of each star is found to agree well with the cool spot areas determined from photometry and Doppler Imaging measurements. The loops are also consistent with the predictions of loop scaling laws. This work was supported by a PPARC studentship at the University of Oxford.