On the Origin of the Bimodal Rotational Velocity Distribution in Stellar
  Clusters: Rotation on the Pre-Main Sequence
  We address the origin of the observed bimodal rotational distribution of
stars in massive young and intermediate age stellar clusters. This bimodality
is seen as split main sequences at young ages and also has been recently
directly observed in the $Vsini$ distribution of stars within massive young and
intermediate age clusters. Previous models have invoked binary interactions as
the origin of this bimodality, although these models are unable to reproduce
all of the observational constraints on the problem. Here we suggest that such
a bimodal rotational distribution is set up early within a cluster's life,
i.e., within the first few Myr. Observations show that the period distribution
of low-mass ($\la 2 M_\odot$) pre-main sequence (PMS) stars is bimodal in many
young open clusters and we present a series of models to show that if such a
bimodality exists for stars on the PMS that it is expected to manifest as a
bimodal rotational velocity (at fixed mass/luminosity) on the main sequence for
stars with masses in excess of $\sim1.5$~\msun. Such a bimodal period
distribution of PMS stars may be caused by whether stars have lost (rapid
rotators) or been able to retain (slow rotators) their circumstellar discs
throughout their PMS lifetimes. We conclude with a series of predictions for
observables based on our model.
