Time-series measurements of chromospheric CA II H and K emission in cool stars and the search for differential rotation

The relative strength of the chromospheric Ca II H and K emission cores has been monitored on a near- nightly basis during several seasons in a variety of cool stars, predominantly those lower-main-sequence stars observed by Wilson for long-term chromospheric activity fluctuations. From initial data obtained in 1980, rotation rates had been inferred from the period of modulation of chromospheric flux. We have analyzed the rotation periods determined from three seasons of Ca II H and K emission strengths in these stars. In 12 stars we find evidence for varying periodicities in different seasons or for multiple periodicities in one season, or both. For about 10 stars, significant peaks in the power spectrum are found at two different frequencies in at least one season. Detailed analysis of the chromospheric emission with time reveals two possibilities consistent with the appearance of dual periodicities in the observed time series: two distinct periods arising from active areas rotating differentially with respect to each other because they are at different latitudes, or the growth and decay of active areas with subsequent birth of active areas occurring at a stellar longitude different from the original site of the activity. Generally, the data from only one season cannot discriminate between these two explanations of dual peaks in the power spectra. In four stars, however, differential surface rotation is a more likely explanation for the observed chromospheric fluctuations with time during the first three seasons. The fractional differential surface rotation would be at least 5% in HD 206860, 10% in HD 101501, 11% in HD 190406 and 21% in HD 114710. The analysis of the data for the GO V star HD 206860, with a relatively rapid rotation period of about 5 days, indicates an active area persisting for three years.