Stellar mergers as the origin of the blue main-sequence band in young
  star clusters
  Recent high-quality Hubble Space Telescope (HST) photometry shows that the
main sequences (MS) stars of young star clusters form two discrete components
in the color-magnitude diagram (CMD). Based on their distribution in the CMD,
we show that stars of the blue MS component can be understood as slow rotators
originating from stellar mergers. We derive the masses of the blue MS stars,
and find that they follow a nearly flat mass function, which supports their
unusual formation path. Our results imply that the cluster stars gain their
mass in two different ways, by disk accretion leading to rapid rotation,
contributing to the red MS, or by binary merger leading to slow rotation and
populating the blue MS. We also derive the approximate merger time of the
individual stars of the blue MS component, and find a strong early peak in the
merger rate, with a lower level merger activity prevailing for tens of Myr.
This supports recent binary formation models, and explains new velocity
dispersion measurements for members of young star clusters. Our findings shed
new light on the origin of the bi-modal mass, spin, and magnetic field
distributions of main-sequence stars.
