A detailed understanding of the rotation-activity relationship using the
  300 Myr old open cluster NGC 3532
  The coeval stars of young open clusters provide insights into the formation
of the rotation-activity relationship that elude studies of multi-age field
populations. We measure the chromospheric activity of cool stars in the 300 Myr
old open cluster NGC 3532 in concert with their rotation periods to study the
mass-dependent morphology of activity for this transitional coeval population.
Using multi-object spectra of the Ca ii infrared triplet region obtained with
the AAOmega spectrograph at the Anglo- Australian Telescope, we measure the
chromospheric emission ratios for 454 FGKM cluster members of NGC 3532. The
morphology of activity against colour appears to be a near-mirror image of the
cluster's rotational behaviour. In particular, we identify a group of
'desaturated transitional rotators' that branches off from the main group of
unsaturated FGK slow rotators, and from which it is separated by an 'activity
gap'. The few desaturated gap stars are identical to the ones in the rotational
gap. Nevertheless, the rotation-activity diagram is completely normal. In fact,
the relationship is so tight that it allows us to predict rotation periods for
many additional stars. We then precisely determine these periods from our
photometric light curves. Our activity measurements show that all fast rotators
of near-solar mass have evolved to become slow rotators, demonstrating that the
absence of fast rotators in a colour-period diagram is not a detection issue
but an astrophysical fact. We also identify a new population of low-activity
stars among the early M dwarfs, enabling us to populate the extended slow
rotator sequence in the colour-period diagram. The joint analysis of
chromospheric activity and photometric time series data thus enables
comprehensive insights into the evolution of the rotation and activity of stars
during the transitional phase between the Pleiades and Hyades age. (Abridged)
