Why do stars rotate the way they do? Nature or Nurture? Is their rotation rate set by their birth conditions or is it the result from evolutionary effects? 
For single stars their rotation rate remain almost unchanged as they evolve along the main sequence, apart from the small effects of stellar winds, expansion and changes in their moment of inertia. In contrast, a star in a binary system may readily be spun up to very high rotation rates due to tides, mass transfer or even a merger.
We investigate the effect of binaries on the distribution of rotation rates of massive stars and conclude for a population characterized by continuous star formation 1 out of 5 massive stars have high projected rotational velocities as a result of binary interaction.
This fraction is similar to what is observed, implies that spin down during star formation is even more efficient than previously thought and raises questions about the interpretation of the surface abundances of rapidly rotating stars as evidence for rotational mixing. Furthermore, our results allow for the possibility that all early-type Be stars result from binary interactions and suggest that evidence for rotation in explosions, such as long gamma-ray bursts, points to a binary origin.
This paper, “The Rotation Rates of Massive Stars: The Role of Binary Interaction through Tides, Mass Transfer, and Mergers”, by de Mink, Langer, Izzard & Sana has just been published in ApJ 764, 166
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