Exploring the stellar rotation of early-type stars in the LAMOST
  Medium-Resolution Survey. II. Statistics
  Angular momentum is a key property regulating star formation and evolution.
However, the physics driving the distribution of the stellar rotation rates of
early-type main-sequence stars is as yet poorly understood. Using our catalog
of 40,034 early-type stars with homogeneous $v\sin i$ parameters, we review the
statistical properties of their stellar rotation rates. We discuss the
importance of possible contaminants, including binaries and chemically peculiar
stars. Upon correction for projection effects and rectification of the error
distribution, we derive the distributions of our sample's equatorial rotation
velocities, which show a clear dependence on stellar mass. Stars with masses
less than $2.5\ {M_\odot}$ exhibit a unimodal distribution, with the peak
velocity ratio increasing as stellar mass increases. A bimodal rotation
distribution, composed of two branches of slowly and rapidly rotating stars,
emerges for more massive stars ($M>2.5\ {M_\odot}$). For stars more massive
than $3.0\ {M_\odot}$, the gap between the bifurcated branches becomes
prominent. For the first time, we find that metal-poor ([M/H] $< -0.2$ dex)
stars only exhibit a single branch of slow rotators, while metal-rich ([M/H] $>
0.2$ dex) stars clearly show two branches. The difference could be attributed
to unexpectedly high spin-down rates and/or in part strong magnetic fields in
the metal-poor subsample.
