Extended main sequence turn-off originating from a broad range of stellar rotational velocities

Star clusters have long been considered to comprise a simple stellar population, but this paradigm is being challenged, since in addition to multiple populations in Galactic globular clusters, a number of younger star clusters exhibit a significant colour spread at the main sequence turn-off. A sequential evolution of multiple generations of stars formed over 100-200 Myr is a natural explanation of this colour spread. Another approach to explain this feature is to introduce the effect of stellar rotation. However, its effectiveness has not yet been proven due to the lack of direct measurements of rotational velocities. Here, we report the distribution of projected rotational velocities (Veqsin i) of stars in the Galactic open cluster M11, measured by Fourier transform analysis. Cluster members display a broad Veqsin i distribution, and fast rotators, including Be stars, have redder colours than slow rotators. Monte Carlo simulations infer that cluster members have highly aligned spin axes and a broad distribution of equatorial velocities biased towards high velocities. Our synthetic cluster simulation further demonstrates how stellar rotation affects the colours of cluster members, suggesting that the colour spread observed in populous clusters can be understood in the context of stellar evolution without introducing multiple stellar populations