Rotation and binary interaction play important roles in understanding the nature of massive stars ( \gtrsim 8 M _ { \odot } ) .
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 \mbox { { $v _ { \textrm { e } } \sin { i } $ } } \sim 540 km s ^ { -1 } , which is \sim 100 km s ^ { -1 } 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 \lambda 4471 absorption line is blueshifted and asymmetric , while its He II \lambda 4686 and H \alpha 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 \mbox { { $v _ { \textrm { e } } \sin { i } $ } } \sim 610 km s ^ { -1 } ) 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 \sim 13.9 mag ) enough to allow future high-resolution spectroscopic follow-ups .