Effects of close binary evolution on the main-sequence morphology of
  young star clusters
  Star clusters are the building blocks of galaxies. They are composed of stars
of nearly equal age and chemical composition, allowing us to use them as
chronometers and as testbeds for gauging stellar evolution. It has become clear
recently that massive stars are formed preferentially in close binaries, in
which mass transfer will drastically change the evolution of the stars. This is
expected to leave a significant imprint in the distribution of cluster stars in
the Hertzsprung-Russell diagram. Our results, based on a dense model grid of
more than 50,000 detailed binary-evolution calculations, indeed show several
distinct, coeval main-sequence (MS) components, most notably an extended MS
turnoff region, and a group of near-critical rotating stars that is spread over
a large luminosity range on the red side of the classical MS. We
comprehensively demonstrate the time evolution of the features in an animation,
and we derive analytic expressions to describe these features. We find
quantitative agreement with results based on recent photometric and
spectroscopic observations. We conclude that while other factors may also be at
play, binary evolution has a major impact on the MS morphology of young star
clusters.
