CHROMOSPHERIC ROTATIONAL MODULATION IN SOLAR-LIKE STARS .2. MULTICOMPONENT MODELING AND ROTATIONAL PERIOD OF ALPHA-CENTAURI-B FROM CA-II-H SPECTROSCOPIC VARIABILITY

The K1V late-type dwarf alpha Cen B has been monitored to search for s pectroscopic variability in the Ca II H chromospheric line during its rotational period. The observations were made with the Coude Echelle S pectrograph with the 1.4 Coude Auxiliary Telescope at ESO, with spectr al resolution 80 000. Variability in flux up to 29 % and profile asymm etries are explained as modulation caused by discrete structures such as solar-like plages on the star surface. An analysis of the visibilit y and velocity of these structures gives a rotational period of 43 day s, in agreement with the Rossby number derived from the Ca II chromosp heric flux. A multi-component model is used to estimate the positions and filling factors of such emissive structures. A coherence time up t o 6 months is found for the plage signature, but gradual phase shifts and overall flux variations are observed from year to year, providing an information on the emergence/disappearance of active regions and on cyclic effects operating on global activity.