What produces the spin spread behind extended cluster turnoffs?
In plain words
In star clusters a few hundred million to two billion years old, the stars that are just exhausting their core hydrogen do not lie on one thin line in the color-brightness diagram. This is now mostly blamed on stars spinning at different rates, but why some stars spin slowly and others fast is unclear.
Precise statement
Intermediate-age and young clusters in the Magellanic Clouds and the Milky Way show extended main-sequence turnoffs and split main sequences (first reported for NGC 1846, Mackey and Broby Nielsen 2007), attributed mainly to a spread in stellar rotation rather than an age spread. Determine the origin of the bimodal rotation distribution (initial conditions set during star formation, tidal braking in binaries, or mergers producing slow rotators) and whether any residual age spread of order $1e8\,\mathrm{yr}$ is required.
What would settle it
Spectroscopic rotation velocities and binary fractions for complete samples of turnoff stars in several clusters, compared with population models that include binary evolution and rotation.
Status in the literature
Spectroscopic studies since about 2017 link turnoff position to measured rotation; the cause of the slow-rotating population remains unsettled.