  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.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. 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 log R'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. 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 log 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. 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.
