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Context. Chromospheric activity monitoring of a wide range of cool stars can provide valuable information on stellar magnetic activity and its dependence on fundamental stellar parameters such as effective temperature and rotation.
Aims. We compile a chromospheric activity catalogue of 4454 cool stars from a combination of archival HARPS spectra and multiple other surveys, including the Mount Wilson data that have recently been released by the NSO. We explore the variation in chromospheric activity of cool stars along the main sequence for stars with different effective temperatures. Additionally, we also perform an activity-cycle period search and investigate its relation with rotation.
Methods. The chromospheric activity index, S-index, was measured for 304 main-sequence stars from archived high-resolution HARPS spectra. Additionally, the measured and archived S-indices were converted into the chromospheric flux ratio logR'_HK. The activity-cycle periods were determined using the generalised Lomb-Scargle periodogram to study the active and inactive branches on the rotation - activity-cycle period plane.
Results. The global sample shows that the bimodality of chromospheric activity, known as the Vaughan-Preston gap, is not prominent, with a significant percentage of the stars at an intermediate-activity level around R'_HK=-4.75. Independently, the cycle period search shows that stars can lie in the region intermediate between the active and inactive branch, which means that the active branch is not as clearly distinct as previously thought.
Conclusions. The weakening of the Vaughan-Preston gap indicates that cool stars spin down from a higher activity level and settle at a lower activity level without a sudden break at intermediate activity. Some cycle periods are close to the solar value between the active and inactive branch, which suggests that the solar dynamo is most likely a common case of the stellar dynamo.