The open cluster NGC 3532: A new benchmark cluster to understand the transition from fast to slow rotation

A very rich cluster intermediate in age between the Pleiades (150 Myr) and the Hyades (600 Myr) is needed to probe the rotational evolution, especially the transition between fast and slow rotation that occurs between the two ages.

To fill this gap, we present unique rotation periods and stellar activity measurements for members of the 300 Myr-old open cluster NGC 3532. Our observations probe directly for the first time the evolution from fast to slow rotation at this key age.

Both the colour-period and the colour-activity diagram show well populated and structured sequences, enabling a one-to-one match of the stars in each diagram. Beyond, we predict rotation periods from the activity measurement and recover them from noisy light curves, highlighting the synergies created by combining light curves, rotation periods, and activity measurements. Through activity measurement, we are able to identify the extended slow rotator sequence extending to P \approx 32d. Their spin-down is comparable to earlier-type slow rotators as a comparison to younger open clusters shows. Our activity measurements also confirms that all fast rotators of near-solar mass (F-G type) 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 identify several short-comings in rotational evolution models. Amongst others, stars on the evolved fast rotator sequence are not spun down strong enough and our observations suggest a significant shorter crossing time for the rotational gap.

The joint analysis of chromospheric activity and photometric time series thus enables comprehensive insights into the evolution of the rotation and activity of stars during the transitional phase between the Pleiades and Hyades ages. The rapid, yet well constrained evolution from fast to slow rotation allows for accurate age-ranking of open clusters in between these ages.