PHOTOSPHERIC METAL ABUNDANCES IN ACTIVE STELLAR ATMOSPHERES

With a sophisticated analysis we determine the photospheric metal abun dances of active stars to investigate the abundance stratification in active stellar atmospheres. For selected single stars and RS CVn binar ies we have taken high-resolution, high-SN and partially phase-resolve d spectra of the entire visible range. With a purely spectroscopic, se lf-consistent method based on line profile synthesis and an equivalent width analysis, we derive the effective temperature T-eff, surface gr avity log g, microturbulence xi(t) and the abundances of Fe, Mg and Si with high internal accuracy (Delta T-eff similar or equal to 70K, Del ta log g similar or equal to 0.1 dex, Delta xi(t) similar or equal to 0.1 km/s, Delta[Fe/H], Delta[Mg/H], Delta[Si/H] similar or equal to 0. 05 dex). Stellar parameters and metal abundances are obtained for the single stars beta Get, kappa Cet and pi(1) UMa as well as for the RS C Vn binaries AY Get, VY Ari, EI Eri, IM Peg, lambda And and II Peg. The effect of stellar activity features (spots and plages) on the derived parameters is investigated by phase-resolved spectroscopy; we find th at one out of four systems shows a significant variation of the Mg and Si abundances with rotational phase. The results help solving the puz zle about the photospheric Fe abundances of RS CVn binaries; specifica lly, we obtain [Fe/H] greater than or equal to -0.4 for all systems, i ndicating that they definitely are not metal-poor. In II Peg and lambd a And, the metals are strongly depleted in the corona relative to the photosphere; in beta Cet and pi(1) UMa, Fe and Si are weakly depleted, but Mg is enhanced (with 1 sigma significance). The abundance stratif ications are discussed in terms of the FIP-effect and the hydrostatic equilibrium stratification.