  Rotation and binary interaction play important roles in understanding the
nature of massive stars (>8 Msun). Some interesting transients, such as the
long-duration gamma-ray bursts, are thought to be originated from fast-rotating
massive stars. Because the strong stellar wind can effectively spin down a
metal-rich massive star with fast rotation, it is very hard to find single
massive stars rotating critically in the Galaxy. In the present work reported
is the discovery of the fastest rotator in the Galaxy, LAMOST
J040643.69+542347.8, with a projected rotational velocity vsini ~540 km/s,
which is ~ 100 km/s faster than that of the previous record holder HD 191423.
The star has a spectral type of O6.5 Vnnn(f)p. Its He I 4471 absorption line is
blueshifted and asymmetric, while its He II 4686 and Ha have central absorption
reversals in their emissions. It is also a runaway star, which implies an
origin in a close binary interaction. Compared to VFTS 285 and VFTS 102 (their
vsini ~ 610 km/s) in the Large Magellanic Cloud, LAMOST J040643.69+542347.8 has
its own peculiar spectral characteristics and earlier spectral type. Moreover,
LAMOST J040643.69+542347.8 is bright (B ~ 13.9 mag) enough to allow future
high-resolution spectroscopic follow-ups.
