CHROMOSPHERIC ACTIVITY IN POPULATION-II BINARIES

We analyse Ca II and Halpha observations for a sample of 27 Pop II bin aries to investigate their degree of chromospheric activity. We find t hat Pop II binaries having periods shorter than round 10 days possess active chromospheres. On the other hand, binaries with longer periods appear to be chromospherically quiet. Only one object clearly deviates from this trend and our high resolution spectroscopic observations re veal that this system contains an evolved star. We interpret these res ults as a clear sign that the nature of chromospheric activity in Pop II binaries is of magnetic origin, powered by the tidal forces acting in the short period systems. In this context activity can be a powerfu l tool to investigate the dynamical evolution of binary systems. The r apid decay of chromospheric activity with increasing orbital period ma y suggest that a minimum rotational velocity is necessary to trigger t he onset of a powerful dynamo. Ca II H and K observations can also be used to determine more precise orbits, in fact in two systems previous ly classified as single line spectroscopic binaries, the Ca II emissio n of the unseen companion is detected. Wilson-Bappu widths are measure d for 12 stars which may be used to derive additional information on t he components of the system.