Binary-driven stellar rotation evolution at the main-sequence turn-off in star clusters

ABSTRACT The impact of stellar rotation on the morphology star cluster colour–magnitude diagrams is widely acknowledged. However, physics driving distribution equatorial velocities main-sequence turn-off stars as yet poorly understood. Using Gaia Data Release 2 photometry and new Southern African Large Telescope medium-resolution spectroscopy, we analyse intermediate-age ($\sim 1\text{-}{\rm Gyr}$-old) Galactic open clusters NGC 3960, 6134, IC 4756 develop a novel method to derive their distributions based SYCLIST models. Combined with literature data for 5822 2818, find tight correlation between number ratio slow rotators clusters’ binary fractions. blue-main-sequence in at least two our are more centrally concentrated than red-main-sequence counterparts. origin its evolution remains unidentified. observed sample suggests binary-driven formation mechanism.