The time dependence of chromospheric decay for solar type stars

For solar type stars in clusters ranging from 10 million to 6 billion years in age, a lower resolution (2.5 A) analog of the Mount Wilson S index of stellar chromospheric activity has been measured. This was done in an attempt to surmount the problem of cluster star faintness. The new index is found by forming the ratio of the residual flux at the bottoms of H and K to the height of the local pseudo-continuum as defined by flux maxima near 3775-3815 and 4010-4030 A. The lower resolution analog is just the quantity (1 - KH), where KH is a measure of the calcium H and K line depths. A temperature-corrected analog (1 - HK0) is also formed. Because of the uncertainties in assigning accurate ages and the scarcity of data for both intermediate and old age clusters, the data do not yet identify a unique form for the time dependence of chromospheric decay. However, they do place far more restrictive constraints on theory than previously available. Further observations of intermediate and old clusters are required to distinguish between the two empirical models developed.